INTENSIVE MONITORING OF SPRING DISCHARGE AND WETLAND HYDROLOGY TO UNDERSTAND AQUIFER DYNAMICS AND THE CONSEQUENCES FOR EXCESSIVE GROUND-WATER WITHDRAWAL

In response to increasing pressures on water resources in Utah’s sparsely populated West Desert, the UGS has initiated a long-term program to monitor ground-water resources within Snake Valley, Utah. This program includes monitoring of water-table levels in carbonate and basin-fill aquifers, stream-flow and spring-discharge rates, and water levels in spring-fed wetlands. Our goals are to better understand the inter-annual variability and long-term trends of water table elevations, and the linkages between ground-water discharge and wetland structure and function. While springs and spring-fed wetlands are common in many desert valleys, the surface-water component of these systems is terminal in the West Desert, and the size of spring-fed wetlands is controlled by ground-water discharge rates and the density of springs within the valley. Desert spring-fed wetlands serve as “critical habitat” for several threatened species in Snake Valley, which may be sensitive to reductions in ground-water discharge due to excessive withdrawals.

Here we report on the first full year of spring discharge and wetland water table elevation measurements. Spring-discharge rates are measured as near as possible to the spring orifices using flumes, weirs, and flow meters, and data are recorded at 15-minute intervals. Seven separate spring-fed wetland complexes are instrumented with 60 piezometers that record water levels at 1-hour intervals. Interestingly, there was little natural variation in spring-discharge rates on an annual basis, suggesting that the spring water sources are derived from regional carbonate aquifers rather than local ground-water systems. In contrast, the hydrographs of dominant wetland types displayed up to 3 feet (1.0 m) of change in water-table levels between winter and late summer. While water availability is a principal factor controlling the distribution of different wetland types, these results suggest that both climatic factors and evapotranspiration through wetland vegetation play an important role in the hydrologic variability of these systems. Ongoing work seeks to quantify water requirements for distinct vegetation types in these desert systems, which may assist in predicting the potential consequences of water-table decline on a regional basis.