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<br />OQ3H4 <br /> <br />2 <br /> <br />( <br /> <br />The concepl of drought differs in humid and semiarid or arid regions. A <br /> <br />humid region resident would cenainly consider most of the southwestern United <br /> <br />States to be in a stale of permanent drought. This is indeed aue if drought is mea- <br /> <br />sured in lerms of agricultural crops such as corn, soybeans or alfalfa rather rhan <br /> <br />native vegeration. Agricultural crops must be irrigated in the southwest, so the <br /> <br />major concern is whether there will be enough water, either from surface or <br /> <br />groundwaler sources, for this pUIpose. Most of the surface waler available for irri- <br /> <br />gation comes from winter precipitation, largely from snowmelt in the spring, and <br />must be stored in reservoirs for use during the growing season. Unless storage fa- <br /> <br />cilities are large enough to supply needs for several years, surface water users are <br /> <br />more vulnerable to shon-duration droughts than those who depend on ground wa- <br /> <br />teL In rime, however. accumulated moisture deficiencies will affecr ground waler <br /> <br />availability and creare oroblems of supply. It should be emphasized that the amount <br /> <br />of precipitation alone C2nnm be used to define drought in practicallerms. Rather ir <br /> <br /> <br />is the discrepancy between the amou.nt and distribution of precipitation and that re- <br /> <br />turned to the atmosphere by evaporation and transpiration. Precipitation must be <br /> <br />sufficient in time and space to maintain soil moisture at levels adequale for plant <br /> <br />growth and maruriry. <br />It is also imponant to recognize that annual precipitation. or even monthly <br /> <br />totals, can be misleading in assessing drought conditions. Seasonal amounts, espe- <br /> <br />cially during the growing season, are perhaps a more realistic measure. This does <br /> <br />not minimize the importance of winter precipitation that recharges soil profiles and <br /> <br /> <br />is vital to long-term yields. Within seasons, the characteristics of individual storms <br /> <br /> <br />can influence plant growth and yields. Monthly tOtals made up of high intensity <br /> <br />( <br /> <br />storms that result largely in surface runoff, do little to replenish soil moisture. <br />Likewise, a series of small storms may not even penetrate the vegetation canopy <br />