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