Models accurately representing the underlying hydrological processes and sediment dynamics in the Nile Basin are necessary for optimum use of water resources. Previous research in the Abay (Blue Nile) has indicated that direct runoff is generated either from saturated areas at the lower portions of the hillslopes or from areas of exposed bedrock. Thus, models that are based on infiltration excess processes are not appropriate. Furthermore, many of these same models are developed for temperate climates and might not be suitable for monsoonal climates with distinct dry periods in the Nile Basin. The objective of this study is to develop simple hydrology and erosion models using saturation excess runoff principles and interflow processes appropriate for a monsoonal climate and a mountainous landscape. We developed a hydrology model using a water balance approach by dividing the landscape into variable saturated areas, exposed rock and hillslopes. Water balance models have been shown to simulate river flows well at intervals of 5 days or longer when the main runoff mechanism is saturation excess. The hydrology model was developed and coupled with an erosion model using available precipitation and potential evaporation data and a minimum of calibration parameters. This model was applied to the Blue Nile. The model predicts direct runoff from saturated areas and impermeable areas (such as bedrock outcrops) and subsurface flow from the remainder of the hillslopes. The ratio of direct runoff to total flow is used to predict the sediment concentration by assuming that only the direct runoff is responsible for the sediment load in the stream. There is reasonable agreement between the model predictions and the 10-day observed discharge and sediment concentration at the gauging station on Blue Nile upstream of Rosaries Dam at the Ethiopia-Sudan border

Understanding the basic runoff processes in the Ethiopian highlands is vital for effective management and utilization of water resources and soil conservation planning. An important question for judging effectiveness of conservation practices is whether runoff is affected by ecology (mainly type of crop) or topography (landscape). A study was conducted in the 113 ha watershed of Maybar, located in the highlands. This watershed has long-term records of rainfall and discharge. To study runoff processes, piezometers were installed in eight transects up and down the slope. In addition, infiltration rates (measured earlier) were compared with rainfall intensities. The results show that the amount of runoff at the test plots was greater for cropland located on mild to intermediate slopes than for grass and wood lands on the steeper slopes. Water tables were closer to the surface on cropland for the mild to intermediate slopes than on grass and in woods for the steeper slopes. Thus, although water table depths and plot runoff were inconclusive on the type of runoff mechanisms, infiltration rates that were generally in excess of the rainfall rates imply that any ecological effect on the amount of surface runoff is small. This is because water infiltration is independent of crop type. Only in cases where the soil was saturated does runoff occur. Piezometer readings show that saturation occurs at the foot of the steep slopes and therefore demonstrates that topographic processes are dominant. Ecology becomes important when infiltration rates are in the same order as the rainfall intensities.

Despite millions of dollars invested in soil and water conservation (SWC) practices in the (sub) humid Ethiopian highlands and billions of hours of food-for-work farm labor, sediment concentration in rivers is increasing. This paper reports on the research to reverse the current trend. Based on the understanding of the hydrology of highlands, we provide evidence on sources of surface runoff and sediment and on mechanisms that govern the erosion processes and approaches and how they affect SWC practices. We suggest that priority in landscape interventions should be given to re-vegetation of the degraded areas so as to reduce the sediment concentration contributions originating from these areas. Additionally, efforts should be directed to gully rehabilitation in the saturated bottom landscape that may consist of vegetating shallow gullies and stabilizing head cuts of deeper gullies. Finally, rehabilitation efforts should be directed to increase the rain water infiltration in the upland areas through the hardpan layer by connecting the land surface to the original deep flow paths that exist below about 60 cm. It will reduce the direct runoff during the rainy season and increase baseflow during the dry season.

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Hardpan is a major cause of land degradation that affects agricultural productivity in developing countries. However, relatively, little is known about the interaction of land degradation and hardpans. The objective of this study was, therefore, to investigate soil degradation and the formation of hardpans in crop/livestock‐mixed rainfed agriculture systems and to assess how changes in soil properties are related to the conversion of land from forest to agriculture. Two watersheds (Anjeni and Debre Mewi) were selected in the humid Ethiopian highlands. For both watersheds, 0–45 cm soil penetration resistance (SPR, n = 180) and soil physical properties (particle size, soil organic matter, pH, base ions, cation exchange capacity, silica content, bulk density and moisture content) were determined at 15 cm depth increments for three land uses: cultivated, pasture and forest. SPR of agricultural fields was significantly greater than that of forest lands. Dense layers with a critical SPR threshold of ≥2000 kPa were observed in the cultivated and pasture lands starting at a depth of 15–30 cm but did not occur in the undisturbed forest land. Compared with the original forest soils, agricultural fields were lower in organic matter, cation exchange capacity, and exchangeable base cations; more acidic; had a higher bulk density and more fine particles (clay and silt); and contained less soluble silica. Overall, our findings suggest that soil physical and chemical properties in agricultural lands are deteriorated, causing disintegration of soil aggregates, resulting in greater sediment concentration in infiltration water that clogged up macro‐pores, thereby disconnecting deep flow paths found in original forest soils. Copyright © 2016 John Wiley & Sons, Ltd.