Response of Riparian Evapotranspiration to Groundwater Fluctuations

Water management in the western United States is becoming increasingly sophisticated in order to adhere to complicated physical, legal, and environmental constraints. The desire to improve water accounting techniques requires improved tools for quantifying the components of the water budget with the greatest degrees of uncertainty. For example, conveyance losses attributed to seepage to groundwater and riparian evapotranspiration (ET), can potentially play a substantial role in water accounting. More specifically, the research described in this paper was motivated an intense water accounting scheme being undertaken on the Truckee River in Nevada and California. The Truckee River is one of the most intensely regulated systems in the U.S. and the federal, state, and local authorities are entering into a new era of flow management known as the Truckee River Operations Act (TROA). TROA is designed to add flexibility to water management in the basin, but as a result, requires intense water accounting, which will be achieved by the Federal Water Master using a RiverWare model. One of the stipulations of TROA is the quantification of incremental changes in conveyance losses, due to channel seepage and ET, resulting from changes in streamflow and stage. The objective of the research presented here was to determine if there is a detectable change in riparian phreatophytic ET as a function of river stage. Due to the high degree of uncertainty associated with most ET measuring techniques, riparian ET was monitored for this study using a sap flow technique which is based on an energy balance. Sap flow has been shown to correlate well with individual plant transpiration. Stream stage and local depth to groundwater were also measured using pressure transducers. In order to parse the influence of groundwater depth from meteorological influences, a weather station was deployed to determine reference ET. Our preliminary results indicate that ET is not substantially influenced by depth to groundwater. Fluctuations in ET appear to be more strongly correlated with reference ET conditions. The results are consistent with previous studies that have shown that phreatophytic plants are more strongly impacted by the rate of change of groundwater depth rather than absolute changes. The root architectures of riparian plants, such as willows and cottonwoods, are strongly influenced by historic groundwater conditions and the plants develop major roots in the vicinity of the capillary fringe. Hence, small variations in depth to groundwater within the historical range of root development, as is the case in a highly regulated system like the Truckee River, are not likely to impact plant transpiration rates, as was shown in this study.