T. Ferré, D. Maddock, (UA-HWR)

Riparian plant communities can exist only where and when their roots have access to plentiful supplies of surface water or groundwater. Due to the population increases in the semi-arid Southwest, groundwater pumping exceeds the rate of recharge in many basins. Therefore, sustainable development of this area requires an improved understanding of the response of these systems to increased water extraction through improved hydrologic models.

Science question and approach

In coordination with an EPA/NSF project managed by Tom Maddock and Kate Baird (UA) studying the effect of lowering the water table on cottonwood ET, drainage in the unsaturated zone due to pumping was monitored by Gerd von Glinski in late May/early June 2001, when cottonwood growth was strong, and in September 2001, when growth had subsided using borehole ground-penetrating radar (BGPR). Monitoring soil moisture drainage as well as re-saturation after recovery of the water table provided a picture over time (one week) as well as over distance.

Results

The focus of efforts during the past 10 months has been analysis of the field measurements. The BGPR-measured change in water content provides detail that cannot be achieved with any other existing water content measurement method. Figure 3-2 shows the evolution of a drained region above the water table (horizontal dashed line) during pumping and refilling during recovery (periods separated by a vertical dashed line). The results can be combined to determine an instantaneous specific yield as a function of time. This shows the time associated with delayed drainage (approximately 1 day) after which the specific yield assumes a reasonable value of 0.35. After pumping, the specific yield rises as high as 0.67. Numerical modeling shows that this is not due to hysteresis, but rather due to air entrapment during recovery. This monitoring method may allow for far more accurate analysis of unconfined aquifer pumping tests, improving the characterization of aquifer properties for water resource and subsurface hydrologic investigations. Variably saturated flow modeling is being conducted in coordination with researchers at the U.S. Salinity Lab (Simunek). These researchers have modified existing inverse numerical models to allow for improved interpretation of soil hydraulic properties from BGPR measurements.

Plans

• Coupled with numerical modeling of drainage, this work will be the basis of von Glinski's master's thesis.



• The remainder of the summer will be spent finalizing a manuscript to submit for publication in the Vadose Zone Journal.



• The method will be used in monitoring root-soil water exchange and deep recharge, undertaken in the San Pedro Basin in cooperation with Goodrich and Scott.



• The method will be used to ground-truth ongoing efforts to map changes in subsurface water storage with Electrical Resistance Tomography (Warrick) and gravity methods (Pool).