Case Study on Extreme Flood Forecasting Based on Precipitation Ensemble Forecast Products in QingJiang Basin of the Yangtse River

The quantitative precipitation forecasting products has larger random, and brings greater uncertainty into flood forecasting. The emergence of ensemble forecast provides a new idea for flood forecasting. A set of forecast results can be obtained by inputting the ensemble precipitation forecast products into the hydrological model, which can avoid the mistake of "single" deterministic numerical forecast results. Therefore, it is a hot and difficult issue for hydro-meteorologists how to introduce precipitation ensemble forecasting to improve the accuracy of flood forecasting and reduce the uncertainty of flood forecasting. In this paper, taking flood process in the Qingjiang River in June 2016 as an example, flood forecasting experiments are carried out based on ensemble precipitation forecasting products.

From 8:00 on July 18 to 8:00 on July 20, the whole Qingjiang Basin was attacked by heavy rain, the accumulated rainfall of all stations is almost 100 mm. The data used in the experiment mainly include: the hourly inflow data of Shuibuya reservoir, the rain data of 70 surface rian stations in the study basin from 2009 to 2016, and the ensemble precipitation prediction of 50 members of the European Numerical Forecasting Center.

According to the humid climate characteristics of the basin, Xin`anjiang model, which is better applied in hydrological forecasting at home and abroad, is selected as the hydrological model of the basin. Combined with the rainstorm and flood process of Shuibuya Reservoir, 30-40 flood processes were selected to carry out hydrological simulation tests to determine the parameters of hydrological model, and seven flood processes during 2008-2012 were used to test the parameters.

During the study, the precipitation forecast results of 50 members in the basin are extracted and processed. The hydrological ensemble forecasting experiment is carried out with the input of 50 members into the hydrological model. The results show that the ensemble precipitation forecasting products are applied to hydrology. It can provide more precipitation forecasting information than deterministic forecasting. It can enrich the input information of hydrological model and calculate the possible range of flood peak and the arrival time of flood peak in river basin, can transform single forecasting into probabilistic forecasting, which can better meet the needs of risk information for flood control and disaster reduction.