15 The increasing pressure on wetland resources continues to threaten the role wetlands play in 16 maintaining the ecological balance of watersheds. The Cuvette Centrale of the Congo is the 17 greatest intertropical peatland in the world. To fully understand its role in water resources and 18 ecological services linked to the quality of water and life in the basin, we first need to quantify 19 its role in the hydrological dynamics. To achieve this aim, we used the Soil and Water 20 Assessment Tool model (SWAT)-modified for tropical environments-in combination with 21 monthly discharge data. We analyzed water fluxes entering and flowing out of the Cuvette 22 Centrale of the Congo River Basin on a monthly time scale for the 2000-2012 period. The 23 model was calibrated, validated, and compared with discharge from gauging stations and 24 surface water elevation from radar altimetry. Results showed that upland runoff from the 25 Congo River was the highest contributor to the Cuvette Centrale (33 percent) followed closely 26 by efficient precipitation inside the Cuvette Centrale (31 percent) with right bank and left bank 27 tributaries contributing 25 percent and 11 percent respectively. We simulated monthly mean 28 interannual inflows of approximately 34,150 m 3 s-1 (88 billion m 3) with the main flood peaking 29 in November (45,310 m 3 s-1) and total outflows averaging around 39,860 m 3 s-1 (100 billion 30 m 3) peaking at 52,430 m 3 s-1 in December for the simulation period. We subsequently estimated 31 a negative monthly mean interannual variation of storage in the Cuvette Centrale wetlands in 32 the order of 5,700 m 3 s-1 suggesting that the Cuvette Centrale supplies the river during low 33 water periods. This highlights the important regulatory function of the Cuvette Centrale and 34 the need for protection of groundwater resources in order to maintain wetland water quantities 35 and quality. 36

The Mekong River, one of the world’s great rivers, is facing the disruption of its sediment balance with anthropic reductions in its sediment load and resultant impacts on nutrient fluxes, aquatic ecology and evolution of its river channel, floodplain and delta. Using long-term monitoring data from 1993-2018, we estimated the temporal variability of sediment loads in Tonle Sap and Lower Mekong Rivers in Cambodia, assessing the sediment linkage between Tonle Sap Lake and Mekong River, which are connected by a seasonally reversing flow through the Tonle Sap River. We used data from three monitoring stations established in Cambodia in 1993, from the Mekong at Kratie (upstream) downstream to the Mekong at Chroy Changvar (just upstream of the Tonle Sap confluence), and the Tonle Sap River at Prek Kdam (about 40 km upstream of the Mekong confluence). We estimated the annual sediment in the main Mekong River was 72±38 Mt/yr at Kratie and 78±22 Mt/yr at Chroy Changvar from 1993-2018. Our calculated sediment load for the Lower Mekong River is lower than reported in older studies (prior to the 2000s), which is consistent with sediment trapping by dams on Upper Mekong mainstream and major tributaries built since 1993, and consistent with other recent estimates of sediment load on the Lower Mekong. Our analysis of water discharge and sediment concentration indicates that Tonle Sap Lake provided 0.65±0.6 Mt of sediment annually to the Lower Mekong River from 1995 to 2000. However, since 2001, Tonle Sap Lake has become a sink for sediment, accumulating an average of 1.35±0.7 Mt annually. Net storage of sediment in Tonle Sap Lake reduces the annual sediment transport to the Mekong delta, further compounding the effects of sediment delivery to the Delta resulting from upstream dam construction and instream sand mining.