1 MEDIUM RANGE RAINFALL AND FLOOD FORECASTING FOR RESERVOIR SYSTEM OPERATION IN THE CA RIVER BASIN (VIETNAM) Hoang Thanh Tung 1) , Roberto Ranzi 2) , Stefano Barontini 2) , Vu Minh Cat 3) 1) Faculty of Hydrology and Water Resources, Water Resources University, Tay Son Street 175, Dong Da District, Hanoi, Vietnam; email: httung@wru.edu.vn 2) University of Brescia, Via Branze 43, Brescia 25123, Italy 3) Faculty of Coastal Engineering, Water Resources University, Tay Son Street 175, Dong Da District, Hanoi, Vietnam Abstract Flood damage and environmental impact mitigation of reservoir operation, as well as the improvement of socio-economic conditions in central provinces of Vietnam has become an urgent matter, especially since water hazards appear to be more frequent and severe today than in the past. This article presents results achieved from a research on medium range rainfall and flood forecasting for operation of the reservoir system in the Ca River basin of Vietnam, with a catchment area of 23830 km 2 . A combination of rainfall and flood forecasting with flood propagation and reservoir control models has been implemented and applied for that basin. A combination of a Numerical Weather Prediction with conceptual hydrological schemes integrated with ARIMA and BPNN-Back Propagation Neural Network models was applied. A Nash-Sutcliffe efficiency between 0.68 and 0.77 was achieved with the conceptual modelling system, while BPNN reached values between 0.85 and 0.90 . Operation rules for reservoir operation are proposed for flood control in the basin, based on weather types and flood forecasting. Keywords: Ca River, Reservoir system, Flood control, Flood forecasting 1. INTRODUCTION In recent years, floods and droughts appear to occur more and more frequently and seriously today than in the past in the central provinces of Vietnam. For instance in 2007, a series of five floods occurred in only one month period causing huge damage on both people and properties in the central districts of Que Phong, Quy Chau, Nghia Dan of Nghe An province in the Ca River Basin; also in october 2010 about 50 casualties and widespread inundation occurred in the same area. Pollution and water shortage in river runoff are also main issues in central provinces of Vietnam. This is not only because of objective factors such as weather and climate variability, but also of subjective factors such as poor capacity of medium range (2 to 5 days ahead) and long term (more than 10 days) rainfall and runoff forecast as well as poor operation of reservoirs in river basins in the central provinces. In the Ca River basin, there are some reservoirs being operated or under construction such as Sao reservoir on the Sao River, Ban Mong reservoir on the Hieu River, Ban Ve reservoir and Khe Bo reservoir on the Ca River, and Thac Muoi reservoir on the Giang River (Fig. 1). The operation of these reservoirs has to meet flood control, hydropower generation, water supply for irrigation, industrial, domestic and environmental demand. Operation rules are set for single reservoirs, mainly for dam safety reasons, and no regulation practices are set for the reservoir system. For mitigation of damages caused by floods, as well as to take into account socio-economic and environmental effects of reservoirs operation, it is necessary to extend research on both medium range rainfall and runoff forecast and to set a comprehensive scientific basis for operation of the reservoir system. Thus, a main objective of this research is to investigate potentials and limitations of medium range rainfall and flood forecast for operation of reservoir system in view of dam safety, flood control and hazard mitigation in the Ca River basin. In the next section the investigated area is described, while methods adopted are outlined in the third section. Results of the modelling scheme adopted are discussed in the following section, before conclusions. 2. THE CA RIVER BASIN The area investigated in this research is the Ca River Basin in Vietnam territory, from Vietnam – Laos boundary to the confluence of the Ca river and the La river at Cho Trang. This corresponds to the coloured area in Fig. 1 which measures 14360 km 2 in size out of the 23830 km 2 of the whole Ca river