Abstract. The Mekong River Basin is a key regional resource in Southeast Asia for sectors that include agriculture, fisheries and electricity production. Here we explore the potential impacts of climate change on freshwater resources within the river basin. We quantify uncertainty in these projections associated with GCM structure and climate sensitivity, as well as from hydrological model parameter specification. This is achieved by running patternscaled GCM scenarios through a semi-distributed hydrological model (SLURP) of the basin. Pattern-scaling allows investigation of specific thresholds of global climate change including the postulated 2 • C threshold of "dangerous" climate change. Impacts of a 2 • C rise in global mean temperature are investigated using seven different GCMs, providing an implicit analysis of uncertainty associated with GCM structure. Analysis of progressive changes in global mean temperature from 0.5 to 6 • C above the 1961-1990 baseline (using the HadCM3 GCM) reveals a relatively small but non-linear response of annual river discharge to increasing global mean temperature, ranging from a 5.4 % decrease to 4.5 % increase. Changes in mean monthly river discharge are greater (from −16 % to +55 %, with greatest decreases in July and August, greatest increases in May and June) and result from complex and contrasting intra-basin changes in precipitation, evaporation and snow storage/melt. Whilst overall results are highly GCM dependent (in both direction and magnitude), this uncertainty is primarily driven by differences in GCM projections of future precipitation. In contrast, there is strong consistency between GCMs in terms of both increased potential evapotranspiration and a shift to an earlier
General circulation models (GCMs) currently perform vertical water and energy balances at 20-or 30-min time intervals for grid points 2o-4 ø apart but generally contain no information on the land-phase transfer of water between grid points or within watersheds. As a result, they operate on an incomplete hydrological cycle. This study combines a hydrological model with a GCM for a macroscale watershed. A water balance was carried out at 12-hour time intervals for a 10-year period using the Canadian Climate Centre GCM II data set for grid points within and surrounding the 1.6 x 10 6 km 2 Mackenzie River Basin in northwestern Canada. The water surpluses from each relevant grid point were accumulated to provide a simulated hydrograph at the outlet of the Mackenzie River. A hydrological model was calibrated and verified using 5 years of recorded climatological and hydrometric data as well as land cover data from classified National Oceanic and Atmospheric Administration advanced very high resolution radiometer images. The climatological outputs from the GCM (precipitation, temperature, and evaporation) were then used as inputs to the hydrological model, generating a second hydrograph for the Mackenzie River. The results show that using the hydrological model with the GCM data produces a better representation of the recorded flow regime. The study provides a means of verifying the performance of the GCM and is a first step in developing a continental-scale hydrological model which will, ultimately, form a part of a full model of the global hydrological cycle. 1. Introduction Atmospheric general circulation models (GCMs) are tools derived from numerical weather prediction models. They are based on the physical laws of conservation of energy and mass for a rotating sphere subject to an external heat source and are used to simulate the behavior of the atmosphere over long periods of time. These models include descriptions of both large-scale atmospheric processes and smaller-scale processes such as cloud distribution; precipitation generation; surface transfers of energy, moisture, and momentum; and land surface hydrology [Boer eta!., 1984]. The models operate at time intervals of about 20 min and at resolutions of the order of 20-4 ø latitude and longitude.
While the atmospheric components of the GCMs are often very sophisticated (dividing the atmosphere into many layers) and the land-phase components may be very detailed (including multiple soil layers, frozen water, vegetation types, varying albedo, and so on), Henderson-Sellers and Pitman [ 1993] have shown that the land-phase parameterizations in current GCMs do not agree on predictions of energy fluxes, snow depth, or water excesses, even when all atmospheric forcings are identical.There is an even more important problem with many GCMs from a hydrological point of view. The problem is that most GCMs contain no lateral transfer of water within the land phase. Such models carry out a vertical water distribution at each grid point at each time interval using precipita-...
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