Since GISs also manage nonspatial attribute databases that are linked to the spatial data, information such as soil characteristics can be made available to each spatial location (each cell) or other desired spatial model input parameter. Thus, numerous data sets on a variety of characteristics can be combined into a coordinated, georeferenced database. GISs can be used to derive additional data layers from original source materials. Functions are available that can combine two or more data files, either spatially or by recomputing attributes.
This concept of derived data simply reflects the fact that required parameters often have not been measured for a specific site or region, are not at the scale of the assessment, or are not identified as specific data parameters. These parameters must be estimated from derived database(s), or other measurements or interpolated and/or extrapolated from data points. Most process-based approaches include some parameters that are difficult or impossible to measure directly and so must be calculated or estimated by other means. Among the reasons:
It may be physically impossible to measure the parameter (e.g., diffusion coefficient).
Limited resources may preclude obtaining the number of measurements needed for the scale and/or time period of interest.
Point measurements may not have sufficient coverage for regional assessments, considering issues of spatial variability.
A number of automated procedures are available for estimating specific model parameters from existing databases. The most commonly used procedures estimate soil parameters from existing soil databases; examples are DBAPE, SOILPROP, and GRASS Waterworks. Data Base Analyzer and Parameter Estimator (DBAPE) (Imhoff et al. 1990), an U.S. Environmental Protection Agency product, is an interactive system designed to allow users to estimate soil parameters and develop meteorologic input for the EPA PRZM (Carsel et al. 1984) and RUSTIC (Dean et al. 1989) models using soils and meteorological databases. The soils database is a selected subset of the SCS national archival data system, NSSAD/SIRS (USDA 1985), that focuses only on agricultural soils. County soils information is included for almost 8,000 soil series with information on soil layering, soil texture class, percent sand, percent clay, bulk density, percent organic matter, available water, hydrologic soil group, and potential crops. From these basic data extracted from NSSAD/SIRS, DBAPE provides procedures for calculating model parameters for wilting point, water content at field capacity (0.1 and 0.33 bar tension), residual water content, saturated hydraulic conductivity, and soil water retention parameters from correlations developed by van Genuchten (1978), Rawls and Brakensiek (1985), and others.