The physical soil properties are summarized in the file soils.sol. The physical properties of the soil govern the movement of water and air through the profile and have a major impact on the cycling of water within the HRU.

The contents of nutrients and constituents in the soil are initialized using the file soil_plant.ini, which references the files nutrients.sol for the initial nutrient contents and the files pest_hru.ini and salt_hru.ini for the initial contents of pesticides and salts in the soil.

If no maximum rooting depth is specified, the model assumes that the roots can develop throughout the entire depth of the soil profile.

Most soil minerals are negatively charged at normal pH and the net interaction with anions such as nitrate is a repulsion from particle surfaces. This repulsion is termed negative adsorption or anion exclusion. Anions are excluded from the area immediately adjacent to mineral surfaces due to preferential attraction of cations to these sites. This process has a direct impact on the transport of anions through the soil for it effectively excludes anions from the slowest moving portion of the soil water volume found closest to the charged particle surfaces (Jury et al., 1991). In effect, the net pathway of the anion through the soil is shorter than it would be if all the soil water had to be used (Thomas and McMahon, 1972).

References

Jury et al., 1991

Thomas and McMahon, 1972

This data is not processed by the model, but the column may not be left blank.

To accurately predict surface runoff and infiltration in in soils where crack flow is relevant, the temporal change in soil volume must be quantified. Bronswijk (1989, 1990) outlines methods used to determine the maximum crack volume.

References

Bronswijk (1989, 1990)

The plant available water content, also referred to as the available water capacity, is calculated by subtracting the fraction of the water content at permanent wilting point from the water content at field capacity, AWC = FC - WP. Available water capacity is estimated by determining the amount of water released between in situ field capacity (the soil water content at soil matric potential of -0.033 MPa) and the permanent wilting point (the soil water content at soil matric potential of -1.5 MPa).

The percent of soil particles that are < 0.002 mm in equivalent diameter.

When defining organic carbon content by soil weight, the soil is defined as the portion of the sample that passes through a 2 mm sieve.

The percent of the sample that has a particle diameter > 2 mm, i.e. the percent of the sample that does not pass through a 2 mm sieve.

The soil bulk density expresses the ratio of the mass of solid particles to the total volume of the soil, ρb = MS /VT. In moist bulk density determinations, the mass of the soil is the oven dry weight and the total volume of the soil is determined when the soil is at or near field capacity.

The saturated hydraulic conductivity relates soil water flow rate (flux density) to the hydraulic gradient and is a measure of the ease of water movement through the soil. It is the reciprocal of the resistance of the soil matrix to water flow.

The structure of the file soils.sol is different than that of most other SWAT+ input files. Depending on the number of soil layers, the file contains two to ten lines per soil. The first line for each soil specifies the variables that apply to the entire soil profile (see first table below). The remaining soil variables are layer-specific and are specified in one line per layer starting one line below the general soil variables (see second table below). The figure below illustrates the structure of a soils.sol file with three soils. Please note that due to space restrictions not all layer-specific variables are included.

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. The definitions for the different classes are:

A. Soils having high infiltration rates even when thoroughly wetted, consisting chiefly of sands or gravel that are deep and well to excessively drained. These soils have a high rate of water transmission and a low runoff potential.

B. Soils having moderate infiltration rates when thoroughly wetted, chiefly moderately deep to deep, moderately well to well drained, with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission and a moderate runoff potential.

C. Soils having slow infiltration rates when thoroughly wetted, chiefly with a layer that impedes the downward movement of water or of moderately fine to fine texture and a slow infiltration rate. These soils have a slow rate of water transmission and a high runoff potential.

D. Soils having very slow infiltration rates when thoroughly wetted, chiefly clay soils with a high swelling potential; soils with a high permanent water table; soils with a clay pan or clay layer at or near the surface; and shallow soils over nearly impervious materials. These soils have a very slow rate of water transmission and a very high runoff potential.

*These criteria are guidelines only. They are based on the theory that the minimum permeability occurs within the uppermost 50 cm. If the minimum permeability occurs between a depth of 50 to 100 cm, then the Hydrologic Soil Group is increased one group. For example, C to B. If the minimum permeability occurs below a depth of 100 cm, the Hydrologic Soil Group is based on the permeability above 100 cm, using the rules previously given.

**Shrink-swell potential is assigned to a profile using the following guidelines: Low: All soils with sand, loamy sand, sandy loam, loam or silt loam horizons that are at least 50 cm thick from the surface without a clay horizon within 100 cm of the surface. Medium: All soils with clay loam horizons within 50 cm of the surface or soils with clay horizons from 50 to 100 cm beneath the surface. High: All soils with clay horizons within 50 cm of the surface. Lower the shrink-swell potential one class when kaolinite clay is dominant.

The name of the soil is a primary key referenced by in .

The percentage of soil particles that have an equivalent diameter between 0.05 and 0.002 mm.

The ratio of the amount of solar radiation reflected by a body to the amount incident upon it. The value for albedo should be reported when the soil is at or near field capacity.

The percentage of soil particles that have an equivalent diameter between 2.0 and 0.05 mm

The pointer to the pesticide initialization file is a foreign key referencing name in pest_hru.ini.

Some soils erode more easily than others even when all other factors are the same. This difference is termed soil erodibility and is caused by the properties of the soil itself. Wischmeier and Smith (1978) define the soil erodibility factor as the soil loss rate per erosion index unit for a specified soil as measured on a unit plot. A unit plot is 22.1-m (72.6-ft) long, with a uniform length-wise slope of 9%, in continuous fallow, tilled up and down the slope. Continuous fallow is defined as land that has been tilled and kept free of vegetation for more than 2 years. The units for the USLE soil erodibility factor in the MUSLE are numerically equivalent to the traditional English units of 0.01 (ton acre hr)/(acre ft-ton inch). Wischmeier and Smith (1978) noted that a soil type usually becomes less erodible with decrease in silt fraction, regardless of whether the corresponding increase is in the sand fraction or clay fraction.

Direct measurement of the erodibility factor is time consuming and costly. Wischmeier et al. (1971) developed a general equation to calculate the soil erodibility factor when the silt and very fine sand contents make up less than 70% of the soil particle size distribution. Williams (1995) proposed an alternative equation.

References

Wischmeier and Smith (1978)

Wischmeier et al. (1971)

Williams (1995)

The name of the soil and plant initialization is a primary key referenced by in .

The pointer to the nutrient initialization file is a foreign key referencing in .

The name of the soil nutrient record is a primary key referenced by in .