19 August 2009

Loosing Groundwater at the Grand Scale

This week, an intriguing study, in which scientists used satellites to determine groundwater depletion at the continental scale, put the spotlight on groundwater withdrawals in India. [Original article in Nature (for those with access via their library), a review in the Washington Post and in the ScienceBlog]. Very timely article for Americans worried about dwindling groundwater resources in the High Plains Aquifer, in the arid Southwest, or in California. The article, by Matthew Rodell and others, reported that, over a six-year period, the average annual loss of groundwater across three states in northwestern India (not just a single small basin) was about 4 cm (1.6 inches) equivalent height of water or nearly 18 cubic kilometers per year across the 450,000 sq.km (170,000 sq.miles) region. At 12% specific yield (the proportion of the ground actually occupied by groundwater), this means an average annual decline of 0.33 m (1 foot) across that entire region. During the period of observation (2002-2008), the precipiation in Northwest India was reported to be about average.

What is interesting about this number is that this is not an untypical rate of long-term groundwater level decline in semi-arid/arid regions with irrigated agriculture or intensive urban groundwater use: water level declines on the order of 30 cm to over 1 meter per year. The North China Plain, a 140,000 sq.km region with both, high population (more than 110 million) and intensive irrigated agriculture (a lot of maize and winter wheat double cropping), lack of surface water and intensive groundwater exploitation have led to annual groundwater level declines exceeding 1m per year. In many overdrafted aquifers of the U.S., including the High Plains aquifer (about the same size at the area in India investigated, but with declining water levels more localized), the alluvial aquifer supplying the City of Tucson (0.5 - 1 m/yr), and some portions of the southern Central Valley aquifer system in California, we have seen similar rates of decline over the past 50-100 years. The result being that unconfined aquifer water levels that used to be within a few feet of the land surface are now 50 to 150 feet deep, and sometimes even more. During drought periods, water level declines can be much larger (check out the figure in my May 14 blog below).

The other part of this study that is noteworthy - because it confirms observations elsewhere and is an important characteristic of aquifers - is the observation of large year-to-year variations in groundwater storage. Compare their results, for example, to the figure in my May 14, 2009 blog: Year-to-year variations averaged across Northwestern India can be as high as 20 cm (equivalent water height), about as much as we saw in our Tule River Basin study in the southern San Joaquin Valley.

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