MONITORING BANK STORAGE IN THE SAN PEDRO RIPARIAN NATIONAL
CONSERVATION AREA, ARIZONA
M.P.L. Whitaker (1), T. Maddock III (1), B. MacNish (1), D. Goodrich (2) and B. Goff (2)
1. The University of Arizona, Dept. Of Hydrology & Water Resources, Tucson, Arizona, USA
2. USDA-ARS U.S. Southwest Water shed Research Center, Tucson, Arizona, USA
* Corresponding author: Univ. of Arizona, Dept. of Hydrology and Water Resources, Tucson, AZ 85721 e-mail: mplw@hwr.arizona.edu
1. INTRODUCTION
This paper describes the instrumentation, data collection and preliminary results for monitoring bank storage at the Lewis Springs site within the San Pedro Riparian National Conservation Area (SPRNCA), located in southeastern Arizona (see Figure 2 in Goodrich et al., 1.1, this issue). Bank storage is a critical variable for accurately modeling the water budget in semi-arid riparian systems, but is particularly difficult to assess and quantify. It is especially essential for understanding ground-water/surface-water interactions.
2. INSTRUMENTATION
In order to monitor and measure bank storage in the SPRNCA, an intensive
network of vadose zone monitoring equipment was installed: ten water content
reflectometery (WCR) probes, eight thermocouples, and three nests of four
tensiometers each were installed on either side of the San Pedro River
at Lewis Springs (see Figure 1). 
The WCR probes were installed
in trenches which were subsequently backfilled. The location of these trenches,
and the location of this instrumentation transect in general, may be identified
as the trench sites in Figure 4 of Maddock et al. (this issue).
The tensiometers and WCR probes were installed at the water table interface and in the capillary fringe to monitor diurnal, seasonal and flood-event-induced fluctuations in soil moisture and soil matric potential within the cottonwood-willow forest gallery (i.e near-stream riparian biome). A submersible pressure transducer was installed in the river along this intensively instrumented transect to measure stream stage elevation data and provide stream stage fluctuations which correspond to the soil moisture and soil matric potential data.
The tensiometers were fitted with pressure transducers, and along with the WCR probes and thermocouples, were wired to a datalogger to collect measurements every 30 minutes. The thermocouples were installed to monitor soil temperature adjacent to the WCR probes, as the WCR probes are temperature-sensitive. Additionally, six neutron probe access tubes (NPATs) were installed on either side of the San Pedro River at Lewis Springs, in two different transects to collect manual measurements of soil moisture to supplement the automated datalogger information. The locations of these NPATs are identified in Figure 4 of Maddock et al. (this issue).
Only select WCR probe soil moisture data and select tensiometer matric potential data are available for presentation at this time.
3. PRELIMINARY RESULTS
Preliminary data showing bank storage response to diurnal fluctuations and Arizona=s 1997 monsoon flood-events are presented. Select WCR probe soil moisture data and tensiometer matric potential data show both immediate and delayed responses to increases in stream stage. The data for two high-flow events are shown: a 38 cm stream stage increase on Julian Day 219 (August 7, 1997), and a 90 cm stream stage increase on Julian Day 221 (August 9, 1997).
Figure 2 shows changes in soil moisture measured using WCR probes.
These probes were selected from the west trench (see Figure 1). Diurnal
fluctuations as well as an immediate response to an increase in stream
stage are evident. Although the stream stage decreases sharply within 24
hours, the riverbank maintains a higher soil moisture for several days.
The strong diurnal fluctuations exhibited by the WCR probes may reflect the evapotranspiration cycles of the cottonwood and willow trees in the SPRNCA. However, the apparently diurnal pattern may also be an artifact of the probes= sensitivity to temperature. The data from the adjacent thermocouples have not yet been used to normalize the soil moisture data to 20E C. Additionally, the probes have not yet been calibrated for the soils in which they are installed; a standard polynomial provided by the manufacturer was used in the interim to discern relative soil moisture differences.
Figure 3 shows fluctuations in soil matric potential measured using
select tensiometer data from nests D, E and F located on the east bank
of the San Pedro River (see Figure 1). 
Diurnal fluctuations
are evident, but are considerably more subtle than those measured with
the WCR probes. At least two factors may account for these differences.
First, the pressure transducers associated with the tensiometers are calibrated
to account for non-isothermal conditions using the method described by
Lacher (1996). While both WCR probes and tensiometers= pressure transducers
are installed below the ground surface to minimize temperature fluctuations,
temperature may still affect the calibration of both instruments (Lacher,
1996 and Warrick et al., submitted 1997) Accordingly, temperature may still
affect these instruments. However, the instruments= proximity to different
plant roots may reflect variation in evapotranspiration of the vegetation
in the cottonwood-willow forest gallery of the SPRNCA.
The response of the tensiometers to increases in stream stage appears slower than the WCR probes, but the two graphs cannot be compared directly because they represent data from opposing river banks. However, it can be noted from Figure 3 that the 6.0 feet-deep tensiometers (6.0 D and E) - regardless of their lateral proximity to the river - respond more quickly to the rises in river stage than the tensiometers at 6.5 feet depth (6.5 D and F). This may be a function of soil texture, which was recorded during the installation process, but has not yet been factored into the interpretation of the results. Note that tensiometer 6.0 D showed a response to the increased stream stage prior to 6.0 E, which is further from the river by approximately 1.5 meters. A similar response can be noted in tensiometers 6.5 D and 6.5 F, respectively.
4. SUMMARY
The response of soil moisture and soil matric potential to increases in stream stage indicate a prolonged increase in bank storage following a high flow event that increases the stream stage by as little as 90 cm. Additionally, the apparent diurnal response of soil moisture and matric potential provide information about the evapotranspiration patterns in the cottonwood-willow forest gallery. Additional soil moisture data, matric potential data and hydraulic head data, collected in coordination with high flow events, will provide crucial information on the evapotranspiration cycles of the cottonwood and willow tree species within the SPRNCA. This data will be used to improve models which consider bank storage as an important variable in the hydrologic cycle.
5. ACKNOWLEDGMENTS
Financial support from the US Bureau of Land Management, the USDA-ARS Global Change Research Program and NASA grant W-18, 997 is gratefully acknowledged. Special thanks are extended to the USDA-ARS staff in Tombstone, Arizona for their diligent efforts. We also wish to extend our sincere thanks to the many ARS and University of Arizona staff and students and local volunteers who generously donated their time and expertise to make this project a success.
6. REFERENCES
Goodrich, D.C. et al. 1997. An overview of the 1997 activities of the Semi-Arid Land Surface-Atmosphere (SALSA) program (1.1, this issue).
Lacher, L. J. 1996. Recharge characteristics of an effluent dominated stream near Tucson, Arizona. Ph.D. Dissertation, Dept. Of Hydrology and Water Resources, Univ. Of Arizona, 229 p.
Maddock, T., R.D. Mac Nish, D.C. Goodrich, D.G. Williams, W.J. Shuttleworth, B.A. Goff, R.L. Scott, M.S. Moran, D.I. Cooper, L.E. Hipps, A.G. Chebouni. 1997. The overview of atmospheric and surface water coupling to regional groundwater models in semi-arid basins. (1.10, this issue).
Warrick, A., P.J. Wierenga, M.H. Young, and S.A. Musil. Submitted August 1997. Diurnal fluctuations of tensiometric readings due to surface temperature changes. Water Resources Research.
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