Conjunctive groundwater use: a ‘lost opportunity’ for water management in the developing world?

Abstract

Conjunctive groundwater use with surface-water resources has only occasionally been considered in the hydrogeological literature and at scientific conferences (e.g. Bredehoeft and Young 1983; Sahuquillo 2002), although in terms of practical water management it represents one of the most important responses to improving drought water-supply security and for long-term climate-change adaptation, and in terms of underpinning science its design requires a refined understanding of resource interconnectivity (both naturally and perturbed by irrigation works or practices) and of aquifer salinisation processes. The aim of this essay is to provide an overview of current conjunctive use in the developing world for both irrigated agriculture and urban water-supply, and to highlight the great potential that planned ‘conjunctive management’ has as a climate-change adaptation strategy. It is primarily relevant to larger alluvial plains, which often have important rivers, large-scale irrigation systems and major aquifers in close juxtaposition—although the potential for conjunctive management can be present in a wider range of settings. There is no rigorous definition for ‘conjunctive use’; however, for the present purpose it is proposed to consider only situations where both groundwater and surface water are developed (or co-exist and can be developed) to supply a given urban area or irrigation canal-command—although not necessarily using both sources continuously over time nor providing each individual water user from either source. Adopting this rather narrow definition excludes consideration here of artificial recharge of aquifers with surface runoff or by rain-water harvesting (without direct supply from the surface-water source), use of groundwater pumping to support river baseflows (without direct supply from water wells) and catchment-scale integrated water-resources management embracing everything from flood protection to wastewater reuse (because of insufficient finance as yet to apply this in the developing world)—although it is recognised that all such techniques can play an important role in water-resources management. A key characteristic of conjunctive use is that it deploys the large natural groundwater storage associated with most aquifers to buffer the high flow variability and drought propensity of many surface watercourses (Foster et al. 2010a), and is thus capable (at varying levels of efficiency) of achieving: (1) much greater water-supply security—by taking advantage of natural aquifer storage, (2) larger net water-supply yield than would generally be possible using one source alone, (3) better timing of irrigation-water delivery—since groundwater can be rapidly deployed to compensate for any shortfall in canal-water availability at critical times for crop growth, (4) reduced environmental impact—by counteracting land water-logging and salinisation, and (5) excessive river-flow depletion or aquifer overexploitation. These benefits have been the driving force for spontaneous conjunctive use of shallow aquifers in irrigation-canal commands worldwide.