WSP08775 - Laserfiche WebLink

Home Browse Search

4 t ';f . " HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT 95 mi south of Oklahoma City, Okla., for the 15-year period 1958-72 was only 70 percent of that for the ll-year period 1930-40. The im- proved farming practices and other factors re- duced the impact on the public so that the more recent droughts received less notoriety, but losses still ran into billions of dollars. The drought of 1942-56 in the southwestern United States is documented in a series of Geological Survey Professional Papers, Nos. 372A through 372G, by Gatewood or Thomas and other co-authors. The effects of this drought in an arid area fluctuated during the period at many localities, but it was severe enough that 282 counties in three states, main- ly in Texas, plus all of Arizona and New Mex- ico and most of Utah were declared disaster areas. Nace and Pluhowski (1965) reported that the rare occurrence of a succession of drought-producing meteorologic events during 1952-56 caused critical water deficiencies in much of the southern half of the nation. The accumulated precipitation deficiencies during the 5-year drought period, expressed in per- centage of the average precipitation for 1 year, ranged from 25 to 225 percent. Low- flow frequency data for eastern Kansas indi- cate that the drought had a recurrence inter- val of more than 50 years. Statistical studies of long-term precipitation records for the southern Great Plains indicate that drought of equivalent severity has a recurrence interval of about 140 years in parts of the area. Ground-water levels declined steadily in much of the Midcontinent, and levels were reduced by tens of feet in some places. The decline in water supplies caused considerable financial loss and many personal hardships. The 1961-67 drought in the Northeast was the longest and most severe in the history of the region, and it affected human activities mainly by its impact on water resources re- lated to agriculture and water supplies (Barks- dale and others, 1966). The Water Resources Council (1966) estimated that in the Northeast a drought of this duration occurs, on the ave- rage, once in about 160 years. Though drought conditions eased at different times in different parts of the Northeast, the drought was not over until the spring of 1967. At least four of the six definitions of droughts suggested by the World Meteorologi- cal Organization (see p. 5 ) are applicable to each of the droughts briefly described above. Meteorologically, absolute amounts of precipi- tation were deficient for specific durations. Climatologically, precipitation, expressed as percentages of normal values, was significant- ly less than 100 percent. Atmospheric condi- tions, namely wind, relative humidity, and temperature, were contributing factors in causing the Dust Bowl. Agriculturally, soil moisture was reduced or depleted and the growth of vegetation and crops was hindered. Hydrologically, stream flows and ground-water levels were low and storage in lakes and reser- voirs was reduced. In some areas, water- management practices had not progressed enough to provide integrated water-supply sys- te ms and surface or subsurface storage to equalize the water supply seasonally or from year to year. The reader must remember that construc- tion equipment used to build surface struc- tures, well-drilling equipment and methods, and pumps were not as advanced 40 or 50 years ago as they are today. Hence, water manage- ment's ability to deliver water or develop ground-water supplies during the earlier droughts should not be judged solely by today's standards. Also, farm practices have im- proved, and hardier corn, wheat, and other crops have been developed since the 1930's; therefore, better production can be achieved now under drought conditions. All these de- velopments and improvements make it easier to adapt to a drought now. Therefore, the per- ceived severity of a drought also depends on how well adaptations work. In the context of a meteorological drought and using the seasonal (July 1 to June 30) precipitation at San Francisco, Calif., as an example, there have been four periods prior to 1976 when precipitation was below average for several years in succession. These periods are tabulated below along with the average defic- iency per season and the total deficiency, as well as similar data for the period 1974-77. Number Deficiency Total of (inches deficiency years per year) (inches) 7 3.74 26.20 7 5.18 36.25 4 5.04 20.14 8 5.25 42.03 3 9.83 29.49 Period 1897-1904 1927-1934 1946-1950 1958-1966 1974-1977