Impacts of ENSO events and land-use changes on water balance of Chi river basin

Historical data of annual rainfall, seasonal and annual flow during 1951 to 1994 collected by Royal Irrigation Department (RID) and Electric Generating Authority of Thailand (EGAT) corresponding to land use changes derived from the landsat imageries during 1973 to 1995 interpreted by Royal Forest Department, Faculty of Forestry, Land Development Department (LDD) and Office of Agricultural Statistics were employed to analyze relationships between land-use changes and water balance of 9 sub-watersheds in Chi river basin. SST and SOI anomaly obtained from National Oceanic and Atmospheric Administration (NOAA) were used to determine ENSO effect on different designed periods of rainfall and runoff of the main basin. It was found that during the past 23 years (1973-1995) forest area in the main basin decreased from 35 percent to 14 percent while the mixed crop land increased from about 1.2 to 32 percent, upland agriculture (maize, cassava, sugar cane) fluctuated from more than 36 percent to 12 percent with a remarkably dropped of cassava from 30 percent to 9.6 percent. The paddy increased from 21 percent to 37.8 percent while urban and water resources were ranged in 0.12-0.88 percent and 0.97 to 1.44 percent respectively. ENSO effect on annual runoff and rainfall was not obvious. Drought was in the ENSO year and tended to occur in the following next year. ENSO phenomena affected rainfall and discharge in Chi Watershed for the short period when there was a SST/SOI strong level of index but its effect was not obvious in statistics. Rainfall only decreased in earliness of rainy season but increased in others. There wasn't much impact of ENSO on discharge and it could be interpreted that strong ENSO in October-March effected drought in the dry season. According to Chi Watershed is in Monsoon zone so drought is not severe even in the ENSO year. Although, the annual rainfall of all sub-basins tended to decrease, there was insignificant relationship between land-use change, particularly forest area depletion, on annual rainfall. Also insignificant relation between water budget components and land use changes was found with rather small effect on seasonal and annual flows of all studied sub-basins and the main basin.