A MODELING EVALUATION OF A METHOD FOR ESTIMATING GROUNDWATER USAGE BY PHREATOPHYTES

A prerequisite to sustainable management of water resources is accurate quantification of ecosystem needs. In semi-arid regions throughout the world, a major ecosystem need is groundwater to support riparian galleries of phreatophytes. In these environments, daily patterns of groundwater usage are often observable as diurnal water table fluctuations. White (1932) introduced a method to quantify phreatophytic consumption of groundwater based on the magnitude of these daily fluctuations, the storage properties of the sediment near the water table, and the rate of water table rise during nighttime hours when transpiration is assumed to be zero. Although the White method has existed for over seventy years and has been applied sporadically, a rigorous evaluation of the approach using numerical modeling has not been reported.

The USGS Variably Saturated 2-Dimensional (VS2D) model was used to simulate water flow in the vicinity of the water table as influenced by daily transpiration patterns. The simulated water-table fluctuations were analyzed using the White method. We found that this method can accurately quantify groundwater consumption if an appropriate value for the readily available specific yield is used. Results indicate that the readily available specific yield is generally less than the difference between the saturated water content and the residual water content for two reasons. First, the time scale over which the fluctuations are observed does not allow the soil moisture profiles to reach equilibrium. Second, in cases where the water table is near the ground surface, the thin vadose zone cannot release as much water as a thicker unsaturated zone. We determined that typical values of readily available specific yield can be obtained based on the textual characteristics of the sediment. An example application of the use of the White method for estimating groundwater consumption by transpiration in a riparian zone in west central Kansas will be presented.