Spatial variations in evapotranspiration-driven fluctuations in the water table in and near vegetated riparian zones

Background/Question/Methods

Hydrographs from shallow wells in vegetated riparian zones frequently display a distinctive pattern of diurnal fluctuations. At most sites, these diurnal water-table fluctuations can be considered a diagnostic indicator of ground water consumption by phreatophytes. Thus, water level records from shallow wells show considerable promise for studying linkages between above- and below-ground processes in ecohydrologic investigations. Although the methodology for measuring these fluctuations is now well established, relatively little attention has been given to understanding the major controls on these fluctuations and interpreting the information embedded in water level records. In riparian zones, there is a particular need for an improved understanding of how water-table fluctuations at a given location are influenced by spatial variations in ground water consumption across and outside of the riparian zone. The objective of this study was to examine the influence of spatial variations in consumptive water use on diurnal water-table fluctuations within and in the immediate vicinity of vegetated riparian zones. An analytical solution was obtained for the mathematical model of water-table fluctuations produced by evapotranspirative consumption of groundwater. The solution was developed for a typical western Kansas stream setting consisting of a dry or water-filled stream channel, parallel strips containing phreatophytic vegetation, and surrounding cropland and pasture. Spatial variation in ground water consumption was achieved by assigning to each zone a different amplitude for the periodic forcing function used to simulate consumptive water use.

Results/Conclusions

Solution results show that lateral ground water flow, induced by spatial variations in the rate of ground water consumption, can alter both the amplitude and phase of diurnal water-table fluctuations. For a given location within the riparian zone, the extent to which diurnal water-table fluctuations are altered is a function of aquifer hydraulic properties, the width of the vegetated strips, and the degree of spatial variation in consumptive water use. Across the riparian zone, there exist regions where the fluctuations are damped and regions where the fluctuations are amplified in magnitude. Likewise, there exist regions where the fluctuations are delayed in phase and regions where they are advanced in phase. The results are used to present guidelines for optimal well placement to minimize the influence of spatial variations in consumptive ground-water use on the amplitude and phase of diurnal water-table fluctuations at a given location.