|Water balance of global aquifers revealed by groundwater footprint|
Groundwater is a life-sustaining resource that supplies water to billions of people, plays a central part in irrigated agriculture and influences the health of many ecosystems. Most assessments of global water resources have focused on surface water, but unsustainable depletion of groundwater has recently been documented on both regional and global scales. It remains unclear how the rate of global groundwater depletion compares to the rate of natural renewal and the supply needed to support ecosystems. The groundwater footprint is the first tool suitable for consistently evaluating the use, renewal and ecosystem requirements of groundwater at an aquifer scale.
In their letter in Nature, Tom Gleeson, Yoshihide Wada, Marc F. P. Bierkens and Ludovicus van Beek define the groundwater footprint as the area required to sustain groundwater use and groundwater-dependent ecosystem services. They show that humans are overexploiting groundwater in many large aquifers that are critical to agriculture, especially in Asia and North America. The authors estimate that the size of the global groundwater footprint is currently about 3.5 times the actual area of aquifers and that about 1.7 billion people live in areas where groundwater resources and/or groundwater-dependent ecosystems are under threat. That said, 80 per cent of aquifers have a groundwater footprint that is less than their area, meaning that the net global value is driven by a few heavily overexploited aquifers.
The groundwater footprint is the first tool suitable for consistently evaluating the use, renewal and ecosystem requirements of groundwater at an aquifer scale. It can be combined with the water footprint and virtual water calculations, and be used to assess the potential for increasing agricultural yields with renewable groundwater. The method could be modified to evaluate other resources with renewal rates that are slow and spatially heterogeneous, such as fisheries, forestry or soil.
The groundwater footprint is a powerful and hydrologically grounded tool for groundwater analysis and policy that complements and extends the ecological footprint, water footprint and virtual water methods. It is an advance on previous work on groundwater depletion as it explicitly includes environmental flows, it considers aquifers as a hydrologically grounded scale of analysis, it is more intuitive to water managers and the general public than depletion volumes, and it is based on improved estimates of recharge and abstraction. As exemplified above, the groundwater footprint refocuses the discussion to solutions, making it a valuable water management and policy tool. Practically, it allows short-term water resource monitoring and management measures to focus on the handful of aquifers with egregious groundwater footprints rather than dissipating efforts across all aquifers. Additionally, the groundwater footprint can be used to assess the potential to achieve increased agricultural yields with sustainable groundwater. Also, as satellite-based groundwater depletion data sets (GRACE) are emerging, the groundwater footprint offers a useful framework for analysing these global depletion data sets in a broader framework of groundwater resource use, availability and environmental flows. Last, because the groundwater footprint method is flexible and spatially distributed, it could be modified for other resources whose renewal is slow and spatially heterogeneous, such as fisheries, forestry or soil.
» Water balance of global aquifers revealed by groundwater footprint (letter in Nature, vol 488, no 7410, pp 197–200)
» Demand for water outstrips supply. Groundwater use is unsustainable in many of the world’s major agricultural zones.