• Evaluating Soil Water Models on Western Rangelands

      Cooley, K. R.; Robertson, D. C. (Society for Range Management, 1984-11-01)
      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 sufficiently 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 Plains, 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.
    • Variability of Infiltration within Large Runoff Plots on Rangelands

      Devaurs, M.; Gifford, G. F. (Society for Range Management, 1984-11-01)
      In this study we investigated the variability of infiltration on native rangeland sites. A rainfall simulator was used to collect data on runoff from small (0.37 m2) plots located within large plot boundaries (32.5 m2). Three range sites were sampled and data were collected from unfenced, fenced, and rototilled conditions on each site. In addition data were collected on vegetation, antecedent moisture, bulk density, soil texture, and organic matter as possible explanations for variations in hydrologic response on small and large plots. The field study demonstrated large variability in measured infiltration and soil physical properties on relatively uniform rangeland sites, suggesting that inherent variability patterns need to be examined to provide appropriate confidence intervals for single parameter values that may be applied to larger areas. No set of factors consistently explained the observed variability within large plots.