Impacts of Groundwater Withdrawals for Irrigation
Where surface water supplies are insufficient, groundwater is often used for irrigation (Figure 4.2.3). Agriculture uses about 70% of the groundwater pumped for human use globally and about 53% of the groundwater pumped in the US (USGS: Groundwater use in the United States). In some parts of the world, groundwater is pumped at a faster rate than natural processes recharge the stored underground water. Groundwater use where pumping exceeds recharge is non-renewable and unsustainable.
Another problem that may occur in some aquifers with excessive groundwater pumping is a compaction of the aquifer and subsidence of the ground surface. When the water is pumped from the pore spaces in the aquifer, the pore spaces compress. The compression of millions of tiny pore spaces in hundreds of meters of aquifer material manifests on the surface as subsidence. The ground elevation actually decreases. Subsidence from groundwater pumping is irreversible and leaves the aquifer in a condition where it cannot be recharged to previous levels.
Our reliance on and depletion of groundwater resources is becoming a global concern as aquifers are being pumped at unsustainable rates in the US (Figure 4.2.4) and all over the world. Enhanced irrigation efficiencies and conservation measures are being implemented when possible to prolong the life of some aquifers. Unfortunately, groundwater is often the water resource that we turn to in times of drought or when other surface-water resources have been depleted. For example, in California during the recent drought, farmers drilled wells and used groundwater to save their crops when surface water resources were not available.
This map of the U.S. shows total groundwater withdraws by state, in millions of gallons per day. California has the highest at 20,000 - 60,000. Nebraska follows at 10,000 - 20,000. Texas and Arkansas are 5,000 - 10,000, Mississippi, Florida, Colorado, Kansas, Arizona, Oregon, and Idaho are each 2,000 - 5,000. The rest of the country is 0 - 2000.
Read the following article:
Rosenberg, David M., Patrick McCully, and Catherine M. Pringle. "Global-scale environmental effects of hydrological alterations: introduction." BioScience 50.9 (2000): 746-751.