HENRY DARCY DISTINGUISHED LECTURE: WHAT THE HECK IS A PHREATOPHYTE: A FIELD INVESTIGATION OF ECOHYDROLOGIC PROCESSES IN STREAM-AQUIFER SYSTEMS

This presentation will be an overview of a multidisciplinary investigation of water use by phreatophytes (plants that utilize ground water) in semi-arid riparian zones. A particular emphasis will be placed on the ecohydrologic information embedded in water-table fluctuations. Hydrographs from shallow wells in vegetated riparian zones frequently display a distinctive pattern of diurnal water-table fluctuations produced by diurnal variations in plant water use. Water-level data from two sites in the Great Plains of the United States will be used to show that these fluctuations are a product of phreatophyte activity and, in conjunction with other hydrologic and ecologic information, to demonstrate the ecohydrologic information that can be gleaned from diurnal water-table fluctuations. These data reveal that the depth at which the evapotranspirative consumption of ground water becomes negligible is strongly controlled by the past hydrologic conditions experienced by the riparian-zone vegetation, and is often unrelated to the physiologic limitations of the vegetation. Ground-water consumption by non-native phreatophytes, such as salt cedar (Tamarix spp.) and Russian olive (Elaeagnus angustifolia), is an issue of considerable concern in the western United States and elsewhere. These data reveal that the reductions in ground-water consumption achieved by removal of these non-native phreatophytes may be quite small, depending on the depth to water, sediment characteristics, and other factors. This investigation demonstrates that diurnal water-table fluctuations can be considered a diagnostic indicator of ground-water consumption by phreatophytes at most sites, so the information embedded within these fluctuations should be more widely exploited in ecohydrologic studies.