Catchment management in the developing world rarely include detailed hydrological components. Here, changes in the hydrological response of a 200-ha catchment management in north Ethiopia are investigated. The management included various soil and water conservation measures such as the construction of dry masonry stone bunds and check dams, the abandonment of post-harvest grazing and the establishment of woody vegetation. Measurements at the catchment outlet indicated a runoff depth of 5 mm or a runoff coefficient (RC) of 1.6 % in the rainy season of 2006. Combined with runoff measurements at plot scale, this allowed calculating the runoff Curve Number (CN) for various land uses and land management techniques. The pre-implementation runoff depth was then predicted using the CN values and a ponding adjustment factor, representing the abstraction of runoff induced by the 242 check dams in gullies. Using the 2006 rainfall depths, the runoff depth for the 2000 land management situation was predicted to be 26.5 mm (RC = 8 %), in line with current RCs of nearby catchments. Monitoring of the ground water level indicated a rise after catchment management. The yearly rise in water table after the onset of the rains (∆T) relative to the water surplus (WS) over the same period increased between 2002-2003 (∆T/WS = 3.4) and 2006 (∆T/WS >11.1). Emerging wells and irrigation are other indicators for improved water supply in the managed catchment. Cropped fields in the gullies indicate that farmers are less frightened for the destructive effects of flash floods. Due to an increased soil water content, the crop growing period is prolonged. It can be concluded that this catchment management has resulted in a higher infiltration rate and a reduction of direct runoff volume by 81 % which has had a positive influence on the catchment water balance.
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.
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