Drastic decline of flood pulse in the Cambodian floodplains (Mekong River and Tonle Sap system)

Abstract

The Cambodian floodplains experience a yearly flood pulse that is essential to sustain fisheries and the agricultural calendar. Sixty years of data, from 1960-2019, are used to track the changes to the flood pulse there. We find that minimum water levels over 2010-2019 increased by up to 1.55m at Kratie and maximum water levels decreased by up to 0.79m at Prek Kdam when compared to 1960-1991 levels, causing a reduction of the annual flood extent. Concurrently, the duration of the flooding season has decreased by about 26d (Kampong Cham) and 40d (Chaktomuk), with the season starting later and ending much earlier. Along the Tonle Sap River, the average annual reverse flow from the Mekong to the Tonle Sap Lake has decreased by 56.5%, from 48.7km3 in 1962-1972 to 31.7km3 in 2010-2018. As a result, wet-season water levels at Tonle Sap Lake dropped by 1.05m in 2010-2019 compared to 1996-2009, corresponding to a 20.6% shrinkage of the lake area. We found that upstream contributors such as current hydropower dams cannot fully account for the observed decline in flood pulse. Instead, local anthropogenic causes such as irrigation and channel incision are important drivers. We estimate that water withdrawal in the Cambodian floodplains is occurring at a rate of (2.1 0.3)km3yr-1. Sediment decline and ongoing sand-mining operations have also caused channel erosion. As the flood pulse is essential for the ecological habitats, fisheries and livelihoods of the region, its reduction will have major implications throughout the basin, from the Tonle Sap system to the Vietnamese Mekong Delta downstream.