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The U.S. Natural Resource Conservation Service (NRCS) classifies soils into four hydrologic groups based on infiltration characteristics of the soils. NRCS Soil Survey Staff (1996) defines a hydrologic group as a group of soils having similar runoff potential under similar storm and cover conditions. Soil properties that influence runoff potential are those that impact the minimum rate of infiltration for a bare soil after prolonged wetting and when not frozen.
These properties are depth to seasonally high water table, saturated hydraulic conductivity, and depth to a very slowly permeable layer.
Soil may be placed in one of four groups, A, B, C, and D, or three dual classes, A/D, B/D, and C/D.
Definitions of the classes are:
A: (Low runoff potential). The soils have a high infiltration rate even when thoroughly wetted. They chiefly consist of deep, well drained to excessively drained sands or gravels. They have a high rate of water transmission.
B: The soils have a moderate infiltration rate when thoroughly wetted. They chiefly are moderately deep to deep, moderately well-drained to well-drained soils that have moderately fine to moderately coarse textures. They have a moderate rate of water transmission.
C: The soils have a slow infiltration rate when thoroughly wetted. They chiefly have a layer that impedes downward movement of water or have moderately fine to fine texture. They have a slow rate of water transmission.
D. (High runoff potential). The soils have a very slow infiltration rate when thoroughly wetted. They chiefly consist of clay soils that have a high swelling potential, soils that have a permanent water table, soils that have a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material. They have a very slow rate of water transmission.
Dual hydrologic groups are given for certain wet soils that can be adequately drained. The first letter applies to the drained condition, the second to the undrained. Only soils that are rated D in their natural condition are assigned to dual classes.
Surface runoff occurs whenever the rate of water application to the ground surface exceeds the rate of infiltration. When water is initially applied to a dry soil, the infiltration rate is usually very high. However, it will decrease as the soil becomes wetter. When the application rate is higher than the infiltration rate, surface depressions begin to fill. If the application rate continues to be higher than the infiltration rate once all surface depressions have filled, surface runoff will commence.
SWAT+ provides two methods for estimating surface runoff: the SCS curve number procedure (SCS, 1972) and the Green & Ampt infiltration method (1911).
The SCS runoff equation is an empirical model that came into common use in the 1950s. It was the product of more than 20 years of studies involving rainfall-runoff relationships from small rural watersheds across the U.S. The model was developed to provide a consistent basis for estimating the amounts of runoff under varying land use and soil types (Rallison and Miller, 1981).
The SCS curve number equation is (SCS, 1972):
2:1.1.1
where is the accumulated runoff or rainfall excess (mm HO), is the rainfall depth for the day (mm HO), is the initial abstractions which includes surface storage, interception and infiltration prior to runoff (mm HO), and is the retention parameter (mm HO). The retention parameter varies spatially due to changes in soils, land use, management and slope and temporally due to changes in soil water content. The retention parameter is defined as:
2:1.1.2
where is the curve number for the day. The initial abstractions, , is commonly approximated as and equation 2:1.1.1 becomes
2:1.1.3
Runoff will only occur when . A graphical solution of equation 2:1.1.3 for different curve number values is presented in Figure 2:1-1.
SCS defines three antecedent moisture conditions:
I—dry (wilting point), II—average moisture, and III—wet (field capacity). The moisture condition I curve number is the lowest value the daily curve number can assume in dry conditions. The curve numbers for moisture conditions I and III are calculated with the equations:
2:1.1.4
2:1.1.5
where is the moisture condition I curve number, is the moisture condition II curve number, and is the moisture condition III curve number.
The moisture condition II curve numbers provided in the tables are assumed to be appropriate for 5% slopes. Williams (1995) developed an equation to adjust the curve number to a different slope:
2:1.1.12
where is the moisture condition II curve number adjusted for slope, is the moisture condition III curve number for the default 5% slope, is the moisture condition II curve number for the default 5% slope, and is the average fraction slope of the subbasin. SWAT+ does not adjust curve numbers for slope. If the user wishes to adjust the curve numbers for slope effects, the adjustment must be done prior to entering the curve numbers in the management input file.
Table 2:1-1: SWAT+ input variables that pertain to surface runoff calculated with the SCS curve number method.
Variable Name | Definition | Input File |
---|---|---|
The Green & Ampt equation was developed to predict infiltration assuming excess water at the surface at all times (Green & Ampt, 1911). The equation assumes that the soil profile is homogenous and antecedent moisture is uniformly distributed in the profile. As water infiltrates into the soil, the model assumes the soil above the wetting front is completely saturated and there is a sharp break in moisture content at the wetting front. Figure 2:1-2 graphically illustrates the difference between the moisture distribution with depth modeled by the Green & Ampt equation and what occurs in reality.
Mein and Larson (1973) developed a methodology for determining ponding time with infiltration using the Green & Ampt equation. The Green-Ampt Mein-Larson excess rainfall method was incorporated into SWAT+ to provide an alternative option for determining surface runoff. This method requires sub-daily precipitation data supplied by the user.
The Green-Ampt Mein-Larson infiltration rate is defined as:
When the rainfall intensity is less than the infiltration rate, all the rainfall will infiltrate during the time period and the cumulative infiltration for that time period is calculated:
2:1.2.5
The change in volumetric moisture content across the wetting front is calculated at the beginning of each day as:
For each time step, SWAT+ calculates the amount of water entering the soil. The water that does not infiltrate into the soil becomes surface runoff.
Table 2:1-2: SWAT+ input variables that pertain to Green & Ampt infiltration calculations.
The peak runoff rate is the maximum runoff flow rate that occurs with a given rainfall event. The peak runoff rate is an indicator of the erosive power of a storm and is used to predict sediment loss. SWAT+ calculates the peak runoff rate with a modified rational method.
The rational method is widely used in the design of ditches, channels and storm water control systems. The rational method is based on the assumption that if a rainfall of intensity begins at time and continues indefinitely, the rate of runoff will increase until the time of concentration, , when the entire subbasin area* is contributing to flow at the outlet. The rational formula is:
2:1.3.1
where is the peak runoff rate (), is the runoff coefficient, is the rainfall intensity (mm/hr), Area is the subbasin area (km) and 3.6 is a unit conversion factor.
The SCS curve number is a function of the soil’s permeability, land use and antecedent soil water conditions. Typical curve numbers for moisture condition II are listed in tables 2:1-1, 2:1-2 and 2:1-3 for various land covers and soil types (SCS Engineering Division, 1986). These values are appropriate for a 5% slope.
Table 2:1-1: Runoff curve numbers for cultivated agricultural lands (from SCS Engineering Division, 1986)
[1] Poor: < 50% ground cover or heavily grazed with no mulch; Fair: 50 to 75% ground cover and not heavily grazed; Good: > 75% ground cover and lightly or only occasionally grazed
[2] Poor: < 50% ground cover; Fair: 50 to 75% ground cover; Good: > 75% ground cover
[3] Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning; Fair: Woods are grazed but not burned, and some forest litter covers the soil; Good: Woods are protected from grazing, and litter and brush adequately cover the soil.
[1] Poor: < 50% ground cover or heavily grazed with no mulch; Fair: 50 to 75% ground cover and not heavily grazed; Good: > 75% ground cover and lightly or only occasionally grazed
[2] Poor: < 50% ground cover; Fair: 50 to 75% ground cover; Good: > 75% ground cover
[3] Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning; Fair: Woods are grazed but not burned, and some forest litter covers the soil; Good: Woods are protected from grazing, and litter and brush adequately cover the soil.
[2] Poor: < 50% ground cover or heavily grazed with no mulch;
[3] Fair: 50 to 75% ground cover and not heavily grazed;
[4] Good: > 75% ground cover and lightly or only occasionally grazed
The runoff coefficient is the ratio of the inflow rate, , to the peak discharge rate, . The coefficient will vary from storm to storm and is calculated with the equation:
2:1.3.15
where is the surface runoff (mm HO) and is the rainfall for the day (mm HO).
2:1.2.1
where is the infiltration rate at time (mm/hr), is the effective hydraulic conductivity (mm/hr), is the wetting front matric potential (mm), is the change in volumetric moisture content across the wetting front (mm/mm) and is the cumulative infiltration at time (mm HO).
2:1.2.2
where is the cumulative infiltration for a given time step (mm HO), is the cumulative infiltration for the previous time step(mm HO), and is the amount of rain falling during the time step (mm HO).
The infiltration rate defined by equation 2:1.2.1 is a function of the infiltrated volume, which in turn is a function of the infiltration rates in previous time steps. To avoid numerical errors over long time steps, is replaced by in equation 2:1.2.1 and integrated to obtain
2:1.2.3
Equation 2:1.2.3 must be solved iteratively for , the cumulative infiltration at the end of the time step. A successive substitution technique is used.
The Green-Ampt effective hydraulic conductivity parameter, , is approximately equivalent to one-half the saturated hydraulic conductivity of the soil, (Bouwer, 1969). Nearing et al. (1996) developed an equation to calculate the effective hydraulic conductivity as a function of saturated hydraulic conductivity and curve number. This equation incorporates land cover impacts into the calculated effective hydraulic conductivity. The equation for effective hydraulic conductivity is:
2:1.2.4
where is the effective hydraulic conductivity (mm/hr), is the saturated hydraulic conductivity (mm/hr), and is the curve number.
Wetting front matric potential, , is calculated as a function of porosity, percent sand and percent clay (Rawls and Brakensiek, 1985):
where is the porosity of the soil (mm/mm), is the percent clay content, and is the percent sand content.
2:1.2.6
where is the change in volumetric moisture content across the wetting front (mm/mm), is the soil water content of the entire profile excluding the amount of water held in the profile at wilting point (mm HO), is the amount of water in the soil profile at field capacity (mm HO), and is the porosity of the soil (mm/mm). If a rainfall event is in progress at midnight, is then calculated:
2:1.2.7
Variable Name | Definition | Input File |
---|
IEVENT
Rainfall, runoff, routing option.
.bsn
ICN
Daily curve number calculation method: 0 calculate daily CN value as a function of soil moisture; 1 calculate daily CN value as a function of plant evapotranspiration
.bsn
CNCOEF
: Weighting coefficient used to calculate the retention coefficient for daily curve number calculations dependent on plant evapotranspiration
.bsn
PRECIPITATION
: Daily precipitation (mm HO)
.pcp
CN2
: Moisture condition II curve number
.mgt
CNOP
: Moisture condition II curve number
.mgt
With SWAT+, users are allowed to select between two methods for calculating the retention parameter. The traditional method is to allow the retention parameter to vary with soil profile water content. An alternative added in SWAT+ allows the retention parameter to vary with accumulated plant evapotranspiration. Calculation of the daily CN value as a function of plant evapotranspiration was added because the soil moisture method was predicting too much runoff in shallow soils. By calculating daily CN as a function of plant evapotranspiration, the value is less dependent on soil storage and more dependent on antecedent climate.
When the retention parameter varies with soil profile water content, the following equation is used:
2:1.1.6
where is the retention parameter for a given day (mm), is the maximum value the retention parameter can achieve on any given day (mm), is the soil water content of the entire profile excluding the amount of water held in the profile at wilting point (mm HO), and and are shape coefficients. The maximum retention parameter value, , is calculated by solving equation 2:1.1.2 using .
The shape coefficients are determined by solving equation 2:1.1.6 assuming that
the retention parameter for moisture condition I curve number corresponds to wilting point soil profile water content,
the retention parameter for moisture condition III curve number corresponds to field capacity soil profile water content, and
the soil has a curve number of 99 (S = 2.54) when completely saturated.
2.1.1.7
2.1.1.8
where is the first shape coefficient, is the second shape coefficient, is the amount of water in the soil profile at field capacity (mm HO), is the retention parameter for the moisture condition III curve number, is the retention parameter for the moisture condition I curve number, is the amount of water in the soil profile when completely saturated (mm HO), and 2.54 is the retention parameter value for a curve number of 99.
When the retention parameter varies with plant evapotranspiration, the following equation is used to update the retention parameter at the end of every day:
2:1.1.9
where is the retention parameter for a given day (mm), is the retention parameter for the previous day (mm), is the potential evapotranspiration for the day (mm d), is the weighting coefficient used to calculate the retention coefficient for daily curve number calculations dependent on plant evapotranspiration, is the maximum value the retention parameter can achieve on any given day (mm), Rday is the rainfall depth for the day (mm HO), and is the surface runoff (mm HO). The initial value of the retention parameter is defined as
When the top layer of the soil is frozen, the retention parameter is modified using the following equation:
2:1.1.10
where is the retention parameter adjusted for frozen conditions (mm), is the maximum value the retention parameter can achieve on any given day (mm), and is the retention parameter for a given moisture content calculated with equation 2:1.1.6 (mm).
The daily curve number value adjusted for moisture content is calculated by rearranging equation 2:1.1.2 and inserting the retention parameter calculated for that moisture content:
2:1.1.11
where is the curve number on a given day and is the retention parameter calculated for the moisture content of the soil on that day.
The time of concentration is the amount of time from the beginning of a rainfall event until the entire subbasin area is contributing to flow at the outlet. In other words, the time of concentration is the time for a drop of water to flow from the remotest point in the subbasin to the subbasin outlet. The time of concentration is calculated by summing the overland flow time (the time it takes for flow from the remotest point in the subbasin to reach the channel) and the channel flow time (the time it takes for flow in the upstream channels to reach the outlet):
2:1.3.2
where is the time of concentration for a subbasin (hr), is the time of concentration for overland flow (hr), and is the time of concentration for channel flow (hr).
The rainfall intensity is the average rainfall rate during the time of concentration. Based on this definition, it can be calculated with the equation:
2:1.3.16
where is the rainfall intensity (mm/hr), is the amount of rain falling during the time of concentration (mm HO), and is the time of concentration for the subbasin (hr).
An analysis of rainfall data collected by Hershfield (1961) for different durations and frequencies showed that the amount of rain falling during the time of concentration was proportional to the amount of rain falling during the 24-hr period.
2:1.3.17
where is the amount of rain falling during the time of concentration (mm HO), is the fraction of daily rainfall that occurs during the time of concentration, and is the amount of rain falling during the day (mm HO).
For short duration storms, all or most of the rain will fall during the time of concentration, causing to approach its upper limit of 1.0. The minimum value of would be seen in storms of uniform intensity (). This minimum value can be defined by substituting the products of time and rainfall intensity into equation 2:1.3.17
2:1.3.18
Thus, falls in the range
SWAT+ estimates the fraction of rain falling in the time of concentration as a function of the fraction of daily rain falling in the half-hour of highest intensity rainfall.
2:1.3.19
where is the fraction of daily rain falling in the half-hour highest intensity rainfall, and is the time of concentration for the subbasin (hr). The determination of a value for is discussed in Chapters 1:2 and 1:3.
Potential evapotranspiration (PET) was a concept originally introduced by Thornthwaite (1948) as part of a climate classification scheme. He defined PET is the rate at which evapotranspiration would occur from a large area uniformly covered with growing vegetation that has access to an unlimited supply of soil water and that was not exposed to advection or heat storage effects. Because the evapotranspiration rate is strongly influenced by a number of vegetative surface characteristics, Penman (1956) redefined PET as “the amount of water transpired ... by a short green crop, completely shading the ground, of uniform height and never short of water”. Penman used grass as his reference crop, but later researchers (Jensen et al., 1990) have suggested that alfalfa at a height of 30 to 50 cm may be a more appropriate choice.
Numerous methods have been developed to estimate PET. Three of these methods have been incorporated into SWAT+: the Penman-Monteith method (Monteith, 1965; Allen, 1986; Allen et al., 1989), the Priestley-Taylor method (Priestley and Taylor, 1972) and the Hargreaves method (Hargreaves et al., 1985). The model will also read in daily PET values if the user prefers to apply a different potential evapotranspiration method.
The three PET methods included in SWAT+ vary in the amount of required inputs. The Penman-Monteith method requires solar radiation, air temperature, relative humidity and wind speed. The Priestley-Taylor method requires solar radiation, air temperature and relative humidity. The Hargreaves method requires air temperature only.
Studies in canopy resistance have shown that the canopy resistance for a well-watered reference crop can be estimated by dividing the minimum surface resistance for a single leaf by one-half of the canopy leaf area index (Jensen et al., 1990):
2:2.2.8
where is the canopy resistance (s m), is the minimum effective stomatal resistance of a single leaf (s m), and is the leaf area index of the canopy.
The distribution of stomates on a plant leaf will vary between species. Typically, stomates are distributed unequally on the top and bottom of plant leaves. Plants with stomates located on only one side are classified as hypostomatous while plants with an equal number of stomates on both sides of the leaf are termed amphistomatous. The effective leaf stomatal resistance is determined by considering the stomatal resistance of the top (adaxial) and bottom (abaxial) sides to be connected in parallel (Rosenburg et al., 1983). When there are unequal numbers of stomates on the top and bottom, the effective stomatal resistance is calculated:
2:2.2.9
where is the minimum effective stomatal resistance of a single leaf (s m), is the minimum adaxial stomatal leaf resistance (s m), and is the minimum abaxial stomatal leaf resistance (s m). For amphistomatous leaves, the effective stomatal resistance is:
2:2.2.10
For hypostomatous leaves the effective stomatal resistance is:
2:2.2.11
Leaf conductance is defined as the inverse of the leaf resistance:
2:2.2.12
where is the maximum effective leaf conductance (m s). When the canopy resistance is expressed as a function of leaf conductance instead of leaf resistance, equation 2:2.2.8 becomes:
2:2.2.13
where is the canopy resistance (s m), is the maximum conductance of a single leaf (m s), and is the leaf area index of the canopy.
For climate change simulations, the canopy resistance term can be modified to reflect the impact of change in CO concentration on leaf conductance. The influence of increasing CO concentrations on leaf conductance was reviewed by Morison (1987). Morison found that at CO concentrations between 330 and 660 ppmv, a doubling in CO concentration resulted in a 40% reduction in leaf conductance. Within the specified range, the reduction in conductance is linear (Morison and Gifford, 1983). Easterling et al. (1992) proposed the following modification to the leaf conductance term for simulating carbon dioxide concentration effects on evapotranspiration:
2:2.2.14
where is the leaf conductance modified to reflect CO effects (m s) and CO is the concentration of carbon dioxide in the atmosphere (ppmv).
Incorporating this modification into equation 2:2.2.8 gives
2:2.2.15
SWAT+ will default the value of CO concentration to 330 ppmv if no value is entered by the user. With this default, the term reduces to 1.0 and the canopy resistance equation becomes equation 2:2.2.8.
When calculating actual evapotranspiration, the canopy resistance term is modified to reflect the impact of high vapor pressure deficit on leaf conductance (Stockle et al, 1992). For a plant species, a threshold vapor pressure deficit is defined at which the plant’s leaf conductance begins to drop in response to the vapor pressure deficit. The adjusted leaf conductance is calculated:
if 2:2.2.16
if 2:2.2.17
where is the conductance of a single leaf (m s), is the maximum conductance of a single leaf (m s), is the rate of decline in leaf conductance per unit increase in vapor pressure deficit (m s kPa), is the vapor pressure deficit (kPa), and is the threshold vapor pressure deficit above which a plant will exhibit reduced leaf conductance (kPa). The rate of decline in leaf conductance per unit increase in vapor pressure deficit is calculated by solving equation 2:2.2.16 using measured values for stomatal conductance at two different vapor pressure deficits:
2:2.2.18
where is the rate of decline in leaf conductance per unit increase in vapor pressure deficit (m s kPa), is the fraction of the maximum stomatal conductance, , achieved at the vapor pressure deficit , and is the threshold vapor pressure deficit above which a plant will exhibit reduced leaf conductance (kPa). The threshold vapor pressure deficit is assumed to be 1.0 kPa for all plant species.
For wind speed in m s, Jensen et al. (1990) provided the following relationship to calculate :
2:2.2.19
where is the mean air temperature for the day (˚C).
To calculate potential evapotranspiration, the Penman-Monteith equation must be solved for a reference crop. SWAT+ uses alfalfa at a height of 40 cm with a minimum leaf resistance of 100 s m for the reference crop. Using this canopy height, the equation for aerodynamic resistance (2:2.2.3) simplifies to:
2:2.2.20
The equation for canopy resistance requires the leaf area index. The leaf area index for the reference crop is estimated using an equation developed by Allen et al. (1989) to calculate as a function of canopy height. For nonclipped grass and alfalfa greater than 3 cm in height:
2:2.2.21
where is the leaf area index and is the canopy height (cm). For alfalfa with a 40 cm canopy height, the leaf area index is 4.1. Using this value, the equation for canopy resistance simplifies to:
2:2.2.22
The most accurate estimates of evapotranspiration with the Penman-Monteith equation are made when evapotranspiration is calculated on an hourly basis and summed to obtain the daily values. Mean daily parameter values have been shown to provide reliable estimates of daily evapotranspiration values and this is the approach used in SWAT+. However, the user should be aware that calculating evapotranspiration with the Penman-Monteith equation using mean daily values can potentially lead to significant errors. These errors result from diurnal distributions of wind speed, humidity, and net radiation that in combination create conditions which the daily averages do not replicate.
The Penman-Monteith equation combines components that account for energy needed to sustain evaporation, the strength of the mechanism required to remove the water vapor and aerodynamic and surface resistance terms. The Penman-Monteith equation is:
2:2.2.1
where is the latent heat flux density (MJ m d), is the depth rate evaporation (mm d), is the slope of the saturation vapor pressure-temperature curve, (kPa ˚C), is the net radiation (MJ m d), is the heat flux density to the ground (MJ m d), is the air density (kg m), is the specific heat at constant pressure (MJ kg ˚C), is the saturation vapor pressure of air at height (kPa), is the water vapor pressure of air at height (kPa), is the psychrometric constant (kPa ˚C), is the plant canopy resistance (s m), and is the diffusion resistance of the air layer (aerodynamic resistance) (s m).
For well-watered plants under neutral atmospheric stability and assuming logarithmic wind profiles, the Penman-Monteith equation may be written (Jensen et al., 1990):
2:2.2.2
where is the latent heat of vaporization (MJ kg), is the maximum transpiration rate (mm d), is a dimension coefficient needed to ensure the two terms in the numerator have the same units (for in m s, = 8.64 x 104), and is the atmospheric pressure (kPa).
The calculation of net radiation, , is reviewed in Chapter 1:1. The calculations for the latent heat of vaporization, , the slope of the saturation vapor pressure-temperature curve, , the psychrometric constant, , and the saturation and actual vapor pressure, and , are reviewed in Chapter 1:2. The remaining undefined terms are the soil heat flux, , the combined term , the aerodynamic resistance, , and the canopy resistance, .
Soil heat storage or release can be significant over a few hours, but is usually small from day to day because heat stored as the soil warms early in the day is lost when the soil cools late in the day or at night. Since the magnitude of daily soil heat flux over a 10- to 30-day period is small when the soil is under a crop cover, it can normally be ignored for most energy balance estimates. SWAT+ assumes the daily soil heat flux, , is always equal to zero.
Once total potential evapotranspiration is determined, actual evaporation must be calculated. SWAT+ first evaporates any rainfall intercepted by the plant canopy. Next, SWAT+ calculates the maximum amount of transpiration and the maximum amount of sublimation/soil evaporation using an approach similar to that of Ritchie (1972). The actual amount of sublimation and evaporation from the soil is then calculated. If snow is present in the HRU, sublimation will occur. Only when no snow is present will evaporation from the soil take place.
Evapotranspiration is a collective term that includes all processes by which water at the earth’s surface is converted to water vapor. It includes evaporation from the plant canopy, transpiration, sublimation and evaporation from the soil.
Evapotranspiration is the primary mechanism by which water is removed from a watershed. Roughly 62% of the precipitation that falls on the continents is evapotranspired. Evapotranspiration exceeds runoff in most river basins and on all continents except Antarctica (Dingman, 1994).
The difference between precipitation and evapotranspiration is the water available for human use and management. An accurate estimation of evapotranspiration is critical in the assessment of water resources and the impact of climate and land use change on those resources.
Water that enters the soil may move along one of several different pathways. The water may be removed from the soil by plant uptake or evaporation. It can percolate past the bottom of the soil profile and ultimately become aquifer recharge. A final option is that water may move laterally in the profile and contribute to streamflow. Of these different pathways, plant uptake of water removes the majority of water that enters the soil profile.
IEVENT | Rainfall, runoff, routing option. | .bsn |
IDT | file.cio |
PRECIPITATION | .pcp |
SOL_K | .sol |
CN2 | .mgt |
CNOP | .mgt |
SOL_BD | .sol |
CLAY | .sol |
SAND | .sol |
The channel flow time of concentration, , can be computed using the equation
2:1.3.7
where is the average flow channel length for the subbasin (km), is the average channel velocity (m s), and 3.6 is a unit conversion factor.
The average channel flow length can be estimated using the equation
2:1.3.8
where is the channel length from the most distant point to the subbasin outlet (km), and is the distance along the channel to the subbasin centroid (km). Assuming , the average channel flow length is
2:1.3.9
The average velocity can be estimated from Manning’s equation assuming a trapezoidal channel with 2:1 side slopes and a 10:1 bottom width-depth ratio.
2:1.3.10
where is the average channel velocity (m s), is the average channel flow rate (), is the channel slope (m m), and is Manning’s roughness coefficient for the channel. To express the average channel flow rate in units of mm/hr, the following expression is used
2.1.3.11
where is the average channel flow rate (mm hr), is the subbasin area (km), and 3.6 is a unit conversion factor. The average channel flow rate is related to the unit source area flow rate (unit source area = 1 ha)
2:1.3.12
where is the unit source area flow rate (mm hr), is the subbasin area (km), and 100 is a unit conversion factor. Assuming the unit source area flow rate is 6.35 mm/hr and substituting equations 2:1.3.11 and 2:1.3.12 into 2:1.3.10 gives
2:1.3.13
Substituting equations 2:1.3.9 and 2:1.3.13 into 2:1.3.7 gives
2:1.3.14
where is the time of concentration for channel flow (hr), is the channel length from the most distant point to the subbasin outlet (km), n is Manning’s roughness coefficient for the channel, Area is the subbasin area (km), and is the channel slope (m m).
Although some of the assumptions used in developing equations 2:1.3.6 and 2:1.3.14 may appear liberal, the time of concentration values obtained generally give satisfactory results for homogeneous subbasins. Since equations 2:1.3.6 and 2:1.3.14 are based on hydraulic considerations, they are more reliable than purely empirical equations.
The land phase of the hydrologic cycle is based on the water balance equation:
where is the final soil water content (mm H2O), is the initial soil water content (mm H2O), t is the time (days), is the amount of precipitation on day i (mm H2O), is the amount of surface runoff on day i (mm H2O), is the amount of evapotranspiration on day i (mm H2O), is the amount of percolation and bypass flow exiting the soil profile bottom on day i (mm H2O), and is the amount of return flow on day i (mm H2O).
In large subbasins with a time of concentration greater than 1 day, only a portion of the surface runoff will reach the main channel on the day it is generated. SWAT+ incorporates a surface runoff storage feature to lag a portion of the surface runoff release to the main channel.
Once surface runoff is calculated with the Curve Number or Green & Ampt method, the amount of surface runoff released to the main channel is calculated:
2:1.4.1
where is the amount of surface runoff discharged to the main channel on a given day (mm HO), is the amount of surface runoff generated in the subbasin on a given day (mm HO), is the surface runoff stored or lagged from the previous day (mm HO), is the surface runoff lag coefficient, and is the time of concentration for the subbasin (hrs).
The expression in equation 2:1.4.1 represents the fraction of the total available water that will be allowed to enter the reach on any one day. Figure 2:1-3 plots values for this expression at different values for and .
Figure 2:1-3: Influence of and on fraction of surface runoff released.
Note that for a given time of concentration, as decreases in value more water is held in storage. The delay in release of surface runoff will smooth the streamflow hydrograph simulated in the reach.
Table 2:1-6: SWAT+ input variables that pertain to surface runoff lag calculations.
Equation Variable | Input Parameter | Input File | Source Code Variable |
---|---|---|---|
The modified rational formula used to estimate peak flow rate is obtained by substituting equations 2:1.3.15, 2:1.3.16, and 2:1.3.17 into equation 2:1.3.1
2:1.3.20
where is the peak runoff rate (), is the fraction of daily rainfall that occurs during the time of concentration, is the surface runoff (mm HO), is the subbasin area (km), is the time of concentration for the subbasin (hr) and 3.6 is a unit conversion factor.
Table 2:1-5: SWAT+ input variables that pertain to peak rate calculations.
Variable Name | Definition | Input File |
---|---|---|
The overland flow time of concentration, tov, can be computed using the equation
2:1.3.3
where is the subbasin slope length (m), is the overland flow velocity (m s) and 3600 is a unit conversion factor.
The overland flow velocity can be estimated from Manning’s equation by considering a strip 1 meter wide down the sloping surface:
2:1.3.4
where is the average overland flow rate (), is the average slope in the subbasin (m ), and is Manning’s roughness coefficient for the subbasin. Assuming an average flow rate of 6.35 mm/hr and converting units
2:1.3.5
Substituting equation 2:1.3.5 into equation 2:1.3.3 gives
2:1.3.6
The aerodynamic resistance to sensible heat and vapor transfer, , is calculated:
2:2.2.3
where is the height of the wind speed measurement (cm), is the height of the humidity (psychrometer) and temperature measurements (cm), is the zero plane displacement of the wind profile (cm), is the roughness length for momentum transfer (cm), is the roughness length for vapor transfer (cm), is the von Kármán constant, and is the wind speed at height (m s).
The von Kármán constant is considered to be a universal constant in turbulent flow. Its value has been calculated to be near 0.4 with a range of 0.36 to 0.43 (Jensen et al., 1990). A value of 0.41 is used by SWAT+ for the von Kármán constant.
Brutsaert (1975) determined that the surface roughness parameter, , is related to the mean height () of the plant canopy by the relationship = or 8.15 where e is the natural log base. Based on this relationship, the roughness length for momentum transfer is estimated as:
when 2:2.2.4
when 2:2.2.5
where mean height of the plant canopy () is reported in centimeters.
The roughness length for momentum transfer includes the effects of bluff-body forces. These forces have no impact on heat and vapor transfer, and the roughness length for vapor transfer is only a fraction of that for momentum transfer. To estimate the roughness length for vapor transfer, Stricker and Brutsaert (1978) recommended using:
2:2.2.6
The displacement height for a plant can be estimated using the following relationship (Monteith, 1981; Plate, 1971):
2:2.2.7
The height of the wind speed measurement, , and the height of the humidity (psychrometer) and temperature measurements, , are always assumed to be 170 cm.
Users are now able to read in daily measured or estimated potential evapotranspiration (PET) values from an external data source for each climate station. However, the user still has the option to calculate PET using the Penman Monteith, Priestley-Taylor, or Hargreaves method. The equations used to generate PET data in SWAT+ are reviewed in Chapter 2:2.2.
Using measured or estimated PET allows the model to use real observed data, which can improve accuracy and reliability. By integrating measured data, the model can account for local variations in climatic conditions that might not be captured by theoretical methods. By incorporating this functionality, the model enhances its capability to simulate real-world conditions, making it a valuable tool for hydrological studies and water resource management. SWAT+ input variables that pertain to measured or estimated PET are summarized in Table 2:2-3.
Table 2:2-3: SWAT+ input variables that pertain to measured or estimated PET.
Description | Source Name | Input Name | Input File |
---|
Priestley and Taylor (1972) developed a simplified version of the combination equation for use when surface areas are wet. The aerodynamic component was removed and the energy component was multiplied by a coefficient, = 1.28, when the general surroundings are wet or under humid conditions
2:2.2.23
where is the latent heat of vaporization (MJ kg), is the potential evapotranspiration (mm d), is a coefficient, is the slope of the saturation vapor pressure-temperature curve, (kPa ˚C), is the psychrometric constant (kPa ˚C), is the net radiation (MJ m d), and is the heat flux density to the ground (MJ m d).
The Priestley-Taylor equation provides potential evapotranspiration estimates for low advective conditions. In semiarid or arid areas where the advection component of the energy balance is significant, the Priestley-Taylor equation will underestimate potential evapotranspiration.
If the Penman-Monteith equation is selected as the potential evapotranspiration method, transpiration is also calculated with the equations summarized in Section 2:2.2.1. For the other potential evapotranspiration methods, transpiration is calculated as:
when 2:2.3.5
when 2:2.3.6
where is the maximum transpiration on a given day (mm HO), is the potential evapotranspiration adjusted for evaporation of free water in the canopy (mm HO), and is the leaf area index. The value for transpiration calculated by equations 2:2.3.5 and 2:2.3.6 is the amount of transpiration that will occur on a given day when the plant is growing under ideal conditions. The actual amount of transpiration may be less than this due to lack of available water in the soil profile. Calculation of actual plant water uptake and transpiration is reviewed in Chapters 5:2 and 5:3.
Any free water present in the canopy is readily available for removal by evapotranspiration. The amount of actual evapotranspiration contributed by intercepted rainfall is especially significant in forests where in some instances evaporation of intercepted rainfall is greater than transpiration.
SWAT+ removes as much water as possible from canopy storage when calculating actual evaporation. If potential evapotranspiration, , is less than the amount of free water held in the canopy, , then
2:2.3.1
2:2.3.2
where is the actual amount of evapotranspiration occurring in the watershed on a given day (mm HO), is the amount of evaporation from free water in the canopy on a given day (mm HO), is the potential evapotranspiration on a given day (mm HO), is the initial amount of free water held in the canopy on a given day (mm HO), and is the final amount of free water held in the canopy on a given day (mm HO). If potential evapotranspiration, , is greater than the amount of free water held in the canopy, , then
2:2.3.3
2:2.3.4
Once any free water in the canopy has been evaporated, the remaining evaporative water demandis partitioned between the vegetation and snow/soil.
The Hargreaves method was originally derived from eight years of cool-season Alta fescue grass lysimeter data from Davis, California (Hargreaves, 1975). Several improvements were made to the original equation (Hargreaves and Samani, 1982 and 1985) and the form used in SWAT+ was published in 1985 (Hargreaves et al., 1985):
2:2.2.24
where is the latent heat of vaporization (MJ kg), is the potential evapotranspiration (mm d), is the extraterrestrial radiation (MJ m d), is the maximum air temperature for a given day (°C), is the minimum air temperature for a given day (°C), and is the mean air temperature for a given day (°C).
Table 2:2-2: SWAT+ input variables used in potential evapotranspiration calculations summarized in this section.
Definition | Source Name | Input Name | Input File |
---|
The plant canopy can significantly affect infiltration, surface runoff and evapotranspiration. As rain falls, canopy interception reduces the erosive energy of droplets and traps a portion of the rainfall within the canopy. The influence the canopy exerts on these processes is a function of the density of plant cover and the morphology of the plant species.
When calculating surface runoff, the SCS curve number method lumps canopy interception in the term for initial abstractions. This variable also includes surface storage and infiltration prior to runoff and is estimated as 20% of the retention parameter value for a given day (see Chapter 2:1). When the Green and Ampt infiltration equation is used to calculate surface runoff and infiltration, the interception of rainfall by the canopy must be calculated separately.
SWAT+ allows the maximum amount of water that can be held in canopy storage to vary from day to day as a function of the leaf area index:
2:2.1.1
where is the maximum amount of water that can be trapped in the canopy on a given day (mm HO), is the maximum amount of water that can be trapped in the canopy when the canopy is fully developed (mm HO), is the leaf area index for a given day, and is the maximum leaf area index for the plant.
When precipitation falls on any given day, the canopy storage is filled before any water is allowed to reach the ground:
and
when 2:2.1.2
and
when 2:2.1.3
where is the initial amount of free water held in the canopy on a given day (mm HO), is the final amount of free water held in the canopy on a given day (mm HO), is the amount of precipitation on a given day before canopy interception is removed (mm HO), is the amount of precipitation on a given day that reaches the soil surface (mm HO), and is the maximum amount of water that can be trapped in the canopy on a given day (mm HO).
Table 2:2-1: SWAT+ input variables used in canopy storage calculations.
Definition | Source Name | Input Name | Input File |
---|
Many semiarid and arid watersheds have ephemeral channels that abstract large quantities of streamflow (Lane, 1982). The abstractions, or transmission losses, reduce runoff volume as the flood wave travels downstream. Chapter 19 of the SCS Hydrology Handbook (Lane, 1983) describes a procedure for estimating transmission losses for ephemeral streams which has been incorporated into SWAT+. This method was developed to estimate transmission losses in the absence of observed inflow-outflow data and assumes no lateral inflow or out-of-bank flow contributions to runoff.
The prediction equation for runoff volume after transmission losses is
2:1.5.1
where is the volume of runoff after transmission losses (), is the regression intercept for a channel of length and width (), is the regression slope for a channel of length and width , , is the volume of runoff prior to transmission losses (), and is the threshold volume for a channel of length and width (). The threshold volume is
2:1.5.2
The corresponding equation for peak runoff rate is
2:1.5.3
where is the peak rate after transmission losses (/s), is the duration of flow (hr), is the regression intercept for a channel of length and width (), is the regression slope for a channel of length and width , is the volume of runoff prior to transmission losses (), is the peak rate before accounting for transmission losses (/s). The duration of flow is calculated with the equation:
2:1.5.4
where is the duration of runoff flow (hr), is the surface runoff (mm HO), is the area of the subbasin (km), is the peak runoff rate (m/s), and 3.6 is a conversion factor.
In order to calculate the regression parameters for channels of differing lengths and widths, the parameters of a unit channel are needed. A unit channel is defined as a channel of length = 1 km and width = 1 m. The unit channel parameters are calculated with the equations:
2:1.5.5
2:1.5.6
2:1.5.7
where is the decay factor ( k), is the unit channel regression intercept (), is the unit channel regression slope, is the effective hydraulic conductivity of the channel alluvium (mm/hr), is the duration of runoff flow (hr), and is the initial volume of runoff (). The regression parameters are
2:1.5.8
2:1.5.9
Transmission losses from surface runoff are assumed to percolate into the shallow aquifer.
Table 2:1-7: SWAT+ input variables that pertain to transmission loss calculations.
Once the maximum amount of sublimation/soil evaporation for the day is calculated, SWAT+ will first remove water from the snow pack to meet the evaporative demand. If the water content of the snow pack is greater than the maximum sublimation/soil evaporation demand, then
2:2.3.10
2:2.3.11
2:2.3.12
where is the amount of sublimation on a given day (mm HO), is the maximum sublimation/soil evaporation adjusted for plant water use on a given day (mm HO), is the amount of water in the snow pack on a given day prior to accounting for sublimation (mm HO), is the amount of water in the snow pack on a given day after accounting for sublimation (mm HO), and is the maximum soil water evaporation on a given day (mm HO). If the water content of the snow pack is less than the maximum sublimation/soil evaporation demand, then
2:2.3.13
2:2.3.14
2:2.3.15
Evapotranspiration (ET) is estimated as the sum of plant transpiration and evaporation rates. SWAT+ inherits the SWAT model simulating potential evapotranspiration (PET) rates using the Priestley-Taylor method (Priestley and Taylor, 1972), the Penman-Monteith method (Monteith, 1965), or the Hargreaves method (Hargreaves and Samani, 1985). The estimated daily PET serves as the maximum daily actual ET amount. Thus, daily ET is assumed to be the same as PET if the sum of transpiration (EP) and soil evaporation (ES) is higher than PET. In this case, the soil evaporation rate is adjusted as ES = PET – EP. In rice paddies and wetlands where standing water exists, daily ET is calculated as the sum of EP and EVP, which can exceed the calculated daily PET. However, shades of mature rice straws can limit water evaporation (Choi et al., 2017; Sakaguchi et al., 2014b).
2:2.3.22
2:2.3.23
where is the evaporation of standing water, LAI is the dimensionless leaf area index, and PET is the potential evapotranspiration.
The SWAT+ soil evaporation equation substitutes Equation 2:2.3.23 if standing water is entirely depleted by transmission losses.
For an HRU with a seasonal high water table, if the soil profile becomes saturated to the point that percolation for upper soil layers to lower soil layers is inhibited, water will pond in the soil profile and create a perched water table.
SWAT+ allows the user to define the depth to an impervious layer for the HRU. If the depth to the impervious layer is in the soil profile, no water is allowed to percolate out of the soil profile. If the impervious layer is defined below the soil profile, percolation out of the soil profile is adjusted from the value determined with equation 2:3.2.3 using:
2:3.4.1
where is the amount of water percolating out of the soil profile on a given day (mm HO), is the amount of water percolating out of the soil profile on a given day calculated with equation 2:3.2.3 (mm HO), and is the distance from the bottom of the soil profile to the impervious layer (m).
Water builds up in the soil profile from the bottom of the profile. After the bottom layer of the profile reaches saturation, any water exceeding the storage capacity of the bottom layer is allowed to fill the overlying layer. This continues upward until all the excess water has been distributed.
The height of the perched water table is calculated:
2:3.4.2
where is the height of the water table (mm), is the water content of the soil profile (mm HO), is the water content of the soil profile at field capacity (mm HO), is the porosity of the soil profile (mm), is the air-filled porosity expressed as a fraction, and is the depth to the impervious layer (mm).
Table 2:3-5: SWAT+ input variables used in perched water table calculations.
Variable Name | Definition | File Name |
---|
Percolation is calculated for each soil layer in the profile. Water is allowed to percolate if the water content exceeds the field capacity water content for that layer and the layer below is not saturated. When the soil layer is frozen, no water flow out of the layer is calculated.
The volume of water available for percolation in the soil layer is calculated:
if 2:3.2.1
if 2:3.2.2
where is the drainable volume of water in the soil layer on a given day (mm HO), is the water content of the soil layer on a given day (mm HO) and is the water content of the soil layer at field capacity (mm HO).
The amount of water that moves from one layer to the underlying layer is calculated using storage routing methodology. The equation used to calculate the amount of water that percolates to the next layer is:
2:3.2.3
where is the amount of water percolating to the underlying soil layer on a given day (mm HO), is the drainable volume of water in the soil layer on a given day (mm HO), is the length of the time step (hrs), and is the travel time for percolation (hrs). If the HRU has a seasonal high water table, percolation is not allowed when where is the water content of the underlying soil layer (mm HO), is the water content of the underlying soil layer at field capacity (mm HO), and is the amount of water in the underlying soil layer when completely saturated (mm HO). The water will instead stay ponded in the upper layer.
The travel time for percolation is unique for each layer. It is calculate
2:3.2.4
where is the travel time for percolation (hrs), is the amount of water in the soil layer when completely saturated (mm HO), is the water content of the soil layer at field capacity (mm HO), and is the saturated hydraulic conductivity for the layer (mm∙h).
Water that percolates out of the lowest soil layer enters the vadose zone. The vadose zone is the unsaturated zone between the bottom of the soil profile and the top of the aquifer. Movement of water through the vadose zone and into the aquifers is reviewed in Chapter 2:4.
Table 2:3-2: SWAT+ input variables used in percolation calculations.
Variable Name | Definition | File Name |
---|
In large subbasins with a time of concentration greater than 1 day, only a portion of the lateral flow will reach the main channel on the day it is generated. SWAT+ incorporates a lateral flow storage feature to lag a portion of lateral flow release to the main channel.
Once lateral flow is calculated, the amount of lateral flow released to the main channel is calculated:
2:3.5.10
where is the amount of lateral flow discharged to the main channel on a given day (mm HO), is the amount of lateral flow generated in the subbasin on a given day (mm HO), is the lateral flow stored or lagged from the previous day (mm HO), and is the lateral flow travel time (days).
The model will calculate lateral flow travel time or utilize a user-defined travel time. In the majority of cases, the user should allow the model to calculate the travel time. If drainage tiles are present in the HRU, lateral flow travel time is calculated:
2:3.5.11
where is the lateral flow travel time (days) and is the drain tile lag time (hrs). In HRUs without drainage tiles, lateral flow travel time is calculated:
2:3.5.12
where is the lateral flow travel time (days), is the hillslope length (m), and is the highest layer saturated hydraulic conductivity in the soil profile (mm/hr).
The expression in equation 2:3.5.10 represents the fraction of the total available water that will be allowed to enter the reach on any one day. Figure 2:3-5 plots values for this expression at different values of .
Figure 2:3-5: Influence of on fraction of lateral flow released.
The delay in release of lateral flow will smooth the streamflow hydrograph simulated in the reach.
Table 2:3-6: SWAT+ input variables used in lateral flow calculations.
Variable Name | Definition | File Name |
---|
The amount of sublimation and soil evaporation will be impacted by the degree of shading. The maximum amount of sublimation/soil evaporation on a given day is calculated as:
2:2.3.7
where is the maximum sublimation/soil evaporation on a given day (mm HO), is the potential evapotranspiration adjusted for evaporation of free water in the canopy (mm HO), and is the soil cover index. The soil cover index is calculated
2:2.3.8
where is the aboveground biomass and residue (kg ha). If the snow water content is greater than 0.5 mm HO, the soil cover index is set to 0.5.
The maximum amount of sublimation/soil evaporation is reduced during periods of high plant water use with the relationship:
2:2.3.9
where is the maximum sublimation/soil evaporation adjusted for plant water use on a given day (mm HO), is the maximum sublimation/soil evaporation on a given day (mm HO), is the potential evapotranspiration adjusted for evaporation of free water in the canopy (mm HO), and is the transpiration on a given day (mm HO). When is low . However, as approaches , , .
Within the saturated zone of groundwater, regions of high conductivity and low conductivity will be found. The regions of high conductivity are made up of coarse-grained particles with a large percentage of macropores that allow water to move easily. The regions of low conductivity are made up of fine-grained particles with a large percentage of mesopores and micropores that restrict the rate of water movement.
An aquifer is “a geologic unit that can store enough water and transmit it at a rate fast enough to be hydrologically significant” (Dingman, 1994). An unconfined aquifer is an aquifer whose upper boundary is the water table. A confined aquifer is an aquifer bounded above and below by geologic formations whose hydraulic conductivity is significantly lower than that of the aquifer. Figure 2:4-1 illustrates the two types of aquifers.
Figure 2:4-1: Unconfined and confined aquifers (After Dingman, 1994).
Recharge to unconfined aquifers occurs via percolation to the water table from a significant portion of the land surface. In contrast, recharge to confined aquifers by percolation from the surface occurs only at the upstream end of the confined aquifer, where the geologic formation containing the aquifer is exposed at the earth’s surface, flow is not confined, and a water table is present.
Topography exerts an important influence on groundwater flow. The flow of groundwater in an idealized hilly upland area is depicted in Figure 2:4-2. The landscape can be divided into areas of recharge and areas of discharge. A recharge area is defined as a portion of a drainage basin where ground water flow is directed away from the water table. A discharge area is defined as a portion of the drainage basin where ground water flow is directed toward the water table. The water table is at or near the surface in discharge areas and surface water bodies are normally located in discharge areas.
Figure 2:4-2: Groundwater flow net in an idealized hilly region with homogenous permeable material resting on an impermeable base (After Hubbert, 1940)
Streams may be categorized by their relationship to the groundwater system. A stream located in a discharge area that receives groundwater flow is a gaining or effluent stream (Figure 2:4-3a). This type of stream is characterized by an increase in discharge downstream. A stream located in a recharge area is a losing or influent stream. This type of stream is characterized by a decrease in discharge downstream. A losing stream may be connected to (Figure 2:4-3b) or perched above (Figure 2:4-3c) the groundwater flow area. A stream that simultaneously receives and loses groundwater is a flow-through stream (Figure 2:4-3d)
Figure 2:4-3: Stream-groundwater relationships: a) gaining stream receiving water from groundwater flow; b) losing stream connected to groundwater system; c) losing stream perched above groundwater system; and d) flow-through stream (After Dingman, 1994).
SWAT+ simulates two aquifers in each subbasin. The shallow aquifer is an unconfined aquifer that contributes to flow in the main channel or reach of the subbasin. The deep aquifer is a confined aquifer. Water that enters the deep aquifer is assumed to contribute to streamflow somewhere outside of the watershed (Arnold et al., 1993).
The water balance for the shallow aquifer is:
2:4.2.1
where is the amount of water stored in the shallow aquifer on day (mm HO), is the amount of water stored in the shallow aquifer on day (mm HO), is the amount of recharge entering the shallow aquifer on day (mm HO), is the groundwater flow, or base flow, into the main channel on day (mm HO), is the amount of water moving into the soil zone in response to water deficiencies on day (mm HO), is seepage from the shallow aquifer to the deep aquifer on day (mm HO) and is the amount of water removed from the shallow aquifer by pumping on day (mm HO).
Length of time step (min): =IDT/60
: Precipitation during time step (mm HO)
: Saturated hydraulic conductivity of first layer (mm/hr)
: Moisture condition II curve number
: Moisture condition II curve number
: Moist bulk density (Mg/): =1 - / 2.65
: % clay content
: % sand content
where is the regression intercept for a channel of length and width (), is the regression slope for a channel of length and width, is the decay factor ( k), is the channel length from the most distant point to the subbasin outlet (km), is the average width of flow, i.e. channel width (m) is the unit channel regression intercept (), and is the unit channel regression slope.
Variable Name | Definition | Input File |
---|
SUB_KM
Area of the subbasin (km)
.sub
HRU_FR
Fraction of subbasin area contained in HRU
.hru
SLSUBBSN
: Average slope length (m)
.hru
HRU_SLP
: Average slope steepness (m/m)
.hru
OV_N
: Manning’s “n” value for overland flow
.hru
CH_L(1)
: Longest tributary channel length in subbasin (km)
.sub
CH_S(1)
: Average slope of tributary channels (m/m)
.sub
CH_N(1)
: Manning’s “n” value for tributary channels
.sub
A fraction of the total daily recharge can be routed to the deep aquifer. The amount of water than will be diverted from the shallow aquifer due to percolation to the deep aquifer on a given day is:
2:4.2.4
where is the amount of water moving into the deep aquifer on day (mm HO), is the aquifer percolation coefficient, and is the amount of recharge entering both aquifers on day (mm HO). The amount of recharge to the shallow aquifer is:
2:4.2.5
where is the amount of recharge entering the shallow aquifer on day (mm HO).
Water may move from the shallow aquifer into the overlying unsaturated zone. In periods when the material overlying the aquifer is dry, water in the capillary fringe that separates the saturated and unsaturated zones will evaporate and diffuse upward. As water is removed from the capillary fringe by evaporation, it is replaced by water from the underlying aquifer. Water may also be removed from the aquifer by deep-rooted plants which are able to uptake water directly from the aquifer.
SWAT+ models the movement of water into overlying unsaturated layers as a function of water demand for evapotranspiration. To avoid confusion with soil evaporation and transpiration, this process has been termed ‘revap’. This process is significant in watersheds where the saturated zone is not very far below the surface or where deep-rooted plants are growing. Because the type of plant cover will affect the importance of revap in the water balance, the parameters governing revap are usually varied by land use. Revap is allowed to occur only if the amount of water stored in the shallow aquifer exceeds a threshold value specified by the user, .
The maximum amount of water than will be removed from the aquifer via ‘revap’ on a given day is:
2:4.2.15
where is the maximum amount of water moving into the soil zone in response to water deficiencies (mm HO), is the revap coefficient, and is the potential evapotranspiration for the day (mm HO). The actual amount of revap that will occur on a given day is calculated:
if 2:4.2.16
if 2:4.2.17
if 2:4.2.18
where is the actual amount of water moving into the soil zone in response to water deficiencies (mm HO), is the maximum amount of water moving into the soil zone in response to water deficiencies (mm HO), is the amount of water stored in the shallow aquifer at the beginning of day (mm HO) and is the threshold water level in the shallow aquifer for revap to occur (mm HO).
The water balance for the deep aquifer is:
2:4.3.1
where is the amount of water stored in the deep aquifer on day (mm HO), is the amount of water stored in the deep aquifer on day (mm HO), is the amount of water percolating from the shallow aquifer into the deep aquifer on day (mm HO), is the groundwater flow, or base flow, into the main channel on day (mm HO) and is the amount of water removed from the deep aquifer by pumping on day (mm HO). The amount of water percolating into the deep aquifer is calculated with the equations reviewed in section 2:4.2.4. If the deep aquifer is specified as the source of irrigation water or water removed for use outside the watershed, the model will allow an amount of water up to the total volume of the deep aquifer to be removed on any given day.
Water entering the deep aquifer is not considered in future water budget calculations and can be considered to be lost from the system.
Groundwater is water in the saturated zone of earth materials under pressure greater than atmospheric, i.e. positive pressure. Remember that in the soil profile water is held at a negative pressure due to the attraction between negatively charged clay particles and water. The groundwater table is the depth at which the pressure between water and the surrounding soil matrix is equal to atmospheric pressure. Water enters groundwater storage primarily by infiltration/percolation, although recharge by seepage from surface water bodies may occur. Water leaves groundwater storage primarily by discharge into rivers or lakes, but it is also possible for water to move upward from the water table into the capillary fringe, a zone above the groundwater table that is saturated.
Water that moves past the lowest depth of the soil profile by percolation or bypass flow enters and flows through the vadose zone before becoming shallow and/or deep aquifer recharge. The lag between the time that water exits the soil profile and enters the shallow aquifer will depend on the depth to the water table and the hydraulic properties of the geologic formations in the vadose and groundwater zones.
An exponential decay weighting function proposed by Venetis (1969) and used by Sangrey et al. (1984) in a precipitation/groundwater response model is utilized in SWAT+ to account for the time delay in aquifer recharge once the water exits the soil profile. The delay function accommodates situations where the recharge from the soil zone to the aquifer is not instantaneous, i.e. 1 day or less.
The recharge to both aquifers on a given day is calculated:
2:4.2.2
where is the amount of recharge entering the aquifers on day (mm HO), is the delay time or drainage time of the overlying geologic formations (days), is the total amount of water exiting the bottom of the soil profile on day (mm HO), and is the amount of recharge entering the aquifers on day (mm HO). The total amount of water exiting the bottom of the soil profile on day is calculated:
2:4.2.3
where is the total amount of water exiting the bottom of the soil profile on day (mm HO), is the amount of water percolating out of the lowest layer, , in the soil profile on day (mm HO), and is the amount of water flow past the lower boundary of the soil profile due to bypass flow on day (mm HO).
The delay time, , cannot be directly measured. It can be estimated by simulating aquifer recharge using different values for and comparing the simulated variations in water table level with observed values. Johnson (1977) developed a simple program to iteratively test and statistically evaluate different delay times for a watershed. Sangrey et al. (1984) noted that monitoring wells in the same area had similar values for , so once a delay time value for a geomorphic area is defined, similar delay times can be used in adjoining watersheds within the same geomorphic province.
The shallow aquifer contributes base flow to the main channel or reach within the subbasin. Base flow is allowed to enter the reach only if the amount of water stored in the shallow aquifer exceeds a threshold value specified by the user, .
The steady-state response of groundwater flow to recharge is (Hooghoudt, 1940):
2:4.2.6
where is the groundwater flow, or base flow, into the main channel on day (mm HO), is the hydraulic conductivity of the aquifer (mm/day), is the distance from the ridge or subbasin divide for the groundwater system to the main channel (m), and is the water table height (m).
Water table fluctuations due to non-steady-state response of groundwater flow to periodic recharge is calculated (Smedema and Rycroft, 1983):
2:4.2.7
where is the change in water table height with time (mm/day), is the amount of recharge entering the shallow aquifer on day (mm HO), is the groundwater flow into the main channel on day (mm HO), and is the specific yield of the shallow aquifer (m/m).
Assuming that variation in groundwater flow is linearly related to the rate of change in water table height, equations 2:4.2.7 and 2:4.2.6 can be combined to obtain:
2:4.2.8
where is the groundwater flow into the main channel on day (mm HO), is the hydraulic conductivity of the aquifer (mm/day), is the specific yield of the shallow aquifer (m/m), is the distance from the ridge or subbasin divide for the groundwater system to the main channel (m), is the amount of recharge entering the shallow aquifer on day (mm HO) and is the baseflow recession constant or constant of proportionality. Integration of equation 2:4.2.8 and rearranging to solve for yields:
if 2:4.2.9
if 2:4.2.10
where is the groundwater flow into the main channel on day (mm HO), is the groundwater flow into the main channel on day (mm HO), is the baseflow recession constant, is the time step (1 day), is the amount of recharge entering the shallow aquifer on day (mm HO), is the amount of water stored in the shallow aquifer at the beginning of day (mm HO) and is the threshold water level in the shallow aquifer for groundwater contribution to the main channel to occur (mm HO).
The baseflow recession constant, , is a direct index of groundwater flow response to changes in recharge (Smedema and Rycroft, 1983). Values vary from 0.1-0.3 for land with slow response to recharge to 0.9-1.0 for land with a rapid response. Although the baseflow recession constant may be calculated, the best estimates are obtained by analyzing measured streamflow during periods of no recharge in the watershed.
When the shallow aquifer receives no recharge, equation 2:4.2.9 simplifies to:
if 2:4.2.11
if 2:4.2.12
where is the groundwater flow into the main channel at time (mm HO), is the groundwater flow into the main channel at the beginning of the recession (time =0) (mm HO), is the baseflow recession constant, and t is the time lapsed since the beginning of the recession (days), is the amount of water stored in the shallow aquifer at the beginning of day (mm HO) and is the threshold water level in the shallow aquifer for groundwater contribution to the main channel to occur (mm HO). The baseflow recession constant is measured by rearranging equation 2:4.2.11.
2:4.2.13
where is the baseflow recession constant, is the time lapsed since the start of the recession (days), is the groundwater flow on day (mm HO), is the groundwater flow at the start of the recession (mm HO).
It is common to find the baseflow days reported for a stream gage or watershed. This is the number of days for base flow recession to decline through one log cycle. When baseflow days are used, equation 2:4.2.13 can be further simplified:
2:4.2.14
where is the baseflow recession constant, and is the number of baseflow days for the watershed.
If the shallow aquifer is specified as the source of irrigation water or water removed for use outside the watershed, the model will allow an amount of water up to the total volume of the shallow aquifer to be removed on any given day. Detailed information on water management may be found in Chapter 6:2.
SUB_KM | .sub |
HRU_FR | Fraction of total subbasin area contained in HRU | .hru |
CH_K(1) | .sub |
CH_W(1) | .sub |
CH_L(1) | .sub |
PET method; set to = 3 for reading in measured PET. | pet |
Name of PET station | pet |
File with PET measured/simulated data | petfile | pet_file | pet.cli |
: Potential evapotranspiration method | pet | pet |
: Wind speed (m/s) | windav | wind_ave |
: Carbon dioxide concentration (ppm) | co2 | co2 |
: Daily maximum temperature (°C) | tmpmax | tmp_max_ave |
: Daily minimum temperature (°C) | tmpmin | tmp_min_ave |
: maximum leaf conductance (m s−1−1) | gsi | stcon_max |
: Fraction of maximum leaf conductance achieved at the vapor pressure deficit specified by | gmaxfr | frac_stcon |
: Vapor pressure deficit corresponding to value given for (kPa) | vpdfr | vpd |
: maximum canopy storage | canmx | can_max |
DEP_IMP | : Depth to impervious layer (mm) | .hru |
DEPIMP_BSN | : Depth to impervious layer (mm) | .bsn |
SOL_K | : Saturated hydraulic conductivity (mm/hr) | .sol |
IWATABLE | High water table code: 0-no water table in soil profile 1-seasonal high water table present in profile | .hru |
SLSOIL | .hru |
SOL_K | .sol |
HRU_SLP | .hru |
LAT_TTIME | .mgt |
GDRAIN | .mgt |
When an evaporation demand for soil water exists, SWAT+ must first partition the evaporative demand between the different layers. The depth distribution used to determine the maximum amount of water allowed to be evaporated is:
2:2.3.16
where is the evaporative demand at depth (mm HO), is the maximum soil water evaporation on a given day (mm HO), and is the depth below the surface. The coefficients in this equation were selected so that 50% of the evaporative demand is extracted from the top 10 mm of soil and 95% of the evaporative demand is extracted from the top 100 mm of soil.
The amount of evaporative demand for a soil layer is determined by taking the difference between the evaporative demands calculated at the upper and lower boundaries of the soil layer:
2:2.3.17
where is the evaporative demand for layer (mm HO), is the evaporative demand at the lower boundary of the soil layer (mm HO), and is the evaporative demand at the upper boundary of the soil layer (mm HO).
Figure 2:2-1 graphs the depth distribution of the evaporative demand for a soil that has been partitioned into 1 mm layers assuming a total soil evaporation demand of 100 mm.
As mentioned previously, the depth distribution assumes 50% of the evaporative demand is met by soil water stored in the top 10 mm of the soil profile. With our example of a 100 mm total evaporative demand, 50 mm of water is 50%. This is a demand that the top layer cannot satisfy.
SWAT+ does not allow a different layer to compensate for the inability of another layer to meet its evaporative demand. The evaporative demand not met by a soil layer results in a reduction in actual evapotranspiration for the HRU.
A coefficient has been incorporated into equation 2:2.3.17 to allow the user to modify the depth distribution used to meet the soil evaporative demand. The modified equation is:
2:2.3.18
where is the evaporative demand for layer (mm HO), is the evaporative demand at the lower boundary of the soil layer (mm HO), is the evaporative demand at the upper boundary of the soil layer (mm HO), and is the soil evaporation compensation coefficient. Solutions to this equation for different values of including for are shown in Figure 2:2-1.
As the value for is reduced, the model is able to extract more of the evaporative demand from lower levels.
When the water content of a soil layer is below field capacity, the evaporative demand for the layer is reduced according to the following equations:
when 2:2.3.19
when 2:2.3.20
where is the evaporative demand for layer adjusted for water content (mm HO), is the evaporative demand for layer (mm HO), is the soil water content of layer (mm HO), is the water content of layer at field capacity (mm HO), and is the water content of layer at wilting point (mm HO).
In addition to limiting the amount of water removed by evaporation in dry conditions, SWAT+ defines a maximum value of water that can be removed at any time. This maximum value is 80% of the plant available water on a given day where the plant available water is defined as the total water content of the soil layer minus the water content of the soil layer at wilting point (-1.5 MPa).
2:2.3.21
where is the amount of water removed from layer by evaporation (mm HO), is the evaporative demand for layer adjusted for water content (mm HO), is the soil water content of layer (mm HO), and is the water content of layer at wilting point (mm HO).
Table 2:2-3: SWAT+ input variables used in soil evaporation calculations.
Although SWAT+ does not currently print groundwater height in the output files, the water table height is updated daily by the model. Groundwater height is related to groundwater flow by equation 2:4.2.6.
2:4.2.19
where is the groundwater flow into the main channel on day (mm HO), is the hydraulic conductivity of the aquifer (mm/day), is the distance from the ridge or subbasin divide for the groundwater system to the main channel (m), is the water table height (m), is the specific yield of the shallow aquifer (m/m), and is the baseflow recession constant. Substituting this definition for into equation 2:4.2.9 gives
2:4.2.20
where is the water table height on day (m), is the water table height on day (m), is the baseflow recession constant, is the time step (1 day), is the amount of recharge entering the aquifer on day (mm HO), and is the specific yield of the shallow aquifer (m/m).
Table 2:4-1: SWAT+ input variables used in shallow aquifer calculations.
Variable Name | Definition | File Name |
---|---|---|
One of the most unique soil orders is the Vertisols. These soils are characterized by a propensity to shrink when dried and swell when moistened. When the soil is dry, large cracks form at the soil surface. This behavior is a result of the type of soil material present and the climate. Vertisols contain at least 30% clay with the clay fraction dominated by smectitic mineralogy and occur in areas with cyclical wet and dry periods.
Vertisols are found worldwide (Figure 2:3-1). They have a number of local names, some of which are listed in Table 2:3-3.
Figure 2:3-1: Soil associations of Vertisols (After Dudal and Eswaran, 1988)
Table 2:3-3: Alternative names for Vertisols or soils with Vertic properties (Dudal and Eswaran, 1988).
One criteria used to classify a soil as a Vertisol is the formation of shrinkage cracks in the dry season that penetrate to a depth of more than 50 cm and are at least 1 cm wide at 50 cm depth. The cracks can be considerably wider at the surface—30 cm cracks at the surface are not unusual although 6-15 cm cracks are more typical.
To accurately predict surface runoff and infiltration in areas dominated by soils that exhibit Vertic properties, the temporal change in soil volume must be quantified. Bouma and Loveday (1988) identified three soil moisture conditions for which infiltration needs to be defined (Figure 2:3-2).
Figure 2:3-2: Diagram showing the effect of wetting and drying on cracking in Vertisols (After Bouma and Loveday, 1988)
Traditional models of infiltration are applicable to soils in which cracks have been closed by swelling and the soil acts as a relatively homogenous porous medium (Condition 3 in Figure 2:3-2). Condition 1 in Figure 2:3-2 represents the driest state with cracks at maximum width, a condition present at the end of the dry season/beginning of the rainy season. Condition 2 in Figure 2:3-2 represents the crack development typical with an actively growing crop requiring multiple irrigation or rainfall events to sustain growth. Bypass flow, the vertical movement of free water along macropores through unsaturated soil horizons, will occur in conditions 1 and 2. Bypass flow (finf,2 in Figure 2:3-2) occurs when the rate of rainfall or irrigation exceeds the vertical infiltration rate into the soil peds (finf,1 in Figure 2:3-2).
When bypass flow is modeled, SWAT+ calculates the crack volume of the soil matrix for each day of simulation by layer. On days in which precipitation events occur, infiltration and surface runoff is first calculated for the soil peds (finf,1 in Figure 2:3-2) using the curve number or Green & Ampt method. If any surface runoff is generated, it is allowed to enter the cracks. A volume of water equivalent to the total crack volume for the soil profile may enter the profile as bypass flow. Surface runoff in excess of the crack volume remains overland flow.
Water that enters the cracks fills the soil layers beginning with the lowest layer of crack development. After cracks in one layer are filled, the cracks in the overlying layer are allowed to fill.
The crack volume initially estimated for a layer is calculated:
2:3.3.1
where is the initial crack volume calculated for the soil layer on a given day expressed as a depth (mm), is the maximum crack volume possible for the soil layer (mm), is an adjustment coefficient for crack flow, is the water content of the soil layer at field capacity (mm HO), and is the water content of the soil layer on a given day (mm HO). The adjustment coefficient for crack flow, , is set to 0.10.
When the moisture content of the entire profile falls below 90% of the field capacity water content for the profile during the drying stage, the crack volume for a given day is a function of the crack volume estimated with equation 2:3.3.1 and the crack volume of the layer on the previous day. When the soil is wetting and/or when the moisture content of the profile is above 90% of the field capacity water content, the crack volume for a given day is equal to the volume calculated with equation 2:3.3.1.
when and 2:3.3.2
when or 2:3.3.3
where is the crack volume for the soil layer on a given day expressed as a depth (mm), is the lag factor for crack development during drying, is the crack volume for the soil layer on the previous day (mm), is the initial crack volume calculated for the soil layer on a given day using equation 2:3.3.1 (mm), is the water content of the soil profile on a given day (mm HO), and is the water content of the soil profile at field capacity (mm HO).
As the tension at which water is held by the soil particles increases, the rate of water diffusion slows. Because the rate of water diffusion is analogous to the coefficient of consolidation in classical consolidation theory (Mitchell, 1992), the reduction in diffusion will affect crack formation. The lag factor is introduced during the drying stage to account for the change in moisture redistribution dynamics that occurs as the soil dries. The lag factor, , is set to a value of 0.99.
The maximum crack volume for the layer, , is calculated:
2:3.3.4
where is the maximum crack volume possible for the soil layer (mm), is the potential crack volume for the soil profile expressed as a fraction of the total volume, is the depth from the soil surface to the bottom of the soil layer (mm), and is the depth of the soil layer (mm). The potential crack volume for the soil profile, , is input by the user. Those needing information on the measurement of this parameter are referred to Bronswijk (1989; 1990).
Once the crack volume for each layer is calculated, the total crack volume for the soil profile is determined.
2:3.3.5
where is the total crack volume for the soil profile on a given day (mm), is the crack volume for the soil layer on a given day expressed as a depth (mm), is the layer, and is the number of layers in the soil profile.
After surface runoff is calculated for rainfall events using the curve number or Green & Ampt method, the amount of runoff is reduced by the volume of cracks present that day:
if 2:3.3.6
if 2:3.3.7
where is the accumulated runoff or rainfall excess for the day (mm HO), is the initial accumulated runoff or rainfall excess determined with the Green & Ampt or curve number method (mm HO), and is the total crack volume for the soil profile on a given day (mm). The total amount of water entering the soil is then calculated:
2:3.3.8
where is the amount of water entering the soil profile on a given day (mm HO), is the rainfall depth for the day adjusted for canopy interception (mm HO), and is the accumulated runoff or rainfall excess for the day (mm HO).
Bypass flow past the bottom of the profile is calculated:
2:3.3.9
where is the amount of water flow past the lower boundary of the soil profile due to bypass flow (mm HO), is the total crack volume for the soil profile on a given day (mm), is the crack volume for the deepest soil layer () on a given day expressed as a depth (mm), and is the depth of the deepest soil layer () (mm).
After is calculated, each soil layer is filled to field capacity water content beginning with the lowest layer and moving upward until the total amount of water entering the soil, , has been accounted for.
Table 2:3-4: SWAT+ input variables used in bypass flow calculations.
Lateral flow will be significant in areas with soils having high hydraulic conductivities in surface layers and an impermeable or semipermeable layer at a shallow depth. In such a system, rainfall will percolate vertically until it encounters the impermeable layer. The water then ponds above the impermeable layer forming a saturated zone of water, i.e. a perched water table. This saturated zone is the source of water for lateral subsurface flow.
SWAT+ incorporates a kinematic storage model for subsurface flow developed by Sloan et al. (1983) and summarized by Sloan and Moore (1984). This model simulates subsurface flow in a two-dimensional cross-section along a flow path down a steep hillslope. The kinematic approximation was used in its derivation.
This model is based on the mass continuity equation, or mass water balance, with the entire hillslope segment used as the control volume. The hillslope segment has a permeable soil surface layer of depth and length with an impermeable soil layer or boundary below it as shown in Figure 2:3-3. The hillslope segment is oriented at an angle αhill to the horizontal.
Figure 2:3-3: Conceptual representation of the hillslope segment.
The kinematic wave approximation of saturated subsurface or lateral flow assumes that the lines of flow in the saturated zone are parallel to the impermeable boundary and the hydraulic gradient equals the slope of the bed.
Figure 2:3-4: Behavior of the water table as assumed in the kinematic storage model.
From Figure 2:3-4, the drainable volume of water stored in the saturated zone of the hillslope segment per unit area, , is
2:3.5.1
where is the drainable volume of water stored in the saturated zone of the hillslope per unit area (mm HO), is the saturated thickness normal to the hillslope at the outlet expressed as a fraction of the total thickness (mm/mm), is the drainable porosity of the soil (mm/mm), is the hillslope length (m), and 1000 is a factor needed to convert meters to millimeters. This equation can be rearranged to solve for :
2:3.5.2
The drainable porosity of the soil layer is calculated:
2:3.5.3
where is the drainable porosity of the soil layer (mm/mm), is the total porosity of the soil layer (mm/mm), and is the porosity of the soil layer filled with water when the layer is at field capacity water content (mm/mm).
A soil layer is considered to be saturated whenever the water content of the layer exceeds the layer’s field capacity water content. The drainable volume of water stored in the saturated layer is calculated:
if 2:3.5.4
if 2:3.5.5
where is the water content of the soil layer on a given day (mm HO) and is the water content of the soil layer at field capacity (mm HO).
The net discharge at the hillslope outlet, , is given by
2:3.5.6
where is the water discharged from the hillslope outlet (mm HO/day), is the saturated thickness normal to the hillslope at the outlet expressed as a fraction of the total thickness (mm/mm), is the velocity of flow at the outlet (mm∙h), and 24 is a factor to convert hours to days.
Velocity of flow at the outlet is defined as
2:3.5.7
where is the saturated hydraulic conductivity (mm∙h) and is the slope of the hillslope segment. The slope is input to SWAT+ as the increase in elevation per unit distance () which is equivalent to . Because , equation 2:3.5.3 is modified to use the value for the slope as input to the model:
2:3.5.8
Combining equations 2:3.5.2 and 2:3.5.8 with equation 2:3.5.6 yields the equation
2:3.5.9
where all terms are previously defined.
Soil is comprised of three phases—solid, liquid and gas. The solid phase consists of minerals and/or organic matter that forms the matrix or skeleton of the soil. Between the solid particles, soil pores are formed that hold the liquid and gas phases. The soil solution may fill the soil pores completely (saturated) or partially (unsaturated). When the soil is unsaturated, the soil solution is found as thin films along particle surfaces, as annular wedges around contact points of particles and as isolated bodies in narrow pore passages.
The soil’s bulk density defines the relative amounts of pore space and soil matrix. Bulk density is calculated:
2:3.1.1
where is the bulk density (Mg m), is the mass of the solids (Mg), and is the total volume (m). The total volume is defined as
2:3.1.2
where is the volume of air (m), is the volume of water (m), and is the volume of solids (m). The relationship between soil porosity and soil bulk density is
2:3.1.3
where is the soil porosity expressed as a fraction of the total soil volume, is the bulk density (Mg m), and is the particle density (Mg m). The particle density, or density of the solid fraction, is a function of the mineral composition of the soil matrix. Based on research, a default value of 2.65 Mg m is used for particle density.
Storage, transport and availability of soil solution and soil air are not nearly as dependent on the total amount of porosity as they are on the arrangement of pore space. Soil pores vary in size and shape due to textural and structural arrangement. Based on the diameter of the pore at the narrowest point, the pores may be classified as macropores (narrowest diameter > 100 ), mesopores (narrowest diameter 30-100 ), and micropores (narrowest diameter < 30 ) (Koorevaar et al, 1983). Macropores conduct water only during flooding or ponding rain and drainage of water from these pores is complete soon after cessation of the water supply. Macropores control aeration and drainage processes in the soil. Mesopores conduct water even after macropores have emptied, e.g. during non-ponding rain and redistribution. Micropores retain soil solution or conduct it very slowly.
When comparing soils of different texture, clay soils contain a greater fraction of mesopores and micropores while sand soils contain mostly macropores. This is evident when the hydraulic conductivities of clay and sand soils are compared. The conductivity of a sand soil can be several orders of magnitude greater than that for a clay soil.
The water content of a soil can range from zero when the soil is oven dried to a maximum value () when the soil is saturated. For plant-soil interactions, two intermediate stages are recognized: field capacity and permanent wilting point. Field capacity is the water content found when a thoroughly wetted soil has drained for approximately two days. Permanent wilting point is the water content found when plants growing in the soil wilt and do not recover if their leaves are kept in a humid atmosphere overnight. To allow these two stages to be quantified more easily, they have been redefined in terms of tensions at which water is held by the soil. Field capacity is the amount of water held in the soil at a tension of 0.033 MPa and the permanent wilting point is the amount of water held in the soil at a tension of 1.5 MPa. The amount of water held in the soil between field capacity and permanent wilting point is considered to be the water available for plant extraction.
Table 2:3-1 lists the water content for three soils as a fraction of the total volume for different moisture conditions. Note that the total porosity, given by the water content at saturation, is lowest for the sand soil and highest for the clay soil.
The sand soil drains more quickly than the loam and clay. Only 15% of the water present in the sand soil at saturation remains at field capacity. 58% of the water present at saturation in the loam remains at field capacity while 68% of the water present at saturation in the clay soil remains at field capacity. The reduction of water loss with increase in clay content is cause by two factors. As mentioned previously, clay soils contain more mesopores and micropores than sand soils. Also, unlike sand and silt particles, clay particles possess a net negative charge. Due to the polar nature of water molecules, clay particles are able to attract and retain water molecules. The higher water retention of clay soils is also seen in the fraction of water present at permanent wilting point. In the soils listed in Table 2:3-1, the volumetric water content of the clay is 0.20 at the wilting point while the sand and loam have a volumetric water content of 0.02 and 0.05 respectively.
The plant available water, also referred to as the available water capacity, is calculated by subtracting the fraction of water present at permanent wilting point from that present at field capacity.
2:3.1.4
where is the plant available water content, is the water content at field capacity, and is the water content at permanent wilting point. For the three soil textures listed in Table 2:3-1, the sand has an available water capacity of 0.04, the loam has an available water capacity of 0.24 and the clay has an available water capacity of 0.21. Even though the clay contains a greater amount of water than the loam at all three tensions, the loam has a larger amount of water available for plant uptake than the clay. This characteristic is true in general.
SWAT+ estimates the permanent wilting point volumetric water content for each soil layer as:
2:3.1.5
where is the water content at wilting point expressed as a fraction of the total soil volume, is the percent clay content of the layer(%),and is the bulk density for the soil layer(Mg m). Field capacity water content is estimated
2:3.1.6
where is the water content at field capacity expressed as a fraction of the total soil volume, is the water content at wilting point expressed as a fraction of the total soil volume, and is the available water capacity of the soil layer expressed as a fraction of the total soil volume. is input by the user.
Water in the soil can flow under saturated or unsaturated conditions. In saturated soils, flow is driven by gravity and usually occurs in the downward direction. Unsaturated flow is caused by gradients arising due to adjacent areas of high and low water content. Unsaturated flow may occur in any direction.
SWAT+ directly simulates saturated flow only. The model records the water contents of the different soil layers but assumes that the water is uniformly distributed within a given layer. This assumption eliminates the need to model unsaturated flow in the horizontal direction. Unsaturated flow between layers is indirectly modeled with the depth distribution of plant water uptake (equation 5:2.2.1) and the depth distribution of soil water evaporation (equation 2:2.3.16).
Saturated flow occurs when the water content of a soil layer surpasses the field capacity for the layer. Water in excess of the field capacity water content is available for percolation, lateral flow or tile flow drainage unless the temperature of the soil layer is below 0°C. When the soil layer is frozen, no water movement is calculated.
Table 2:3-1: SWAT+ input variables used in percolation calculations.
Area of the subbasin (km)
: effective hydraulic conductivity (mm/hr)
: average width of tributary channel (m)
: Longest tributary channel length in subbasin (km)
: Hillslope length (m)
: Saturated hydraulic conductivity (mm/hr)
: Average slope of the subbasin (m/m)
: Lateral flow travel time (days)
: Drain tile lag time (hrs)
Definition | Source Name | Input Name | Input File |
---|---|---|---|
Variable Name | Definition | File Name |
---|---|---|
Variable Name | Definition | File Name |
---|---|---|
: soil evaporation compensation coefficient
esco
esco
GW_DELAY
: Delay time for aquifer recharge (days)
.gw
GWQMN
: Threshold water level in shallow aquifer for base flow (mm HO)
.gw
ALPHA_BF
: Baseflow recession constant
.gw
REVAPMN
: Threshold water level in shallow aquifer for revap (mm HO)
.gw
GW_REVAP
: Revap coefficient
.gw
RCHRG_DP
: Aquifer percolation coefficient
.gw
GW_SPYLD
: Specific yield of the shallow aquifer (m/m)
.gw
ICRK
Bypass flow code: 0-do not model bypass flow; 1-model bypass flow
.bsn
SOL_CRK
: Potential crack volume for soil profile
.sol
CLAY
: Percent clay content
.sol
SOL_BD
: Bulk density (Mg m)
.sol
SOL_AWC
: available water capacity
.sol