Temporal Variability of Water Discharge and Sediment Load in the Mekong River at Kratie and Chroy Changvar
Annual water discharge and computed sediment loads on the mainstem show higher flows and sediment loads at Kratie than at Chroy Changvar (Figure 6). On an annual basis, 80% of the flow occurred during the rainy season (May to October) and 20% during the dry season (September to April). On average, the annual water discharge in Kratie was 404,000 Million cubic meters (Mm3/yr), 36,000 Mm3/yr higher than the annual water discharge at Chroy Changvar (368,000 Mm3/yr). This pattern is similar to the pattern of the flow in Kratie and Stung Treng (Cambodia-Lao border, about 150 km upstream of Kratie). MRC (2019) reported in their observed flows for the Mekong mainstream stations over the period 2000-2017, Stung Treng discharge is found higher than the downstream at Kratie. After Kratie the Mekong enters into its delta; the water flow in this reach is very complex (due to downstream backwater effects, the overbank flows, the temporary water storage function of the floodplain) especially during the flood season when hydraulic conditions define the flow distribution between different river branches. The downstream reduction in gaged flow at Kratie and Chroy Changvar occurred mainly at higher flows and can be attributed to overland flow from the Mekong River traversing the floodplain to the Tonle Sap Lake and by flow into major distributaries between Kratie and Chroy Changvar. During the flood season when the flow of the Mekong River, water starts to spillover both banks of the Mekong River between upstream of Kompong Cham (150 km upstream of Chroy Changvar) and Chroy Changvar station. Part of the spillover the right bank reaches the Tonle Sap Lake as overland flow. This overland flow was reported on average 2,500 Mm3/yr by Kummu et al. (2014). On the left bank of the main river at Kampong Cham, part of the Mekong flow is partly diverted into the Tonle Toch River, which then discharges back into the Mekong further downstream of Chroy Changvar in the Mekong Delta. The bypath discharge by this river have no previously reported yet. The trend analysis confirmed a decreasing trend in annual water discharge at Chroy Changvar (statistically significant (p<0.05) ), but we found there was no significant trend at upstream Kratie (Table 6).
The average annual sediment load in Kratie was 72±26 Mt/yr and 78±22 Mt/yr at Chroy Changar (Figure 6). The annual sediment kept decrease from 123 Mt/yr in 1995 and reached low (40 Mt/yr) in recent years at Kratie. The sediment load at Chroy Changar was strong fluctuated over the study period. The maximum was found in 128 Mt in 2011, and the lowest sediment was 32 Mt in dry year 1998. The MK test found clearly decreasing trends in sediment load at Kratie in Mekong River. Sediment load at Kratie experienced a decrease trend with significance (statistically significant (p <0.05)), while there was no trend in sediment load at Chroy Changvar