To do this, surface run-off simulated using the curve number method was compared with observed runoff in numerous rainfall run-off events in three small tropical watersheds located in the Upper Blue Nile basin, Ethiopia. The curve number method generally performed well in simulating surface run-off in the studied watersheds. There was no difference in the performance of the curve number method in simulating surface run-off under low and high antecedent rainfall.

It was also found that the method accurately estimated surface run-off at high rainfall intensity. However, at low rainfall intensity, the curve number method repeatedly underestimated surface run-off. This was possibly due to low infiltrability and valley bottom saturated areas typical of many tropical soils, indicating that there is scope for further improvements in the parameterization/representation of tropical soils in the curve number method for run-off estimation, to capture low rainfall-intensity events.

In this study the retention parameter was linked to the soil moisture content, which seems to be an appropriate approach to account for antecedent wetness conditions in the tropics.

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