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Land use and land cover (LULC) have been continuously changing, through human activities, leading to variations in hydrological cycle. In this study, we applied SWAT model to investigate potential impacts of LULC on water budget of the Chi river basin in Thailand. Five plausible scenarios of land use change were evaluated, including an conversion of forested area, expansion of farmland, switching of rice paddy fields to energy crops and two scenarios involving conversion of farmland to rice and sugarcane plantation. The results indicated that different land use scenarios contributed to various effects in annual and seasonal water yield and evapotranspiration (ET). Conversion of forested area and farmland showed slightly small changes on water flows and ET. Substitution of paddy fields by sugarcane plantation showed clearly reduced water flows and increased ET by almost 5.0% in dry season. Particularly, in case of expansion of rice paddy fields to farmland, small changes occur on annual flow and ET but more significant effect occur on seasonal flows. The results showed decrease in ET by nearly 12.0%, leading to increase of water yield by up to 5.1% during dry season. Finally, conversion of farmland to sugarcane plantation for biofuel production showed significant effect on seasonal ET, mainly decreasing in dry season by 4.5 % but small changes were detected on water yields.

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In the hydrological cycle and in water use, evapotranspiration is one of the most important components because it represents a loss of usable water from the hydrological supply. The purposes of this research are to examine the temporal trend of reference evapotranspiration and pan evaporation for the upper region, the lower region, and the whole catchment of the Chao Phraya River Basin, and to consider the spatial and temporal distribution of monthly reference evapotranspiration. To achieve these purposes, daily reference evapotranspiration from 1971 to 2002 was computed for each weather station using the FAO Penman-Monteith method. The average of monthly reference evapotranspiration in each weather station was calculated. Thereafter, this monthly reference evapotranspiration reference point for evapotranspiration was interpolated using the Co-Kriging interpolation to obtain a spatial value. From the results of this research it can be concluded that the trends of both reference evapotranspiration and pan evaporation in all the regions have decreased from 1971 to 2002 and that the spatial and temporal distribution of mean monthly reference evapotranspiration are affected by the changes in the meteorological variables for each month and for each different region.