Groundwater recharge and flow in a small mountain catchment in northern Ethiopia

Abstract

The hydrodynamic behaviour of a sloped phreatic aquifer in the Tigray Highlands in northern Ethiopia is described. The aquifer is situated in the soils of a plateau on top of a basalt sequence and lies on steep slopes; the latter lead to hydraulic gradients that can cause high discharge fluxes. Distinct wet and dry seasons characterize the climate of the Tigray Highlands and recharge is absent during the dry season. Because of the fertile vertisols that have developed, the plateau is heavily cultivated and thus has great local economic, and hence social, importance. Water for land irrigation is almost exclusively delivered by rainfall, which is largely restricted to the period June-September. During the dry season, the water table drops dramatically and the aquifer drains nearly completely, under the strong gravity-driven, sustained discharges. This study strives to give insights into recharge and discharge mechanisms of the aquifer, in order to improve the effectiveness of the implemented water conservation measures.Key words groundwater recharge; integrated water balance; water management; runoff; soil moisture balance; MODFLOW; Ethiopia Recharge et écoulement hydrogéologiques dans un petit bassin versant de montagne en Ethiopie du nordRésumé L'article décrit le comportement hydrodynamique d'un aquifère phréatique incliné dans les Tigray Highlands en Ethiopie du nord. L'aquifère se situe dans les sols d'un plateau aux pentes raides, au sommet d'une séquence basaltique. L'escarpement cause des gradients hydrauliques conduisant à des écoulements importants. Les saisons humide et sèche bien différenciées caractérisent le climat des Tigray Highlands, et la recharge est nulle pendant la saison sèche. A cause des vertisols fertiles, le plateau est intensément cultivé et présente par conséquent une valeur locale économique et sociale importante. L'eau d'irrigation provient presque exclusivement de la pluie, qui est largement limitée à la période Juin-Septembre. Pendant la saison sèche, la nappe descend énormément, et l'aquifère est presque complètement drainé, en raison des flux gravitaires continus importants. Cette étude continue à la compréhension des mécanismes de recharge et d'écoulement de l'aquifère, dans le but d'améliorer l'efficacité des mesures implémentées de conservation de l'eau.

Groundwater and climate in Africa—a review

“…Although an improved understanding of this relationship is strongly constrained by limited observational data sets (Carter & Parker, 2009;Howard & Griffith, 2009;MacDonald et al, 2009), a nascent but developing knowledge of the interactions between climate and groundwater is emerging and exemplified by contributions to this Special Issue (Mahé, 2009;Mileham et al, 2009;Nyenje & Batelaan, 2009;Olago et al, 2009;Walraevens et al, 2009;).…”

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.…”

Climate Change and Sustainable Development of Water: Sub-Saharan Africa

“…The relationship between rainfall and recharge is not necessarily linear, with the possibility of recharge occurring sporadically on regional and temporal scales. In the Ethiopian highlands, for instance, recharge from monsoonal rains is released by spring and seeps in the dry season but this monsoonal recharge does not necessarily contribute to inter-annual groundwater storage (Walraevens et al, 2009). Contributing to the non-linearity of the relationship between rainfall and recharge, some studies suggest that a certain threshold of rainfall (annual or event-driven) must be reached before any recharge to groundwater occurs.…”

“…The decline of groundwater levels has been observed during periods of sustained annual reduction of rainfall such as during drought. Complete dewatering of certain aquifers was observed in Ethiopia during drought (Walraevens et al, 2009) and decline of groundwater levels observed during drought in Burkina Faso (Thiery et al, 1993) and in the upper Niger Basins (Bamba et al, 1996). It is suggested that the volume of groundwater extracted should not be greater than the volume recharged (Kinzelbach et al, 2003), and probably much less.…”