
This map shows changes in river streamflow worldwide between 1948 and 2004. Bluish colors show more runoff, reddish colors show less. (Graphic courtesy of Journal of Climate, modified by UCAR.)
Several articles published recently discuss the potential impact of declining water supplies on food production as temperatures rise and rainfall declines. Agriculture uses far more freshwater than any other human endeavor, typically from 70 to 90 percent of freshwater supply.
Irrigation water typically comes from rivers and lakes, but dependence on groundwater aquifers is increasing as surface water declines. Water tables in several places -- the Ogallala Aquifer in the U.S.,the Saudi aquifer and the China North Plain -- are all declining as groundwater pumping tries to compensate for loss or overuse of surface water in rivers and lakes.
This tight link between water supply and food supply becomes something to ponder in light of research results showing decline in runoff from several major river systems (graphic above), and a report published in April in PNAS on projected shrinkage of Colorado River water deliveries from decreased precipitation, rising temperatures, and more evaporation.
Lake Mead in March 2008 behind Hoover Dam on the Colorado River. Levels then were down about 100 feet from full pool. Levels are now at 44-year lows. Photo: Neil McIntosh/flickr

Lester Brown discusses implications in the May issue of Scientific American. He describes the potential for food supply collapse caused by falling water tables, disappearing soils, and rising temperatures. Brown says the biggest threat for disruption comes from poor countries with fragile or non-existent infrastructure and governments.
These reports follow on a paper published in Science earlier this year detailing projected declines in food production across large areas of the sub-tropical and tropical regions because of heat waves and drought.
That paper by Battisti and Naylor projected that hot summers and declining crop yields will become common over a broad area where nearly half of the world's population lives. Most of the farmers in this region are subsistence farmers and incomes of less than $2 a day. They largely depend on rainfall for growing crops and do not have the capital, technology, or know-how to store water and irrigate.
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