Evaluation of Precipitation Forecast from Global Forecast System over Nile Basin

Abstract:

This study evaluates the short-to-medium range (1-day, 5-day, 10-day and 15-day) accumulated precipitation forecasts from Global Forecast System for seven sub-basins within the Nile Basin against GPM IMERG “Early” and “Final” products. The sub-basins are defined by the seven major dams in the Nile River basin: (1) High Aswan Dam; (2) Owen Falls Dam; (3) Sennar Dam; (4) Jebel Aulia Dam; (5) Khashm El Gibra Dam; (6) Roseires Dam; and (7) Millennium Dam. In terms of temporal variability, there was a good agreement between the forecasted and observed precipitation on a 10-day basis. When compared to IMERG Final, the 10-day accumulated GFS precipitation of High Aswan Dam scored as high as 0.79 in terms of NSE. Thus, GFS products provide relatively reliable accumulated precipitation forecasts. However, the GFS precipitation forecasts were mostly biased: they tend to overpredict rainfall for Jebel Aulia Dam, Khashm El Gibra Dam, Roserires Dam and Millennium Dam, underestimate rainfall for High Aswan Dam and produce almost unbiased estimates for Owen Falls Dam and Sennar Dam. Additionally, GFS forecasts have a general tendency to underpredict the area of precipitation across all sub-basins. GFS is able to capture the general evolution of rainfall events both temporally and spatially. Although the performance of GFS varies at different locations, the GFS precipitation forecasts can be a good reference to dam operators in Africa.