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10 .~..... '" HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT season. These conditions are depicted in a general way by the patterns in figure 3. Another way to recognize a drought is to compare ground-water levels and rates of de- cline with historic records. When precipitation is below normal, natural recharge is less, and ground-water levels may drop. This response is usually slow; that is, 2 or more years may pass before the effect of reduced recharge is evident in a given well. The major problem in trying to quantify this effect is that the water level in the vast majority of wells is affected by pumping in the vicinity, and any response to a change in recharge is masked by the fluc- tuations caused by the pumping. One observation well that registers the water-table level in the Snake River basin about 130 mi east of Boise, Idaho is not af- fected by pumping or return flows from irri- gation. The hydrograph for this well is plotted in figure 4, and the decline of the water sur- face because of reduced natural recharge is clearly shown. The normal rise of 1.3 to 1.7 ft usually between October and May was only 0.45 ft in the winter of 1976-77. And the annual decline, which had ranged between 1.0 and 2.2 ft, was 2.9 ft in 1977. If ground-water use increases to offset deficiencies in the surface-water supply, the rate of decline accelerates, and shallower wells may go dry. The magnitude of the effect of increased pumping is illustrated in figure 5 for a well in the Central Valley of California. The seasonal fluctuations from pumping are 946 :: 947 o ui~ to ~ 948 >-~ :::0 u. w 949 ~~ .~ ~ ~ 950 >", ~o 0:: Z 951 w~ >-oJ ~ s: 952 1972 1973 1976 1977 1974 1975 Figure 4. Water level decline caused by reduced recharge during the drought. Well is in the Snake River basin 130 miles east of Boise, Idaho. superimposed on a rising trend that started in 1968 when water was first imported to thE' area and pumping was reduced. The water level prior to the importation reached an all time low of 618 ft below land surface in 1967. Water-level records on this well have been obtained since 1960 when the level was 495 ft below land surface. The decline of 123 ft was caused by the overdraft during the 7 years, 1961-67. In the ensuing 9 years the water level rose 266 ft, but in only 8 months in 1977 the increased pumping lowered the water level 165 ft which is 62 percent of the rise in just 7 percent of the time or 1.3 times the decline in the 7 years prior to the importation of water. Still another indication of drought condi- tions is a deterioration of water quality. Both surface water and ground water are subject to changes in quality when new stresses are applied to a water system during a drought, and in most cases the change is for the worse. Reduced freshwater flow into an estuary usually allows the saltwater to encroach far- ther than normal into the estuary or even into the tidal reaches of the tributary streams. The Sacramento-San Joaquin Delta in Cali- fornia was one of the areas that had problems because of saltwater encroachment. In some coastal areas, freshwater is in- jected into wells near the ocean to establish a hydraulic gradient that will prevent saltwater intrusion into the aquifer. During a drought, there may not be enough freshwater available to maintain the freshwater-saltwater interface at an acceptable position, and salinity prob- lems might develop. Lower streamflows during a drought usually mean higher water temperatures with adverse effects on fish and greater aquatic growths that cause increased eutrophication of water bodies. Special releases and spills at dams on the Columbia River were made in 1977 to help preserve the fishery resource during the mi- gration period. The National Oceanic and Atmospheric Administration, Environmental Data Service, uses a drought index developed by W. C. Palm- er (1965) to classify drought severity. Briefly, the method is directed at a quantitative 1978 assessment of periods of prolonged meteoro- logical anomalies by considering antecedent precipitation, the precipitation during a given period, and the duration and magnitude of the abnormal moisture deficiency. The difference