CHAPTER FIVE
5.0 MODEL APPLICATION, DATA PRESENATTION AND INTERPRETATION
RESULTS
5.1 Generalities on the Model application
The HATWAB model runs and simulates the water balance for each
of the 62500 grid cells of the study area using inputs datasets previously
developed and presented in GIS dataset formats. The GIS-database includes the
hydro-climatic data sets which are potential precipitation (PPT) and potential
Evapotranspiration (PET). It includes also the soil texture, the Vegetations,
and rooting depth, which are used to derive the Soil moisture capacities per
unit volume: Field capacities (FC), Wilting point (WP), Available water content
(AWC). All the data sets are incorporated in the model at a 6X6 minutes (12X12
km) grid cell spatial resolution.
A raster file having an integer value of 1 inside the basin
area and 0 outside the basin is assigned to the model as masking file
delineating and forcing the basin boundary to the entire output of the model
simulations.
Other important characteristic of the model is the facility to
simulate the water balance for a specified area: a single grid cell, a
sub-watershed (example: Kasai, Ubangi, Bangui, Ouesso, Lualaba sub Congo) or
the entire Congo basin.
For each grid cell, the model run for a time loop of 12
months, using PPT and PET data sets available for a period of the 30 years
(1961-1990) until a dynamic steady state is achieved in SM, AET and TRO with
tolerance error of 0.01% in all these variables. If the steady state is not
achieved the model proceed on a second, third, and so on, until the steady is
achieved.
Finally, monthly and annual averages for Soil moisture (SM),
Actual Evapotranspiration (AET), Runoff (TRO), Vertical Integrated moisture
coverage (C), and the Imbalance (I) are outputted for the entire basin.
A spreadsheet was developed to accumulate the total Runoff for
each sub-watershed, and to average the annual Soil moisture and
Evapotranspiration. These results will be presented in the following
sections.
5.2 Initial soil moisture
In order to solve the water balance differential equation
governing the soil moisture Equation (31), the initial soil moisture is
determined in accordance with Alemaw (2003). In most hydrological water
balance, it is assumed that at the end of a wet season the soil moisture
remains at the state of its field capacity attained during the wet season.
Therefore, the simulation starts at the end of the wet season when it is
assumes that the soil is at field capacity. Soil water stocks are then depleted
during the dry season in accordance with the moisture retention function.
For a large watershed, the ratio PPT/PET can be used as a good
indicator of seasonal
distribution of the wetness of the dryness at a
specific geographic location in a region: a
wet season is characterised by a
ratio PPT/PET greater than the unity, while a value less
than the unity corresponds to the dry season. The month where
PPT/PET just starts to fall under the unity is selected by the model as the
starting month of the simulation.
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