WATER is one of the resources in increasingly short supply. We'll talk more about this in a later lecture (click "Land degradation" to move to that lecture now, if you wish), but a brief summary of some of the trends is included here
Much irrigated land relies on groundwater (subterranean water, as in aquifers), and in many areas, this groundwater being withdrawn faster than it is recharged.
As an example, the US, India, Libya and Saudi Arabia all rely on groundwater for irrigating large areas, and all are overpumping many of their aquifers.
One famous example is the Ogallala Aquifer in the central US. It runs from northwest Texas to southern S. Dakota, and contains about as much water as Lake Michigan. The Ogallala supports about 1/5 of the irrigated farmland in the US, but it is being pumped much faster than it is being recharged. In fact, the annual overdraft from the Ogallala is about equal to total yearly flow of the Colorado River! This is essentially fossil water that is being mined, as it is recharged only very slowly. Overdrafts are so severe that areas in the Texas panhandle (in the southern-most reach of the Ogallala) have actually reverted to dryland farming, as pumping the water has become so expensive. In fact, between 1982 and 1992, farmers using the Ogallala lost three times more irrigated acreage than they gained! In all, five of the high plains states and three in the western US cut their irrigated area by nearly 10% over those 10 years as aquifer levels dropped. Rain-watered crops can still be grown in some of this area, but at lower yields. Water conservation efforts in some areas irrigated by the Ogallala's water, however, have helped its levels actually increase slightly (Eastern and Central Nebraska). Conservation has included terracing, crop rotation, use of more efficient center-pivot and drip irrigation systems, and simply decreasing the area under irrigation.
The problem of overpumping groundwater is by no means restricted to the US. In India, water tables are dropping more than 1 meter per year in several states, including Punjab (India's "breadbasket"). These trends may make it impossible to keep double cropping rice and wheat (which has been important in boosting grain production) and may necessitate switches from rice to crops like millet or sorghum.
Surface water is also problematic. Excessive diversion of rivers for irrigation has adverse effects on the rivers themselves and on the seas into which they flow. For example, there have been major reductions in levels of the Aral and Caspian Seas (described in "Re-examining the world food prospect," which is on the supplementary reading list for this unit). A town that used to be a port and fishing town on the Aral Sea is now 50km from its shores! (BUT, we'll see later that the Aral Sea is actually recovering -- one of those bright spots, folks!) Competition from cities for water is also intensifying.
For information on stream flows (historical and current) in the US, you can visit the USGS's site, WaterWatch.
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