18 GIS Applications to the Basin-Scale Assessment of Soil Salinity and Salt Loading to Groundwater Dennis L. Coxwin, James D. Rhoades, and Peter J. Vaughan USDA-ARS, U.S. Salinity Laboratory Riverside, California ABSTRACT An overview is presented of previously published work by Corwin and his col- leagues concerning the application of a geographic information system (GIS) to non-point source (NPS) pollutants in the vadose zone. Two different GIS-based approaches are described for the prediction of the area1 distribution of a NPS pollutant, specifically salin- ity, at a basin scale. The first approach couples a regression model of salinity development to a GIS of soil salinity development factors (i.e., permeability, leaching fraction, and groundwater electrical conductivity) for the Wellton-Mohawk Irrigation District near Yuma, AZ, during the study period 1968 to 1973. The regression model predicts the com- posite salinity of the root zone (i.e., top 60 cm.). Areas of low, medium, and high salin- ization potential are delineated for the entire 44 000 ha irrigation district. Measured salin- ity data verified that 86% of the predicted salinity categories were accurately predicted. The second approach loosely coupled the one-dimensional, transient-state solute transport model, TETrans, to the geographic information system ARC/INFO. Slightly less than 2400 ha of the Broadview Water District located on the west side of central California’s San Joaquin Valley are being used as the test site to evaluate the integrated GIS/transport model during the study period 1991 to 1996. TETrans uses the GIS as a spatial database from which to draw its input data. Preliminary simulations are presented for the main growing season of 1991. Display maps show spatial distributions of soil salinity profiles to a depth of 1.2 m, irrigation efficiencies, drainage amounts, and salt loading to ground- water over the 2396 ha study area. These maps provide a visual tool for making irrigation management decisions to minimize the environmental impact of salinity on soil and groundwater. The first approach is best suited for areas where steady-state conditions are approximated, while the second approach can be used under transient-state conditions. Limited surface water resources and continued contamination of surface water supplies, have increased the reliance upon groundwater to meet growing water demands from agricultural, industrial, and domestic consumers. Already, ground- water accounts for one-half of the drinking water and 40% of the irrigation water Copyright 0 1996 Soil Science Society of America, 677 S. Segoe Rd., Madison, WI 53711, USA. Application of GIS to the Modeling of Non-Point Source Pollutants in the Vadose Zone, SSSA Spe- cial Publication 48. Purchased by USDA for Official Use 296 CORWIN ET AL. used in the USA. The degradation of soil and water resources by non-point source (NPS) pollutants, such as salinity, poses a tremendous threat because of the area1 extent of their contamination and the difficulty of effective remediation once soils and groundwater are contaminated. Salinity within irrigated soils clearly limits productivity in vast areas of the USA and other parts of the world. In spite of the fact that salinity buildup on irri- gated lands is responsible for the declining resource base for agriculture, the answers to a number of fundamental questions are still unknown. For instance, it is not known beyond speculated assessments what the area1 extent of the salt- affected soil is, where the location of contributory sources of salt loading to the groundwater are, to what degree agricultural productivity is being reduced by salinity, or whether the trend in soil salinity development is increasing or decreas- ing? Suitable regional-scale inventories of soil salinity do not exist, nor do prac- tical techniques to monitor salinity or to assess the impacts of changes in man- agement upon soil salinity and salt loading at a regional scale. The primary rea- son for the lack of answers to these questions stems from the area1 extent of the problem, and the spatially complex and dynamic nature of salinity in soil. The ability to locate sources of salt loading within irrigated landscapes and to model the migration and accumulation of salts in the vadose zone to obtain an estima- tion of their loading to the groundwater is an essential tool in combating the degradation of our groundwater. Groundwater quality affected by salinity depends on the spatially distributed properties that influence contaminant trans- port. The phenomenon of salt transport through the vadose zone is affected by the temporal variation in irrigation water quality, and the spatial variability of plant water uptake and of chemical and physical properties of soil. Coupling a geo- graphic information system (GIS) to a deterministic model potentially offers a means of dealing with the complex spatial heterogeneity of soils which influence the intricate biological, chemical and physical processes of transient-state solute transport in the vadose zone. Modeling the movement and accumulation of a NPS pollutant such as soil salinity is a spatial problem well suited for the integration of a deterministic model of solute transport with GIS. GIS serves as a spatial database to organize, manipulate, and display the complex spatial data used by a deteminisitic model to describe the regional-scale distribution of soil salinity and salt loading to groundwater. The coupling of the spatial data handling capabilities of GIS with a one-dimensional solute transport model offers the advantage of utilizing the full information content of the spatially distributed data to analyze solute movement on a field scale in three dimensions. As a visualization and analysis tool, GIS is capable of manipulating both spatially-referenced input and output parameters of the model. The first applications of GIS to NPS pollution were for surface water resources in the mid 1980s (Hopkins & Clausen, 1985; Pelletier, 1985; Potter et al., 1986). These consisted of land evaluation and water quality models of runoff and soil erosion. The first application of GIS for assessing the impact of NPS pol- lutants in the vadose zone occurred in the late 1980s. Corwin et al. (1988, 1989), and Corwin and Rhoades (1988) first applied the use of GIS to delineate areas of accumulation of salinity in the vadose zone by coupling a GIS of the Well- GIS APPLICATIONS 297 ton-Mohawk Irrigation District to a regression model of soil salinity develop- ment. For an in-depth review of GIS applications of deterministic models for regional-scale assessment of NPS pollutants in the vadose zone the reader is referred to Corwin (1996, this publication). It is the objective of this chapter to review the work of Corwin and his col- leagues regarding the development of practical methodologies for delineating area1 distributions of soil salinity and estimating salt-loading on irrigated agri- cultural land at a regional scale. Two approaches have been developed over the past 7 yr that use the spatial data handling and visualization capabilities of a GIS. One approach couples a regression model of salinity development to a GIS of soil salinization factors to model the accumulation of soil salinity in the root zone (top 60 cm) under steady-state conditions (Corwin et al., 1988, 1989; Corwin & Rhoades, 1988. This is referred to in this chapter as the Steady-State GIS/Salini- ty Model. The second approach integrates a one-dimensional solute transport model with a GIS of solute transport parameters to model the movement of salts through the vadose zone under transient-state conditions (Corwin et al., 1993a, b; Vaughan et al., 1993; Vaughan & Corwin, 1995). This approach is referred to as the Transient-State GIS/Salinity Model. Both approaches result in the capability of producing maps of soil salinity accumulation within the root zone. METHODS AND MATERIALS Steady-State GIS/Salinity Model A regression model of salinity development was formulated upon com- monly known cause-and-effect salinization factors (Corwin et al., 1988, 1989; Corwin & Rhoades, 1988). In arid climates, the development of soil salinity on irrigated lands can be conceptually related to several general factors: irrigation water quality, physical edaphology, groundwater characteristics and irrigation management. Figure 18-1 illustrates the interacting dynamics of these saliniza- tion factors. The interaction of the factors illustrated in Fig. 18-1 causes the buildup of salts in the root zone. Evapotranspiration, which results in the selective removal of water by plant roots leaving behind any salts naturally present in the irrigation water, is the process primarily responsible for the accumulation of salt in the root zone. Under steady-state conditions with a net downward water flux, salt con- centration will increase with depth through the root zone. All things being equal, irrigation water of poor quality (i.e., high salt concentration) results in higher soil salinity profiles. Edaphic factors such as soil permeability are potentially influ- ential upon the accumulation of salt in soil due to the retarding effect upon water flow which reduces any leaching of salts. A shallow water table and high salini- ty groundwater also are likely to be influential in the development of soil salini- ty as a result of the increased potential for upward movement of salts (Ayers & Westcot, 1976; Shih, 1983). Figure 18-2a shows the relationship between the sur- face evaporation rate and depth to the groundwater, while Fig. 18-2b shows the accumulation of salt near the soil surface due to the upward movement of salts carried by water rising to meet the evaporative demand. As the depth to ground- 298 CORWIN ET AL. SALT ACCUMULATION ON IRRIGATED, ARID-ZONE SOIL EVAPORATION AND TRANSPIRATION REMOVE WATER AND LEAVE SALTS BEHIND LESS THAN EVAPORATION LEGEND :,$$.:; INDICATES SALTS +INDICATES FLOW OF WATER Fig. 18-1. Schematic illustrating the processes involved in the salinization of irrigated, agricultural lands.