SOIL HYDRAULIC PARAMETERS FOR REGIONAL SCALE APPLICATIONS OF MECHANISTIC MODELS

Authors: Timlin, Dennis; Williams, Robert; AHUJA, LAJPAT - 5402-15-00

Submitted to: Soil Science Society of America Special Publication Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 6/20/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Geographic Information Systems (GIS), have tremendous potential to manage spatially distributed data. In GIS systems, data on soil properties, landscape characteristics, vegetation, etc. can be associated with spatial coordinates. Data from different sources can then be aggregated and/or overlaid for various sizes of land areas. One use of GIS is to couple it with a water and solute transport computer simulation model. Input data can be organized on the basis of soil mapping units for example and the model run for each GIS cell (in this case a soil mapping unit) using data for the soil type corresponding to that cell. GIS maps of pesticide distribution or pollution hazards can be generated from the model output and management decisions made on the basis of the maps. One drawback to the use of these solute and water transport models, however, is the difficultly of measuring all the soil properties that they require on the scale used by GIS models. These properties include the soil's ability to infiltrate water and release water to plants. As an alternative, these properties can often be estimated from more easily available soil properties such as soil texture. This paper summarizes the different methods that can be used for this purpose. The information in this paper can make it easier for GIS users to obtain data for water and solute transport models they may couple with a GIS system.

Technical Abstract: The use of mechanistic models at regional scales requires parameters that represent the soil hydraulic properties of the area and their variability. Because of the large number of samples required to characterize an extensive area such as a watershed, methods to estimate soil hydraulic properties from simpler data or from a small number of measurements are alternatives to making extensive field measurements. Readily available data that can be used to obtain hydraulic parameters include soil texture, bulk density, porosity, and 33 kPa water content. The choice of estimation method depends on the type of data available and the possibility of making field measurements. Examples of estimating saturated hydraulic conductivity, and hydraulic conductivity and water content as functions of matric potential are presented. After choosing an estimation procedure, a method to assess the quality of the estimations must be chosen. Because a simulation model integrates the effects of soil hydraulic properties with climate variables and, if simulated, with plant factors, the targeted output should be the basis of comparisons in addition to comparing measured and estimated soil hydraulic properties. The results of simulations carried out using the soybean model GLYCIM show that estimated soil properties may only be useful if long term averages of soybean yield are desired.