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<br />donei however, good results have not always been obtained. A commonly used <br />procedure is that of the U,S. Soil COnservation Service (SCS) which permits <br />generalizing loss-rate functions through the SCS curve number technique. The <br />curve number reflects land use, cover, and soil types with allowances for <br />antecedent moisture conditions (U.S. Department Df Agriculture, SCS, 1957). <br /> <br />Unit hydrographs fDr ungaged basins (also known as synthetic unit <br />hydrographs) are usually developed in two steps. First, an equation or <br />procedure is devised that will allow a unit hydrograph to be computed, <br />Second, the procedure (or equation) must be related to definable basin <br />characteristics. The simplest and most direct method is to transfer a unit <br />hydrograph from an adjacent gaged basin Df similar hydrological and <br />meteorological characteristics with simple adjustments, This technique is <br />fairly cOl1l11Oni however, it is usually difficult to locate "similar" basins. <br />MDre complicated and more general procedures include deriving parameters that <br />describe the unit hydrograph, and then transferring these parameters with <br />some adjustments. The two most common synthetic unit hydrograph methods used <br />in the COrps of Engineers are the procedures of Clark (1945) and Snyder <br />(1938) , <br /> <br />An alternative to the unit hydrograph approach is the simulation of the <br />most significant watershed processes, such as interception and infiltration, <br />overland flow, and channel flow, using small elements of the watershed to <br />trace the movement of water through the basin, The various watershed <br />elements are linked together to produce a model of a complete watershed, The <br />Stanford Watershed Model and the MITCAT Model (MIT Catchment Model) are two <br />well-known examples of this type of model. Models of this type have been <br />designated as "distributed-parameter" models in contrast to "lumped- <br />parameter" models such as the unit hydrograph. <br /> <br />The distributed-parameter approach is very attractive for modeling <br />ungaged basins, because it is based in principle on the use of physically- <br />based parameters for describing the response of the basin to rainfall. The <br />required data include lengths of overland flDW paths and overland flow <br />resistance coefficients, channel geometry and roughness, channel lengths, and <br />loss~rate parameters. The channel parameters can be obtained quite readilYi <br />however, it is somewhat more difficult to estimate the overland flow <br /> <br />6 <br /> <br />e <br /> <br />e <br /> <br />e <br />