Temporal Variability of Water Discharge and Sediment Load in the Tonle Sap River at Prek Kdam
Cumulative seasonal flow volumes are presented in Figure 8,with negative values indicating reverse flow from the Mekong to Tonle Sap Lake, and positive values for outflow from the lake to the Mekong River. The seasonal outflow from the lake averaged 68,000 Mm3, equivalent to 18% of the annual discharge of Mekong River at Chroy Changvar. The seasonal reverse flow into the lake from the Mekong River averaged 36,000 Mm3, about 10% of the annual discharge of the Mekong River at Chroy Changvar. Thus, the outflow from the lake was almost twice the reverse flow from the Mekong River. Some of this difference can be attributed to runoff from the 760,000 km2 drainage basin of the Tonle Sap Lake, some to overbank flow from the mainstem Mekong across floodplains into the Lake. The annual average overland flow into the lake was estimated to be 2,600 Mm3 while the average overland flow from the lake was approximately similar magnitude, being 2,700 Mm3 (Kummu et al., 2014). Looking at temporal trends, reverse flows from the Mekong to the Lake showed decreases over the 24-year period, but these were not statistically significant (p , 0.05); outflows also showed decreases, which were significant (p <0.05) (Table 7) .
Net annual sediment loads averaged 3.7 Mt in reverse flows into Tonle Sap Lake, equivalent to 4.8% of annual average sediment load of the mainstem Mekong at Chroy Changvar. (Figure 9). The sediment load from the lake to Mekong River was 2.9 Mt/yr, which is equal to 3.7% of average annual sediment load in Chroy Changvar. Thus, we can estimate that Tonle Sap Lake gained an average of 0.8 Mt of sediment annually from the Mekong river, but as this term is calculated as a residual, it must be treated with curation (as such residual terms incorporate and hide errors in other terms) (G. M. Kondolf & Matthews, 1991). Sediment loads in reverse flow increased over the observed period reverse flow being statistically significant (p<0.05 ), while the lake outflow sediment load was found no trend.