Variability of Infiltration within Large Runoff Plots on Rangelands.

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 m*) plots located within large plot boundaries (32.5 mr). 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. Historically it has been assumed that watersheds are homogeneous. The only data collected were streamflow measurements, which were assumed to integrate hydrologic processes on an entire watershed. Use of rainfall simulators made it possible to assess the impacts of management practices on specific parts of a watershed. That capability led to an emphasis upon characterizing the spatial variability of hydrologic properties to eventually link point and area1 measurements. For simplicity, many hydrologic models still assume that a watershed is homogeneous. Such models ignore the spatial variability of hydrologic properties and lump watershed characteristics as area1 averages. However, numerous recent investigations of hydrologic responses (Achouri 1982, Blackburn 1975, Grah 1983, Gifford 1976, Springer and Gifford 1980, Lyford and Qashu 1969, Merzougui 1982, Murabayashi and Fok 1979, Sharma et al. 1980, Rogowski 1980, Tricker 198 I, Vieira et al. 198 1) have illustrated great spatial variability of hydrologic properties within short distances. To estimate the effect of spatial variability of infiltration on hydrologic models’ assumptions of homogeneous sites or watersheds, area1 loss rates must be contrasted with point infiltration rates. Three theoretical explanations of this functioning of natural watersheds have been proposed. Hawkins (1981) submits that watersheds act as a collection of runoff elements, each with independent, uniform hydrologic characteristics. Computer simulation has been used by Smith and Hebbert (1979) and Cundy (1982) to investigate the effects of spatial variability on plot and hillslope performance. Smith and Hebbert developed a model that considered only the spatial variability of rainfall excess. The Cundy model considers variability in soil properties, initial moisture, and rainfall intensity, and routes rainfall excess to downslope using the kinematic wave technique. Authors are graduate research assistant and professor and chairman, Watershed Science Unit, Range Science Department, Utah State University, UMC 52, Logan 84322. Gifford’s current address is Dept. of Range, Wildlife,and Forestry, University of Nevada, Reno 895 12. Ms. Devaurs is currently with Los Alamos National Lab. Los Alamos, NM 87545. This project was supported through the Northwest Watershed Research Center, USDA, ARS, Boise, Ida. ,and the Utah Agricultural Experiment Station, Projects 749 and 771. Tech. Pap. 2877, Utah Agricultural Experiment Station, Logan 84322. Manuscript accepted February 7, 1984. JOURNAL OF RANGE MANAGEMENT 37(6), November 1964 These models implicitly transfer point infiltration properties to a watershed-wide application. They are significant developments, yet they are computer simulations and not adequately field validated. In this field study, we evaluated the variability of infiltration characteristics and soil properties on small (0.61 m X 0.61 m) runoff plots within large (3.05 m X 10.67 m) runoff plots on “homogeneous*’ semiarid rangeland sites.