aquifers contributing baseflow to these

streams are contaminated with low concentrations of atrazine and are a nonpoint source for contamination of streams during baseflow periods. This result also underscores the need for further study of the exchange of water and contaminants between surface- and ground-water systems.

Atrazine was the most frequently detected of the 11 herbicides and 2 metabolites measured during each of the three sampling periods (see table). The second most frequently detected compound was the atrazine metabolite deethylatrazine. This compound is not applied as an herbicide but is largely derived from the breakdown of atrazine by soil microorganisms. Notably, little information is available at this time about the toxicity and health effects of deethylatrazine or other atrazine metabolites. The ratio of deethylatrazine to the parent compound, atrazine, is lowest in late spring and early summer after herbicide application and highest in fall when the principal source of water in streams is from ground water. This ratio may prove to be useful in determining sources and mechanisms for atrazine contamination of surface and ground water.

USGS hydrologists are continuing research on the occurrence, distribution, and fate of atrazine and other agricultural chemicals in water resources of the Midwest. Current research includes studies of the temporal distribution of several herbicides, including atrazine and its metabolites, in spring and summer storm runoff; the occurrence, distribution, and deposition rates of atrazine in atmospheric wet deposition; and the regional distribution of atrazine and other agricultural chemicals in ground water.

Resources (DWR) personnel classified the 1987, 1988, and 1990 water years as critically dry.

A critically dry year is that which occurs about 1 year in 10 in the record of a particular region. Runoff during the 1989 water year was below average but was sufficient to prevent the year from being classified as critically dry. As late as March (the last month of California's winter storm season), watersupply forecasts showed a greater than 50 percent chance that 1989 would be a critically dry year. A series of March storms, however, brought plentiful rain and snow over the northern half of the State and temporarily moderated the severity of the drought.

Historic Perspective

Droughts are a recurring feature of California's climate. To help put the current drought in perspective, hydrologists from the USGS water resources office in Sacramento, in cooperation with DWR personnel, examined streamflow records from the Sacramento River basin and from unregulated long-term stations located throughout California. When considered individually, no single year of the current drought ranks as extreme. When considered as a whole, however, the drought aspect of the last 4 years is more evident.

For the Sacramento River basin, two consecutive critically dry years have occurred only twice before, from 1933 to 1934 and from 1976 to 1977. Three consecutive critically dry years are unprecedented in the historical record, and three critically dry years during a 4-year period have occurred only once before, from 1931 to 1934, during the Dust Bowl Era. The current 4-year drought has three critically dry years, two of which are consecutive (1987-88, 1990). Even though the drought is less severe in southern California, that area's dependence on transfers of water from the northern part of the State gives the current drought a statewide importance.

Prior to the drought of 1987–90, the droughts of 1928-37 and 1976–77 were considered to be the most severe in the State's history. Comparing the current drought to these historic droughts provides perspective, but comparisons are difficult because the droughts have different durations. The severity of multiyear droughts can be measured by the accumulated deficit in streamflow (departure below the mean) during the drought. Periods of drought of the same duration can be directly compared by ranking their accumulated deficits.

Ranked by accumulated deficits, the droughts of 1928-37, 1976-77, and 1987-90 (in the central part of the State) are equivalent in severity. The drought of 1928-37 is

the State's most severe drought, however, when the duration, the magnitude and areal extent of accumulated deficits, and the severity of individual years are taken into account. The current drought will have to continue for several more years before it would rival the drought of 1928-37.

Impact of the Drought

Because of the recurrent nature of droughts in California, considerable effort has been made to lessen their effects. Water supplies have been developed and are managed to provide dependable supplies to major agricultural areas and population centers and, more recently, for the protection of environmental quality. Isolated dry years and the first year of protracted droughts have only limited. adverse effects on human activities. The most seriously affected areas are wildlands and nonirrigated agricultural lands. Water-supply problems were minimal in 1987, however, because surface-water storage was carried over from 1986.

During 1988 and early 1989, water shortages affected about one-third of California's population and more than 40 percent of the State's irrigated agriculture. Many areas had insufficient rainfall for dry-farmed crops, and ranchers from 42 counties were accepted into Federal emergency feed programs. Drought emergencies were declared in 14 counties. Many urban areas instituted mandatory or voluntary water conservation measures.

Where available, ground water was used to compensate for deficiencies in surface-water supplies. In general, ground-water supplies were adequate, but water shortages occurred in localized areas of excessive drawdown and in some upland and coastal areas where ground-water reservoirs are small.

In 1989, managers in both of the State's two major water projects, the State Water Project and the Federal Central Valley Project, announced anticipated reductions in water deliveries of as much as 50 percent. The wet weather in March 1989 brought relief to many water users, however, and permitted full delivery of agricultural water supplies. Drought restrictions were eventually lifted in most areas of the State. Water restrictions continued along the central coast, which did not benefit from the March rains and is not a part of any of the State's large water projects.

In 1990, no relief from spring rains was forthcoming. Deliveries of State and Federal project water are reduced as much as 50 percent for agricultural customers, and some municipal and industrial contractors have lesser reductions. Only once before (1977) in the history of the water projects were such reductions necessary.

In response to the reductions in surface water, farmers are modifying irrigation practices, turning to ground-water supplies (often an expensive alternative), taking land out of production, and changing crop rotation. Reduction in irrigation water causes salt to

accumulate in the soil and has a long-term impact on agricultural production. Reduced agricultural production causes substantial economic losses in many areas. Agricultural advisors are warning orchard crop growers that attempting to stretch water supplies by inadequate irrigation may kill trees.

Most urban areas of the State are instituting drought restrictions. Southern California was little affected during the first 3 years of the drought because of diverse sources of water supply-northern California, the eastern Sierra Nevada, and the Colorado River basin. In 1990, however, this area is also faced with possible shortages.

The response of urban water users to the drought of 1976-77 demonstrated that major reductions in urban water use are possible. Conservation measures include voluntary and mandatory rationing, educational programs, the use of water-saving devices in homes, decreased irrigation of landscapes, reuse of industrial water, altered rate (price) structures, and penalties (including the threat of jail sentences) for misuse. Santa Barbara is probably the most adversely affected community in the State, and a strict mandatory water conservation program has been enacted to cut water use by 45 percent. It is now illegal to water lawns in Santa Barbara.

The reduction in streamflow has caused a hydroelectric-power generation decrease, which has forced the use of alternative and more costly sources of power. Because utilities expect hydroelectric-power generation to be 40 percent below normal, rate increases may be necessary to offset reduced production from hydroelectric sources.

As the drought continues, demands on ground-water sources increase as surface supplies in many areas become stressed. The resulting drawdown leads to serious overdraft in some basins. Using the 1976-77 drought as an indicator, ground-water use during the current drought can be expected to increase about 50 percent above the amount used during a normal year. The increased withdrawals in many areas are already causing excessive drawdown, water-quality problems, and seawater intrusion into some coastal basins. The result is that public water supplies in some areas may not meet recommended standards for drinking water; depleted ground water has increased the potential for land subsidence and associated reduction in storage capacity of aquifers because of compaction; and some areas now prohibit drilling new wells and restrict pumping from present wells.

Recreational activities, and the associated economic benefits for many businesses and

communities, are also adversely affected by the drought. Low streamflows and reservoir levels restrict boating and fishing. Extreme fire danger has led to the closing or restricted use of wildlands. The drought has been extremely hard on fish and wildlife. Decreased releases of water downstream from reservoirs and high air temperatures have resulted in a fourth straight year of poor anadromous fish spawn. For some species, the problem is now compounding: reduced populations that hatched during the first year of drought are returning to even worse conditions. Reservoir fisheries are also being adversely affected by low water levels. The flooding of some waterfowl refuges is incomplete, and forage and cover for upland wildlife is reduced.

The drought has had a devastating effect on California's timberlands. Trees weakened by the drought are being killed by insects and disease. In the Lake Tahoe basin, about 20 percent of the trees are already dead, and even more trees are in jeopardy this year.

The danger of forest fires is exacerbated by the presence of this added dry fuel. In Yosemite National Park, California's famed sequoias were sprayed with a fire-retardant to protect them from threat of fire.

Forecast for 1991

DWR forecasters estimate that if runoff is 70 percent of normal next year, there is a 65 percent chance that most of the State's water needs would be met. Carryover storage in reservoirs is well below normal, however, so reservoir storage would not recover from depleted levels. The forecasters also estimate that if runoff is 5 percent above normal, reservoir storage levels would return to normal and that most of the State's water need would be met as well. Because the winter storm season brings most of the State's annual precipitation, it may be spring 1991 before we know if California is in for a fifth straight year of drought or if the dry spell will be broken.

Continuing Evolution in Mapping Technology

By Lindy Mann

The initial cycle of detailed topographic mapping covering the conterminous 48 States, begun more than 50 years ago, was completed by the USGS during fiscal year 1990. Having accomplished this significant milestone, the USGS National Mapping Division is focusing on updating the more than 57,000 maps generated and making them available in digital form. New automated methods for processing spatially related data are streamlining map production techniques as well as revolutionizing the application of this information to land and resource management issues.

The conterminous United States and Hawaii are mapped as quadrangles, each covering 7.5 minutes of latitude and longitude. The 7.5-minute mapping program began in the late 1930's in a cooperative effort with the Tennessee Valley Authority. Most of the 7.5minute quadrangle maps are printed at a scale of 1:24,000 (1 inch on the map represents 2,000 feet on the ground). For a few areas, 1:25,000-scale maps are published in which contours are depicted in meters.

The mainland of Alaska has been mapped at a scale of 1:63,360 (1 inch represents 1 mile). A few areas of the Alaska peninsula and most of the Aleutian Islands are mapped with very general reconnaissance maps because the persistent cloud cover over these areas makes it difficult to acquire the aerial photography needed for map compilation. The final map completed in the first cycle of the national program was the Seneca quadrangle south of Canyon City in Grant County, Oreg.

One of the key components in the evolution of the National Mapping Program is the conversion of the 1:24,000- and 1:25,000-scale topographic maps to digital form. The resulting data will be made available to Federal, State, and public users through the National Digital Cartographic Data Base, which the USGS is assembling as the largest repository of map information on the United States and its territories.