DIURNAL FLUCTUATIONS IN THE SHALLOW WATER TABLE DUE TO EVAPOTRANSPIRATION AND INTERACTIONS WITH GROUNDWATER FLOW PATHS

Interaction between plants and a shallow water table is important for the functioning of ecosystems and biodiversity of wetlands. Groundwater discharge and recharge to and from the shallow aquifer is studied by comparing elevation of water table to the adjacent surface water levels. The study site is adjacent to the Irwin Prairie wetland within a shallow unconfined sand aquifer, Lucas County, Ohio. Groundwater discharge and recharge to and from the shallow aquifer is studied by comparing elevation of water table to the adjacent surface water levels. Diurnal fluctuations of the shallow water table are monitored to calculate evapotranspiration being withdrawn directly from the groundwater (ETg). Along with the regional flow paths to and from the wetland, the rate of evapotranspiration (ET) is a major process affecting shallow water table level adjacent to the wetland. Water levels were measured manually in piezometers (wells IP1 through IP7). Data loggers were installed in wells IP1 and IP5 to measure the water levels for every hour and monitor effects of ETg. Water table level measurements were also used to monitor groundwater recharge, discharge and flow direction. Diurnal fluctuations observed over a dry period between the precipitation events, showed a higher ETg at IP5 than at IP1. The average ETg over a period of three months (May – July) at IP1 is 0.021 inches and 0.043 inches at IP5. IP 5 tended to have a higher ETg as the water table is closer to the surface and more easily accessed by phreatophytes. A plot of water-table elevation as a function of distance from the wetland shows mounding near IP1, indicating the presence of a groundwater divide. Groundwater flows in either direction from groundwater divide to the wetland and to ditch, i.e, groundwater flows from areas of higher head to areas of lower head. During wet periods when water table is mounded, groundwater discharges into the wetland. While in a dry period, wetland recharges or drains to the shallow unconfined aquifer and is observed to be flowing away from the wetland throughout the study site.