Water Budget
The hydroperiod in the Great Dismal Swamp is very different from what people have assumed about swamps. The Great Dismal Swamp doesn't have standing water year round. The vegetation wouldn't be able survive in such conditions if that were the case. Historically the Great Dismal's Swamp hydroperiod was heavily influenced by the change in seasons (FWS,2006).
The Great Dismal Swamp's water budget is shaped by natural input-output events. Before the Great Dismal Swamp hydrology was altered, water entered the swamp through a number of different pathways.
The Great Dismal Swamp also had a number of different pathways that water left.
The Great Dismal Swamp's water budget is shaped by natural input-output events. Before the Great Dismal Swamp hydrology was altered, water entered the swamp through a number of different pathways.
- Precipitation is the major source of water and was highest during the summer months, accounting for 127 cm/yr.
- Surface inflow is the next source of water input, where 89% of surface water inflow occurred during November- April, where 22 billion gallons of water came into the swamp in the form of sheet flow and streams (FWS, 2006).
- Groundwater was the last source of input into the swamp. The swamp received inflow from drainage basins and was stored in the shallow aquifer that was under the swamp. These inputs especially precipitation influenced the water level changes in the peat on a rapid basis (Phipps et al., 1979).
The Great Dismal Swamp also had a number of different pathways that water left.
- Evapotranspiration is the largest portion of water removal. The annual evapotranspiration loss was 39 inches and caused a lowering of the water levels in the swamp throughout the summer.
- Surface water runoff was also another source of outflow. The Northwest Pasquotank, Elizabeth River, and Shingle Creek were the sites of drainage. During the winter, this discharge came in the form of sheet flow and even when there was low flow periods, the water still followed the random channels cut by the high flow (FWS, 2006).
- Groundwater is the last water output. If the upper layer that restricted the shallow aquifer was gone, then the ground water would well up into the peat and would be released by the peat by evapotranspiration. Groundwater was released directly into the lake by seepage and also by surface flow through uncontrolled outlet channels where the aquifer had been broken opened (FWS, 2006).
Below (Figure 1) illustrates what the water budget looked like before human activities, where P= precipitation, I= interception, Pn= total precipitation, ET= evapotranspiration, Si = Surface inflow, Gi=groundwater inflow, So=surface outflow, and Go= groundwater outflow . Although the swamp had output flows, some of that water was stored underground and in the peat.The net change in volume per unit time which had a value of about +74 shows that a good amount of water was stored in the swamp.
Figure 1- Historical Water budget in the Great Dismal Swamp
- The greatest impact on the hydrologic cycle in the Great Dismal Swamp has and still is the Dismal Swamp Canal. The Dismal Swamp Canal divided the swamp from north to south and formed a barrier that hinders the eastward flow of surface water. Therefore the western dike that was built stores all that water in the western region. Basically the land to the east of the canal slowly dried up, while the west side had increased wetness as well as flooding (Phipps et al., 1979).
- The Dismal Swamp Company believed that the groundwater seepage would have sustained the water level in the canal, but that was not the case. As a result Lake Drummond was used as a water resource and the Feeder Ditch was built between the lake and the Dismal Swamp Canal.It's purpose was to channel the surface water from the swamp which would end up in the Dismal Swamp Canal. In addition to the building of ditches and canals, roadways were constructed, which also acted as a barrier to the flow of surface water across the ditches. The wide sheet of water that use to flow eastward across the swamp, as well as north of the lake, no longer is present. All of the water is intercepted by the ditch system and then is channelized into the lake (Phipps et al, 1979).
- It's really difficult to understand exactly what is happening with the hydroperiod in the Great Dismal Swamp, however general ideas can be drawn. The amount of surface inflow and it's rate has increased drastically. This is mainly due to upland uses. The aquifer's water recharge is now caught and diverted into the refuge as surface water. This increase surface water volume explains the higher surface water levels during storm events and winter. It also may be responsible for the low water levels in the swamp during the dry summer periods (FWS, 2006).
- Above (Figure 2) illustrates what the water budget currently looks like as a result of human activities. The swamp is no longer storing water underground or in the peat and the dryness is what aids these wildfires. It is either being released out of the swamp or it goes to waste because the lake can only hold but so much water. The net change in volume per unit time of -43 indicates that water storage has really decreased.
Figure 2- Current Water budget in the Great Dismal Swamp