Submitted to: Soil Science Society of America Special Publication Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 1/1/1995
Publication Date: N/A
Citation: Interpretive Summary: Agricultural chemicals and salts are dissolved in the water present in many agricultural soils. These chemicals are carried downwards when water in the soil flows downwards towards the water table. Downwards movement occurs when it rains or when irrigation water is applied. This paper concerns a computer model of transport of chemicals that predicts how much water and chemicals will enter the groundwater. Many variables need to be specified for this prediction, among them are: amount of irrigation water applied per day; existing concentrations of each chemical; and evapotranspiration of water from plants. The computer model combining these various kinds of data makes a calculation that runs over a period of time such as the summer growing season. For each day, the model predicts amount of irrigation water enters the groundwater and, for several chemicals, how much of each enters the groundwater. The model is coupled to geographic information system software in order to use the map-making capability of the software. Maps of the various results mentioned above can be made.
Technical Abstract: The Unsatchem model of Simunek and Suarez (1994) was interfaced to a geographic information system (GIS). The one-dimensional version of Unsatchem is a finite-element model of water flow in the vadose zone coupled with multicomponent solute transport, production and transport of CO2 and heat in addition to a major ion model of chemical speciation and kinetics. The new version of this program, Unsatchemgeo, runs in a geographic context in association with the GIS such that data required by the model are obtained directly from database table and computed results are written to other database tables. Support programs were also created for initial generation of the INFO database and input of data in text form. In a simulation, Unsatchemgeo computed vertical water flow at eight locations within a single field located on the west side of the San Joaquin Valley. Observed cumulative drainage flow from the tile drain system is greater than calculated drainage through the lower boundary of the model for all times. The onset of increased tile drain flow occurs prior to the start of irrigations. These results and observations indicate the existence of water sources for tile drainage other than irrigation of this field.