Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on water resources

Water Use Trends in the Southwestern United States 1950-1990

Michael O'Donnell
Jonathan Rademaekers
U. S. Geological Survey

Water usage in the United States is important for examining depletions of ground- water aquifers and pollution impacts of both surface- and ground-water sources. Water use data has been collected by the USGS Water Resources Division starting in 1950 and has continued at five-year intervals since. The data presented here were extracted from these documents and compiled into digital format for use in a geographic information system (GIS). We present here data for several selected water-use categories by state in order to show general trends. However, comparisons for ratios of ground- and surface-water withdrawals as well as other attributes included in the water-use surveys can be used for greater focus.

Rural use and Public Supplies

Of the different water uses, rural uses have the least influence on withdrawals and has changed very little compared to irrigation and the total water use in the U.S. Since 1950, water uses for irrigation are greatest west of the 100th meridian and have increased greatly with time. Compared to Western states, the Eastern and Midwestern states use very little water for irrigation. For the past forty years, the usage of water for public supplies has dramatically increased for only a few specific states, including California, Texas, Illinois, Michigan, Ohio, Pennsylvania, New York, Washington, Georgia, North Carolina, and New Jersey. Public water use in these states increased from as little as 0 to 800 mgd to as much as 52,000 mgd. The total water withdrawal has generally increased for all states between 1950 and 1990 and conclusively is attributed to population growth, land use changes, climate changes, and social changes.

Domestic Water Use

A significant factor affecting domestic water use has been an increasing number of single-family homes with fewer members in those households (Schefter, 1990). Households with a higher income bracket tend to use more water, but average incomes have only slightly risen after adjusting for inflation. It is not thought that incomes have affected water withdrawals for public supplies. Costs of water in households have fluctuated and are not influenced by inflation. In general, domestic water use per household0 changed very little between 1960 and 198. The domestic water use per capita has increased over time, but usage per household has fluctuated only slightly (Schefter, 1990). In addition, between 1960 and 1985, public water utility withdrawals increased by 1.5 times the rate of buyers, whereas 2/3 of water withdrawal was used for domestic use (Schefter, 1990). Costs of water to households include delivery and disposal, but in spite of this, water was cheaper in 1985 than in 1965. Fluctuations of water use can not necessarily be attributed to costs because data were not collected over time and sometimes .the costs were subsidized or paid for by taxes.

Water Use for Irrigation

Irrigation strongly increases crop productivity. For instance, in 1982, irrigated farms accounted for 12 percent of all farms produced and almost 1/3 of the total value for crop production (Moore and others, 1990). The United States is typically divided into those states exceeding an average of 20 inches on the eastern U.S. and those producing less than 20 inches on the western U.S. which is why western states require a significant amount of water for crop production compared to eastern U.S. (fig. 1). Cropland in humid regions is also often irrigated to halt crop production variability during droughts. Areas in which such practices occur include Florida, Arkansas, and Louisiana.

Figure 1. The line of 20 inches of average annual rainfall is the boundary between humid and dry regions (Moore and others, 1990).

The amount of irrigated land has increased between 1900 and 1935 as a result of massive improvements in the hydrologic and technological infrastructure that occurred around the turn of the century (fig. 2). This soon declined, and irrigation leveled off for approximately thirty years, eventually decreasing in the 1960s. But the increase in irrigated land in the 70s was a result of increasing agricultural trade and rising farm-commodity prices (Moore and others, 1990). Overall, the amount of land being irrigated has increased steadily from 1900 to 1980, and has approximately doubled between 1950 and 1978, when the acreage of harvested cropland declined by 10 percent (Moore and others, 1990).

Figure 2. Trends in the number of farms irrigated and the acres of irrigated farms (Moore and others, 1990).

Farm and ranch development has been promoted in the western United States by subsidizing irrigation through such policies as the Reclamation Act of 1902. This was accomplished by the U.S. Bureau of Reclamation financing hydrologic projects interest free and collecting funds from hydrologic power revenues.

Between 1950 and 1985, water use for irrigation increased about 54 percent from an estimated 89,000 Mgal/d in 1950 to 137,000 Mgal/d in 1985 (Moore and others, 1990). Water use changes can be influenced by climatic changes as well. For instance, in 1985, an average of 54 percent (73,8000 Mgal/d) of the water withdrawn for irrigation was consumptively used by evaportranspiration or incorporated into crops and 17 percent (23,6000 Mgal/d) was lost in conveyance (evaporation or leakage to the groundwater system). The remaining 29 percent of the water withdrawn was returned to streams or infiltrated into the soil to reach the ground water system (Moore and others, 1990).

Table 1. Average acre-feet of irrigation water applied per acre, 1984. (Moore and others, 1990)
state/regionirrigation systemcorn wheatcottonalfalfa
gravity flow3.1
sprinkler1.9 NANANA
gravity flow1.1 0.90.8NA
gravity flow3.3
sprinklerNA 1.0NA2.0
gravity flow2.2 1.6NA1.6
gravity flow1.6
humid region
gravity flow0.9 0.50.4NA




Figure 3. Sources of irrigation water between 1950 and 1985. (Moore and others, 1990)

Water Use Issues in the Southwest

Of the water used for irrigation in the U.S., 2/3 is extracted from surface water and one third from the ground (fig. 3). The western U.S. depends on surface waters for irrigation, but areas such as Lower Colorado River basin and California require greater ground water resources than the other western states. The Plains region extracts water from the Ogallala aquifer primarily for irrigation purposes. Of the irrigated crops grown in the western states, alfalfa requires the greatest amount of water, followed by cotton, corn and wheat (Table 1).

Another issue also involved with water use is water quality. Irrigation, which does not permit improvements of water quality after usage before returning to a source, as municipal/sewage water would, changes the content of dissolved salts and adds agricultural chemicals and eroded sediments. For a typical soil, water returning to a source after irrigation is more saline than prior to an irrigated application. This problem is associated with one-fourth of the irrigated lands in the U.S., primarily between Arizona, Eastern Utah and Western Colorado, as well as isolated areas in the Imperial-Coachella Valleys of California and Yakima Valley of Washington. Such problems have led to needed desalinization stipulations set by US-Mexico accords. Salt deposits accumulate as water evaporates, which leads to increased needs for water and/or the loss of agricultural land to unsuitable environments in which not even salt-resistant crops can survive.

Definitions of extracted water uses for map interpretations:

Click to see animation of water for irrigation from 1950 to 1990

Water withdrawal for irrigation: All water including surface and ground withdrawals, sewage, and conveyance loss.

Click to see animation water for public supplies from 1950 to 1990

Water withdrawal for public supplies: Applies to uses by public and private water suppliers.

Click to see animation of total withdrawal from 1950 to 1990

Total water withdrawal: Includes public supply, commercial, domestic, industrial, mining, livestock, and irrigation, but does not include water used for water power.

References Cited

Moore, M.R., Crosswhite, W.M., and Hostetler, J.E., 1990, Agricultural Water Use in the United States, 1950 - 85, in: National Water Summary 1987 - Hydrologic Events and Water Supply and Use, U.S. Geological Survey Water-Supply Paper 2350, p. 93-108.

Schefter, J.E., 1990, Domestic Water Use in the United States, 1960 - 85, in: National water summary 1987 - Hydrologic Events and Water Supply and Use U.S. Geological Survey Water-Supply Paper 2350, p. 71-80.

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