A not-so-paradoxical groundwater rise in semi-arid environments

Semi-arid areas are usually associated with decreasing water levels due to groundwater overexploitation, that threatens the sustainability of the water resource and causes negative environmental impacts. More surprisingly, semi-arid areas also host situations of long-term rising groundwater levels, due to anthropization. Changes in land use and land cover, surface application of groundwater pumped from deeper aquifers, inflow of external resources or insufficient sanitation may generate a significant rise in groundwater levels. Consequences on the local water balance may be not less dramatic than those of overexploitation, as rising groundwater tables compromise i) medium term agriculture, via waterlogging and salinization of soils, ii) urban development, via the saturation of drainage systems and water networks and iii) the environment, via the increase in groundwater discharges towards low-topography areas or surface water masses. Qualitative consequences include the death of crops by hypoxia, the increase of water diseases, or the eutrophication of surface water masses. Illustrated by lessons learnt from Mauritania, Niger, Algeria, Australia and Spain, this paper proposes a typology of contexts and associated consequences. It also illustrates implemented or proposed remediation actions, acting from the causes (land-use restrictions, implementation of drip irrigation, enhancement of sewage networks) to the effects (green belt around cities, drainage of agricultural plots for downstream or more remote rejection, groundwater abstraction at regional scale for further treatment). In a wider scale, this paper highlights a less visible but crucial aspect of the need to anticipate anthropogenic and climatic impacts on groundwater evolution at several timescales