The terms dryland and dry farming are used in the United States to characterize agriculture existing on an average annual precipitation of from 10 to 20 inches. A high variability in the annual amount is common in these relatively low rainfall areas—less than half the longtime average can occasionally be expected. On the basis of seasonal water availability for vegetation, dryland farming extends from areas where economically useful vegetation is barely sustained, to areas where drought is only an intermittent problem.