Soil type
Soil types used in the Horton (ILSAX) hydrological model in DRAINS follow the U.S. Soil Conservation Service system adopted in the ILLUDAS model from which ILSAX was developed. There are four soil types involving different infiltration characteristics:
- Type 1 (or A) low runoff potential, high infiltration rates (sand and gravels),
- Type 2 (or B) moderate infiltration rates and moderately well-drained,
- Type 3 (or C) slow infiltration rates (may have layers that impede downward movement of water),
- Type 4 (or D) soils with high runoff potential, very slow infiltration rates (consisting of clays with a permanent high water table and a high swelling potential).
Users can specify a number between 1 and 4. DRAINS will interpolate between the standard infiltration factors applying to values of 1, 2, 3 or 4. The infiltration curves for these standard soil types are illustrated below.
If you require soil characteristics that are not covered by the standard types, you can specify your own parameters. To do this, you must click the You specify box in the Horton (ILSAX) hydrological model property sheet called from the hydrological model menu. Additional entry spaces will appear for seven parameters describing the new soil type.
The numbered dots on the infiltration curve refer to antecedent moisture conditions that indicate the catchment wetness prior to a storm.
The effects of varying soil types can be determined by sensitivity analysis, using techniques similar to those for examining AMCs and sub-catchment surface roughnesses.
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