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<br />, <br /> <br />, <br />, <br /> <br />INTRODUCTION <br /> <br />Prediction of overland flow generated by precipitation has been the <br />object of intense study by hydrologists for many years. Although methods of <br />prediction vary widely, the most commonly used method probably has been to <br />relate flow characteristics to measurable features of drainage basins, such <br />as area, relief, drainage density, and vegetative cover, and to extrapolate <br />these relationships to larger areas of similar characteristics. Such studies <br />necessarily require a long period to establish flow characteristics of the <br />index watershed. <br /> <br />Advent of the high-speed digital computer has made possible the solution <br />of numerous rainfall-runoff models that have been developed. One such model <br />developed and being used by the U.S. Geological Survey is a physically based <br />model known as PRMS (Precipitation-Runoff Modeling System) (Leavesley and <br />others, written commun., 1983). PRMS is a modular design deterministic <br />distributed-parameter modeling system developed to evaluate the impacts of <br />various combinations of precipitation, climate, and land use on streamflow, <br />sediment yields, and general basin hydrology. Surface runoff in this model <br />is generated by precipitation excess resulting from application of the <br />Green-Ampt infiltration equation. <br /> <br />For many years, researchers have used various types of infiltrometers to <br />determine infiltration characteristics of soils. Most of this work has been <br />concentrated on agricultural land, where the effect of soil treatments on <br />water uptake is important for crop production. In 1971, the U.S. Geological <br />Survey began development of a rainfall simulator that could be used on range- <br />land, to determine the effects of different land treatments on runoff and <br />erosion (Lusby, 1977). This simulator was designed like a rainfall-runoff <br />facility constructed at Colorado State University (Holland, 1969). The <br />facility at Colorado State University was a permanently installed system, <br />designed to study processes of runoff from an impervious surface, upon which <br />various controlling factors could be imposed. The rainfall design of this <br />system was adapted for use as a portable unit. One product of the simulation <br />runs is determination of effective average infiltration, precipitation minus <br />runoff, over areas of about 2,500 ft2. These data need to be used on a <br />broader scale than simple comparison of individual results; the use we <br />describe is the definition of parameters used in the PRMS. <br /> <br />PURPOSE AND SCOPE <br /> <br />The primary purpose of the study is to determine if data on infiltration <br />and runoff obtained from the U.S. Geological Survey rainfall simulator, are <br />useful in defining parameters used in the pRMS, which is used to predict run- <br />off from larger watersheds. Incidental to this purpose are determination of <br />the spatial variability of the parameters measured over the simulation plots <br />and over the watershed, and sensitivity of these parameters in predicting <br />runoff. <br /> <br />2 <br />