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Waterfowl Production on the Woodworth Station
in South-central North Dakota, 1965-1981

Wetland Water Budget

Eisenlohr and Sloan (1968), Shjeflo (1968), Eisenlohr at al. (1972), Sloan (1972), and Winter and Carr (1980) present descriptions and discussions of factors influencing wetland (pothole) water budgets in this area. Reference lists supplement these presentations. The following summary of a generalized pothole water budget was compiled from these papers, many of which were based on data gathered on or near the WSA.

Sources of water for a pothole are precipitation on its surface, overland flow or runoff, and underground or seepage inflow.

Rainfall on wetlands and their basins is the greatest direct addition of water in the WSA. The net amount of direct winter precipitation varies from pothole to pothole because of wind transportation of snow. Potholes with tall, emergent vegetation tend to accumulate drifting snow.

Snowmelt water flowing over frozen ground is probably the largest source of runoff water to prairie potholes because runoff adds to pothole water only when water can flow over the ground without being absorbed; the ground must be either frozen or saturated. Size of the watershed into a basin also affects the amount of runoff to pothole basins.

Seepage inflow of groundwater depends on configuration of the adjacent water table and hydraulic conductivity of the glacial drift. Movement of groundwater to and from potholes in glacial till is relatively slow but is significant because it increases or decreases the rate of water loss and water levels in potholes. Winter and Carr (1980) showed that a complex interrelation seems to occur between wetlands and groundwater. In their study, some wetlands seemed to recharge groundwater, some were flow-through types where groundwater enters one side and surface water seeps into the ground on the other side, and some were discharge points for ground water. They also found that water flowage in or out of wetlands varied throughout the year.

Natural loss of water from potholes occurs by transpired evaporation, surface overflow, and seepage outflow of groundwater. Water loss also results from drainage, seepage to irrigation wells, and pumping and siphoning.

Evapotranspiration of water directly from potholes is generally the cause of most water loss. Average seasonal evapotranspiration losses for both clear and vegetated potholes was found to be about 0.6 m for the season May-October (Shjeflo 1968). Presence of emergent aquatic vegetation increases the evapotranspiration rate. Average annual lake evaporation for the area is about 81 cm(Kohler et al. 1959) and occurs during the ice-free period, April-September. Evaporation is greatest from potholes free of emergent vegetation (Eisenlohr and Sloan 1968).

Seepage outflow of groundwater, like groundwater inflow, depends on configuration of the adjacent water table and hydraulic conductivity of the glacial drift.

Overflow loss of pothole water occurs when the water supply exceeds the pothole basin holding capaciy. Overflow loss of water is usually relative to an individual pothole because other potholes lower in the watershed may absorb the overflow from higher basins. Much of the water supply to bigger potholes is dependent on overflow water from smaller potholes. Such overflow is usually of short duration for most potholes, and many large basin potholes do not overflow. Frequency of overflow is related to the amount of precipitation and spring runoff.

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