Distributed discharge and sediment concentration predictions in the sub-humid Ethiopian highlands: the Debre Mawi watershed Seifu A. Tilahun, 1 * Christian D. Guzman, 2 Assefa D. Zegeye, 2,4 Dessalegn C. Dagnew, 1 Amy S. Collick, 3 Birru Yitaferu 5 and Tammo S. Steenhuis 1,2 1 School of Civil and Water Resources Engineering, Bahir Dar University, P.O.Box 26, Bahir Dar, Ethiopia 2 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, USA 3 US Department of Agriculture - Agricultural Research Service, University Park, Pennsylvania, USA 4 Adet Research Center, ARARI, P.O. Box 08, Bahir Dar, Ethiopia 5 Amhara Region Agricultural Research Institute (ARARI), P.O. Box 527, Bahir Dar, Ethiopia 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. KEY WORDS erosion; saturation excess runoff; sediment transport; gully erosion; simulation models; Blue Nile; Ethiopia Received 16 November 2013; Accepted 14 July 2014 INTRODUCTION Increasingly, the study of overland flow and erosion mechanisms in the Ethiopian highlands is becoming an active research area (e.g. Balthazar et al ., 2013; Gebremicael et al., 2013). Although Hortonian and saturated overland flow processes both occur with various intensity in watersheds around the country, studies have only recently begun to note the importance of saturation excess overland flow in light of the high infiltration capacity of some Ethiopian soils. Zeleke (2001) suggested that runoff in the northwestern highlands could be controlled more by matrix potential and saturation than by hydraulic conductivity. This trend was later echoed by other studies observing that soil moisture storage had to be filled before surface runoff was generated in Ethiopian highland watersheds (Bewket and Sterk, 2003). In semi- arid areas such as Tigray, although infiltration excess by heavy storms is still important during the first phase of rainy period, saturation excess runoff dominates the second part of the rainy season (Descheemaeker et al., 2009; Walraevens et al., 2009; Zenebe et al., 2013). Further studies have shown that soil saturation in (semi) humid experimental watersheds is the dominant mecha- nism for overland flow (Liu et al., 2008; Collick et al., 2009; Steenhuis et al., 2009; Bayabil et al., 2010; Tesemma et al., 2010; Engda et al., 2011) and for erosion (Tilahun et al., 2013a,b). Spatially, soil saturation (and *Correspondence to: S. A. Tilahun, School of Civil & Water Resources Engineering, Bahir Dar University, Bahir Dar, Ethiopia. E-mail: satadm86@gmail.com HYDROLOGICAL PROCESSES Hydrol. Process. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10298 Copyright © 2014 John Wiley & Sons, Ltd.