Resilience of shallow groundwater resources and their potential for use in small-scale irrigation : a study in Ethiopia

Abstract

Groundwater use for small-scale irrigation in sub-Saharan Africa is low, though is expected to increase in the near future. There is currently limited understanding of shallow groundwater resources, which are most likely to be exploited by poor rural communities due to their accessibility. This PhD study aimed to determine the potential for use of shallow groundwater for small-scale irrigation and the resilience of the resources to increased abstraction, land-use change and climate variability. Research was conducted principally at a study site in northwest Ethiopia with seasonal rainfall and a predominance of rainfed agriculture. The shallow aquifer comprises a thin weathered regolith above largely impermeable basalt. Hydrochemistry analyses suggested little connection between the shallow aquifer and a deep fractured aquifer. To fill gaps in formal hydrometeorological monitoring, a community-based monitoring programme was initiated. Statistical comparisons confirmed that the datasets were of as high or higher quality as those from formal networks, remote sensing and reanalyses. A recharge assessment estimated annual recharge of 280-430 mm, confirming that a sufficient renewable shallow groundwater resource is available for small-scale irrigation. Four nested catchments were modelled using SHETRAN, a physically-based spatially-distributed modelling program. The modelling identified the foot of hillslopes and narrow valleys as showing the greatest potential for irrigated agriculture as groundwater in those locations remained available and accessible for the longest periods. Potential future scenarios were run in the SHETRAN models considering likely climate variability, land use change and increasing abstraction. Around 35% of arable land in the modelled catchments had shallow groundwater available throughout the dry season. During simulated multi-year droughts, a significant percentage of arable land still had sufficient groundwater available for irrigation of a second growing season. Conversion of pasture and scrubland to cultivated land did not have a significant impact on water resources while degradation of highlands to bareground had a positive impact. The severest impact on water resources resulted from increased coverage of Eucalyptus. Notably, simulation of increased abstraction and irrigation at smallholder levels had little impact on surface and groundwater availability. This study demonstrates the potential for greater exploitation of shallow groundwater for small-scale irrigation by rural communities and the resilience of the resource to climate variability, land use change and increasing abstraction.