Distributed discharge and sediment concentration predictions in the sub‐humid Ethiopian highlands: the Debre Mawi watershed

Abstract

Experimental research in the Ethiopian highlands found that saturation excess induced runoff and erosion are common in the sub‐humid conditions. Because most erosion simulation models applied in the highlands are based on infiltration excess, we, as an alternative, developed the Parameter Efficient Distributed (PED) model, which can simulate water and sediment fluxes in landscapes with saturation excess runoff. The PED model has previously only been tested at the outlet of a watershed and not for distributed runoff and sediment concentration within the watershed. In this study, we compare the distributed storm runoff and sediment concentration of the PED model against collected data in the 95‐ha Debre Mawi watershed and three of its nested sub‐watersheds for the 2010 and 2011 rainy seasons. In the PED model framework, the hydrology of the watershed is divided between infiltrating and runoff zones, with erosion only taking place from two surface runoff zones. Daily storm runoff and sediment concentration values, ranging from 0.5 to over 30 mm and from 0.1 to 35 g l−1, respectively, were well simulated. The Nash Sutcliffe efficiency values for the daily storm runoff for outlet and sub‐watersheds ranged from 0.66 to 0.82, and the Nash–Sutcliffe efficiency for daily sediment concentrations were greater than 0.78. Furthermore, the model uses realistic fractional areas for surface and subsurface flow contributions, for example between saturated areas (15%), degraded areas (30%) and permeable areas (55%) at the main outlet, while close similarity was found for the remaining hydrology and erosion parameter values. One exception occurred for the distinctly greater transport limited parameter at the actively gullying lower part of the watershed. The results suggest that the model based on saturation excess provides a good representation of the observed spatially distributed runoff and sediment concentrations within a watershed by modelling the bottom lands (as opposed to the uplands) as the dominant contributor of the runoff and sediment load. Copyright © 2014 John Wiley & Sons, Ltd.

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

“…Specifically, the most erosive flow events may result from the cumulative effect of long duration rainfall events rather than soil infiltration capacities (Bayabil et al, 2010;Tilahun et al, 2015Tilahun et al, , 2016. Additionally, relying on one runoff generation mechanism may complicate incorporation of contributions such as drainage of roads.…”

“…Shanko Bahir was monitored during the three rain seasons (wet period) in each of the respective years 2010, 2011, and 2012 since runoff is only generated during these times in its ephemeral streams. Tilahun et al (2015) recently modeled the hydrological and erosion patterns for the total surrounding watershed area of Debre Mawi and other nearby sub-watersheds. The Shanko Bahir sub-watershed, however, had to be modeled separately as this investigation demonstrates due to flow contributions from a road that borders and intersects the Debre Mawi watershed.…”

“…The sub-humid climate patterns, clay soils, and agricultural cultivation patterns, described below, are similar to the nearby watersheds surrounding the Adet station of the Amhara Regional Agricultural Research Institute (ARARI) which has a long-standing presence researching hydrology and erosion in this strategic teff (Eragrostis tef) growing area of the Amhara region. The catchments experience a warm, sub-humid, semimonsoonal climate with a unimodal rainfall pattern and an average of 1,100 mm of rainfall, 80% of which falls in this location from the months of June to September (Mekonnen and Melesse, 2011;Teshome et al, 2013;Tilahun et al, 2015). Due to its elevation of between 2,200 to 2,300 m a.s.l.…”

“…it is classified in the Weyna Dega agro-ecological belt (Hurni, 1998). The main outlet for the 95 ha portion of the Debre Mawi watershed, studied by Tilahun et al (2015Tilahun et al ( , 2016, was denoted as "Weir 5" and is jointly monitored by Bahir Dar University and ARARI. The 14 ha Shanko Bahir sub-watershed is named after the village within it and corresponds to one of the four gauged subwatersheds of the 95 ha northern portion, referred to as "the sub-watershed at Weir 2" in the previous investigation .…”

“…The nearest sub-watershed to Shanko Bahir in that study was adjacently located just to the west, represented by "Weir 1" (Figure 1) (Tilahun et al, , 2016, and was used for model parameter fitting and comparison. For more information on the gauging of the larger encompassing watershed and surrounding sub-watersheds refer to Tilahun et al (2015Tilahun et al ( , 2016.…”