Seasonal Streamflow Conditions and Historic Droughts in Delaware
In a typical year, highest streamflows occur during the winter months, decreasing through the spring and summer, with lowest streamflows occurring during the fall months. When streamflow is much below normal at any time of year, a drought may be occurring. The effects of most droughts are most obvious in middle-to-late summer, when streamflow is usually low and water demand from evapotranspiration and water use is high.
A simple definition of drought, such as "extended period of dry weather," is an easily understood concept. Droughts, however, differ greatly in their extent, duration, and severity; these differences make quantitative analyses and comparisons among droughts difficult. A drought can affect many States and last 10-15 years, as during the 1960's. However, a drought affecting one or two counties and lasting 3-6 months may be more devastating locally.
In Delaware, water shortages resulting from droughts commonly are most severe in summer or early fall when streamflow and ground-water levels are lowest and demand for water is greatest. However, droughts that become apparent during the growing season may have developed from an antecedent precipitation deficiency; these are delayed or hidden droughts (Parker and others, 1964, p. 19).
Because relatively few data on historical streamflow in Delaware are available, accurate estimation of recurrence intervals for hydrologic droughts before the early 1940's is not possible. However, by using precipitation data and limited streamflow information. Hoyt (1936, p. 66) characterized drought conditions in 1930 as the most extensive since 1894 in humid parts of the United States. Drought conditions persisted through 1934, and effects were manifested chiefly as low streamflow and decreased crop yields. Economic losses probably were greater than those sustained during any previous drought, as use and demand for water had increased considerably by the early 1930's. By the end of summer 1930, several government and private agencies had implemented relief measures in many drought-stricken communities.
The drought of 1953-57 had a substantial effect on water supplies in Delaware. Drought conditions developed statewide in early summer 1953 and were alleviated in the southern part of the State by fall 1956. However, drought conditions persisted in northern Delaware until fall 1957. The drought materially affected agricultural activities, as soil-moisture deficiencies resulted in decreased crop yields. Except for the temporary relief provided by Hurricanes Connie and Diane, flow in streams statewide was less than average during the period. In 1954, average annual streamflow of White Clay Creek near Newark was about 38 ft3/s less than the long-term average flow. Streamflow increased by 1957 but was still about 24 ft3/s less than the long-term average. Recurrence intervals of the 1953-57 drought range from 10 to 25 years.
From 1961 through 1971, the longest and most severe-drought in the history of the region profoundly affected water supplies and agricultural activities in the northeastern United States. Although most water-supply facilities kept pace with demand, many communities utilized emergency supplies and accelerated construction of supplementary facilities. In 1965, however, more than 100 public water supplies in the Northeast were critically short of water or were faced with serious, impending water-supply problems. The President declared a limited national emergency in parts of Delaware, New Jersey, Pennsylvania, and New York, and directed all Federal agencies to assist communities with critical water-supply problems in the drought-stricken area. The U.S. Geological Survey assisted by locating, identifying, and estimating the capacity of sources of emergency supply for the affected communities. Although the accessibility and desirability of emergency supplies differed widely, suitable supplies were located in all instances (Barksdale and others, 1966).
Drought developed statewide in summer 1961 and persisted in southern Delaware until summer 1967, in the northern part of the State until spring 1970, and in central Delaware until summer 1971. In 1966, average annual streamflow at sites 1 and 3-6 (fig. 4) reached record minimum levels. Except for temporary increases caused by the August 1967 floods, streamflows throughout the State remained below average during the period. Minimum daily flows of record were observed in many streams, and, in several instances, streamflow ceased entirely. Recurrence intervals of the 1961-71 droughts ranged from 10 to more than 25 years.
During the drought of 1979-83, less than average streamflow materially affected water supplies and agricultural activities state-wide. Water-rationing programs were implemented at times to conserve dwindling supplies, and crop yields decreased substantially. Drought conditions developed in northern Delaware in winter 1979, progressed to central and southern parts of the State by summer 1980, and persisted statewide until early 1983. In 1981, average annual streamflow of Red Clay Creek at Wooddale was about 31 ft3/s less than the long-term average flow. This is the largest deficiency in steamflow observed since 1944, when recordkeeping began at the site. Recurrence intervals of the 1979-83 droughts ranged from 10 to 25 years.
Like the 1979-83 drought, the drought of 1984-88 had a substantial effect on water supplies and agricultural activities throughout Delaware. In 1988, temporary restrictions on nonessential water uses such as vehicle washing and lawn watering were implemented to conserve diminishing supplies in northern Delaware. Agricultural yields decreased considerably, as many crops withered when soil moisture declined below the wilting point. Streamflow was much less than average in 1985-86 in northern Delaware, and in 1985 and 1988 in the central and southern parts of the State. Recurrence intervals of the 1984-88 drought ranged from 10 to 25 years statewide.
The most recent drought occurred in 1999 following an extended periodof low rainfall and high evapotranspiration during the summer and fall of 1998. A recurrence interval has not been estimated, but the magnitude and extent of the drought approached that of the 1960's. Relief came from the welcome rainfall of Hurricanes Dennis and Floyd in August and September 1999. However, the effects of the 1999 drought may be still be lingering into the present.
Selected References
- Barksdale, H.C., O'Bryan, Deric, and Schneider, W.J., 1966
- Effects of drought on water resources in the Northeast: U.S. Geological Survey Hydrologic Investigations Atlas HA-243.
- Hoyt, J.C., 1936
- Droughts of 1930-34: U.,S. Geological Survey Water-Supply Paper 680, 106p.
- National Oceanic and Atmospheric Administration, 1970-88
- Climatological data, annual summary, Maryland and Delaware: Asheville, N.C., National Climatic Data Center (variously paginated).
- Parker, G.G., Hely, A.G., Keighton, W.B., and others, 1964
- Water resources of the Delaware River basin: U.S. Geological Survey Professional Paper 381, 200 p.
- U.S. Geological Survey
- National Water Summary 1988-89--Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, p. 319-326.
- U.S. Weather Bureau
- 1947-69, Climatological data, Maryland and Delaware: Department of Commerce (variously paginated).
For additional information:
Chief, Delaware Office
U.S. Geological Survey
1289 McD Drive
Dover, DE 19901
Phone : (302)734-2506
Email: info@md.water.usgs.gov
