| The Chao Phraya River basin, the largest basin in Kingdom of Thailand is located in central and northern part of Thailand. This basin has two large-scale dams: Bhumibol Dam in the Ping River and Sirikit Dam in the Nan River. Their large reservoirs have contributed to economic development of the country for supplying increased electricity and irrigated water demands as well as flood mitigation in riparian areas along the Ping, the Nan and the Chao Phraya rivers.Analysis of comparing annual and monthly flow regime downstream of the dams before and after dam construction shows a constant increase in low flow and a drastic decrease about 60% in high flow. As analysis of precipitations shows no long-term increase or decrease tendency of the precipitation, it can be concluded that the reservoir operation is the main cause of the flow regime change.Spectrum of flow was analyzed by using FFT for daily discharges in Nakhon Sawan and immediate downstream of the Bhumibol dam (Royal Irrigation Department’s code P.12) after the Bhumibol dam construction. Flow in Nakhon Sawan has periodic characteristics of 3.5 days and 7 days, and the released water from the Bhumibol dam at P.12 has periodic characteristics of 2.5 days, 3.5 days and 7 days. Reservoir operation affected the hydrological cycle considerably. |
