BNPP/ASB Functional Value of Biodiversity Project – Phase II
· total yield by time at locations upstream from major urban centers and at the coastal zone,
· seasonal variability of total flow related to seasonality of the simulated rainfall data,
· duration of storm events effects on stage height at location upstream from major urban centers.
Mekong. For the Mekong basin, we are utilizing a model specifically designed for analysis of water (and energy) in regional-scale, large river, the Variable Infiltration Capacity (VIC) model. VIC (Liang et al, 1994) is a physically based model, which nonetheless parameterizes small scale processes to allow application to large river basins, which typically are resolved at spatial resolutions from 1/8 degree latitude by longitude (e.g., where the resolution of the precipitation, temperature, radiative, and other surface forcings are available or can be derived) to coarser resolutions such as the 2 degree global application described by Nijssen et al (2002). Previous applications of VIC include such large continental river basins as the Columbia (Nijssen et al., 1997), the Arkansas-Red (Abdulla et al., 1996), and the Upper Mississippi (Cherkauer and Lettenmaier, 1999), among other rivers. VIC has also been applied to the entire area of China (Su and Xie, 2003).
A detailed description of the VIC model can be found in Liang et al. (1994, 1996 and 1998). Briefly, the model has parameterizations to represent the vertical exchange of moisture and energy between the vegetation canopy and the atmosphere, similar in many respects to other Soil-Vegetation-Atmosphere Transfer Schemes (SVATS). Its main distinction from other SVATS is its representation of the effects of spatial variability in soil, topography, and vegetation, and their effects on runoff generation, which is assumed to occur dominantly via the saturation excess mechanism (which is usually a defensible assumption in humid environments). The model also represents a “slow”, or baseflow, runoff response via a nonlinear deep soil drainage parameterization. The VIC model is coupled to a streamflow routing scheme that transports the runoff generated within each grid cell through a specified channel network. The routing model does not account for channel losses, extractions, diversions and reservoir operations (the latter are represented in the water management model). The routing model is described in detail in Lohmann et al. (1996; 1998).
Mae Chaem. Our original commitment was to also use VIC for the Mae Chem work. But we were concerned that the physics as so represented in VIC would not accurately represent the steep topography and finer-scale issues of the Mae Chaem basin. As noted, VIC is conceptualized as a larger, regional scale model. As such, its application to smaller-scale basins and sub-basins, such as the Mae Chaem, is questionable. So to examine problems at this scale, we decided to make the commitment to utilizing our high-resolution hydrologic model, the Distributed Hydrology Soil Vegetation Model (DHSVM, Wigmosta et al, 1994). Unlike VIC, DHSVM is intended for application to small to moderate (typically less than about 1000 km2) drainage areas, over which digital topographic data allows explicit representation of the mechanisms by which water travels over the surface and through the subsurface. Like VIC, it represents runoff generation via the saturation excess mechanism. Unlike VIC, it explicitly represents topographic effects, including the formation of perched water tables, on runoff generation, incident solar radiation (hence net radiation), and explicitly represents the vegetation and its properties (like root depth), as well as soil properties, on a pixel-by-pixel basis. The model grid resolution typically is 30-150 m, several orders of magnitude higher than VIC. However, because of the large computational burden (and data limitations), DHSVM is restricted to relatively small catchments. We have conducted some limited experiments comparing DHSVM sensitivity, for instance, to vegetation and vegetation change (Van Shaar et al, 2002). Although the macroscale performance of the two models is similar in gross features (e.g., ability to reproduce seasonal fluctuations in runoff), there are important differences in predicted runoff and other surface fluxes, especially at shorter time scales.
Our
charge is to produce model simulations using a “standard climatology and
either a ‘standard’ rainfall, or a ‘typical actual rainfall
record’ (ie. last 10 years).”
Mekong.
The standard climatology for the VIC is our 20-year climate record, with
accompanying discharge. To examine the overall model and data trade-offs,
and to establish the most robust (and hence trustworthy) product, we will
conduct a set of model runs which will move progressively though the
assumptions inherent in model scaling and data sets. From these results,
we will determine the “best” combination of parameters to do the
scenarios (Activity 2.4) with.
Mae Chem. The available data (including elevation effects) for the climatology of the Mae Chaem are much more restricted than for the Mekong. Hence we will (be forced to) use the 2-year dataset developed under Activity 2.2.
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