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This study examined seasonal and annual trends of rainfall and streamflow data in the Mae Klong Basin, Thailand. Monthly data of eight key rainfall stations and ten streamflow stations were analyzed to detect trends using the non-parametric Mann-Kendall test, whilst the magnitude of the trends was determined by Sen’s slope method for the period 2000-2015. For 75% of the analyzed stations, rainfall was found to increase in the wet season and decrease in the dry season. Station 130013 situated in the lower region showed a statistically significant increasing trend with a trend slope of 16.02 mm/yr in the wet season, while station 130042- also located in the lower region of the basin- showed a statistically significant decreasing trend, with a trend slope of 23.60 mm/yr in the dry season. On an annual basis, 63% of the analyzed stations showed increasing rainfall trends, particularly in the central and lower regions of the Mae Klong Basin; however, rainfall trends in the upper region were found to be decreasing, which reflected water contributions to two main reservoirs in the upper part. The trends of naturalized inflow of Srinagarind and Vajiralongkorn Reservoirs were found to be decreasing on both seasonal and annual bases, while two naturalized streamflow stations located in Lam Taphoen and Lampachi sub-basins in the central and lower regions, respectively, showed increasing trends in both dry and wet seasons. The trends of regulated stream-flow stations downstream of 4 main dams which were a result of reservoir operation were found to mostly decrease on an annual scale. Results of this study can help water resources managers enhance accuracy of assessment and effective planning of water resources management in the basin.
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