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).