Progress:In year 3 of a 3-year grant. Partners Nebraska Environmental Trust, Lower Loup NRD and USGS
Understanding the spatial and temporal characteristics of groundwater/surface-water interaction is critical to the integrated management of groundwater and surface-water resources. Streams in the Loup River basin begin in the Sand Hills and flow easterly through the dissected silt plains where the primary land-use type changes from undeveloped rangeland to irrigated agriculture. The influence of land cover and land use on focused groundwater discharge patterns is not well understood and is a data gap that needs to be addressed. Streams typically warm and cool seasonally while groundwater typically remains at a constant temperature year-round. Temperature differences between the groundwater and surface-water are greatest during the summer and winter months. By mapping stream surface temperatures, it is possible to identify and delineate areas of focused groundwater discharge. A growing body of work suggests that the use of airborne thermal infrared imagery is useful in assessing groundwater/surface-water interaction in streams and lakes. There have been advancements in data collection and processing that have improved the precision and resolution of thermal data since Rundquist and others (1985) studied groundwater discharge patterns in the Crescent Lake National Wildlife Refuge area of Nebraska. Current technology can resolve temperature differences to within 0.1 degrees Celsius in grids as small as one square foot. Digital georeferenced airborne thermal infrared imagery will be interpreted with a geographic information system (GIS) and will allow for an assessment of stream/aquifer interaction for stream reaches tens of miles long. Airborne thermal data will be verified with continuous water-temperature logging at existing stream-gaging stations and with self-logging thermistors. Mapped thermal anomalies will be investigated with a variety of techniques including water-temperature, potentiomanometer, and seepage meter measurements and ground-based thermal photography.
The temporal variability of groundwater discharge will be investigated through the use of coupled real-time streambank observation wells and stream gages. Coupled groundwater/surface-water gages consist of a streamflow gaging station coupled with an observation well completed below the elevation of the streambed and instrumented with a water-level recorder. The comparison of continuous groundwater levels and stream stage can allow for the calculation of hydraulic gradient which indicates whether a stream is gaining or losing near the gaging location. Continuous water temperature is measured both in the stream and the shallow observation well providing further support to groundwater/surface-water interaction trends. The information provided by a network of coupled groundwater/surface-water gages allows scientists and engineers to analyze streamflow and groundwater discharge patterns, both temporally and spatially. This will provide water managers a better understanding of the impacts of irrigation and climate on instream flows, which agriculture, fish and wildlife, recreation, and public and domestic supplies depend upon.
The Loup River NRDs plan to cooperate with the U.S. Geological Survey (USGS) to coordinate collection of airborne thermal infrared data. Additional data will be collected to ground-truth the thermal data and to investigate anomalies that may indicate focused groundwater discharge. Four stream reaches, totaling approximately 320 river miles, have been identified by the Loup River NRDs as priority streams where additional groundwater/surface-water interaction information is needed. The airborne thermal data collection and subsequent ground-truthing will be carried out over two years in order to ensure the work is done efficiently and effectively. Within the four-targeted stream reaches four additional coupled groundwater/surface-water gages will be instrumented at existing stream-gaging stations. The coupled gages will further enhance the USGS national stream-gaging network, which provides a continuous, well-documented, archived, and unbiased source of reliable and consistent water data. Because of the nationally consistent, prescribed standards by which the data are collected and processed, the data from individual stations are commonly used for purposes beyond its original intent. Other possible uses include providing data for evaluating groundwater and surface-water interaction, and providing baseline information allowing managers to assess long-term changes in the hydrologic cycle.