CONCLUSIONS

The sharply converging flow paths in the floodplain aquifer at the water table in the upstream end of the study reach may result from lower hydraulic conductivities in this part of the aquifer as compared with the northern part of the study reach. This is supported by the differences in baseflow contributions between the two areas with the northern segments contributing relatively more baseflow per unit length of channel than the southern segments. Another contributing factor may be a possible survey error in the elevation of the shallow ESC piezometer.
A reduction of 15% in the downstream gradient in the ground water system between April and June may compare well with a 33% reduction in baseflow contribution to stream flow after data on changes in stream-bottom surface area are incorporated in the analysis.

The greater declines in water levels in the floodplain aquifer near the upstream end of the reach may be related to more phreatophyte activity in that area. When data collected in other experiments on sap flow and moisture fluxes in and above the riparian canopy have been analyzed, those results may explain some of the variations seen in the water level data (Williams et al, 1997).

Seasonal variations in vertical gradients are influenced by the activities of phreatophytes on both diurnal and seasonal scales. Another influence may be exerted by the discharge from the underlying regional aquifer. Though discharge from that aquifer would be expected to be constant under steady-state conditions, the influence of even distant pumpage may be causing another signal that has an effect on vertical gradients in the floodplain aquifer.

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FUTURE LINES OF INVESTIGATION

The SALSA program includes a variety of hydrologic and atmospheric studies that when integrated, will enable a detailed definition of the movements and pathways of water moving through the control volume at the Lewis Springs site.The riparian vegetation along the San Pedro river has three sources of water to draw upon. Precipitation and bank storage from high flow events may together supply as much as half of the corridors ET fluxes. In this section we endeavor to identify the linkages and interdependencies of our investigative plans with those of other SALSA participants.

ACKNOWLEDGEMENTS

Financial support from the USDA-ARS Global Change Research Program, USDA National Research Initiative Grant Program, Arizona Department of Water Resources, and the U. S. Bureau of Land Management is gratefully acknowledged. Assistance was also provided in part by EPA STAR Graduate Student Fellowship Program, National Science Foundation, U.S. Geological Survey, Cochise County Highway and Flood Control Department, and Fort Huachuca. A very special thanks to the ARS staff in Tombstone for their diligent efforts in installing many of the piezometers used in this study, and our sincere thanks to the many ARS and Univeristy of Arizona staff and students, and local volunteers who generously provided their time and efforts to make this project a success.

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REFERENCES

Goodrich, D. C., et al, 1997, An Overview of the 1997 Activities of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program. (1.1, in Proceedings)

Huckleberry, G., 1996, Historical Channel Changes on the San Pedro River, Southeastern Arizona, Arizona Geological Survey Open-File Report 96-15, 21p.

Pool, D. R., and Coes, A. L., 1997, Hydrogeologic Investigations of the Sierra Vista Sub-basin of the Upper San Pedro River Basin, Southeast Arizona U. S. G. S. Water Resources Investigation, Manuscript in review.

Rantz, S. E. and others, 1982, Measurement and Computation of Streamflow - Volume 1. Measurement of Stage and Discharge: U. S. Geological Survey Water-Supply Paper 2175, 284p.

Rantz, S. E., 1982, Measurement and Computation of Streamflow - Volume 2. Computation of Discharge. U. S. Geological Survey Water-Supply Paper 2175, 631p.

Whitaker, M. P. L., et.al., 1997, Monitoring Bank Storage in the San Pedro Riparian National Conservation Area, Arizona. (P2.13, in Proceedings)

Williams, D. G., et.al., 1997, Biotic Control over the Functioning of Desert Riparian Ecosystems. (1.11, in Proceedings)