Assessing impacts of climate change on water resources and agriculture: A case study of Tonle Sap basin, Cambodia : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Water Resource Management at Lincoln University

Abstract

Cambodia’s Tonle Sap Lake is the largest permanent freshwater body not just in Cambodia, but in Southeast Asia, and is also one of the world’s richest lacustrine-wetland ecosystems. Agriculture and fisheries provide the primary livelihoods of people living in the area. Despite a high abundance of natural resources, the area around Tonle Sap Lake is known to be one of the poorest areas in Cambodia, where most people derive their livelihoods directly from the resources provided by the lake. This study aims to assess the impacts caused by climate change on water resources and agricultural production in the basin by looking into future changes of streamflow of the tributary rivers, flood pulse and the paddy rice areas supported by the Mekong River’s flood pulse. For this study, six climate change scenarios were employed to assess future change in rainfall and river flow. They are the result of the combinations between three global circulation models (GFDL-CM3, GISS-E2-R-CC and IPSL-CM5A-MR) and two representative concentration pathways (RCP4.5 and RCP8.5). HEC-HMS was used for simulating rainfall and runoff for 11 sub-basins that feed the Tonle Sap Lake. HEC-RAS was used for computing inundation areas around the lake. Both models were calibrated and validated using data for the year 2000–2005 and 2006–2007, respectively. The results from HEC-RAS were exported in a format that enabled further analysis in GIS to examine changes to paddy rice areas at both the basin- and sub-basin scales. Both HEC-HMS and HEC-RAS model performances were evaluated using statistical indices NSE and R2. The indices indicated satisfactory performance for both simulation models with NSE > 0.40 and R2 > 0.60 for HEC-HMS and NSE > 0.60 and R2 ≥ 0.90 for HEC-RAS. The main findings in the study were the reduction of annual streamflow that is projected to occur in almost every sub-basin under all climate change scenarios up to the year 2030. The Dauntri sub-basin is projected to experience the highest streamflow decrease, up to 62.53%, while streamflow in the Sen and Boribor sub-basins showed a slight increase. The results from HEC-RAS suggest a decrease of flood pulse extent under all climate change scenarios. The magnitudes of decrease are almost the same for each scenario with an average decrease of around 10%. The Sen sub-basin showed the greatest reduction of flooded areas (13.82%) while Sangker was projected to decrease the least (2.00%). Though the Sen and Boribor catchments show an increase in streamflow, the increase is offset by the reduced flows in the remaining catchments, thus contributing less flow to the lake overall, leading to its reduced area. The results also suggest a decrease in paddy rice areas supported by the flood pulse. Stuang is the sub-basin with the highest reduction of paddy rice areas of up to 28.36%, while Sangker remains the sub-basin with the least reduction (2.67%). Some noteworthy implications arise from the main findings. The decrease of flows in the tributary rivers suggests an increase in drought risk and consequences for household water supply and surface irrigation that divert or extract water from those rivers. The change of the extent of flood pulse suggests that there will be lower nutrients and sediment loads and this would substantially impact the ecosystems in the lake and other connected parts. The reduction of paddy rice areas underscores the potential implications for social and economic development such as food insecurity, unemployment and economic impacts.