Water balance analysis for the Tonle Sap Lake–floodplain system M. Kummu, 1 * S. Tes, 2 S. Yin, 2 P. Adamson, 3 J. Józsa, 4,† J. Koponen, 5 J. Richey 6 and J. Sarkkula 7 1 Water and Development Research Group, Aalto University, Finland 2 Department of Hydrology and River Works, Ministry of Water Resources and Meteorology, Phnom Penh, Cambodia 3 School of Mathematical SciencesAdelaide University, South, Australia 4 Department of Hydraulic and Water Resources Engineering, Budapest University of Technology and Economics, Hungary 5 Environmental Impact Assessment Centre of Finland, EIA Ltd., Espoo, Finland 6 School of Oceanography, University of Washington, Seattle, WA, 98195, USA 7 SYKE, Finland Environment Institute, Helsinki, Finland Abstract: The Tonle Sap Lake of Cambodia is the largest freshwater body of Southeast Asia, forming an important part of the Mekong River system. The lake has an extremely productive ecosystem and operates as a natural floodwater reservoir for the lower Mekong Basin, offering flood protection and assuring the dry season flow to the Mekong Delta. In light of the accelerating pace of water resources development within the Mekong Basin and the anticipation of potentially significant hydrological impacts, it is critical to understand the overall hydrologic regime of Tonle Sap Lake. We present here a detailed water balance model based on observed data of discharges from the lake’s tributaries, discharge between Mekong and the lake through the Tonle Sap River, precipitation, and evaporation. The overland flow between the Mekong and lake was modelled with the EIA 3D hydrodynamic model. We found that majority (53.5%) of the water originates from the Mekong mainstream, but the lake’s tributaries also play an important role contributing 34% of the annual flow, while 12.5% is derived from precipitation. The water level in the lake is mainly controlled by the water level in the Mekong mainstream. The Tonle Sap system is hence very vulnerable, from a water quantity point of view, to possible changes in the Mekong mainstream and thus, development activities in the whole Mekong basin. From a biogeochemical point of view, the possible changes in the lake’s own catchment are equally important, together with the changes in the whole Mekong Basin. Based on our findings, we recommend of continuing the monitoring programmes in lake’s tributaries and urgently starting of groundwater measurement campaign within the floodplain, and including the groundwater modelling to be part of the hydrodynamic models applied for the lake. Copyright © 2013 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. KEY WORDS flood pulse; water balance model; floodplain; hydrology; Tonle Sap Lake; Mekong Received 11 June 2012; Accepted 11 January 2013 INTRODUCTION Floodplains are one of the most important parts of aquatic ecosystems. They provide important services for human- kind, as they are rich in biodiversity and highly productive in terms of fishes and other aquatic animals (Finlayson and Spiers, 1999; Millennium Ecosystem Assessment, 2005). Maintenance of the hydrological regime of a floodplain and its natural variability is necessary to sustain its ecological and biodiversity characteristics (Millennium Ecosystem Assessment, 2005). The most extensive wetland habitats of the Mekong River Basin are located in the heart of Cambodia, the Tonle Sap Lake (Figure 1). The lake and its surrounding floodplains form the largest freshwater body in Southeast Asia. The lake is reported to be very productive (Rainboth, 1996; Sverdrup- Jensen, 2002), driven by an annual mono-modal flood pulse (Junk et al., 2006; Lamberts, 2006). The lake functions as a natural floodwater reservoir for the Mekong system during the dry season (November–April), when approximately half of the discharge to the Mekong Delta in Vietnam originates from the lake (Fuji et al., 2003). The ecosystem is driven by a flood pulse regime, supporting a fishery and aquaculture that provides approximately up to 80% of the protein consump- tion of Cambodia (e.g. Ahmed et al., 1998; Hortle, 2007). Although there are over 70 million people living in the Mekong Basin (Pech and Sunada, 2008) and more than 30 existing large dams (Mekong River Commission, 2009), the Mekong is still considered to be one of the few large river basins in which the flow regime has remained in rather natural conditions (Mekong River Commission, 2005b). The Mekong is, however, facing rapid development, including deforestation (FAO, 2006; Shi, 2008), large irrigation schemes (Hori, 2000; Kummu et al., 2009), and construction of hydropower dams with large reservoirs (e.g. Hori, 2000; Dore and Yu, 2004; King et al., 2007; Mekong River Commission, 2008; ICEM, 2010; Kummu et al., 2010; Grumbine and Xu, 2011). Recent hydrological impact assessment studies (Adamson, 2001; ADB, 2004; World Bank, 2004; ICEM, 2010; Mekong River Commission, 2010; Lauri et al., 2012; Räsänen et al., 2012) conclude that due to the planned development, the dry season water levels would rise, and wet season water levels would become lower, relative to current conditions. The magnitude of predicted *Correspondence to: M. Kummu, Water and Development Research Group, Aalto University, Finland. E-mail: matti.kummu@iki.fi † Present address: MTA-BME Water Management Research Group, Hungary HYDROLOGICAL PROCESSES Hydrol. Process. 28, 1722–1733 (2014) Published online 14 February 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9718 Copyright © 2013 John Wiley & Sons, Ltd.