Effects of El Niño on Flow of Hongshui River

The Hongshui River, with the length of 659 km and the catchment area of 3.3×104 km2, is the biggest tributary of the Pearl River in China. Considering 26 hydropower projects within the Hongshui River basin, it is crucial to predict the discharge flow for the basin. The discharge flow data of five hydrological gauge stations (1951-2015), situated in the Hongshui River basin, were selected, and the effects of El Niño on the flow of the basin was investigated by means of a comprehensive index MEI – C-MEI, constructed by extreme MEI, average MEI and duration month MEI, presented in this work. The results showed that, the discharge flow for the basin is related to both the intensity of C-MEI and the period of duration month MEI. For the El Niño episode with strong C-MEI and short period, the flow increases obviously in the strengthening duration of the El Niño episode. Taking for example of the El Niño episode in 1982-1983 and 1997-1998 (15 duration months), the average flow increment of over 100% was obtained and that of about 200% occurred in some hydrological gauge stations. For the weakening duration of the El Niño event, the flow decreases, the maximum decrement reaching -50% in the same event in 1982-1983 and 1997-1998. Comparing the El Niño episode with strong C-MEI and short period, the level of the flow change, in either strengthening or weakening duration, was raised in the El Niño episode with strong C-MEI and long period. With respect to the El Niño episode in 1986-1988 (19 months) and 1991-1993 (32 months), due to much longer duration months than those in 1982-1983 and 1997-1998, the flow increment approximates 100% in their strengthening duration and the decrement reaches -80% in the weakening duration, even the relative low intensity of the El Niño episode.