Laboratory and field methods for determining the hydraulic properties of soil are costly and time consuming. A more tractable approach for many applications is the determination of soil hydraulic properties from conventionally measured soil property information, including soil texture, particle-size distribution, bulk density, and organic matter content.

Many researchers have developed formulations based on the soil water retention curve and the underlying statistical pore size distribution theory in an attempt to relate commonly observed soil information (e.g., soil texture) to desired soil hydraulic properties. Clapp and Hornberger (1978), for example, examined the correlation of soil texture with a power function model of the soil moisture characteristic. Using desorption data from 1446 different soils, they tested the power function and found considerable variation in saturation suction within and among textural classes. Gradual air entry was determined to have a significant effect on the determination of the wetting front suction. They also discovered that the power function coefficient is highly correlated with soil texture, as represented by mean clay fraction within the appropriate USDA soil texture class.

Clapp and Hornberger's work has been the basis for most of the soil-property parameterizations used by the SVATS modeling community. The table below is an example of the type of "lookup table" that is currently used in many of these models. Until recently, there has been a dearth of actual soil texture information over regional scales with which to apply any of these approaches. A wide range of schemes, amounting to little more than poorly-directed guesswork, have been used to generate soil information for model input.

4/19/99