Ensemble forecasting and flood warning in Dez Dam Reservoir using GEFSv12 Re-forecast temperature and precipitation data

It is necessary to forecast the volume of inflow to reservoir operation in flood conditions, which is of double importance in mountainous basins with a rain-snow regime. The main purpose of the present study was to evaluate the accuracy of GEFSv12 reforecast data as an input to the HEC-HMS precipitation-runoff conceptual model in forecasting and warning issuance for floods entering the Dez dam resrvoir, southwest Iran. For this purpose, after deriving precipitation and temperature parameters (including control values and ensemble members) up to 10-days lead time, the flood ensemble forecast was produced. In the study, in case of exceeding the threshold discharge (1620 and 2640 m3/sec with return period of 2 and 5 years, respectively) by a relative majority of forecasting members, the flood warning was issued. Although, flood forecast in basins with the rain-snow regime is very complicated, the results of evaluation indexes showed that a considerable improvement of forecast accuracy might be expected 5 days ahead (5-days lead time). Then again, there was no direct relationship between the reduction of forecast lead time and the enhancement of forecast accuracy. The results of the probabilistic study of the members’ agreement percent in exceeding the threshold discharge of 2640 m3/sec indicated warning issuance within the forecast lead time of 4-7 days ahead.