A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Water Engineering and Management

Thesis (M. Eng.) - Asian Institute of Technology, 2017

This study examined the combined and separated impacts of climate and land use change on streamflow, sediment yield and nitrate-nitrogen in Songkhram river basin Thailand. The climate data predicted by ACCESS, CNRM and MPI for three future time periods (2010{u2013}2039, 2040{u2013}2069, 2070{u2013}2099) were prepared by using linear scaling bias correction method under RCP 4.5 and RCP 8.5. Three calibrated RCM showed that temperature is continuously increasing in the study area, however, future precipitation is highly complex and uncertain, there were significant differences among various RCM under RCP scenario. The future land use were simulated using the Conservation of Land Use and its Effects at small regional extent (Dyna-CLUE) model by establishing logistic regression model for seven land use types. The regression model Load Estimator (LOADEST) were applied to developed regression equations and to estimate nutrient and sediment loading at three site of Songkhram River basin from 1990 to 2014. Estimated loading shows increasing trend during past, however, suspended solid showed decreasing trend during historical period. Non-parametric Mann-Kendall test was used to check the significance of trend. Three scenarios (climate change only, land use change only and combined) were developed and the streamflow, sediment yield, and nitrate yield in future periods under these scenarios was simulated by using Soil and Water Assessment Tools (SWAT) model. Under climate change only annual and seasonal, streamflow, sediment yield, and nitrate yield is in decreasing rate with more in later period. Whereas land use change increase the flows and sediment yield during all periods and under both scenarios. But LULC change decreased nitrate yield. The results for combined scenario was similar to that of the climate change only scenario. In the future, climate changes tend to affect the hydrological regimes, sediment yield and nitrate yield much more prominently than the land use change, leading significant decreasing in streamflow, sediment yield and nitrate yield. Nevertheless, the role of land use change should not be overlooked, especially if the climate becomes drier in future, as in this case it may magnify the hydrological responses.