DYNAMICS OF WATER-TABLE FLUCTUATIONS IN AN UPLAND BETWEEN 2 PRAIRIE-POTHOLE WETLANDS IN NORTH-DAKOTA

Data from a string of instrumented wells located on an upland of 55 m width between two wetlands In central North Dakota, USA, indicated fre quent changes in water-table configuration following wet and dry perio ds during 5 years of investigation, A seasonal wetland is situated abo ut 1.5 m higher than a nearby semipermanent wetland, suggesting an ave rage ground water-table gradient of 0.02. However, water had the poten tial to flow as ground water from the upper to the lower wetland durin g only a few instances, A water-table trough adjacent to the lower sem ipermanent wetland was the most common water-table configuration durin g the first 4 years of the study, but it is likely that severe drought during those years contributed to the longevity and extent of the wat er-table trough, Water-table mounds that formed In response to rainfal l events caused reversals of direction of flow that frequently modifie d the more dominant water-table trough during the severe drought. Rapi d and large water-table rise to near land surface in response to inten se rainfall was aided by the thick capillary fringe. One of the wettes t summers on record ended the severe drought during the last year of t he study, and caused a larger-scale water-table mound to form between the two wetlands. The mound was short in duration because it was overw helmed by rising stage of the higher seasonal wetland which spilled in to the lower wetland. Evapotranspiration was responsible for generatin g the water-table trough that formed between the two wetlands, Estimat ion of evapotranspiration based on diurnal fluctuations in wells yield ed rates that averaged 3-5 mm day(-1). On many occasions water levels in wells closer to the semipermanent wetland indicated a direction of Bow that was different from the direction indicated by water levels in wells farther from the wetland. Misinterpretation of direction and ma gnitude of gradients between ground water and wetlands could result fr om poorly placed or too few observation wells, and also from infrequen t measurement of water levels in wells.