Historical Change in the Flow Duration Curve for the Upper Nan River Watershed, Northern Thailand

Abstract

　In recent years, large-scale deforestation has increased in Nan Province, northern Thailand, and the impact on the hydrological environment due to deforestation has become a concern. In this study, historical changes in the rainfall duration curve and flow duration curve (FDC) were examined by statistical analysis using hydrological data for a 42-year period (1974- 2015) obtained from the upper Nan River watershed. Daily rainfall did not show any significant increasing trend over the study period. However, low-flow discharge and drought-flow discharge in the FDC both increased significantly. Especially, the drought-flow discharge, from 86 % to 96 % of the annual time flow exceeded (days 314-351), increased with a significance level of 0.001. According to previous research, natural forest decreased by about 41.5 % (from 856,246 to 500,439 ha) and agricultural land increased by about 51.5 % (from 342,997 to 518,257 ha) in Nan Province over a 17-year period (1995-2012). This largescale land-use change is thought to have influenced the change in the FDC.