Impacts of Land Cover Changes and Large Reservoirs Development on Streamflow Regime of the Chao Phraya River Basin and Its Tributaries

Abstract

Flow timing parameters in terms of quarter flow interval (QFI) and half-flow interval (HFI) for wet period and five-percent flow interval (5%FI) and one-percent flow interval (1%FI) for dry season were determined basing on cumulative flow of 5-yr interval average for selected observation stations of Ping (P19A), Wang (W4A), Yom (Y3A), Nan (N5A), and lower Chao Phraya basins at C2, C7A, and C15. The investigation was carried out using secondary data of streamflow from 1956-1993 observed by RFD and forest map prepared by RFD. It was found that land-use changes during the past 3-decades shortened wet flow timing which consequently decreased dry flow in those narrow and long basins such as Wang and Yom rivers especially in the drought year. For those river systems having large reservoirs, changes of flow timing was dependent on reservoir regulation, downstream rainfall patterns and characteristics. The impact of forest conversion on flow regime of large river basin as Chao Phraya were deemed less than that of the operation of huge reservoirs.