UZIG-News - July 1999 - Issue 9

Newsletter of the Unsaturated Zone Interest Group (UZIG) of the US Geological Survey (USGS). The purpose of this newsletter is to enhance communication within the UZIG. It should not be quoted or cited as a publication. Send desired changes in the mailing list to jrnimmo@usgs.gov. Please contact authors or other people mentioned in UZIG-News with any questions, comments, or suggestions.

by Stan Leake (saleake@usgs.gov)

The Southwest Ground-Water Resources Project began October 1, 1998 as the first regional issue study of the U.S. Geological Survey Ground-Water Resources Program. The main emphases are the interaction of ground water and surface water, and the relation of that interaction to sustainability of ground water resources in the Southwest. The study area includes aquifer systems in the arid to semi-arid basins in southwestern states including California, Nevada, Utah, Arizona, and New Mexico. Main focus areas of the project are:

1. Regional synthesis of information on the interaction of ground water and surface water.
2. Assessments of the effects of ground-water development on riparian systems.
3. Assessments of the effects of climate variations on recharge to and discharge from ground-water systems.
4. Development of improved methods to quantify inflow to ground- water systems from streams and application of these methods in the Southwest.
5. Development of improved simulations of the interaction of surface water and ground water.

Many of the other proposals described studies of recharge at various sites around the Southwest. The review team repackaged these into a network of four or five recharge study sites in the Southwest (listed below) at which similar recharge estimation methods would be applied over the course of the five-year Southwest Ground-Water Resources Project.

by Rick Healy (rwhealy@usgs.gov)

A record number of people (69!) attended the 1999 Unsaturated Zone Interest Group (UZIG) Meeting at the USGS Western Regional Center in Menlo Park, CA. Attendees, who included personnel from Water Resources and Geologic Divisions of the USGS as well as USDA-ARS, DOE, Pacific Northwest National Lab (PNNL), Lawrence Berkeley National Lab (LBNL), Lawrence Livermore National Lab (LLNL), the USDA Salinity Lab (USSL), and the Army Cold Regions Reseach Lab (CRREL) were treated to two days of enlightening seminars and lively discussion on January 13 and 14. A half-day field trip to the Santa Clara Valley Water District recharge ponds took place on January 15.

Welcoming addresses were presented by Andrea Alpine, Acting Regional Director, USGS, WR; Peter Anttila, Assistant Regional Hydrologist, USGS-WRD, WR; and Steve Ingebritsen, Chief, Branch of Regional Research, USGS-WRD, WR. Scientific presentations were organized into six sessions. Session I addressed investigations near waste disposal sites and included talks by Lorrie Flint, Alan Flint, Dave Hudson, Dave Stonestrom, and John Nimmo. Phil Meyer of PNNL gave a talk on estimating uncertainty when site-specific data is limited. Session II dealt with hydraulic and transport properties. Rien van Genuchten of USSL made a presentation on hydraulic property characterization and estimation in support of predictive flow/transport modeling. Presentations were also made by Steve Grant of CRREL and Brian Andraski. Session III consisted of tours of USGS Menlo Park labs, poster presentations, and demonstrations of computer software. Session IV was devoted to talks on aquifer recharge. John Risley and Joe Hevesi both made presentations on the use of watershed models to estimate recharge in large basins. Al Rutledge talked about computerized hydrograph separation. Mike Fayer of PNNL and John Izbicki made presentations on studies in arid regions, and Art Baehr discussed work in southern New Jersey. In Session V Ken Hollett discussed recent activities of the USGS Office of Ground Water, and Bill Isherwood of LLNL described DOE's recent Vadose Zone Science Effort. The final session of talks, on liquid, gas, and chemical fluxes in the unsaturated zone, included presentations by Boris Faybishenko of LBNL, Dave Stannard, Hubert Morel-Seytoux, John Rogie, and Mark Reid. The group dinner on Thursday evening at the Su Hong restaurant provided an opportunity to relax and continue discussions.

For the next nationwide UZIG meeting, it was agreed to shoot for the fall of 2000 as a target date. Albuquerque is a possible location.

Thanks go to John Nimmo, Kim Perkins, Angus Lewis, and crew for efforts in organizing and running the highly successful meeting. Thanks also to Peter Martin for arranging the field trip.

[Editor's note: The Menlo Park UZIG meeting was preceeded by the first of four 1999 Ground-Water Recharge Workshops, the others being presented in Baltimore, Tucson, and Atlanta. The article below by Bert Stolp is based on a presentation at the Tucson Workshop.]

by Bert Stolp (bjstolp@usgs.gov)

Tooele Valley covers about 300 square miles in northern Utah, and is located about 30 miles west of Salt Lake City. The climate is semi-arid. Mountains with a humid climatic regime flank the eastern and western sides of the valley. The current best estimate for long-term average annual recharge is 75,000 acre-feet, based on the cumulative efforts of six hydrological studies and the use of both empirical and analytical methods.

In 1913, Everett Carpenter of the U.S. Geological Survey published the results of a rudimentary hydrologic reconnaissance of Tooele Valley. He identified the surrounding mountains as the major source of recharge to the valley aquifers. He suggested that surface water from mountain streams recharges the ground-water system at canyon mouths where flow sinks into the coarse beds of the alluvial slopes. In 1946, H.E. Thomas updated and further investigated the findings of Carpenter. In addition to recharge from stream losses, Thomas identified other recharge sources: (1) subsurface inflow from stream-channel deposits and adjoining areas further south, (2) seepage of unconsumed irrigation water, and (3) infiltration of precipitation on the valley floor. This assessment of recharge mechanisms is based on a synthesis of conceptual and field observations, but neither Carpenter nor Thomas had the data needed to quantify recharge amounts.

Joseph Gates in 1965 made the first attempt to quantify Tooele Valley recharge. His method uses the assumption that recharge in the valley cannot exceed the sum of total precipitation in Tooele Valley and the tributary mountain areas. Recharge was calculated as a residual when evapotranspiration within the mountain areas, streamflow diversions out of the recharge areas, and well withdrawals within the recharge areas were subtracted from total precipitation. The recharge amount was checked using Darcy's Law: ground-water flow was calculated for a reference line that generally separates the discharge areas from the recharge areas in Tooele Valley, and for which aquifer characteristics are known. This combination of empirical and analytical methods gave a recharge estimate of 100,000 acre-feet/year.

Al Razem and Judy Steiger re-examined recharge in Tooele Valley in 1981 by looking more closely at individual components of the water budget. Recharge in the tributary mountains and along the valley margins was determined using a modified version of the Eakin method (State of Nevada, Office of the State Engineer, Water Resources Bulletin No. 12, 1951). Subsurface inflow from areas to the south was also estimated. Discharge processes in the valley were closely examined, phreatophyte areas were carefully delineated, and valley spring discharges were measured and estimated. Using all these factors, recharge to the valley was estimated at about 60,000 acre-feet. This amount was used as the staring point for a two-dimensional ground-water flow model, which was calibrated for Tooele Valley by Al Razem and Scott Barthloma. A recharge of about 57,000 acre-feet/year gave the best calibration of the flow model. Flow modeling is an analytical process; depending on the accuracy of the estimated aquifer characteristics and distribution, it can confirm whether recharge estimates are capable of producing the observed hydraulic gradients.

In 1999, Pat Lambert and Bert Stolp constructed a three-dimensional ground-water flow model for Tooele Valley. Using additional water-level data, land-use information, and updated estimates of irrigation areas and efficiencies, recharge to the valley was revised to approximately 75,000 acre-feet/year. This budget was used as the starting point for calibration of the 3-D flow model. With this recharge rate, and some re-adjustment of distribution and individual components, the flow model reasonably reproduces hydraulic gradients observed in Tooele Valley.

The conceptualization of recharge sources and mechanisms for Tooele Valley has not changed dramatically since initial recognition by Carpenter and Thomas. However, estimates concerning the amounts of long-term average recharge have varied significantly. The use of ground-water flow modeling, where the distribution and amount of recharge must lead to reasonably close prediction of measured hydraulic gradients and water levels, supports the conceptually based recharge estimates. The latest refined estimate of 75,000 acre-feet/year falls in the middle of the 57,000 to 100,000 acre-feet/year range of the historical studies.