Page 298 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

262 part II The Water, Weather, and Climate Systems
F cus Study 9.1 Sustainable Resources High Plains Aquifer Overdraft
north america’s largest known aqui-
fer system is the High Plains aquifer, which underlies a 450 600-km2 area shared by eight states and extending from southern South Dakota to Texas. also known as the Ogallala aquifer, for the principal geologic unit forming the aquifer system, it is composed mainly of sand and gravel, with some silt and clay deposits. The average thickness of the saturated parts of the aquifer is highest in nebraska, southwestern kansas, and the Oklahoma Panhandle (Figure 9.1.1). Throughout the region, groundwa-
ter flows generally from east to west, discharging at the surface into streams and springs. Precipitation, which var- ies widely over the region, is the main source of recharge; annual average precipitation ranges from about
30 cm in the southwest to 60 cm in the northeast. Drought conditions have prevailed throughout this region since 2000.
Heavy mining of High Plains ground- water for irrigation began about 70 years ago, intensifying after World War ii with the introduction of centre-pivot irrigation, in which large, circular devices provide water for wheat, sorghums, cotton, corn, and about 40% of the grain fed to cattle in the United States (Figure 9.1.2). The U.S. geological Survey (USgS) began monitoring this groundwater mining
from a sample of more than 7000 wells
in 1988.
The High Plains aquifer now irrigates about one-fifth of all U.S. cropland, with more than 160 000 wells providing water for 5.7 million hectares. The aquifer also supplies drinking water for nearly 2 mil- lion people. in 1980, water was pumped from the aquifer at the rate of 26 billion cubic metres a year, an increase of more than 300% since 1950. By 2000, withdraw- als had decreased slightly due to declin- ing well yields and increasing pumping
costs, which led to the abandonment of thousands of wells.
The overall effect of groundwater withdrawals has been a drop in the water table of more than 30 m in most of the region. Throughout the 1980s, the water table declined an average of 2 m each year. During the period from predevel- opment (about 1950) to 2011, the level of the water table declined more than 45 m in parts of northern Texas, where the sat- urated thickness of the aquifer is least, and in western kansas (Figure 9.1.3). ris- ing water levels are noted in small areas of nebraska and Texas due to recharge from surface irrigation, a short period of above-normal-precipitation years, and downward percolation from canals and reservoirs. (See ne.water.usgs.gov/ogw/ hpwlms/.)
The USgS estimates that recovery of the High Plains aquifer (those por- tions that have not collapsed) would take at least 1000 years if groundwater mining stopped today. Obviously, bil- lions of dollars of agricultural activity cannot be abruptly halted, but neither can profligate water mining continue. This issue raises tough questions: How do we best manage cropland? Can extensive irrigation continue? Can the region continue to meet the demand to produce commodities for export, or for animal feed? Should we continue high- volume farming of certain crops that are in chronic oversupply?
Scientists now suggest that ir-
rigated agriculture is unsustainable
on the southern High Plains. Present irrigation practices, if continued, will de- plete about half of the High Plains aquifer (and two-thirds of the Texas portion) by 2020. eventually, farmers will be forced
to switch to non-irrigated crops, such as sorghum, and these are more vulnerable to drought conditions (and will also yield smaller economic returns). add to this the
Wyoming
Colorado
New Mexico
South Dakota
In Houston, Texas, the removal of groundwater and crude oil caused land throughout an 80-km radius to subside more than 3 m over the years. In the Fresno area of California’s San Joaquin Valley, after years of inten- sive pumping of groundwater for irrigation, land levels dropped almost 10 m because of a combination of water removal and soil compaction from agricultural activity.
Pollution of Groundwater
If surface water is polluted, groundwater inevitably be- comes contaminated during recharge. Whereas pollu- tion in surface water flushes downstream, slow-moving groundwater, once contaminated, remains polluted virtually forever.
MEXICO
SATURATED THICKNESS metres
120–365 60–120 30–60 0–30
Nebraska
Kansas
Oklahoma
150 KILOMETRES
Texas
▲Figure 9.1.1 Average saturated thickness of the High Plains Aquifer. [after D. e. kromm and S. e. White, “interstate groundwater manage- ment preference differences: The High Plains region,” Journal of Geography 86, no. 1 (January– February 1987): 5.]
approximate 10% loss of soil moisture due to increased evapotranspiration demand caused by climatic warming for the region by 2050, as forecast by computer models, and we have a portrait of a major regional water problem.
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