Detecting Climate Variability and Hydrologic Alteration in the Pasak River Basin, Thailand

Rainfall and temperature are key factors in the hydrologic cycle, which are directly related to streamflow and can determine climate change. Changes in rainfall and temperature are important for understanding the relative of hydrologic alteration, potentially causing flood and drought in the Pasak River Basin, Thailand. The variation characteristics of rainfall and temperature in the Pasak River Basin were detected using Mann-Kendall with Sens slope estimator and the streamflow alteration were determined using Indicators of Hydrologic Alteration (IHA). A least 50 years of annual and monthly meteorological station data were analyzed to identify trends and rates of change while the daily continuous streamflow discharge at the upstream (S.33) and downstream (S.9) gauging stations were used as the input for the IHA software. The results showed that the annual and seasonal maximum and minimum temperatures indicated significant increasing trends at p-value of 0.05, while the rainfall pattern detected insignificant increasing trends except for the reduction of 1.03 mm/year at Lom Sak station which is located in the upstream of the watershed. Considering the streamflow alteration in the Pasak River, the median monthly flow at the upstream station (S.33) tended to decrease from 0.06 to 2.71 m³/sec, correlated with the rainfall trend at Lom Sak station. Meanwhile the pattern of median monthly flow hydrograph at the downstream station (S.9) was altered by lagging of the rising limb from August to September, drop in peak, and increased during the dry season with a rate of 28.07 m³/s. Finally, this study clearly captures significant changes in the key climate variables implying Climate Change, and human-induced streamflow alteration in the Pasak River Basin. This would be the beginning of risk perception and awareness for identifying risk management interventions in the future.