Analytical Solutions for Examining Spatial Variations in Evapotranspiration-Driven Fluctuations in the Water Table in Vegetated Riparian Zones

Diurnal water-table fluctuations observed in shallow wells in vegetated riparian zones are a diagnostic indicator of groundwater consumption by evapotranspiration (ETG). In order to better understand how these fluctuations vary across a riparian zone, analytical solutions for strip-sinks with periodic forcing functions were developed. The periodic forcing functions, which represent the evapotranspirative consumption of groundwater, are finite within the strips and zero outside. Linearity allows superposition of the strip solutions to simulate spatial variations in ETG due to variations in vegetation density within the riparian zone. The solution is applied to data from a field site in a riparian zone along the Arkansas River in west-central Kansas where groundwater levels in shallow wells have been monitored at 15-minute intervals for up to five years. A series of strips were used to represent the largely dry unvegetated river channel, two zones within the vegetated riparian zone, and adjacent pastures and cultivated fields. The spatial variations in the phase and amplitude of the simulated fluctuations were consistent with general patterns observed in the field data during both wet and dry years. Several empirical methods have been developed for estimating ET G using characteristics of the diurnal water-table fluctuations. This solution reveals where best to place wells for use with those methods. Similar solutions have been developed for circular sinks to assess the impact of phreatophyte-control activities on water-table fluctuations at wells in areas of cleared vegetation.