Seasonal Streamflow Conditions and Historic Droughts

June 30, 1999

Streamflows currently are well below the normal range of flows expected for this time of year across Maryland and Delaware. In most years, highest streamflows occur during the winter months, decreasing through the spring and summer, with lowest streamflows occurring during the early fall months. Summer flow in streams is normally sustained by ground-water outflow that was recharged by precipitation in the previous seasons.

Summertime streamflows during 1998 were generally sustained in Maryland and Delaware due to above average rainfall in the previous winter and spring. In fact, winter rainfalls in the two states were among the highest in the Northeast, being 150% and 137% of normal, respectively, according to data from the Northeast Climate Center, However, summer rainfalls in 1998 were only 71% of normal in both states, and by late fall flow volumes in many streams had fallen to the lowest 20% of historical flow and in some cases to the lowest 10%. Streamflows generally remained in the below normal range during the winter of 1999 because precipitation was insufficient to fully recharge the ground-water system. During base-flow conditions (non-storm runoff), streams had about one-third of the flow during the winter of 1999 than they had during the winter of 1998. The already depleted ground-water storage conditions combined with less than normal precipitation during the past 3 months has resulted in continued low streamflows. June flows in many streams are at or near typical annual low flow values and streamflows are expected to continue to decline through the summer, unless sufficient rainfall occurs to reverse the projected trend.

There have been five major droughts since the early 1900's that have affected all or significant parts of the Maryland, Delaware, and D.C. areas. The drought of 1930-1932 was probably the most severe agricultural drought ever recorded in Maryland and D.C., with a recurrence interval estimated as greater than 25 years. That same drought persisted through 1934 in Delaware. The Maryland and D.C. droughts of 1953-1956, 1980-1983, and 1984-1988 and the Delaware droughts of 1953-1957, 1979-1983, and 1984-1988 were less severe. The drought of 1958-1971 in Maryland and D.C. and 1961-1971 in Delaware was also estimated as having a recurrence interval of greater than 25 years, but is particularly notable for its longevity and for its sustained impact of decreasing streamflows throughout the region. More information on historical droughts is available at http://md.water.usgs.gov/publications/wsp-2375/md-dc.

In Maryland and Delaware, current streamflow conditions are generally less than the minimum June flows observed during the 1980-1982 drought. Real-time streamflow data are not available for the District of Columbia, but streamflow conditions there closely follow those in central Maryland. Streamflow conditions are at or near minimum June flows observed during the 1938-1942 and 1962-1971 droughts. Streamflows have not declined to levels observed in the 1930-1932 drought.