Evaluating Hydroperiod Response in the Rainwater Basin Wetlands of South-Central Nebraska

A collection of wetlands in south-central Nebraska make up a region called the Rainwater Basin. This basin contains closed-basin wetlands formed in loess. The wetlands receive water from precipitation and irrigation runoff. Since the early 1900s, wetland area in the basin has decreased dramatically due to intensive agriculture which either altered or removed the wetlands. The Rainwater Basin wetlands provide many ecological services and thus, should be preserved, but are most noted for the resting, breeding, and feeding habitat they provide for millions of migratory birds that is not provided elsewhere in this region along the continental flyway. Given the limited research on some of the physical, chemical, and biological processes that occur within these wetlands, research needed to be conducted on how these wetlands affect groundwater quality of the High Plains Aquifer, on how the restoration practice of sediment removal impacted groundwater quality, and on the effect of sedimentation and hydroperiod on plant and wetland bird communities. In an effort to understand these research goals, this study attempted to define the hydrology of individual, representative wetlands within the basin. The specific goal of this study was to determine and understand seasonal wetland hydroperiods and to determine the magnitude of evapotranspiration (ET) and infiltration and their impact on water loss from the selected sites. Three sites, Lindau WPA, Moger (North) WPA, and Griess WPA, were investigated to better understand these processes. Hydroperiods were determined by stilling well and topographical survey data. Shallow drive-point wells provided information on water movement within the wetland sediments. ET was calculated using the Bowen Ratio Energy Budget (BREB) method. Precipitation was determined by tipping bucket rain gages and with data provided by the High Plains Regional Climate Center (HPRCC). Infiltration was modeled using a water balance approach during periods when precipitation was not occurring and data from surface water storage volumes and ET could be used. Study results show that surface water volumes are highly dependent on the magnitude of precipitation events and the soil water content. In addition, dry, desiccated soils can reduce surface storage during precipitation events because of rapid infiltration into fractures. Fractures can subsequently close after being wetted reducing infiltration rates. ET magnitude was dependent on available energy to a site, but it was also dependent on the exposed surface area of the wetland. Wetlands with contained water volumes and small exposed surface areas such as Moger (North) WPA lost less water to ET than the large exposed water surface area of Lindau WPA. However, with the contained volume and higher surface water head pressures, Moger (North) WPA had larger infiltration rates than Lindau WPA. Overall, based on the modeling, infiltration removed more water by volume from the wetland surface storage than did ET