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In recent years, a great emphasis has been placed on improving and maintaining the quality of national water resources. Pollutant run off together with their sediments are the main problems of these resources. Some mathematical models have been developed for calculating run off volumes and
predicting eroded soil of which ANSWERS is one. ANSWERS is a deterministic model that uses distributed parameters and is event oriented for simulating hydrologic processes in a watershed.In order to test this model, a
24 hectare watershed was selected.Agricultural farms in this watershed are of an average 35 percent slope.Analysing the resulting out put from computer
shows that the volume of observed and estimated run off is almost the same in almost more than 96 percent of the cases. However, the estimated eroded soil
doesn't match the observed one and showes to be up to 2.5 times more. Also it was observed that ANSWERS was more dependable in simulating medium to high rainfalls in comparision with low ones. In this investigation, other
models were used too. It was shown that the volume of the estimated run off estimated by empirical model was close to the observed one but less close as
compared with the ANSWERS model. The eroded soil calculated by USLE showed a better correlation with the observed eroded soil (in low rainfalls) than the ANSWERS model but in high rainlatts they result almost the same.
Niether the estimated volume of run off nor the concentration of sediments, calculated by curve number (CN), showed acceptable correlation with the
observed data. More investigations are needed to make these models applicable to watersheds in Iran.