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age and infiltration using either the Green-Ampt or Phillips equation, or a variation thereof. The PSRM model uses the SCScurve number for determining soil infiltration, but uses soil moisture accounting to determine available stor- age. These models are either continuous or quasi-continuous (soil-moisture accounting is continuous, but routing is only performed only for a specified storm period), but continuous meteorologic data was not made available for this test, thus modelers were required to estimate initial starling conditions for each storm. Soil moisture accounting and infiltration procedures generally are more data-intensive than the SCS curve and HOrlon methods, and require a num- ber of parameters corresponding to physical soil-water storage and infiltration characteristics. Table 1. Characteristics of rainfall-runoff models [Some models have several options for runoff generation and flow routing, but only the option used for this study is identified]. Mod~1 Simulation Runoff Overland Channel Watershed Type Generation Flow Flow Representation CASC2D' Event Soil moisture Cascade Diffusive wave Distributed accounting CUHPt Event HOrlon Unit hydrograph Unit hydrograph . Lumped CUHP/SWMM~ Event HOrlon Unit hydrograph Unit hydrograph Distributed DR3M" Quasi- Soil moisture Kinematic wave Kinematic wave Distributed continuous accounting HEC-I tt Event SCS-curve no. Unit hydrograph Muskingum Distributed HSPF~~ Continuous Soil moisture Kinematic wave Kinematic wave Distributed Accounting PSRM'" Quasi- SCS-curve no.& Cascade Kinematic wave Distributed continuous Soil moisture SWMMttt Event HOrlon Kinematic Wave Kinematic Wave Distributed TR20::: Event SCS curve no. SCS unit SCS unit Lumped hydro graph hydrograph '. CASC2D. Cascade 2-Dimensional (Julian and Saghafian, 1991) t. CUHP . Colorado Unit Hydrograph Procedure (Urban Drainage and Flood Control District., 1984) ~. CUHP/SWMM - Subbasin application ofCUHP linked together with SWMM ". DR3M. Distributed Rainfall Routing RunoffModcl (Alley and Smith, 1982) tt. HEC-I - Hydrologic Engineering Center (1990) H. HSPF - Hydrologic Simulation Progn1m Fortran (Bicknell and others, 1993) .... PSRM. Penn State Runoff~odel (Aron and others. 1996) ttt. SWMM - Storm Water Management Model (Huber and Dickinson, 1988) u:. TR20 - Technical Release No 20 (Soil Conservation Service, 1983) Once excess precipitation is determined, surface runoff is calculated for overland flow and channel flow by one of the following methods; (I) unit hydrograph, (2) SCS triangular unit hydrograph, or (3) by solving equations for flow. The unit-hydrograph procedure derives a hydrograph by assuming a specific shape that represents land-use, soil, and geometry characteristics of the watershed, although techniques are available to derive the unit hydrograph from observed rainfall-runoff data, this data was not made available in this study. The SCS triangular unit hydrograph is an approximation of a nonlinear runoff distribution that is assumed to be constant in a unit hydrograph method. A num- ber of methods exist for solving equations for flow. The Muskingum method is used for channel routing by determi- nation of a wedge-shape channel storage in relation to inflow and outflow channel volume. Overland flow and channel routing is performed in some models by kinematic wave to solve the continuity equation for flow or by diffu- sive wave, which includes an additional pressure-differential term (Miller, 1984). The cascade method is a two- . dimensional kinematic wave approximation for routing overland flow (Julien and others, (1995). Models that use the kinematic or diffusive wave routing differ by how overland flow and channel characteristics are specified. 7-164 f 1 . ! i ~ "j i , ! .. ~ i t . ..