Calibrated Win TR-20 model was validated and simulated to predict the peak runoff rates for 2, 5, 10, 25, 50 and 100 years for a small watershed area of 72 km2 in the southern region of Khyber Pakhtunkhwa of Pakistan. The catchment has a gauged spillway outlet at the downstream. The model was initially calibrated on the available known parameters from the grid survey and its derivatives, watershed physical features and other scales endorsed into the reservoir for monitoring. The calibrated model was tested and validated on physical data collected for duration of three months with a coefficient of determination of 98% among the observed and estimated runoff depths and peak runoff. After confirmation log-Pearson type III distribution was fitted to annual one day maximum rainfall upon which one day maximum rainfalls for 2, 5, 10, 25, 50 and 100 years return periods were simulated as 50, 80, 105, 144, 180 and 223 mm respectively. Run off depths for the same one day maximum rainfall of given returns period were further simulated as 10.34, 21.15, 30.3, 39.78, and 84.03 and 53.14 mm. Moreover the peak runoff rates for the return periods of 2, 5, 10, 25, 50 and 100 years were predicted as 11.3, 39.9, 77.7, 147.5, 221.9, and 320 m3s-1. Hence it was concluded that Win TR-20 provided satisfactory simulation of rainfall and their resultant runoffs and peak runoff rates which can be confidently recommended for use in small watersheds in the specific region of Khyber Pakhtunkhwa.
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