Satellites observations enable the mapping of global groundwater dynamics
An expanding global population, with its accelerated demand for food, industrial products and domestic consumption, drives a commensurate demand for water. While much of this demand is met by surface water, groundwater is increasingly exploited. Globally, irrigated agriculture is the prime user of groundwater.
Half or more of irrigation water, estimated at around 70-80 % of total water consumption, is supplied by groundwater. As irrigated agriculture increases, so does the use of fertilisers, manure and pesticides, which contributes to the pollution and contamination of groundwater, especially from phosphorus, nitrogen and antibiotics.
If withdrawals surpass net recharge, groundwater will be depleted. In fact, based on recent monitoring data, this is, in fact, the case for major aquifers around the globe. Total groundwater depletion is estimated to have doubled during the period 1960 to 2000. The risk for depletion increases in arid and semi-arid areas. Because of the difficulties to assess and monitor at global scales, few studies on groundwater dynamics are available.
However, recent space observations demonstrate that changes in groundwater storage can be estimated. The global groundwater footprint – the theoretical size necessary to sustain current use rates – is currently estimated to be 3.5 times the actual area of aquifers.
For example, the expansion of irrigation in India has resulted in severe depletion and contamination of groundwater reserves. Groundwater storage is important for Indian agriculture as groundwater-based irrigation has been expanded substantially in the past decade. Groundwater pumping for irrigation has been a major contributor to adverse variations in groundwater levels in north-western India, however using the GRACE observations and derived estimates of groundwater changes, it was recently shown that further to anthropogenic causes, precipitation variability played the major role in groundwater changes in north-central and southern India. This knowledge highlights the need and options for adapted water management in current and projected climate conditions.
Groundwater use is difficult to monitor globally
The NASA-DLR GRACE mission provides the first opportunity to directly measure groundwater changes from space. By observing changes in the Earth’s gravity field, scientists can estimate changes in the amount of water stored in a region, which cause changes in gravity. GRACE provides a more than 10-year data record for scientific analysis. This makes a huge difference for scientists and water managers who want to understand trends in how our resources are being consumed over the long term. GRACE has returned data on some of the world’s biggest aquifers and how their water storage is changing.
The GRACE satellites.
Source: NASA, PODAAC (https://podaac.jpl.nasa.gov/gravity).