Reply to Pfister and Hellweg: Water footprint accounting, impact assessment, and life-cycle assessment. Global monthly water scarcity: blue water footprints versus blue water availability. Our Ecological Footprint (New Society Publishers, 1996) Virtual water: a strategic resource, global solutions to regional deficits. Human appropriation of natural capital: a comparison of ecological footprint and water footprint analysis. The Water Footprint Assessment Manual: Setting the Global Standard (Earthscan, 2011)
Contribution of global groundwater depletion since 1900 to sea-level rise. Nonsustainable groundwater sustaining irrigation: a global assessment. Global depletion of groundwater resources. Satellites measure recent rates of groundwater depletion in California's Central Valley. Satellite-based estimates of groundwater depletion in India. Global hydrological cycles and world water resources. Global estimates of water withdrawals and availability under current and future business-as-usual conditions. Global water resources: vulnerability from climate change and population growth. Human appropriation of renewable fresh water. Groundwater use for irrigation - a global inventory. Global groundwater? Issues and solutions. The method could be modified to evaluate other resources with renewal rates that are slow and spatially heterogeneous, such as fisheries, forestry or soil. It can be combined with the water footprint and virtual water calculations 12, 13, 14, and be used to assess the potential for increasing agricultural yields with renewable groundwaterref 15. The groundwater footprint is the first tool suitable for consistently evaluating the use, renewal and ecosystem requirements of groundwater at an aquifer scale. 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. We 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. Here we define the groundwater footprint (the area required to sustain groundwater use and groundwater-dependent ecosystem services) and show that humans are overexploiting groundwater in many large aquifers that are critical to agriculture, especially in Asia and North America. It remains unclear how the rate of global groundwater depletion compares to the rate of natural renewal and the supply needed to support ecosystems. Most assessments of global water resources have focused on surface water 3, 4, 5, 6, but unsustainable depletion of groundwater has recently been documented on both regional 7, 8 and global scales 9, 10, 11. 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 1, 2.
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