Evaluating Soil Water Models on Western Rangelands

Abstract

The soil profile is an important water storage reservoir within the hydrologic cycle. An understanding of the factors affecting daily soil water status is necessary to increase or modify vegetation or water yields. Many mathematical simulation models have been developed to assess soil water status, but none were found that were specifically developed for use on Western rangelands. The purpose of this report was to test soil water models that appeared to be sufftciently general for adaption to rangeland conditions, to determine if they could provide adequate results, and the level of sophistication required. The 2 models selected for evaluation were the Ekalaka Rangeland Hydrology and Yield Model (ERHYM) developed for use during the growing season on grasslands of the northern Great Phtms,and the Soil-Plant-Air-Water model (SPAW), which was developed for use with cultivated crops in the Midwest. Results indicated that both models could be adapted to produce adequate soil water information under rangeland conditions of southwestern Idaho. Overall, the somewhat simpler ERHYM model produced results more closely aligned to observed values than did SPAW. The lack of a snow accumulation and melt routine in SPAW (which could be added) appeared to be the main source of observed differences. These differences were a function of timing rather than a difference in total soil water at the end of each year, where results for the 2 models were very similar. The soil profile is one of the most important water storage reservoirs within the hydrologic cycle. In arid regions, available soil water seldom exists for more than a few months at a time, because it is rapidly extracted by plant transpiration and soil evaporation. Therefore, very little water ever percolates below plant rooting depth. In more humid regions, the magnitude of infiltrated water may be more than adequate for plant needs, and excess water may percolate into ground water reservoirs. An understanding of the factors affecting the day-today soil water status is important when attempting to increase or modify vegetation or water yields. The time distribution of soil water within the root zone is a complex interaction of many variables related to present and historical climate, plants, and parent soil materials. Hildreth ( 1976,1978) indicated that many mathematical simulations or computer models have been developed to assess soil water status. However, most of these models were developed to satisfy a particular need (i.e., spring wheat yield predictions in the northern Great Plains)and may not represent other crops or locations. None of the models were specifically developed for use on Western rangelands. The objective of this study was to determine if existing models with soil water accounting procedures included, representing different levels of complexity and data requirements, could be adapted to particular Western rangeland conditions. Two models were selected for evaluation based on the following criteria: (1) models appeared to be general enough to be adaptable to range conditions; (2) models had been tested against field data (even though for cultivated crops); and (3) documentation was readily available. The models selected were the ERHYM (Ekalaka Rangeland Hydrology and Yield Model) developed by Wight and Neff Authors are hydrologist and hydrologic technician. respectively. Northwest Watershed Research Center, USDA-ARS. 270 South Orchard, Boise, Ida. 83705. Manuscript accepted February 29, 1984. JOURNAL OF RANGE MANAGEMENT 37(6), November 1984 Table 1. Input requirements for running SPAW and ERHYM soil water balance models. SPAW Model ERHYM Model Initial soil water (or default option) Daily precipitation Daily runoff or SCS curve number Daily pan evaporation Monthly pan coefficient Avg. annual pan evaporation Layer thickness A time interval A soil pressure for tolerance parameter Soil water evaporation parameter Soil freezing dates Root distribution Canopy cover curve Canopy susceptibility to water stress Phenology curve Phenology susceptibility to water stress Moisture-stress curves (as provided) Planting & harvest dates Initial soil water Daily precipitation SCS curve number Daily max & min temperature Daily solar radiation