Soil Library (Texture Class)

This screen is used in conjunction with the Plant Growth Model Method for catchment hydrology and crop water requirements.

WEAP comes with a built-in library of soil data for the 12 standard texture classes defined by the US Department of Agriculture (USDA): Clay, Clay loam, Loam, Loamy sand, Sand, Sandy clay, Sandy clay loam, Sandy loam, Silt, Silt loam, Silty clay and Silty clay loam. (A thorough discussion of soil texture and these texture classes can be found at http://edis.ifas.ufl.edu/SS169 . A more general discussion of soil and water can be found at http://www.fao.org/docrep/r4082e/r4082e03.htm .) The van Genuchten parameters (alpha,and N) were derived using a curve fitting process based on the water content at field capacity and wilting point. Theta residual and saturated hydraulic conductivity were extracted from the texture class averages reported in the Rosetta database. The texture class named "Consolidated rock" represents a rocky surface that can hold no water. You can edit this library or add to it. Buttons on the toolbar at the top allow you to add, delete or rename texture classes. The Copy button will create a new texture class as a copy of the highlighted type. You can export all the texture classes to a comma separated value (CSV) file, which can be edited in Excel or a text editor. You can import texture classes from a CSV file -- new texture classes can be added in this way, or data for existing texture classes can be updated. When reading in from the file, WEAP will match the texture class name to determine if the imported data represents a new texture class or changes to an existing texture class. The format for the CSV file to import must be the same format as is created when exporting -- the same columns of data in the same order. Each WEAP area (dataset) has its own copy of the Soil Library. Therefore, changes made to the library in one area will not affect the library of another area. You can use import and export to move values from one area's library to another.

The following columns of information exist for each texture class:

Texture Class

The name of the texture class.

Saturation

Fully saturated water, equivalent to effective porosity of the soil, where all the pore spaces are filled with water. Saturated soil quickly drains until it reaches field capacity.

Field Capacity

Field capacity is defined as the amount of soil moisture or water content held in soil after excess water has drained away and the rate of downward movement has materially decreased. Irrigation or precipitation in excess of field capacity will quickly drain away (deep percolation), and is not available for crop evapotranspiration.

Wilt Point

Minimum soil moisture at which a plant wilts, sometimes referred to as the "permanent wilting point." Once the soil moisture depletion reaches this level, the plant has died.

Available Water Capacity

Available Water Capacity = Field Capacity - Wilt Point Available Water Capacity represents the available water that can be stored in soil and be available for growing crops.

Saturation, Field Capacity, Wilt Point and Available Water Capacity are all represented as the % of the total volume that contains water.

Theta Residual

This parameter is one of the van Genuchten parameters that is used to characterize the soil moisture retention curve for use in the Philip equation for infiltration. This parameter is the residual water content.

Alpha

N

Saturated Hydraulic Conductivity

The saturated hydraulic conductivity is a measure of a soil's ability to transmit water when subjected to a hydraulic gradient. This value can be left blank if the Soil Water Capacity is being specified using the Soil Library. In that case, the conductivity from the soil library will be used in the calculations.

Menu Option: General: Soil Library