Hydrology
The Great Dismal Swamp is sustained from several water sources.
- Precipitation is the dominant source of water within the swamp with an annual rainfall of 100-125 cm in the Norfolk area. Water levels in the peat change rapidly in response to precipitation.
- The swamp also receives water as inflow from the Suffolk Scarp and from numerous drainage basins west of the Suffolk Scarp.This provides a total catchment area estimated to be 300 km2 (Phipps et al.,1979). Discharge from the area is from surface flow and rainfall which feeds the Great Dismal Swamp.
- Groundwater also discharges into the swamp from the shallow aquifer and gathers on the escarpment.Throughout the early spring and late winter, seepage of groundwater has been observed from numerous places beside the base of the scarp. Before the peat in the Great Dismal Swamp formed, the drainage basins were believed to have been a part of the western extensions of the drainage pattern. The buildup of peat eventually changed the pattern for drainage east of the scarp where streams may have existed (Phipps et al.,1979).
Historically, the Great Dismal Swamp's annual hydrologic (water) cycle occurred where the water level changed throughout the year and usually seasonally.
- Fall: The Great Dismal Swamp was very dry and the water table was low. Little water movement occurred downstream and most of the water was lost by evapotranspiration.
- Winter: The opposite occurred in the Great Dismal Swamp. Basically, the amount of rainfall escalated and the water table rose or even reached the surface (FWS, 2006). Evapotranspiration, and temperature declined while, sheetflow or overflow into the Swamp on the east and south sides took place.
- Spring: The swamp would have been flooded to its maximum capacity with minimum lateral water movement. By that time the temperature would have risen again and so would the evapotranspiration rate. Plants would begin to grow and the water table would start to fall beneath the surface. This enabled the vegetation to obtain oxygen because the soils were able to aerate.
- Summer: the swamp would receive additional water by precipitation. Even though the water level changed throughout the year, historically a large amount of the water would be held up in the peat soils. (FWS, 2006).
- Over the past century, the Great Dismal Swamp has been changed by human activities. This is clearly seen by the type of vegetation that succeeded over the years. The plants that were once dominant in the Great Dismal are no longer there and are currently being replaced by other plant species. The changes to the Great Dismal Swamp's hydrology is not fully understood, however certain facts can be drawn.
Currently in today's water cycle, the Great Dismal Swamp is experiencing increased volumes of surface water during the winter or storm events and reduced volumes of water to recharge the swamp during the summer.
- Basically, what is known is that the quantity and rate of surface inflow that gets into the Great Dismal Swamp National Wildlife Refuge has risen as a result of upland use practices, which includes road building, field tilling and housing along the Suffolk Scarp (FWS, 2006). The development of 158 miles of ditches, canals and roadways are the major factors that have altered the Great Dismal Swamp. These structures serve as dams that block overland water flow. The alterations have also removed the layer of the artesian aquifer which was a buffer that reduced the evapotranspiration rate during the spring and summer (FWS, 2006). As a result the peat drys up and wildfires occur more often.