Limits to global groundwater consumption: Effects on groundwater levels and river low flows

Abstract

Freshwater is essential for human existence. With increasing global population and economic growth, freshwater demands almost tripled over the past decades and are expected to rise further. During dry periods, when water in rivers and lakes is scarce and cannot meet human water demands, people often turn to groundwater as an alternative freshwater resource. Yet, when more groundwater is abstracted than recharged groundwater depletion occurs, groundwater levels will drop, and baseflows to rivers decreases. Excessive abstraction for irrigation, where groundwater is slowly renewed, is the main cause of the depletion and climate change has the potential to exacerbate the problem in some regions. The expected increase in water demands and the negative effects of groundwater depletion, poses the urgent question how sustainable current water uses are worldwide and where and when abstractions lead to associated problems, such as drying up of wells and falling river flows. This study assessed the limits to global groundwater consumption,by estimating past and future trends in groundwater levels and river flows as the result of changes in abstractions and climate. In this study it is predicted where and when groundwater levels drop that low that groundwater becomes unattainable for abstractions, or where and when groundwater discharges no longer help to sustain river low flows. In this study a coupled version of the global hydrological model PCR-GLOBWB and a groundwater model using MODFLOW is developed. The model is the first global scale model that simulates lateral groundwater flows, that accounts for groundwater-surface water interactions, and that includes geohydrological information in its parameterization of aquifers, including estimates of aquifer thickness, and likelihood of confining units. Also, a dynamic scheme is introduced to allocate water demands over groundwater and surface water resources based on the water availability in the resource. This new information and novel model approachesare needed to realistically simulate the effects of groundwater and surface water use on groundwater levels and river flows. Water abstractions of the recent past and near future are included (1960-2010). This study is the first that reveals hotspots of groundwater depletion on confined aquifers. Regions where groundwater is already, or almost unattainable for abstraction are found for intensively irrigated regions in dry climates, for example in India, Pakistan, North China and the Mid-West of the US. It is expected that groundwater demands will intensify, particularly for these regions and for semi-arid regions, due to increase of drought frequency and duration reflected in river low flows, combined with population growth, expanding irrigation areas, and rising standards of living. Groundwater depletion will rise worldwide, and new regions experiencing groundwater depletion will develop. For some of these regions groundwater will become unattainable for abstractions. Examples are Southern Europe, parts of Africa, and the Middle East. Global cost to ensure groundwater supply will increase rapidly. Regionally, an increase or decline in exploitation cost will be an essential factor in future economic development and should be carefully considered in order to warrant a reliable groundwater supply under changing climatic conditions.