HSPF

Abstract: A clear understanding of a model is important for its appropriate use. In this article, eleven watershed scale hydrologic and nonpoint-source pollution models are reviewed: AGNPS, AnnAGNPS, ANSWERS, ANSWERS-Continuous, CASC2D, DWSM, HSPF, KINEROS, MIKE SHE, PRMS, and SWAT. AnnAGNPS, ANSWERS-Continuous, HSPF, and SWAT are continuous simulation models useful for analyzing long-term effects of hydrological changes and watershed management practices, especially agricultural practices. AGNPS, ANSWERS, DWSM, and KINEROS are single rainfall event models useful for analyzing severe actual or design single-event storms and evaluating watershed management practices, especially structural practices. CASC2D, MIKE SHE, and PRMS have both long-term and single-event simulation capabilities. Mathematical bases, the most important and critical elements of these mathematical models, were identified and compiled. In this article, a comprehensive summary of the compilation is presented in tabular form. The flow-governing equations and their solution methods used in each of the eleven models are discussed. The compilation of the mathematical bases of these models would be useful to determine the problems, situations, or conditions for which the models are most suitable, the accuracies and uncertainties expected, their full potential uses and limitations, and directions for their enhancements or new developments. AGNPS, AnnAGNPS, DWSM, HSPF, MIKE SHE, and SWAT were found to have all the three major components (hydrology, sediment, and chemical) applicable to watershed-scale catchments. SWAT is a promising model for continuous simulations in predominantly agricultural watersheds, and HSPF is promising for mixed agricultural and urban watersheds. Among the single-event models, DWSM provides a balance between the simple but approximate and the computationally intensive models and, therefore, is a promising storm event model for agricultural watersheds.

Abstract: Strengths and limitations of hydrologic simulation models are used as criteria for selecting a particular modelfor a given water resources application. The performance of the Soil and Water Assessment Tool (SWAT) and the HydrologicSimulation Program-Fortran (HSPF) continuous simulation models was compared on eight nested agricultural watershedswithin the Little Washita River Experimental Watershed (LWREW) and two agricultural watersheds adjacent to the LWREWwithin the Washita River Basin in southwestern Oklahoma. Two subwatersheds within the LWREW were first used to calibrateparameters in both models for a wetter than average period of record. Both models were then applied to six othersubwatersheds within the LWREW and the two adjacent watersheds, under varying climatic conditions. Three quantitativeand two qualitative evaluation criteria were used to assess streamflow simulated by SWAT and HSPF: computation of(1) deviation of streamflow volume, (2) coefficient of efficiency, and (3) prediction efficiency and visual inspection of(4) hydrographs and (5) flow duration curves. A comparison of model performance showed that while HSPF performed betteron the watersheds used for calibration, SWAT gave better results on the validation watersheds. On one of the validationsubwatersheds adjacent to the LWREW, values of deviation of streamflow volume were -38.7%, -13.3%, and -1.3% for SWATand -64.3%, -81.1%, and -8.2% for HSPF under much dryer than average, dryer than average, and near averageclimatic conditions, respectively. Differences in model performance were mainly attributed to the runoff productionmechanism in the two models. Results of this study showed that SWAT exhibited an element of robustness in that it gave moreconsistent results than HSPF in estimating streamflow for agricultural watersheds under various climatic conditions. SWATmay therefore be better suited for investigating the long-term impacts of climate variability on surface-water resources.

Abstract: A modified concept of hydrological response units (HRUs) for regional modelling of river basins using the PRMS/ MMS model is presented. The HRUs are delineated by geographical information system (GIS) analysis from physiographic basin properties such as topography, soils, geology, rainfall and land use using a thorough hydrological systems analysis. The HRUs, once classified by GIS analysis, preserve the three-dimensional heterogeneity of the drainage basin. The River Brol basin (A = 216km 2 ), Rheinisches Schiefergebirge, Germany was selected to apply the concept. In total, 23 HRUs were delineated and tested with the PRMS/MMS model using a 20-year hydrometeorological daily database. The hydrological systems analysis revealed that interflow is the dominant flow process through the basin's slopes and the major contribution to groundwater recharge and river runoff. This was accounted for by parameterizing the HRUs in the model control file to drain their surplus water not used for satisfying the demand of evapotranspiration to a common conceptual subsurface storage. This storage was simulated by interflow drainage to the groundwater aquifer in the valley floor, which in turn drained to the channel network. The PRMS/MMS model simulated the observed daily discharge very well and the fit was described by a daily correlation coefficient of r = 0.91. The NASIM and HSPF models using different means to represent the basin's physiographic heterogeneity were applied to the Brol basin as well, but did not achieve this correlation. The HRU concept was found to be a reliable method for regional hydrological basin modelling and allows spatial up- and downscaling. Future research on this concept will focus on incorporating the variable precipitation distribution into the classification of HRUs and on the hydrodynamic routing of the modelled discharge. Additionally, satellite imagery must be used for classifying land use in macroscale drainage basins.