A simple semi‐distributed water balance model for the Ethiopian highlands

Abstract

The discharge of the Nile River is highly dependent on the flow generated in the highlands of Ethiopia. However, little is known about the local (i.e. small scale) watershed hydrological response, due in part to a lack of long duration, continuous hydrological data. The goal of this paper was to develop a realistic, simple model that is useful as a tool for planning watershed management and conservation activities so that the effects of local interventions on stream flow can be predicted at a larger scale. The developed model is semi-distributed in that it divides the watershed into different regions that become hydrologically active given different amounts of effective cumulative rainfall after the start of the rainy season. A separate water balance is run for each of the hydrologic regions using rainfall and potential evaporation as the major inputs. Watershed parameters that were calibrated included the amount of water required before each region becomes hydrologically active, the fraction of soil water that becomes runoff and subsurface flow, and aquifer characteristics, Model validation indicated that daily discharge values were predicted reasonably well with Nash Sutcliffe values ranging from 0?56 to 0?78. Despite the large distance between the test watersheds, the input parameter values for the watershed characteristic were remarkably similar for the humid highlands, indicating that the model could be used to predict discharge in un-gauged basins in the region. As expected, the watershed in the semi-arid region behaved somewhat differently than the other three watersheds. Good quality precipitation data, even for short durations, were key to the effective modelling of runoff in the highland watersheds

Analyzing runoff processes through conceptual hydrological modeling in the Upper Blue Nile Basin, Ethiopia

“…The hillslopes (medium and steep slope source areas in this paper) generated almost no direct runoff as saturated excess flow. Similar results were obtained by different researchers in the Blue Nile Basin, who identified hillslopes as main recharge areas Collick et al, 2009;Tilahun et al, 2013). Our results contribute to the debate on the relative importance of saturated excess runoff versus infiltration excess runoff (Hortonian overland flow) mechanisms in the Upper Blue Nile Basin, showing that the rainfall-runoff processes are better represented by the soil reservoir methodology.…”

“…Hydrological models that allow for a description of the hydrology of the region play an important role in predicting river discharges from ungauged catchments and understanding the rainfall-runoff processes in the catchments in order to enhance hydrological and water resources analysis. As such, a number of models have been developed and applied to study the water balance, soil erosion, climate and environmental changes in the Blue Nile Basin (e.g., Johnson and Curtis, 1994;Conway, 1997;Mishra and Hata, 2006;Kebede et al, 2006;Kim and Kaluarachchi, 2008;Collick et al, 2009;Steenhuis et al, 2009;Tekleab et al, 2011;Tilahun et al, 2013).…”

A multi basin SWAT model analysis of runoff and sedimentation in the Blue Nile, Ethiopia

“…While these assumptions simplify the processes that govern water movement through porous media (in particular, partly-saturated regions), for a daily model, water balance models have been shown to better capture the observed responses in numerous African watersheds (Guswa et al, 2002). For Ethiopia, water balance models outperform models that are developed in temperate regions (Liu et al, 2008;Collick et al, 2009;Steenhuis et al, 2009;White et al, 2010). For the complete model description see .…”

Modeling sediment concentration and discharge variations in a small Ethiopian watershed with contributions from an unpaved road

“…Additionally, relying on one runoff generation mechanism may complicate incorporation of contributions such as drainage of roads. While some modelers have relied on infiltration-excess, employing an SCS Curve Number approach (Arnold et al, 1998;Haith and Shoemaker, 1987;Krysanova et al, 1998;SCS, 1956;Williams et al, 1984), others use saturation-excess as the principal runoff generating mechanism in catchments (Beven and Kirkby, 1979;Bingner and Theurer, 2007;Buytaert et., 2004;Collick et al, 2009;Dunne and Black, 1970;Liu et al, 2008;Steenhuis et al, 2009). In Ethiopian mountainous basins, both mechanisms likely occur in different watersheds, at different extents, and at different times during a rainy season (Betrie et al, 2011;van Griensven et al, 2012;Tilahun et al, 2016).…”

“…The hydrology sub-model of the PED model ( Figure 4) was developed for the Ethiopian highlands Collick et al, 2009) and employs a Thornthwaite-Mather (TM) procedure to predict recharge and runoff through three distinct portion of the watershed that either infiltrate and contribute to baseflow or produce runoff directly (Steenhuis and Van der Molen, 1986).…”