Memoire Online - Hydrological modeling of the Congo River basin: Asoil-water balance approach

In order to understand water balances, complex hydrologic systems have always been simplified. Thornthwaite and Mather (1957) conceptualized a Catchment water balance model for long term monthly climatic conditions; which then led to many researches on water balance models for similar conditions on a catchment, region or a continent. Typical examples for these models are the GIS-based water balance model for the Southern Africa of Alemaw and Chaoka (2003), the Grid-based model for Latin America of Vorosmarty et al. (1989), Reed et al. (1998), the Grid -based model for Amazone

basin of Marengo (2005), the Rhine flow model of Van Deursch and Kwadijk (1993); and the Large-scale water balance model for the upper Blue Nile in Ethiopia of Conway (1997), and the spatial water balance of Texas (Reed et al., 1998).

Hydrological water balance models can be based on the interactions between the water, atmosphere, and land surface; which is a combination of the atmospheric water balance, Surface Water Balance and the soil Water Balance Studies.

The estimation of hydrologic flux using Atmospheric water balance has been studied by a number of researchers (Reed et al., 1998; Rasmusson, 1967; Brubaker et al., 1994; and Oki et al., 1995; Marengo, 2005), among them. Rasmusson (1967), Brubaker et al. (1994) and Oki et al. (1995) describe atmospheric water balance at river basin, continental, and global scales. Rasmusson (1967) analyzes the characteristics of total water vapor flux fields over North America and the Central American Sea. Reed et al (1998) presents the atmospheric water balance of Texas and Marengo (2005), that of the Amazone basin.

The atmospheric branch of the water balance has always been expressed in the form of a


where -?×Q=C	expresses the vertically integrated moisture convergence, Q is the

water-vapor fluxe, P is the precipitation, E is the Evapotranspiration, and dW/dt represents the atmospheric storage (water vapour) change term which is generally negligible for averages over a month or more (Roads et al., 1994; Eltahir and Bras, 1994; Reed et al., 1998; Curtis and Hastenrath, 1999; Costa and Foley, 1999; Zeng, 1999, Marengo, 2005). Given adequate data, the atmospheric water balance is a promising method for estimating regional evaporation, runoff, and changes in basin storage (Reed et al, 1998).

Although the atmospheric water balance model is efficient to estimate hydrologic flux within an area, significant uncertainties in runoff estimation using atmospheric data exist even at the continental scale (Reed et al., 1998). Comparing their estimated continental runoff with those given by Baumgartner and Reichel (1975) in North America river runoff, Brubaker et al. (1994) and Oki et al. (1995) note the fact that poorly defined continental or basin boundaries may contribute to inaccuracies in runoff estimation. To obtain an accurate runoff form vertically integrated vapour flux convergence, the annual change in atmospheric water storage and surface water storage should be negligible (Reed et al, 1998).

The simple bucket models have been developed by hydrologist to simulate near-surface
hydrological model in the conditions where detailed data about soil layers, depth to

groundwater, and vegetation are not available (Reed et al, 1998). Despite numerous uncertainties associated with the simple soil-water budget model many researchers have applied this type of model to problems ranging from catchment scale studies to the global water balance and climate change scenarios (Thornthwaite, 1948; Thornwaite and Mather, 1957; Manabe, 1969; Mather, 1978; Dunne and Leopold, 1978, Shiklomanov, 1983; Alley, 1984; Willmott et al., 1985; Mintz and Serafini, 1992; Mintz and Walker, 1993, Alemaw and Chaoka, 2003, Sen and Ambro Gieske, 2005, Marengo, 2005).

The "bucket" model approach is attractive because of its simplicity and requires minimal input data: precipitation, potential evapotranspiration, and soil-water holding capacity (Reed et al, 1998). The studies by Willmott et al. (1985), Mintz and Walker (1993), and Mintz and Serafini (1992), are climatology studies that present the global distributions of precipitation, evapotranspiration, and soil moisture. Mintz and Serafini (1992) compare their evapotranspiration estimates for sixteen major river basins throughout the world with those derived from river runoff analysis made by Baumgartner and Reichel (1975) and the values show reasonable agreement (Reed et al, 1998).

At a smaller scale, Mather (1978), quoted by Reed et al (1998), describes the application of a soil-water budget model to several watersheds in the coastal plains of Delaware, Maryland, and Virginia. Comparisons between measured and computed runoff values are rather poor for monthly data, but better for annual data.

In its simplest form, the soil-water budget model does not account for situations where the precipitation rate is greater than the infiltration capacity of the soil. Mather (1978) describes one approach to remedy this problem, that is, to first use the SCS method to estimate direct overland runoff and substract this amount from the precipitation before it is allowed to enter the soil "bucket." This approach appears to yield better results. A similar approach of taking an initial rainfall abstraction before allowing precipitation to enter the soil column for climatological budgeting was used in a study of the Niger Basin (Maidment et al., 1996), in southern Africa region (Alemaw and Chaoka, 2003) and in the Limpopo basin (Alemaw, 2006).

The commonly used method in hydrologic studies, namely the surface water balance, relies on the fact that with the exception of coastal areas, the landscape can often be divided into watershed units from which there is only one surface water outflow point (Reed et al, 1998). If assumed that change in storage is negligible and that there are no significant inter-watershed transfers via groundwater or man-made conveyance structures, providing that the average watershed precipitation and runoff can be measured with reasonable accuracy, the annual evaporative losses from a watershed can be estimated by Empirical relationships which are often used to estimate mean annual or mean monthly flows in ungaged areas; this approach is used in this study (Reed et al, 1998).

Arnell (1995), Lullwitz and Helbig (1995), Reed et al (1998), Alemaw and Chaoka (2003) and Alemaw (2006) describe studies of runoff mapping using a geographic information system (GIS) to manage spatial data at a regional or continental scale.

Arnell (1995) presents five approaches for deriving gridded runoff maps at a 0.5 degree grid resolution; they include: (1) simply averaging the runoff from all stations within each grid cell, (2) statistically interpolating runoff between gages, (3) using an empirical relationship that relates runoff to precipitation, potential evaporation, and temperature, (4) using a soil-water balance type model, and (5) overlaying grid cells onto catchment runoff maps to derive area-weighted runoff estimates. In a study using the 5 approaches to map runoff over a large portion of Western Europe, the results show that method (5) produces the most reasonable estimates. In a study similar to that of Arnell (1995), Lullwitz and Helbig (1995) created 0.5⁰ grid runoff maps for the Weser River in Germany. In both this papers, authors noted that 0.5 degree runoff maps can be useful for validating general circulation models (GCM's). Church et al. (1995), using an interpolation method to create runoff maps, present maps of evapotranspiration (ET) and runoff/precipitation (R/P) ratios for the northeastern United States. A different approach of mapping surface runoff, similar to Arnell's method (Arnell, 1995), combining an empirical rainfall-runoff relationship and watershed runoff balancing was used by Reed et al (1998).

Alemaw and Chaoka (2003) developed a GIS based hydrological model simulating the spatial and temporal distribution of water budget parameters for the Southern Africa region. This model was slightly modified and used for the Limpopo basin (Alemaw, 2006), where monthly runoff is generated from matrix of specific geo-referenced grids.

Hydrological modeling of the Congo River basin: Asoil-water balance approach

3.2 Water Balance Model approaches