Effects of Land Cover Change and Large Reservoir Operation on Water Balance of the Chao Phraya River Basin

Abstract

Periodical and long-term annual water balance of the Chao Phraya river basin and its tributaries were determined by water balance equation using data on annual basin rainfall and runoff during 1954 to 1993 observed by Meteorological Department and Royal Irrigation Department. Land cover change data during the investigation period were provided by Royal Forest Department and Land Development Department with those recorded by the Office of Agricultural Economic. The results revealed that mean annual rainfall of the Chao Phraya basin ranged between 1,034-1,893 mm and potentially declined with decreasing forest area from 67% in 1961 to 37% in 1993 and insignificant difference in the entire Chao Phraya basin and its sub-basins. In overall, runoff had potentiality to be decreased in every sub-basin and the lower basin particularly for Wang, Yom and the lower Chao Phraya at Ang Thong. E_t component was significantly higher in Wang and the lower Chao Phraya basin whereby E_t were up to 90% of average annual rainfall in the last 2 periods (1986-1993) of which the average annual rainfall was decreased. The greater E_t loss at downstream was perhaps due to the extended area of dry-season paddy cultivation including rapid development of industries and urbanization.