Spatial and temporal variability of river flows in the degraded semi-arid tropical mountains of northern Ethiopia

Abstract

Water availability has for long been a critical issue in many developing countries. Despite its enormous potential of water resources, Ethiopia is suffering from a lack of water availability and threatened by the consequences of climate change. Well considered planning to develop these resources is crucial. However, very few observational runoff data exist for this type of environments. Especially runoff data for catchments at the intermediate scale (100 -10,000 km²) are lacking. This study assesses the runoff from 10 mediumsized catchments in the Geba river basin, a subcatchment of the Nile in the semi-arid degraded northern Ethiopian highlands. Flow depth records were automatically obtained every 10 minutes during the rainy seasons (July -September) of 2004-2007 and converted to continuous runoff discharge records.Cumulative annual runoff depths (46 -395 mm) are mainly correlated with rainfall depth. Estimated runoff coefficients (9 -47%) and are negatively correlated with the areal fraction of limestone outcrops in the catchments, indicating runoff transmission losses.Throughout the rainy season, increases in runoff depth and runoff coefficient were observed, which is partly attributed to an increase in baseflow throughout the season. The majority of the runoff occurs during flash floods, i.e. relatively short runoff events with often very high peak discharges. Characteristics of these floods 2 are discussed with some examples, including an exceptionally large flood. Taking into account the difficult conditions for river discharge measurements in a country like Ethiopia, this study provides one of the most comprehensive analyses so far of the magnitude and dynamics of river discharges in Ethiopia.

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

“…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, ). 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 ., ; Walraevens et al ., ; Zenebe et al ., ). Further studies have shown that soil saturation in (semi) humid experimental watersheds is the dominant mechanism for overland flow (Liu et al ., ; Collick et al ., ; Steenhuis et al ., ; Bayabil et al ., ; Tesemma et al ., ; Engda et al ., ) and for erosion (Tilahun et al ., ,).…”

Land Management in the Northern Ethiopian Highlands: Local and Global Perspectives; Past, Present and Future

“…In situ soil conservation and water harvesting (Nyssen et al ., ; Gebreegziabher et al ., ; Nyssen et al ., ; Adimassu et al ., ; Mekonnen et al ., ) as well as the construction of small reservoirs (Haregeweyn et al ., ; Haregeweyn et al ., ) have both led to strongly decreased sediment yield and runoff coefficients at catchment scale, and to increased levels of the water tables (Nyssen et al ., ; Walraevens et al ., ) and have improved livelihood, as reported also elsewhere in Africa (Wildemeersch et al ., ). However, significant differences in runoff coefficients could not be demonstrated between subcatchments of the 5000 km 2 Geba basin, most probably due to the overall implementation of SWC activities (Zenebe et al ., ). Particularly in cases of large conversions to forest, such as the area upstream of Alamata, effects are very clear, especially in terms of decreased downstream flooding and changes of river channel morphology (i.e.…”

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

“…Yet, the exact mechanisms and sediment sources are being scrutinized more recently in the Amhara sub-humid region (Bayabil et al, 2010;Setegn et al, 2010;Tebebu et al, 2010Tebebu et al, , 2015. Comparisons with the drier Tigray region (Nyssen et al, 2006;Vanmaercke et al, 2010;Walraevens et al, 2009;Zenebe et al, 2013) are helping to better understand and evaluate the performance of soil and water conservation measures. The quick generation and increased runoff and sediment in Shanko Bahir relative to the nearby sub-watershed were observable during field work and complicated previous modeling efforts .…”

“…In the semi-arid highlands of Ethiopia, studies have also shown the presence of saturation-excess as a mechanism, however, only during certain parts of the rainy season. In Tigray, infiltration-excess overland flow caused by heavy or high intensity storms has been shown to be common early in the rainy season (Descheemaeker et al, 2009;Walraevens et al, 2009;Zenebe et al, 2013). Thus, although these regimes occur with varying intensity in different watersheds, few attempts had been made to integrate both mechanisms in a satisfactory way.…”