International Conference on Forest Environment in continental river basins; with a focus on the
Mekong River. December 5-7, 2005, Phnom Penh, Cambodia
______________________________________________________________________________
Variations of the Mekong discharge over the half past century
Katia Laval, T. Ngo-duc, J. Polcher
Laboratoire de Météorologie Dynamique,
University of Pierre Marie Curie, Paris, France
Coresponding author: Katia Laval (laval@lmd.jussieu.fr)
Keywords: Land surface scheme, soil water storage, river discharge, flood plains.
1. Introduction
Detection of changes in long time series of hydrological data is important for planning of
future water resource management. Up to present, these studies are principally based on the
historical observed river discharge data, but often experience difficulty because of missing
values and gaps in the observed data. Land Surface Models (LSMs), which simulate river
discharges through a routing scheme, appear thus to be a good tool for completing such
studies.
2. Land surface Model and forcing data
The Land Surface Model, ORCHIDEE (Verant et al., 2004; Krinner et al., 2005) developed
at the Institute Pierre Simon Laplace, France, can be integrated to compute the hydrological
balance of a specific river basin if we can provide atmospheric data to force the model.
ORCHIDEE represents the water and energy exchanges between land, vegetation and
atmosphere. Water, which leaves the soil reservoir either through runoff or drainage, is
stored in the three reservoirs of routine scheme and cascades progressively towards ocean
(Figure 1).
Figure 1. Principle of the river routing scheme
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International Conference on Forest Environment in continental river basins; with a focus on the
Mekong River. December 5-7, 2005, Phnom Penh, Cambodia
______________________________________________________________________________
In order to study the inter annual variability of surface conditions over a larger number of
years, we have built a forcing data set, NCC, based on reanalysis data, NCEP, corrected by
CRU precipitation. We were able to define 53 years of data as forcing to the ORCHIDEE
Model; and compute the evolution of land water storage over the last 53 years (Ngo-duc et
al., 2005a).
3. Results and Discussion
The results of the hydrological budget were validated by observed river discharges of the
world’s 10 largest rivers. The seasonal variations of simulated Mekong discharge are
compared with the observed ones (Figure 2). We take advantage also of the recent GRACE
Mission to evaluate seasonal water storage over the catchment basin (Figure 3). The
evolution of continental water budget is used to analyze trends of this storage (Ngo-duc et
al., 2005 b). According to the simulations, the Mekong discharge variability seems to have
been changed these last 20 years.
The influence of flood plains and irrigation on the water balance over the basin is
investigated; we show that they have a substantial effect on water storage.
Figure 2. Seasonal variations of river discharge at the Pakse station (simulated and
observed)
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International Conference on Forest Environment in continental river basins; with a focus on the
Mekong River. December 5-7, 2005, Phnom Penh, Cambodia
______________________________________________________________________________
Figure 3. Water storage variations simulated by the two version of ORCHIDEE and
evaluated by the GRACE mission (o)
References:
Verant S., K. Laval, J. Polcher and M. de Castro: (2004): Sensitivity of the continental
hydrological cycle to the spatial resolution over the Iberian Peninsula, J. of
Hydrometeorology., 5( 4), 265-283.
Krinner G., Viovy N., de Noblet-Ducoudré N., Ogée J., Polcher J., Friedlingstein P., Ciais
P., Sitch S., Prentice I. C. (2005): A dynamic global vegetation model for studies of the
coupled atmosphere-biosphere system. Global Biogeochem. Cycles, 19(1), GB1015.
Ngo-duc, T, J. Polcher, and K. Laval, 2005a: A 53 year forcing data set for Land Surface
Models, J. Geophys Res., 110, D06116.
Ngo-duc, T, K. Laval, J. Polcher, A. Lombard and A. Cazenave, 2005b: Effect of land
water storage on global mean sea level over the past 50 years, Geophys Res. Letters, 32,
L09704.
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