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

Human impacts on the landscape

Land-Use Trends in the Southwestern United States

Michael O'Donnell
U. S. Geological Survey


Land use changes over time provide the necessary insight for how we have utilized, historically perceived, and what we ultimately perceive to be the most efficient and versatile land practices. In consideration of technological change, supply and demand, and preservation there have been major changes in how we use the resources.

By examining major land uses and crop cultivation, we can estimate impacts by examining and projecting trends. Because of a growing population we must pay more attention to how efficiently we can use and reuse a limited resource in a non-detrimental fashion. The two chosen data sets, major land uses and major crop cultivation, are stepping stones to evaluating and predicting land degradation. To complete the circle, water usage issues (irrigation and resulting erosion), population growth (sensitive areas in relation to where people are congregating), climatic changes, and other anthropogenic and natural influences must also be considered for preserving the balance of ecosystems.

Major Land Uses

Click on images to view full-sized maps for two time periods

Landuse 1945 and 1949
Landuse 1954 and 1959
Landuse 1964 and 1969
Landuse 1974 and 1978
Landuse 1982 and 1987
Landuse 1945 and 1992

Sources of Data

The Economic Research Service, National Agricultural Statistics Service, and United States Department of Agriculture were responsible for compiling the data, or collecting the information from other agencies, into digital format. We then structured the data so that it could be spatially represented using GIS (Geographic Information Systems) techniques. Our purpose is to visually demonstrate the spatial changes found in the U.S. over time, and identify associations with other impacts.

Crop Data
Land Use Data (Search for keyword "89003" to find the data

The major land use surveys began in 1945 and are taken every fourth year. The crop data were compiled every year, but we recompiled it to match the years surveyed in the Census of Agriculture so we could make comparisons between types and amounts of crops grown and the number of acres of land in tillage. The crop data and land use surveys data were listed by state, as were the data for the number of acres used for farms. This will be important for investigating changes since the 1840s, through the depression and the green revolution. Such issues bring forth notions of land destruction due to irrigation, cultivation, livestock practices and chemical uses. In addition, issues of wind erosion, salinization, loss of nutrients, and acid rain due to increased urbanization and industrialization can also be associated with land-use changes.

Cropland use and Irrigation

figure 1
Figure 1. Percentage of irrigated acreage, by crop type. (Source: US Bureau of the Census, 1984).

One of several issues that influences crop production is irrigation. For example, the percentage of harvested acres were greatest for crops irrigated the most in 1982 (Moore and others, 1990)(fig 1). Between 1920 and 1945, the number of irrigated acres increased more than the number of acres harvested, except for 1934, which is possibly a result of the depression (fig. 2). This difference in acres irrigated and acres harvested increased between 1950 and 1970 as a result of federal programs to reduce cropland acreage.

Land-use trends specific to the Southwest

The general trends of land use from 1945 to 1992 in the southwest show significant decreases in forests, increases in special uses (highways, roads, airports, Federal and State parks, wilderness areas, wildlife refuges, national defense and industrial uses and urban areas), decreases in pastures, and either an increase or an unchanged amount of land as cropland.

Figure 2. Percent changes from previous estimates of harvested cropland on all farms and of the number of irrigated acres. (Source: Census of Agriculture).

We have chosen four major crops, for years spanning from 1870 to 1994, which affect the Southwest the greatest. Barley, corn, and all wheat types show a significant increase in yield for the southwestern states. Sorghum increased in CA, AZ, NM, CO, and KS, but decreased for TX, OK, WY, and NE. Examining data for these crop types and their evapotranspiration rates in relation to where we are growing them will help us determine what we can change to increase our output and preserve resources.

Click to see animation of sorghum in 1964 and 1994

Click to see animation of barley from 1870 to 1994

Click to see animation of corn from 1870 to 1994

Click to see animation of wheat from 1870 to 1994

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, 553 p.

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