Runoff and sediment response to deforestation in a large Southeast Asian monsoon watershed

Abstract

Deforestation has been reported to increase annual streamflow, baseflow, and sediment load in many parts of the world. However, studies about the hydrological effects of deforestation are scant in the tropical monsoon region. In this study, these effects were examined in the Upper Chao Phraya River basin (UCPRB) which is a typical tropical monsoon basin and is an important area of water resources in Thailand. A significant breakpoint for annual streamflow and annual precipitation in the entire UCPRB is identified as 1993, and this breakpoint is also identified for annual baseflow in most upstream sub-basins during the past decades (1981–2015). Significant increasing trend is detected in the annual suspended sediment load of the Wang River sub-basin. Land use maps generated by a CA-Markov model suggest that continuous deforestation occurred during the study period. The contributions of climate change and deforestation to the hydrological and sedimentary changes are then attributed within the Budyko Framework. Precipitation increment dominates the annual streamflow increment in the entire UCPRB and the annual baseflow increment in the Ping River sub-basin, and deforestation dominates the annual streamflow increment in the upstream sub-basins, the annual baseflow increment in most of the upstream sub-basins, and the increasing suspended sediment load in the Wang River sub-basin. The hydrological and sedimentary response to deforestation is then quantified. In the entire UCPRB, one percent reduction of the forest cover can increase the annual streamflow by 1.9%. In the upstream sub-basins, one percent reduction of the forest cover can increase the annual streamflow and annual baseflow by 2.5%–5.4% and 2.6%–6.7%, respectively. One percent reduction of the forest cover can increase the annual suspended sediment load by 8.7%. These findings will help develop better understanding of land use management in the UCPRB for water-soil conservation.