Study area
The Mekong River is the 12th longest river in the world with the length of 4,800 km, has the 21stlargest river basin area at 795,000 km2, and the 8th largest average annual runoff, 470 km3. By convention, the Mekong River basin is divided into two sub-basins: The Upper Mekong basin, with 24% of the total drainage area (21% in China, 3% in Myanmar), and the Lower Mekong basin, with 76% of the total drainage area (Laos 25%, Thailand 23%, Cambodia 20% and Vietnam 8%) (Figure 1).
The climate of the Mekong Basin is dominated by the Southwest Monsoon, which generates wet and dry seasons of more or less equal length. The monsoon season usually lasts from May until late September or early October. Annual average rainfalls over the Cambodian floodplain and the Vietnamese delta are less than 1,500 mm. the highest rainfalls occur in the Central Highlands and within the mainstream valley in central Laos. At altitudes above 500 masl, dry season temperatures are lower, though not by much. In the warmest months of March and April, average temperature ranges from 30°C to 38°C. Rainy season mean temperatures decrease significantly from south to north, from 26°C to 27°C in Phnom Penh to 21°C to 23°C in Thailand northern part. The Mekong’s average discharge to the sea is about 15,000 m3/s (Adamson, Rutherfurd, Peel, & Conlan, 2009; A. Gupta & Liew, 2007).
In Cambodia, the Mekong River connects with Tonle Sap Lake (the largest permanent freshwater body in the Southeast Asia) via the Tonle Sap River, at the Chaktomuk confluence at Phnom Penh (Figure 1 ). Tonle Sap Lake (TSL) has a unique hydrological system characterized by a flood pulse from the Mekong River. The Lake is about 120 km long and 35 km wide and covers 2,500 km2 in the dry season, but it expands to about 250 km long and 100 km wide, and covers 17,500 km2 in the wet season because high stages in the mainstem Mekong River drive flow upstream through the Tonle Sap River (Ian Charles Campbell, Say, & Beardall, 2009). From October to April, flow in the mainstem Mekong recedes, and water flows back from Tonle Sap Lake to the Mekong River via the Tonle Sap River (Fujii et al., 2003; Masumoto, 2000). The majority of water in the Lake in the wet-season is from the Mekong mainstem (Kummu et al., 2014), and the delayed release of this water provides important freshwater flow to the Mekong Delta during the dry season, protecting the fertile agricultural lands of the delta from saltwater intrusion from the South China Sea (Hai, Masumoto, & Shimizu, 2008).
We selected three stations with continuous records of discharge and total suspended sediment: the Mekong River at Kratie, the Mekong at Chroy Changvar (near the Chatumuk confluence, just upstream of the Tonle Sap confluence) reflecting discharge and sediment load above interactions with the Tonle Sap, and the Prek Kdam station on the Tonle Sap River, representing the flow-reversal system of Mekong and Tonle Sap. We analyzed flow and sediment records based on the hydrological year: from 1st of May to 30th of April next year (Kummu & Sarkkula, 2008; Kummu et al., 2014).