As water demand rises rapidly, some regions are withdrawing groundwater faster than it can recharge. Now scientists can couple new space-based observations with models and data to quantify global and regional groundwater changes, reports Ninad Bondre.
Globally, groundwater is about a quarter of the total water consumed (Döll 2009). Irrigated agriculture accounts for almost 80 percent of freshwater use, and in India, almost half of the water used for irrigation comes from aquifers. Groundwater can be relied upon during times of low surface-water availability, particularly in regions dominated by strongly seasonal precipitation. And it is an assured source of relatively clean water in regions where surface water sources are highly polluted (Döll 2009). But increases in population, industrialisation and the areas of land brought under irrigation are putting unprecedented pressure on groundwater resources.
A case in point is the northern Indian subcontinent. We knew from well data that groundwater withdrawals here were exceeding recharge, leading to a lowering of water tables. But the scale and seriousness of the problem became apparent in 2009, after two independent groups published their findings. Using observations made by NASA's Gravity Recovery and Climate Experiment (GRACE) satellites, these studies showed a dramatic depletion of groundwater in this region from 2002-2008 (Rodell et al. 2009; Tiwari et al. 2009). Consistently high rates of withdrawal lowered water tables by up to 4 cm/year (see figure). Natural recharge is no longer replenishing the aquifers adequately.
The concerns raised by GRACE-based studies are confirmed by other approaches. A global analysis published last year used a hydrological model to quantify recharge and a variety of records to estimate withdrawal (Wada et al. 2010). The results show that globally, the difference between withdrawal and recharge has more than doubled compared to what it was in 1960.
This is not to say that groundwater is depleting uniformly around the world. Recharge is sufficient to balance withdrawal in many regions, for example in the northeastern United States and much of Europe. But what is worrying about recent results is that groundwater use is unsustainable in precisely those regions that will continue to rely on it the most for irrigation and domestic consumption – highly populated regions like northern India and arid or semi-arid regions like the southwestern United States.
Climate change adds a new twist to the tale. There is some potentially good news for regions like northern India in that models indicate no substantial change in groundwater recharge by 2050, and even a slight increase (Döll 2009). This is because of an expected increase in precipitation in this region as the Earth warms (although the projections of different models regarding precipitation vary widely). But Tiwari et al. (2010) point out that a warmer world will entail greater evapotranspiration, which might balance out the gains in this region due to increased precipitation and recharge. And future demand for groundwater in the region could be different from current demand.
Studies that rely on GRACE data and global hydrological models provide a broad overview of groundwater depletion. This is also the case with simulations of changes in groundwater recharge due to future climate change. Such studies could be invaluable in informing policy decisions at the state or national level. However, they cannot be relied on solely by those entrusted with managing smaller administrative units.
The challenge is to distil the relevance of global findings to local settings – such as those farmers in western India – accounting for a host of variables including changes in population, socio-economic conditions, demand and future availability of surface water. We need to act now to find solutions to ensure that these underground stores of water remain wellsprings for the future.
Famiglietti J et al. (2011) Geophysical Research Letters 38: L03403, doi: 10.1029/2010GL046442.
Döll P (2009) Environmental Research Letters 4: 035006, doi:10.1088/1748-9326/4/3/035006.
Rodell M, Velicogna I and Famiglietti J S (2009) Nature 460: 999-1002, doi:10.1038/nature08238.
Tiwari V M, Wahr J and Swenson S (2009) Geophysical Research Letters 36: L18401, doi:10.1029/2009GL039401.
Wada Y et al. (2010) Geophysical Research Letters 37: L20402, doi: 10.1029/2010GL044571.
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