Seasonal Streamflow Conditions and Historic Droughts in Maryland and the District of Columbia

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.

The 1930-32 drought probably was the most severe agriculture drought ever recorded in Maryland and the District of Columbia. Rainfall during that period was about 40 percent less than average. The year 1930 was the driest year since 1869. Crop losses for 1930 were estimated at $40 million (U.S. Weather Bureau, 1930, v. 35, no.13).

The 1953-56 drought affected almost all of Maryland and the District of Columbia. Drought recurrence intervals exceeded 25 years for those areas of Maryland west of Baltimore. For the remaining parts of Maryland and the District of Columbia, the drought had recurrence intervals of 10-25 years, except for the area north and east of Baltimore where recurrence intervals were less than 10 years.

From 1958 through 1971, a regional drought having recurrence intervals greater than 25 years caused streamflow deficiencies throughout Maryland and the District of Columbia. This drought persisted the longest of any drought since 1930 and was the most severe in terms of annual departure from average streamflow. Streamflow records for the Patuxent River near Unity, Md., indicate a negative departure from average annual streamflow nearly each year of the drought. Even though yearly rainfall totals were less than the long-term yearly average, rainfall was sufficient to prevent major agricultural losses. By 1966, streamflow in the Potomac River--the major source of water supply for Washington, D.C.--had declined to record lows. Withdrawals accounted for 80 percent of the available river flow. Population projections for the Washington metropolitan area indicated that, if a drought of similar magnitude were to recur, river flows would be insufficient to meet human needs and maintain aquatic life. As a result, the District of Columbia and surrounding municipalities in Maryland and Virginia signed water-use agreements limiting the quantity of water that can be withdrawn. In addition, water-supply structures were built in the Potomac River basin to augment streamflow during periods of large consumptive use.

The 1980-83 drought affected all but the westernmost part of Maryland. Recurrence interval of the drought was about 10 to 25 years throughout the affected area. The extent to which streamflow decreased during this drought is similar to that during the 1958-71 drought. No major agricultural drought developed, and water supplies were adequate for public supply use.

Another drought began in the fall of 1984 and continued through the summer of 1988. The drought affected Maryland east and south of Frederick and the District of Columbia. The recurrence interval for this drought ranged from 10 to 25 years. Many counties in Maryland were declared disaster areas because of large agricultural losses during the summers of 1986 and 1987. Losses for 1986, 1987, and 1988 were estimated to be $89, $113, and $100 million, respectively. Water supplies for municipalities did not become critically low, although several towns restricted water use during each summer.

The most recent drought occurred in 1999 following an extended period of 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. The drought was most severe and prolonged in western Maryland, which did not benefit from the welcome rainfall of Hurricanes Dennis and Floyd in August and September 1999. The effects of the 1999 drought may be still be lingering into the present.

Droughts have occurred about once every 10 years since 1930 but differed in severity and duration. Annual departure generally was most severe at the end of the 1958-71 drought. Greater than average streamflow in Maryland and the District of Columbia has alternated with short periods of less than average streamflow.

SELECTED REFERENCES

National Oceanic and Atmospheric Administration, 1970-88

Climatological data, annual summary, Maryland and Delaware: Asheville, N.C., National Climatic Data Center (variously paginated).

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

1930-39, Climatological data, Maryland and Delaware sections: Department of Agriculture (variously paginated).