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Impoundments play an important role in water supply and flood control. SWAT+ models four types of water bodies: ponds, wetlands, depressions/potholes, and reservoirs. Ponds, wetlands, and depressions/potholes are located within a subbasin off the main channel. Water flowing into these water bodies must originate from the subbasin in which the water body is located. Reservoirs are located on the main channel network. They receive water from all subbasins upstream of the water body.
Impoundment structures modify the movement of water in the channel network by lowering the peak flow and volume of flood discharges. Because impoundments slow down the flow of water, sediment will fall from suspension, removing nutrient and chemicals adsorbed to the soil particles.
A reservoir is an impoundment located on the main channel network of a watershed. No distinction is made between naturally-occurring and man-made structures. The features of an impoundment are shown in Figure 8:1.1.
The water balance for a reservoir is:
8:1.1.1
where is the volume of water in the impoundment at the end of the day (m HO), is the volume of water stored in the water body at the beginning of the day (m HO), is the volume of water entering the water body during the day (m HO), is the volume of water flowing out of the water body during the day (m HO), is the volume of precipitation falling on the water body during the day (m HO), is the volume of water removed from the water body by evaporation during the day (m HO), and is the volume of water lost from the water body by seepage (m HO).
The volume of water lost to evaporation on a given day is calculated:
8:1.1.6
where is the volume of water removed from the water body by evaporation during the day (m HO), is an evaporation coefficient (0.6), is the potential evapotranspiration for a given day (mm HO), and is the surface area of the water body (ha).
The surface area of the reservoir is needed to calculate the amount of precipitation falling on the water body as well as the amount of evaporation and seepage. Surface area varies with change in the volume of water stored in the reservoir. The surface area is updated daily using the equation:
8:1.1.2
where is the surface area of the water body (ha), is a coefficient, is the volume of water in the impoundment (m HO), and is an exponent.
The coefficient, , and exponent, , are calculated by solving equation 8:1.1.2 using two known points. The two known points are surface area and volume information provided for the principal and emergency spillways.
8:1.1.3
8:1.1.4
where is the surface area of the reservoir when filled to the emergency spillway (ha), is the surface area of the reservoir when filled to the principal spillway (ha), is the volume of water held in the reservoir when filled to the emergency spillway (m HO), and is the volume of water held in the reservoir when filled to the principal spillway (m HO).
The volume of precipitation falling on the reservoir during a given day is calculated:
8:1.1.5
where is the volume of water added to the water body by precipitation during the day (m HO), is the amount of precipitation falling on a given day (mm HO), and is the surface area of the water body (ha).
The volume of water lost by seepage through the bottom of the reservoir on a given day is calculated:
8:1.1.7
where is the volume of water lost from the water body by seepage (m HO), is the effective saturated hydraulic conductivity of the reservoir bottom (mm/hr), and is the surface area of the water body (ha).
The volume of outflow may be calculated using one of four different methods: measured daily outflow, measured monthly outflow, average annual release rate for uncontrolled reservoir, controlled outflow with target release.
When measured daily outflow (IRESCO = 3) is chosen as the method to calculate reservoir outflow, the user must provide a file with the outflow rate for every day the reservoir is simulated in the watershed. The volume of outflow from the reservoir is then calculated:
8:1.1.8
where is the volume of water flowing out of the water body during the day (m HO), and is the outflow rate (m/s).
When measured monthly outflow (IRESCO = 1) is chosen as the method to calculate reservoir outflow, the user must provide a file with the average daily outflow rate for every month the reservoir is simulated in the watershed. The volume of outflow from the reservoir is then calculated using equation 8:1.1.8.
The surface area of the pond or wetland is needed to calculate the amount of precipitation falling on the water body as well as the amount of evaporation and seepage. Surface area varies with change in the volume of water stored in the impoundment. The surface area is updated daily using the equation:
8:1.2.2
where is the surface area of the water body (ha), is a coefficient, is the volume of water in the impoundment (m HO), and is an exponent.
The coefficient, , and exponent, , are calculated by solving equation 8:1.1.2 using two known points. For ponds, the two known points are surface area and volume information provided for the principal and emergency spillways.
8:1.2.3
8:1.2.4
where is the surface area of the pond when filled to the emergency spillway (ha), is the surface area of the pond when filled to the principal spillway (ha), is the volume of water held in the pond when filled to the emergency spillway (m HO), and is the volume of water held in the pond when filled to the principal spillway(m HO). For wetlands, the two known points are surface area and volume information provided for the maximum and normal water levels.
8:1.2.5
8:1.2.6
where is the surface area of the wetland when filled to the maximum water level (ha), is the surface area of the wetland when filled to the normal water level (ha), is the volume of water held in the wetland when filled to the maximum water level (m HO), and is the volume of water held in the wetland when filled to the normal water level (m HO).
When the average annual release rate (IRESCO = 0) is chosen as the method to calculate reservoir outflow, the reservoir releases water whenever the reservoir volume exceeds the principal spillway volume, . If the reservoir volume is greater than the principal spillway volume but less than the emergency spillway volume, the amount of reservoir outflow is calculated:
if 8:1.1.9
if 8:1.1.10
If the reservoir volume exceeds the emergency spillway volume, the amount of outflow is calculated:
if 8:1.1.11
if 8:1.1.12
where is the volume of water flowing out of the water body during the day (m HO), is the volume of water stored in the reservoir (m HO), is the volume of water held in the reservoir when filled to the principal spillway (m HO), is the volume of water held in the reservoir when filled to the emergency spillway (m HO), and is the average daily principal spillway release rate (m/s).
The volume of precipitation falling on the pond or wetland during a given day is calculated:
8:1.2.7
where is the volume of water added to the water body by precipitation during the day (m HO), is the amount of precipitation falling on a given day (mm HO), and is the surface area of the water body (ha).
When target release (IRESCO = 2) is chosen as the method to calculate reservoir outflow, the reservoir releases water as a function of the desired target storage.
The target release approach tries to mimic general release rules that may be used by reservoir operators. Although the method is simplistic and cannot account for all decision criteria, it can realistically simulate major outflow and low flow periods.
For the target release approach, the principal spillway volume corresponds to maximum flood control reservation while the emergency spillway volume corresponds to no flood control reservation. The model requires the beginning and ending month of the flood season. In the non-flood season, no flood control reservation is required, and the target storage is set at the emergency spillway volume. During the flood season, the flood control reservation is a function of soil water content. The flood control reservation for wet ground conditions is set at the maximum. For dry ground conditions, the flood control reservation is set at 50% of the maximum.
The target storage may be specified by the user on a monthly basis or it can be calculated as a function of flood season and soil water content. If the target storage is specified:
8:1.1.13
where is the target reservoir volume for a given day (m HO), and is the target reservoir volume specified for a given month (m HO). If the target storage is not specified, the target reservoir volume is calculated:
if 8:1.1.14
if or 8:1.1.15
where is the target reservoir volume for a given day (m HO), is the volume of water held in the reservoir when filled to the emergency spillway (m HO), is the volume of water held in the reservoir when filled to the principal spillway (m HO), is the average soil water content in the subbasin (mm HO), is the water content of the subbasin soil at field capacity (mm HO), is the month of the year, is the beginning month of the flood season, and is the ending month of the flood season.
Once the target storage is defined, the outflow is calculated:
8:1.1.16
where is the volume of water flowing out of the water body during the day (m HO), is the volume of water stored in the reservoir (m HO), is the target reservoir volume for a given day (m HO), and is the number of days required for the reservoir to reach target storage.
Once outflow is determined using one of the preceding four methods, the user may specify maximum and minimum amounts of discharge that the initial outflow estimate is checked against. If the outflow doesn’t meet the minimum discharge or exceeds the maximum specified discharge, the amount of outflow is altered to meet the defined criteria.
if 8:1.1.17
if 8:1.1.18
if 8:1.1.19
where is the volume of water flowing out of the water body during the day (m HO), is the initial estimate of the volume of water flowing out of the water body during the day (m HO), is the minimum average daily outflow for the month (m/s), and is the maximum average daily outflow for the month (m/s).
Table 8:1-1: SWAT+ input variables that pertain to reservoirs.
Variable Name | Definition | File Name |
---|---|---|
RES_ESA
: Surface area of the reservoir when filled to the emergency spillway (ha)
.res
RES_PSA
: Surface area of the reservoir when filled to the principal spillway (ha)
.res
RES_EVOL
: Volume of water held in the reservoir when filled to the emergency spillway (10 m HO)
.res
RES_PVOL
: Volume of water held in the reservoir when filled to the principal spillway (10 m HO)
.res
RES_K
:Effective saturated hydraulic conductivity of the reservoir bottom (mm/hr)
.res
IRESCO
Outflow method
.res
RES_OUTFLOW
: Outflow rate (m/s)
resdayo.dat
RESOUT
: Outflow rate (m/s)
resmono.dat
RES_RR
: Average daily principal spillway release rate (m/s)
.res
STARG(mon)
: Target reservoir volume specified for a given month (m HO)
.res
IFLOD1R
: Beginning month of the flood season
.res
IFLOD2R
: Ending month of the flood season
.res
NDTARGR
: Number of days required for the reservoir to reach target storage
.res
OFLOWMN(mon)
: Minimum average daily outflow for the month (m/s)
.res
OFLOWMX(mon)
: Maximum average daily outflow for the month (m/s)
.res
The primary difference between ponds and wetlands is the method in which the outflow is calculated.
Pond outflow is calculated as a function of target storage. The target storage varies based on flood season and soil water content. The target pond volume is calculated:
if 8:1.2.11
if or 8:1.2.12
where is the target pond volume for a given day (m HO), is the volume of water held in the pond when filled to the emergency spillway (m HO), is the volume of water held in the pond when filled to the principal spillway (m HO), is the average soil water content in the subbasin (mm HO), is the water content of the subbasin soil at field capacity (mm HO), is the month of the year, is the beginning month of the flood season, and is the ending month of the flood season.
Once the target storage is defined, the outflow is calculated:
8:1.2.13
where is the volume of water flowing out of the water body during the day (m HO), is the volume of water stored in the pond (m HO), is the target pond volume for a given day (m HO), and is the number of days required for the pond to reach target storage.
The volume of water entering the pond or wetland on a given day is calculated:
8:1.2.8
where is the volume of water flowing into the water body on a given day (m HO), is the fraction of the subbasin area draining into the impoundment, is the surface runoff from the subbasin on a given day (mm HO), is the groundwater flow generated in a subbasin on a given day (mm HO), is the lateral flow generated in a subbasin on a given day (mm HO), is the subbasin area (ha), and is the surface area of the water body (ha). The volume of water entering the pond or wetland is subtracted from the surface runoff, lateral flow and groundwater loadings to the main channel.
The volume of water lost by seepage through the bottom of the pond or wetland on a given day is calculated:
8:1.2.10
where is the volume of water lost from the water body by seepage (m HO), is the effective saturated hydraulic conductivity of the pond or wetland bottom (mm/hr), and is the surface area of the water body (ha).
The volume of water lost to evaporation on a given day is calculated:
8:1.2.9
where is the volume of water removed from the water body by evaporation during the day (m HO), is an evaporation coefficient (0.6), is the potential evapotranspiration for a given day (mm HO), and is the surface area of the water body (ha).
Ponds and wetlands are water bodies located within subbasins that received inflow from a fraction of the subbasin area. The algorithms used to model these two types of water bodies differ only in the options allowed for outflow calculation.
The water balance for a pond or wetland is:
8:1.2.1
where is the volume of water in the impoundment at the end of the day (m HO), is the volume of water stored in the water body at the beginning of the day (m HO), is the volume of water entering the water body during the day (m HO), is the volume of water flowing out of the water body during the day (m HO), is the volume of precipitation falling on the water body during the day (m HO), is the volume of water removed from the water body by evaporation during the day (m HO), and is the volume of water lost from the water body by seepage (m HO). is added to shallow aquifer storage.
In areas of low relief and/or young geologic development, the drainage network may be poorly developed. Watersheds in these areas may have many closed depressional areas, referred to as potholes. Runoff generated within these areas flows to the lowest portion of the pothole rather than contributing to flow in the main channel. Other systems that are hydrologically similar to potholes include playa lakes and fields that are artifically impounded for rice production. The algorithms reviewed in this section are used to model these types of systems.
To define an HRU as a pothole, the user must set IPOT (.hru) to the HRU number. To initiate water impoundment, a release/impound operation must be placed in the .mgt file. The water balance for a pothole is:
8:1.3.1
where is the volume of water in the impoundment at the end of the day (m HO), is the volume of water stored in the water body at the beginning of the day (m HO), is the volume of water entering the water body during the day (m HO), is the volume of water flowing out of the water body during the day (m HO), is the volume of precipitation falling on the water body during the day (m HO), is the volume of water removed from the water body by evaporation during the day (m HO), and is the volume of water lost from the water body by seepage (m HO).
Water entering the pothole on a given day may be contributed from any HRU in the subbasin. To route a portion of the flow from an HRU into a pothole, the variable IPOT (.hru) is set to the number of the HRU containing the pothole and POT_FR (.hru) is set to the fraction of the HRU area that drains into the pothole. This must be done for each HRU contributing flow to the pothole. Water routing from other HRUs is performed only during the period that water impoundment has been activated (release/impound operation in .mgt). Water may also be added to the pothole with an irrigation operation in the management file (.mgt). Chapter 6:2 reviews the irrigation operation.
The inflow to the pothole is calculated:
8:1.3.4
where is the volume of water flowing into the pothole on a given day (m HO), is the amount of water added through an irrigation operation on a given day (m HO), is the number of HRUs contributing water to the pothole, is the fraction of the HRU area draining into the pothole, is the surface runoff from the HRU on a given day (mm HO), is the groundwater flow generated in the HRU on a given day (mm HO), is the lateral flow generated in the HRU on a given day (mm HO), and is the HRU area (ha).
The volume of water lost to evaporation on a given day is calculated:
if 8:1.3.5
if 8:1.3.6
where is the volume of water removed from the water body by evaporation during the day (m HO), is the leaf area index of the plants growing in the pothole, is the leaf area index at which no evaporation occurs from the water surface, is the potential evapotranspiration for a given day (mm HO), and is the surface area of the water body (ha).
The surface area of the pothole is needed to calculate the amount of precipitation falling on the water body as well as the amount of evaporation and seepage. Surface area varies with change in the volume of water stored in the impoundment. For surface area calculations, the pothole is assumed to be cone-shaped. The surface area is updated daily using the equation:
8:1.3.2
where is the surface area of the water body (ha), is the volume of water in the impoundment (m HO), and is the slope of the HRU (m/m).
The volume of precipitation falling on the pothole during a given day is calculated:
8:1.3.3
where is the volume of water added to the water body by precipitation during the day (m HO), is the amount of precipitation falling on a given day (mm HO), and is the surface area of the water body (ha).
The wetland releases water whenever the water volume exceeds the normal storage volume, . Wetland outflow is calculated:
if 8:1.2.14
if 8:1.2.15
if 8:1.2.16
where is the volume of water flowing out of the water body during the day (m HO), is the volume of water stored in the wetland (m HO), is the volume of water held in the wetland when filled to the maximum water level (m HO), and is the volume of water held in the wetland when filled to the normal water level (m HO).
Table 8:1-2: SWAT+ input variables that pertain to ponds and wetlands.
PND_ESA
: Surface area of the pond when filled to the emergency spillway (ha)
.pnd
PND_PSA
: Surface area of the pond when filled to the principal spillway (ha)
.pnd
PND_EVOL
: Volume of water held in the pond when filled to the emergency spillway (10 m HO)
.pnd
PND_PVOL
: Volume of water held in the pond when filled to the principal spillway(10 m HO)
.pnd
WET_MXSA
: Surface area of the wetland when filled to the maximum water level (ha)
.pnd
WET_NSA
: Surface area of the wetland when filled to the normal water level (ha)
.pnd
WET_MXVOL
: Volume of water held in the wetland when filled to the maximum water level (m HO)
.pnd
WET_NVOL
: Volume of water held in the wetland when filled to the normal water level (m HO)
.pnd
PND_FR
: Fraction of the subbasin area draining into the pond
.pnd
WET_FR
: Fraction of the subbasin area draining into the wetland
.pnd
PND_K
: Effective saturated hydraulic conductivity of the pond bottom (mm/hr)
.pnd
WET_K
: Effective saturated hydraulic conductivity of the wetland bottom (mm/hr)
.pnd
IFLOD1
: Beginning month of the flood season
.pnd
IFLOD2
: Ending month of the flood season
.pnd
NDTARG
: Number of days required for the reservoir to reach target storage
.pnd
Water may be removed from the pothole in three different types of outflow. When the volume of water in the pothole exceeds the maximum storage, the excess water is assumed to overflow and enter the main channel in the subbasin. When the retaining wall or berm is removed (this is done with a release/impound operation in the management file), all water stored in the pothole enters the main channel. The third type of flow from the pothole is via drainage tiles installed in the pothole.
Pothole outflow caused by overflow is calculated:
if 8:1.3.10
where is the volume of water flowing out of the water body during the day (m HO), is the volume of water stored in the pothole (m HO), and is the maximum amount of water that can be stored in the pothole (m HO).
When a release operation is scheduled, all water in the pothole becomes outflow:
8:1.3.11
where is the volume of water flowing out of the water body during the day (m HO), and is the volume of water stored in the pothole (m HO).
SWAT+ incorporates a simple mass balance model to simulate the transport of sediment into and out of water bodies. SWAT+ defines four different types of water bodies: ponds, wetlands, reservoirs and potholes. Sediment processes modeled in ponds, wetlands, reservoirs, and potholes are identical.
When calculating sediment movement through a water body, SWAT+ assumes the system is completely mixed. In a completely mixed system, as sediment enters the water body it is instantaneously distributed throughout the volume.
The volume of water lost by seepage through the bottom of the pothole on a given day is calculated as a function of the water content of the soil profile beneath the pothole.
if 8:1.3.7
if 8:1.3.8
if 8:1.3.9
where is the volume of water lost from the water body by seepage (m HO), is the effective saturated hydraulic conductivity of the 1st soil layer in the profile (mm/hr), is the surface area of the water body (ha), is the soil water content of the profile on a given day (mm HO), and is the field capacity soil water content (mm HO). Water lost from the pothole by seepage is added to the soil profile.
When drainage tiles are installed in a pothole, the pothole will contribute water to the main channel through tile flow. The pothole outflow originating from tile drainage is:
if 8:1.3.12
if 8:1.3.13
where is the volume of water flowing out of the water body during the day (m HO), is the average daily tile flow rate (m/s), and is the volume of water stored in the pothole (m HO).
Table 8:1-3: SWAT+ input variables that pertain to potholes.
Variable Name | Definition | File Name |
---|---|---|
IPOT
Number of HRU that is impounding water (that contains the pothole)
.hru
MONTH/DAY or HUSC
Timing of release/impound operation.
.mgt
MGT_OP
Operation code. MGT_OP = 13 for release/impound operation
.mgt
IMP_TRIG
Release/impound action code: 0: impound, 1: release
.mgt
SLOPE
: Slope of the HRU (m/m)
.hru
POT_FR
: Fraction of the HRU area draining into the pothole
.hru
EVLAI
: Leaf area index at which no evaporation occurs from the water surface
.bsn
POT_VOLX
: Maximum amount of water that can be stored in the pothole (mm)
.hru
POT_TILE
: Average daily tile flow rate (mm)
.hru
Assuming that the volume of the water body remains constant over time, the processes described above (inflow, settling, outflow) can be combined into the following mass balance equation for a well-mixed water body:
8:3.2.1
where is the volume of the system (m HO), is the concentration of nutrient in the system (kg/m HO), is the length of the time step (1 day), is the amount of nutrient entering the water body during the day (kg/day), is the rate of water flow exiting the water body (m HO/day), is the apparent settling velocity (m/day), and is the area of the sediment-water interface (m).
The mass balance equation for sediment in a water body is:
8:2.1.1
where is the amount of sediment in the water body at the end of the day (metric tons), is the amount of sediment in the water body at the beginning of the day (metric tons), is the amount of sediment added to the water body with inflow (metric tons), is the amount of sediment removed from the water by settling (metric tons), is the amount of sediment transported out of the water body with outflow (metric tons).
The amount of sediment transported out of the water body on a given day is calculated as a function of the final concentration. The initial suspended solid concentration is:
8:2.3.1
where is the amount of sediment transported out of the water body with outflow (metric tons), is the final sediment concentration in the water body (Mg/m), and is the volume of outflow from the impoundment (m HO).
SWAT+ incorporates a simple empirical model to predict the trophic status of water bodies. For studies that require detailed modeling of lake water quality, SWAT+ has been linked to distributed lake water quality models such as WASP.
SWAT+ defines four different types of water bodies: ponds, wetlands, reservoirs and depressional/impounded areas (potholes). Nutrient processes modeled in ponds, wetlands and reservoirs are identical. Nutrient processes are not yet modeled in potholes.
Incoming sediment is deposited using a modified overflow rate model (EPA 1986, cited in Haan et al., 1994). For each day, the deposition routine begins with the computation of the detention times. The actual detention time is based upon the ratio of the impoundment volume to the outflow rate.
8:2.2.1
Where is detention time (), is an empirical parameter to account for impoundment geometry, hydraulic response, and stratification of the suspended sediment, is the dead storage (the portion of the pond are that does not contribute to settling) (Griffin et al., 1985), Vol is the average impoundment volume over the time step (ft), and is the average outflow rate over the time step (ft s). The detention time required for 100% of the suspended sediment to settle out of suspension is computed form the average impoundment depth (volume / area) and the settling velocity.
The trapping efficiency is calculated as
8:2.2.2
Where is trapping efficiency (fraction), is the settling velocity (m/d), is overflow velocity (m/d). is defined as
8:2.2.3
Where is reservoir outflow in m and is reservoir surface area in .
During days of no sediment inflow the amount of suspended solid settling that occurs in the water body on a given day is calculated as a function of concentration. The initial suspended solid concentration is:
8:2.2.4
where is the initial concentration of suspended solids in the water (Mg/m), is the amount of sediment in the water body at the beginning of the day (metric tons), is the amount of sediment added to the water body with inflow (metric tons), is the volume of water stored in water body or channel at the beginning of the day (m HO), and is the volume of water entering water body on given day (m HO).
Settling occurs only when the sediment concentration in the water body exceeds the equilibrium sediment concentration specified by the user, . The concentration of sediment in the water body at the end of the day is calculated:
if 8:2.2.5
if 8:2.2.6
where is the final sediment concentration in the water body (Mg/m), is the initial concentration of suspended solids in the water body (Mg/m), is the equilibrium concentration of suspended solids in the water body (Mg/m), is the decay constant (1/day), is the length of the time step (1 day), and is the median particle size of the inflow sediment (µm). Assuming 99% of the 1 µm size particles settle out of solution within 25 days, is equal to 0.184.
For ponds, wetlands, and potholes, the median particle size of the inflow sediment is calculated:
8:2.2.7
where is the median particle size of the inflow sediment (m), is percent clay in the surface soil layer in the subbasin, is the percent silt in the surface soil layer in the subbasin, is the percent sand in the surface soil layer in the subbasin. Because reservoirs are located on the main channel network and receive sediment from the entire area upstream, defaulting the sand, silt, and clay fractions to those of a single subbasin or HRU in the upstream area is not appropriate. Instead the user is allowed to set the median particle size diameter to a representative value for reservoirs.
The amount of sediment settling out of solution on a given day is then calculated:
8:2.2.8
where is the amount of sediment removed from the water by settling (metric tons), is the initial concentration of suspended solids in the water body (Mg/m), is the final sediment concentration in the water body (Mg/m), and is the volume of water in the impoundment (m HO).
Table 8:2-1: SWAT+ input variables that pertain to sediment settling.
When calculating nutrient transformations in a water body, SWAT+ assumes the system is completely mixed. In a completely mixed system, as nutrients enter the water body they are instantaneously distributed throughout the volume. The assumption of a completely mixed system ignores lake stratification and intensification of phytoplankton in the epilimnion.
The initial amount of nitrogen and phosphorus in the water body on the given day is calculated by summing the mass of nutrient entering the water body on that day with the mass of nutrient already present in the water body.
8:3.1.1
where is the initial mass of nutrient in the water body for the given day (kg), is the mass of nutrient in the water body at the end of the previous day (kg), and is the mass of nutrient added to the water body on the given day (kg).
In a similar manner, the initial volume of water in the water body is calculated by summing the volume of water entering the water body on that day with the volume already present in the water body.
8:3.1.2
where is the initial volume of water in the water body for a given day (m HO), is the volume of water in the water body at the end of the previous day (m HO), and is the volume of water entering the water body on the given day (m HO).
The initial concentration of nutrients in the water body is calculated by dividing the initial mass of nutrient by the initial volume of water.
Nutrient transformations simulated in ponds, wetlands and reservoirs are limited to the removal of nutrients by settling. Transformations between nutrient pools (e.g. NO3 NO2 NH4) are ignored.
Settling losses in the water body can be expressed as a flux of mass across the surface area of the sediment-water interface (Figure 8:3-1) (Chapra, 1997).
The mass of nutrient lost via settling is calculated by multiplying the flux by the area of the sediment-water interface.
8:3.1.3
where is the mass of nutrient lost via settling on a day (kg), is the apparent settling velocity (m/day), is the area of the sediment-water interface (m), is the initial concentration of nutrient in the water (kg/m HO), and is the length of the time step (1 day). The settling velocity is labeled as “apparent” because it represents the net effect of the different processes that deliver nutrients to the water body’s sediments. The water body is assumed to have a uniform depth of water and the area of the sediment-water interface is equivalent to the surface area of the water body.
The apparent settling velocity is most commonly reported in units of m/year and this is how the values are input to the model. For natural lakes, measured phosphorus settling velocities most frequently fall in the range of 5 to 20 m/year although values less than 1 m/year to over 200 m/year have been reported (Chapra, 1997). Panuska and Robertson (1999) noted that the range in apparent settling velocity values for man-made reservoirs tends to be significantly greater than for natural lakes. Higgins and Kim (1981) reported phosphorus apparent settling velocity values from –90 to 269 m/year for 18 reservoirs in Tennessee with a median value of 42.2 m/year. For 27 Midwestern reservoirs, Walker and Kiihner (1978) reported phosphorus apparent settling velocities ranging from –1 to 125 m/year with an average value of 12.7 m/year. A negative settling rate indicates that the reservoir sediments are a source of N or P; a positive settling rate indicates that the reservoir sediments are a sink for N or P.
A number of inflow and impoundment properties affect the apparent settling velocity for a water body. Factors of particular importance include the form of phosphorus in the inflow (dissolved or particulate) and the settling velocity of the particulate fraction. Within the impoundment, the mean depth, potential for sediment resuspension and phosphorus release from the sediment will affect the apparent settling velocity (Panuska and Robertson, 1999). Water bodies with high internal phosphorus release tend to possess lower phosphorus retention and lower phosphorus apparent settling velocities than water bodies with low internal phosphorus release (Nürnberg, 1984). Table 8:3-1 summarizes typical ranges in phosphorus settling velocity for different systems.
Table 8:3-1: Recommended apparent settling velocity values for phosphorus (Panuska and Robertson, 1999)
SWAT+ input variables that pertain to nutrient settling in ponds, wetlands and reservoirs are listed in Table 8:3-2. The model allows the user to define two settling rates for each nutrient and the time of the year during which each settling rate is used. A variation in settling rates is allowed so that impact of temperature and other seasonal factors may be accounted for in the modeling of nutrient settling. To use only one settling rate for the entire year, both variables for the nutrient may be set to the same value. Setting all variables to zero will cause the model to ignore settling of nutrients in the water body.
After nutrient losses in the water body are determined, the final concentration of nutrients in the water body is calculated by dividing the final mass of nutrient by the initial volume of water. The concentration of nutrients in outflow from the water body is equivalent to the final concentration of the nutrients in the water body for the day. The mass of nutrient in the outflow is calculated by multiplying the concentration of nutrient in the outflow by the volume of water leaving the water body on that day.
Variable Name | Definition | Input File |
---|---|---|
Nutrient Dynamics | Range in settling velocity values (m/year) |
---|---|
Variable Name | Definition | Input File |
---|---|---|
RES_NSED
: Equilibrium sediment concentration in water body (mg/L)
.res
PND_NSED
: Equilibrium sediment concentration in water body (mg/L)
.pnd
WET_NSED
: Equilibrium sediment concentration in water body (mg/L)
.pnd
POT_NSED
: Equilibrium sediment concentration in water body (mg/L)
.hru
CLAY
: Percent clay in the surface soil layer in the subbasin
.sol
SILT
: Percent silt in the surface soil layer in the subbasin
.sol
SAND
: Percent sand in the surface soil layer in the subbasin
.sol
RES_D50
: Median particle size of sediment in a reservoir
.res
Shallow water bodies with high net internal phosphorus flux
Water bodies with moderate net internal phosphorus flux
Water bodies with minimal net internal phosphorus flux
Water bodies with high net internal phosphorus removal
IPND1
Beginning month of mid-year nutrient settling period for pond and wetland modeled in subbasin
.pnd
IPND2
Ending month of mid-year nutrient settling period for pond and wetland modeled in subbasin
.pnd
PSETL1
Phosphorus settling rate in pond during mid-year nutrient settling period (IPND1 month IPND2) (m/year)
.pnd
PSETL2
Phosphorus settling rate in pond during time outside mid-year nutrient settling period ( month < IPND1 or month > IPND2) (m/year)
.pnd
NSETL1
Nitrogen settling rate in pond during mid-year nutrient settling period (IPND1 month IPND2) (m/year)
.pnd
NSETL2
Nitrogen settling rate in pond during time outside mid-year nutrient settling period ( month < IPND1 or month > IPND2) (m/year)
.pnd
PSETLW1
Phosphorus settling rate in wetland during mid-year nutrient settling period (IPND1 month IPND2) (m/year)
.pnd
PSETLW2
Phosphorus settling rate in wetland during time outside mid-year nutrient settling period ( month < IPND1 or month > IPND2) (m/year)
.pnd
NSETLW1
Nitrogen settling rate in wetland during mid-year nutrient settling period (IPND1 month IPND2) (m/year)
.pnd
NSETLW2
Nitrogen settling rate in wetland during time outside mid-year nutrient settling period ( month < IPND1 or month > IPND2) (m/year)
.pnd
IRES1
Beginning month of mid-year nutrient settling period for reservoir
.lwq
IRES2
Ending month of mid-year nutrient settling period for reservoir
.lwq
PSETLR1
Phosphorus settling rate in reservoir during mid-year nutrient settling period (IRES1 month IRES2) (m/year)
.lwq
PSETLR2
Phosphorus settling rate in reservoir during time outside mid-year nutrient settling period ( month < IRES1 or month > IRES2) (m/year)
.lwq
NSETLR1
Nitrogen settling rate in reservoir during mid-year nutrient settling period (IRES1 month IRES2) (m/year)
.lwq
NSETLR2
Nitrogen settling rate in reservoir during time outside mid-year nutrient settling period ( month < IRES1 or month > IRES2) (m/year)
.lwq
Pesticide in a well-mixed water body is increased through addition of mass in inflow, resuspension and diffusion from the sediment layer. The amount of pesticide in a well-mixed water body is reduced through removal in outflow, degradation, volatilization, settling and diffusion into the underlying sediment.
Under favorable conditions of light and temperature, excess amounts of nutrients in water can increase the growth of algae and other plants. The result of this growth is an increase in the rate of eutrophication, which is a natural ecological process of change from a nutrient-poor to a nutrient-rich environment. Eutrophication is defined as the process by which a body of water becomes enriched in dissolved nutrients (as phosphates) that stimulate the growth of aquatic plant life, usually resulting in the depletion of dissolved oxygen (Merriam-Webster, Inc., 1996).
Nutrient enrichment of moving waters and lakes is a normal result of soil weathering and erosion processes. The gradual evolution of Ice Age lakes into marshes and, eventually, organic soils is a result of eutrophication. However, this process can be accelerated by the discharge of wastes containing high levels of nutrients into lakes or rivers. One example of this is Lake Erie, which is estimated to have aged the equivalent of 150 natural years in a 15-year span of accelerated eutrophication.
Excessive plant growth caused by accelerated eutrophication can lead to stagnation of the water. The stagnation is caused by an increased biological oxygen demand created by decaying plant remains. The result of this increased oxygen demand is a tendency toward anaerobic conditions and the inability of the water body to support fish and other aerobic organisms.
Nitrogen, carbon and phosphorus are essential to the growth of aquatic biota. Due to the difficulty of controlling the exchange of nitrogen and carbon between the atmosphere and water and fixation of atmospheric nitrogen by some blue-green algae, attempts to mitigate eutrophication have focused on phosphorus inputs. In fresh-water systems, phosphorus is often the limiting element. By controlling phosphorus loading, accelerated eutrophication of lake waters can be reduced.
In systems where phosphorus is the primary, controllable limiting nutrient of water body eutrophication, the amount of phosphorus present in the water body can be used to estimate the amount of eutrophication present in the water body.
Pesticides will partition into particulate and dissolved forms. The fraction of pesticide in each phase is a function of the pesticide’s partition coefficient and the water body’s suspended solid concentration:
8:4.1.1
8:4.1.2
where is the fraction of total pesticide in the dissolved phase, is the fraction of total pesticide in the particulate phase, is the pesticide partition coefficient (m/g), and is the concentration of suspended solids in the water (g/m).
The pesticide partition coefficient can be estimated from the octanol-water partition coefficient (Chapra, 1997):
8:4.1.3
where is the pesticide partition coefficient (m/g) and is the pesticide’s octanol-water partition coefficient (mg (mg ). Values for the octanol-water partition coefficient have been published for many chemicals. If a published value cannot be found, it can be estimated from solubility (Chapra, 1997):
8:4.1.4
where is the pesticide solubility (moles/L). The solubility in these units is calculated:
8:4.1.5
where is the pesticide solubility (moles/L), is the pesticide solubility (mg/L) and is the molecular weight (g/mole).
Pesticides in both the particulate and dissolved forms are subject to degradation. The amount of pesticide that is removed from the water via degradation is:
8:4.1.6
where is the amount of pesticide removed from the water via degradation (mg pst), is the rate constant for degradation or removal of pesticide in the water (1/day), and is the amount of pesticide in the water at the beginning of the day (mg pst). The rate constant is related to the aqueous half-life:
8:4.1.7
where is the rate constant for degradation or removal of pesticide in the water (1/day), and is the aqueous half-life for the pesticide (days).
SWAT+ incorporates a simple mass balance developed by Chapra (1997) to model the transformation and transport of pesticides in water bodies. The model assumes a well-mixed layer of water overlying a sediment layer. Figure 8:4-1 illustrates the mechanisms affecting the pesticide mass balance in water bodies.
SWAT+ defines four different types of water bodies: ponds, wetlands, reservoirs and depressional/impounded areas (potholes). Pesticide processes are modeled only in reservoirs.
A number of empirically derived equations have been developed to calculate chlorophyll a level as a function of total phosphorus concentration. SWAT+ uses an equation developed by Rast and Lee (1978) to calculate the chlorophyll a concentration in the water body.
8:3.3.1
where is the chlorophyll concentration (g/L) and is the total phosphorus concentration (g/L).
The equation has been modified to include a user-defined coefficient:
8:3.3.2
The user-defined coefficient, , is included to allow the user to adjust the predicted chlorophyll concentration for limitations of nutrients other than phosphorus. When is set to 1.00, equation 8:3.3.2 is equivalent to equation 8:3.3.1. For most water bodies, the original equation will be adequate.
While evaluation of water quality by secchi-disk depth measurements is subjective, some general correlations between secchi-disk depth and public perception of water quality have been made. One such correlation made for Annebessacook Lake in Maine (EPA, 1980) is given in Table 8:3-3.
Table 8:3-3: Relationship between secchi-disk depth and public perception of water quality.
Secchi-disk depth (m) | Public perception of water quality |
---|---|
Table 8:3-4: SWAT+ input variables that impact eutrophication calculations in ponds, wetlands and reservoirs.
Variable Name | Definition | Input File |
---|---|---|
0.0-0.9
gross pollution; water body totally unsuitable for recreation
1.0-1.9
algae blooms still evident; quality unacceptable for most uses
2.0-2.9
some complaints of declining water quality; some impairment of water use
3.0-3.9
satisfactory quality; no impairment of water use
4.0-4.9
excellent water quality; a positive factor encouraging use of lake
5.0+
exceptional quality
CHLA
:variable for calculation of chlorophyll a concentration in a pond
.pnd
CHLAW
:variable for calculation of chlorophyll a concentration in a wetland
.pnd
CHLAR
:variable for calculation of chlorophyll a concentration in a reservoir
.lwq
SECCI
:variable for calculation of secchi-disk depth in a pond
.pnd
SECCIW
:variable for calculation of secchi-disk depth in a wetland
.pnd
SECCIR
:variable for calculation of secchi-disk depth in a reservoir
.lwq
Pesticide in the sediment layer underlying a water body is increased through addition of mass by settling and diffusion from the water. The amount of pesticide in the sediment layer is reduced through removal by degradation, resuspension, diffusion into the overlying water, and burial.
Pesticide in the dissolved phase is available for volatilization. The amount of pesticide removed from the water via volatilization is:
8:4.1.8
where is the amount of pesticide removed via volatilization (mg pst), is the volatilization mass-transfer coefficient (m/day), is the surface area of the water body (m), is the fraction of total pesticide in the dissolved phase, is the amount of pesticide in the water (mg pst), and V is the volume of water in the water body(m HO).
The volatilization mass-transfer coefficient can be calculated based on Whitman’s two-film or two-resistance theory (Whitman, 1923; Lewis and Whitman, 1924 as described in Chapra, 1997). While the main body of the gas and liquid phases are assumed to be well-mixed and homogenous, the two-film theory assumes that a substance moving between the two phases encounters maximum resistance in two laminar boundary layers where transfer is a function of molecular diffusion. In this type of system the transfer coefficient or velocity is:
8:4.1.9
where is the volatilization mass-transfer coefficient (m/day), is the mass-transfer velocity in the liquid laminar layer (m/day), is the mass-transfer velocity in the gaseous laminar layer (m/day), is Henry’s constant (atm m mole), is the universal gas constant (8.206 10 atm m (K mole)), and is the temperature ().
For lakes, the transfer coefficients are estimated using a stagnant film approach:
8:4.1.10
where is the mass-transfer velocity in the liquid laminar layer (m/day), is the mass-transfer velocity in the gaseous laminar layer (m/day), is the liquid molecular diffusion coefficient (m/day), is the gas molecular diffusion coefficient (m/day), is the thickness of the liquid film (m), and is the thickness of the gas film (m).
Alternatively, the transfer coefficients can be estimated with the equations:
8:4.1.11
8:4.1.12
where is the mass-transfer velocity in the liquid laminar layer (m/day), is the mass-transfer velocity in the gaseous laminar layer (m/day), is the oxygen transfer coefficient (m/day), is the molecular weight of the compound, and is the wind speed (m/s). Chapra (1997) lists several different equations that can be used to calculate .
Pesticide is removed from the water body in outflow. The amount of dissolved and particulate pesticide removed from the water body in outflow is:
8:4.1.14
8:4.1.15
where is the amount of dissolved pesticide removed via outflow (mg pst), is the amount of particulate pesticide removed via outflow (mg pst), is the rate of outflow from the water body (m HO/day), is the fraction of total pesticide in the dissolved phase, is the fraction of total pesticide in the particulate phase, is the amount of pesticide in the water (mg pst), and is the volume of water in the water body (m HO).
Table 8:4-1: SWAT+ input variables that pesticide partitioning.
Variable Name | Definition | Input File |
---|
Pesticide in the particulate phase may be removed from the water layer by settling. Settling transfers pesticide from the water to the sediment layer. The amount of pesticide that is removed from the water via settling is:
8:4.1.13
where is the amount of pesticide removed from the water due to settling (mg pst), is the settling velocity (m/day), is the surface area of the water body (m), is the fraction of total pesticide in the particulate phase, is the amount of pesticide in the water (mg pst), and . is the volume of water in the water body (m HO).
Pesticides in both the particulate and dissolved forms are subject to degradation. The amount of pesticide that is removed from the sediment via degradation is:
8:4.2.8
where is the amount of pesticide removed from the sediment via degradation (mg pst), is the rate constant for degradation or removal of pesticide in the sediment (1/day), and is the amount of pesticide in the sediment (mg pst). The rate constant is related to the sediment half-life:
8:4.2.9
where is the rate constant for degradation or removal of pesticide in the sediment (1/day), and is the sediment half-life for the pesticide (days).
As in the water layer, pesticides in the sediment layer will partition into particulate and dissolved forms. Calculation of the solid-liquid partitioning in the sediment layer requires a suspended solid concentration. The “concentration” of solid particles in the sediment layer is defined as:
8:4.2.1
where is the “concentration” of solid particles in the sediment layer (g/m), is the mass of solid particles in the sediment layer (g) and is the total volume of the sediment layer (m).
Mass and volume are also used to define the porosity and density of the sediment layer. In the sediment layer, porosity is the fraction of the total volume in the liquid phase:
8:4.2.2
where is the porosity, is the volume of water in the sediment layer (m) and is the total volume of the sediment layer (m). The fraction of the volume in the solid phase can then be defined as:
8:4.2.3
where is the porosity, is the volume of solids in the sediment layer (m) and is the total volume of the sediment layer (m).
The density of sediment particles is defined as:
8:4.2.4
where is the particle density (g/m), is the mass of solid particles in the sediment layer (g), and is the volume of solids in the sediment layer (m).
Solving equation 8:4.2.3 for and equation 8:4.2.4 for and substituting into equation 8:4.2.1 yields:
8:4.2.5
where is the “concentration” of solid particles in the sediment layer (g/m), is the porosity, and is the particle density (g/m).
Typical values of porosity and particle density for fine-grained sediments are = 0.8-0.95 and = 2.4-2.7 *10 g/m (Chapra, 1997). Assuming = 0.8 and = 2.6*10 g/m, the “concentration” of solid particles in the sediment layer is 5.210 g/m.
The fraction of pesticide in each phase is then calculated:
8:4.2.6
8:4.2.7
where is the fraction of total sediment pesticide in the dissolved phase, is the fraction of total sediment pesticide in the particulate phase, is the porosity, is the particle density (g/m), and is the pesticide partition coefficient (m/g). The pesticide partition coefficient used for the water layer is also used for the sediment layer.
SWAT+ calculates loading of pathogens and indicator bacteria for pathogens from land areas in the watershed. In reservoirs, bacteria die-off is the only process modeled.
LKPST_KOC | : Pesticide partition coefficient (m/g) | .lwq |
LKPST_REA | : Rate constant for degradation or removal of pesticide in the water (1/day) | .lwq |
LKPST_VOL | : Volatilization mass-transfer coefficient (m/day) | .lwq |
LKPST_STL | : Pesticide settling velocity (m/day) | .lwq |
Pesticide in the sediment layer is available for resuspension. The amount of pesticide that is removed from the sediment via resuspension is:
8:4.2.10
where is the amount of pesticide removed via resuspension (mg pst), is the resuspension velocity (m/day), is the surface area of the water body (m), is the amount of pesticide in the sediment (mg pst), and is the volume of the sediment layer (m). The volume of the sediment layer is calculated:
8:4.2.11
where is the volume of the sediment layer (m), is the surface area of the water body (m), is the depth of the active sediment layer (m). Pesticide removed from the sediment layer by resuspension is added to the water layer.
Pesticide in the dissolved phase is available for diffusion. Diffusion transfers pesticide between the water and sediment layers. The direction of movement is controlled by the pesticide concentration. Pesticide will move from areas of high concentration to areas of low concentration. The amount of pesticide that is transferred between the water and sediment by diffusion is:
8:4.2.12
where is the amount of pesticide transferred between the water and sediment by diffusion (mg pst), is the rate of diffusion or mixing velocity (m/day), is the surface area of the water body (m), is the fraction of total sediment pesticide in the dissolved phase, is the amount of pesticide in the sediment (mg pst), is the volume of the sediment layer (m), is the fraction of total water layer pesticide in the dissolved phase, is the amount of pesticide in the water (mg pst), and V is the volume of water in the water body (m HO). If , is transferred from the sediment to the water layer. If , is transferred from the water to the sediment layer.
The diffusive mixing velocity, , can be estimated from the empirically derived formula (Chapra, 1997):
8:4.2.13
where is the rate of diffusion or mixing velocity (m/day), is the sediment porosity, and is the molecular weight of the pesticide compound.
The processes described above can be combined into mass balance equations for the well-mixed water body and the well-mixed sediment layer:
8:4.3.1
8:4.3.2
where is the change in pesticide mass in the water layer (mg pst), is the change in pesticide mass in the sediment layer (mg pst), is the pesticide added to the water body via inflow (mg pst), is the amount of dissolved pesticide removed via outflow (mg pst), is the amount of particulate pesticide removed via outflow (mg pst), is the amount of pesticide removed from the water via degradation (mg pst), is the amount of pesticide removed via volatilization (mg pst), is the amount of pesticide removed from the water due to settling (mg pst), is the amount of pesticide removed via resuspension (mg pst), is the amount of pesticide transferred between the water and sediment by diffusion (mg pst), is the amount of pesticide removed from the sediment via degradation (mg pst), is the amount of pesticide removed via burial (mg pst)
Pesticide in the sediment layer may be lost by burial. The amount of pesticide that is removed from the sediment via burial is:
8:4.2.14
where is the amount of pesticide removed via burial (mg pst), is the burial velocity (m/day), is the surface area of the water body (m), is the amount of pesticide in the sediment (mg pst), and is the volume of the sediment layer (m).
Table 8:4-2: SWAT+ input variables related to pesticide in the sediment.
Variable Name | Definition | Input File |
---|
A first order decay function is used to calculate changes in bacteria concentrations (Bowie et al., 1985).
8:5.1.1
8:5.1.2
where is the amount of less persistent bacteria present in the reservoir on day (#cfu/100mL), is the amount of less persistent bacteria present in the reservoir on day (#cfu/100mL), is the rate constant for die-off of less persistent bacteria in water bodies (1/day), is the amount of persistent bacteria present in the reservoir on day (#cfu/100mL), is the amount of persistent bacteria present in the reservoir on day (#cfu/100mL), and , is the rate constant for die-off of persistent bacteria in water bodies (1/day).
The die-off rate constants are adjusted for temperature using the equations:
8:5.1.3
8:5.1.4
where , is the rate constant for die-off of less persistent bacteria in water bodies (1/day), is the rate constant for die-off of persistent bacteria in water bodies (1/day), is the rate constant for die-off of less persistent bacteria in water bodies at 20C (1/day), is the rate constant for die-off of persistent bacteria in water bodies at 20C (1/day), is the temperature adjustment factor for bacteria die-off/re-growth, and is the water temperature (C).
Table 8:5-1: SWAT+ input variables that pertain to bacteria die-off in the water bodies.
Variable Name | Definition | Input File |
---|
LKPST_KOC | : Pesticide partition coefficient (m/g) | .lwq |
LKSPST_REA | : Rate constant for degradation or removal of pesticide in the sediment (1/day) | .lwq |
LKPST_RSP | :Resuspension velocity (m/day) | .lwq |
LKSPST_ACT | : Depth of the active sediment layer (m) | .lwq |
LKPST_MIX | : Rate of diffusion or mixing velocity (m/day) | .lwq |
LKSPST_BRY | : Pesticide burial velocity (m/day) | .lwq |
WDPRES | : Die-off factor for persistent bacteria in water bodies at 20C (1/day) | .bsn |
WDLPRES | : Die-off factor for less persistent bacteria in water bodies at 20C (1/day) | .bsn |
THBACT | : Temperature adjustment factor for bacteria die-off/growth | .bsn |