In-Situ Determination of Specific Yield Using Soil Moisture and Water Level Changes in the Riparian Zone of the Arkansas River, Kansas.

Specific yield (S_y - drainable porosity) is an important parameter for numerous hydrogeologic investigations, but few reliable methods for its in-situ determination exist. S_y is commonly estimated by saturating and draining soil cores, using soil water characteristics in parametric models, or from pumping tests. However, these methods can introduce large uncertainties due to core disturbance, inappropriate models, and the impact of heterogeneity, respectively. At a site in the riparian zone of the Arkansas River in south-central Kansas, the amount and variability of groundwater consumption by phreatophytes is being assessed using water-table fluctuations. S_y is an important control on the magnitude of these fluctuations, so estimates of its value and its variability in time and space are needed. An in-situ method based on soil-moisture and water-level changes (Skaggs et al., 1978) was used for this purpose because it can provide information at the scale appropriate for such an investigation. Neutron access tubes (0 - 1.82 m in depth) were installed at four locations in the riparian zone to periodically gather soil moisture information. Water level data in the shallow alluvial aquifer were collected every 15 minutes in six wells using pressure transducers calibrated with manual measurements. The method of Skaggs et al. assumes that conditions in the unsaturated zone are at equilibrium, i.e. soil moisture changes are only a product of the rise or fall of the water table. One period for which this assumption appears reasonable was in September and October of 2003 when the water table dropped more than 0.61 m in four weeks. The S_y estimates for this period displayed relatively little variability between sites (0.19-0.21). Total porosity for the depth intervals through which the water table fell varied from 0.28-0.36, so the S_y estimates appear reasonable. S_y estimates from a pumping test, however, displayed much greater variability (0.15-0.31), which is most likely a product of heterogeneity in hydraulic conductivity. Results from a deeper set of neutron access tubes (0-3.05 m in depth) installed in the summer of 2004 will also be presented.