An efficient semi-distributed hillslope erosion model for the subhumid Ethiopian Highlands

Abstract

Erosion modeling has been generally scaling up from plot scale but not based on landscape topographic position, which is a main variable in saturation excess runoff. In addition, predicting sediment loss in Africa has been hampered by using models developed in western countries and do not perform as well in the monsoon climate prevailing in most of the continent. The objective of this paper is to develop a simple erosion model that can be used in the Ethiopian Highlands in Africa. We base our sediment prediction on a simple distributed saturated excess hydrology model that predicts surface runoff from severely degraded lands and from bottom lands that become saturated during the rainy season and estimates interflow and baseflow from the remaining portions of the landscape. By developing an equation that relates surface runoff to sediment concentration generated from runoff source areas, assuming that baseflow and interflow are sediment-free, we were able to predict daily sediment concentrations from the Anjeni watershed with a Nash–Sutcliffe efficiency ranging from 0.64 to 0.78 using only two calibrated sediment parameters. Anjeni is a 113 ha watershed in the 17.4 million ha Blue Nile Basin in the Ethiopian Highlands. The discharge of the two watersheds was predicted with Nash–Sutcliffe efficiency values ranging from 0.80 to 0.93. The calibrated values in Anjeni for degraded (14%) and saturated (2%) runoff source area were in agreement with field evidence. The analysis suggests that identifying the runoff source areas and predicting the surface runoff correctly is an important step in predicting the sediment concentration

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.…”

“…Quantitatively, the depth of water stored in the soil, S(t), evolves over time using the water balance Different studies show that part of the interflow water from the steep hills appears at the hill bottoms during wet periods in the form of increased moisture content or overland flow (Frankenberger et al, 1999;Bayabil et al, 2010;Mehta et al, 2004;Tilahun et al, 2013). These findings reveal that the hill bottoms receive additional inputs to the soil reservoir from the steep upper parts of the hills besides the rainfall.…”

“…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).…”

Improving efficacy of landscape interventions in the (sub) humid Ethiopian highlands by improved understanding of runoff processes

“…As shown above and confirmed by modeling studies with a water balance (Engda et al, 2011;Tilahun et al, 2013a), the total annual outflow measured at the watershed outlet is less than total inflow (i.e., precipitation minus evaporation) into the watershed. Because these watersheds are located in the headwaters, some of the rainfall that infiltrates in the headwaters, either topographically drains toward the outlet becoming deep percolation (i.e., bed rock storage, Chandler, 2006) or appears as springs beyond the gage bypassing measurement and as base flow at the gage after the rainy monsoon phase ends.…”

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

“…Legates et al (2011) argued that such frameworks are essential for understanding hydrology, sediment transport, and nutrient loss. Such approach used by Steenhuis et al (2013), employed for this study, has been useful for previous researchers in these highlands (Tilahun et al, 2013a(Tilahun et al, , b, 2015.…”