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SEASONAL ESTIMATES OF RIPARIAN EVAPOTRANSPIRATION (CONSUMPTIVE
WATER USE) USING REMOTE AND IN-SITU MEASUREMENTS
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| D. Goodrich (1), S. Moran (2), R. Scott (3), J.
Qi (2), D. Williams (3), C. Unkrich (1), S. Schaeffer (3), R. Mac Nish (3),
T. Maddock (3), B. Goff (1), J. Toth (1), L. Hipps (4), D. Cooper (5), J.
Schieldge (6), A. Chehbouni (7), C. Watts (8), J. Shuttleworth (3), O. Hartogensis
(9), H. De Bruin (9), Y. Kerr (10), R. Marsett (1), W. Ni (2)
(1) USDA-ARS, Tucson, Arizona, USA; (2) USDA-ARS, Phoenix, Arizona,
USA; (3) University of Arizona, Tucson, Arizona, USA; (4) Utah St. University,
Logan, Utah, USA; (5) Los Alamos National Lab, Los Alamos, New Mexico, USA;
(6) Jet Propulsion Lab, Pasadena, California, USA; (7) OSTROM/IMADES, Hermosillo,
Sonora, Mexico; (8) IMADES, Hermosillo, Sonora, Mexico; (9) AUW, Wageningen,
The Netherlands; (10) CESBIO, Toulouse, France
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1. INTRODUCTION
In many semi-arid basins, groundwater constitutes the primary
water source that sustains human habitation, agriculture and riparian systems.
Utilizing regional groundwater models as an aid in managing these resources
requires accurate estimates of the basin boundary conditions. A critical
boundary condition closely coupled to atmospheric processes and rarely known
with great certainty is seasonal riparian evapotranspiration (ET).
This quantity can often be a significant factor in the
basin water balance in semi-arid regions, yet is very difficult to estimate
over a large area (Maddock et al., this conference). Better understanding
and quantification of the total annual riparian ET of the Upper San Pedro
River Basin (USPB - see Figure
1) was one of the primary 1997 objectives of the SALSA program. Current
riparian ET estimates for this basin indicate it is not a trivial quantity.
The Arizona Department of Water Resources
Hydrologic Survey Report (1991) concluded it was one of the largest components
of the water budget. The objective of this poster is to provide an overview
of the experimental efforts undertaken in 1997 to estimate annual riparian
ET. |
2. METHODS
A series of intensive measurement campaigns were carried
out over the 1997 riparian growing season to estimate riparian ET as well
as accomplish a number of other research objectives (Goodrich et
al., this conference). The general experimental strategy consists of:
- Carrying out a variety of intensive local scale measurements
- Utilizing remotely sensed data for spatial extrapolation
- Utilizing continuous data and models for temporal interpolation/extrapolation
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2.A. Intensive Local Scale Measurements
The San Pedro riparian system is generally characterized
by three primary vegetation associations: the cottonwood/willow forest gallery
adjacent to the stream, sacaton grassland, and mesquite thickets or bosques.
Intensive local scale measurements were carried out to acquire an improved,
process-based understanding of groundwater, surface water and evapotranspiration
(ET) interactions, as well as fluxes of these quantities into and out of
the riparian system (Figure 2).
 |
 The
Lewis Springs site, located roughly 13 km east of Sierra Vista/Ft. Huachuca,
was selected as the location for the intensive measurements. At this site
(approximately 800 x 700m) detailed vegetation characterization was carried
out and a variety of instrumentation was installed for continuous monitoring
of some variables and intermittent monitoring of others (see Figure 3).
Intermittent, intensive monitoring was carried out via a series of 24 to
56 hour "synoptic" measurement campaigns over the 1997 riparian
growing season. Table 1 contains the dates of the synoptic campaigns, all
of which were coordinated with closely spaced Landsat and ERS-2 overpasses
(see Moran et al., this conference). For a subset of the campaigns, aircraft-based
remotely sensed data was acquired. Table 2 contains a general list of the
types of coordinated measurements made and references with more detail on
these measurements. |
Table 1. 1997 Synoptic Campaign Summaries
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| Date |
General Condition |
Aircraft Remote Sensing* |
| Feb 16 |
Winter |
|
| Mar 20-21 |
Pre- GreenUp |
|
| Apr 19-21 |
Green Up |
Single Channel Thermal |
| Jun 7-9 |
Maximum PET |
|
| Jul 9-10 |
Maximum PET |
Single Channel Thermal |
| Aug 7-19 |
Peak Monsoon |
SingleChannel Thermal, TIMS/ TMS, Visible/Near IR |
| Oct 14-16 |
Die Down |
Single Channel Thermal |
| * All dates bracket closely spaced Landsat and ERS-2 overpasses |
Table 2. Groups of Synoptic and Long-Term Coordinated Measurements
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| General Measurement Class |
References in Proceedings of this
Conference |
| Groundwater/Surface Water |
Maddock et al.; Mac Nish et al. |
| ET/Energy Balance - Overview |
Hipps et al. (paper 1.4) |
| - Mesquite/Grass ET |
Scott et al. |
| - Cottonwood/Willow Sapflow |
Williams et al.; Schaeffer et al. |
| - LIDAR (August) |
Cooper et al. |
| - Scintillometer (August) |
Chehbouni et al. (paper 1.6) |
| Vadose Zone/Bank Storage |
Whitaker et al.; Hymer et al. |
| Water Source/Isotopes |
Williams et al.; Snyder et al. |
| Remote Sensing |
Moran et al. (papers 1.12, 2.14);Qi et al; Schieldge and Kahle |
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2.B. Remotely Sensed Data for Spatial Extrapolation
 Aircraft overflights were designed to cover the majority of the
BLM San Pedro National Riparian Conservation Area (SPNRCA). Surface temperatures
from four single channel thermal imaging flights from April to October will
be the primary remote sensing data used to estimate annual riparian ET.
In addition, a high resolution (5m) map of vegetation types for the entire
corridor was created from several spectral bands from an airborne Deadalus
sensor in May 1996 (Moran et al., this issue). Data from the August 1997
flight of this sensor and the multichannel TIMS thermal sensor will be used
to validate the estimates obtained from the single channel system. The single
channel thermal images from the four missions are shown in Figure 4.
Preliminary analyses by Qi et al. (this conference) combined
the August ground data from the Lewis Springs site with the thermal image
data to develop a relationship between the difference of remotely sensed
surface temperature and air temperature versus hourly averaged ET. Assuming
that water loss rates at Lewis Springs are representative of the entire
corridor, this relationship and the vegetation map were used to extrapolate
the total daily water loss from the corridor (for the region from Lewis
Springs to 6 km north of Fairbank) of 48,270,000 kg (48,270 m3 or 39.1 ac-ft)
of water per day. |
2.C. Temporal Interpolation / Extrapolation
for Seasonal ET Estimates
If the ET estimates from the August campaign are representative
of a 180-day growing season the annual riparian ET would have been approximately
8,689,000 m3 (7,040 ac-ft). This covers roughly 60% of the area for which
remotely sensed data was acquired and approximately 50% of the SPNRCA.
This simple analysis assumed water availability and atmospheric
conditions were uniform over the corridor. The next refinement will use
the thermal aircraft data to map differential ET via differences in remotely
sensed surface temperature. This is essential as it is known that during
July and August the San Pedro dried up in the vicinity of Boquillas, increasing
water stress and decreasing tree transpiration, as confirmed by sapflow
measurements in the ephemeral Escapule wash area adjacent to the main riparian
corridor.
This will only address spatial variations in ET when there
is a remotely sensed image. A further refinement would estimate temporal
variations between remote sensing flights to obtain ET throughout the growing
season. A potentially usefull continuous measure is stream discharge. Stream
discharge is a large-area integrative measure and is arguably one of the
most accurate hydrologic measurements that can be made. Stream baseflow,
greatly reduced during the growing season, reflects the effects of riparian
ET on groundwater gradients.
Atmospheric measures are also promising estimators of riparian
ET. In Figure 5 stream discharge from the USGS Charleston gage and air temperature
at Lewis Springs are illustrated for portions of October 1996. A remarkable
correlation between these two variables is apparent. Although ET is not
directly measured in this case we can assume that variations in baseflow
are attributable to riparian ET. It is also interesting to note that when
the first hard freeze occurs on Oct. 21 the diurnal streamflow pattern ceases
and discharge increases, suggesting that freezing had stopped riparian ET.
However, temperature-only ET-estimation methods are not recommended unless
this is the only available data source (Shuttleworth, 1992). Shuttleworth
also notes that preferred methods for estimating ET require a value of the
difference between the saturated vapor pressure and the ambient vapor pressure
or vapor pressure deficit (VPD). VPD and whole-tree transpiration are plotted
for several days of the
August campaign in Figure 6. A reasonable correlation exists, indicating
a simple model which incorporates VPD could be used to estimate temporal
variations in ET .
 
Williams et al. (this conference) and Schaeffer and Williams
(also this conference) noted the strong influence of photosynthetically
active radiation (PAR) on the diurnal variations in whole tree transpiration.
Their plots of PAR versus whole-tree transpiration for a portion of the
same time shown in Figure 6 are even more clearly correlated than the VPD
plot. |
3. CONCLUSIONS - FUTURE DIRECTIONS
Additional variables and models, as well as remotely sensed
data, can potentially be used to better estimate the spatial and temporal
variations in riparian ET between intensive measurement periods. With this
information it is expected that reasonable estimates of the seasonal variation
in riparian ET as well as the total consumptive water use can be obtained. |
4. ACKNOWLEDGMENTS
Financial support from the USDA-ARS Global Change Research Program, NASA
grant W-18,997, NASA Landsat Science Team, grant #S-41396-F, USDA National
Research Initiative Grant Program, Electrical Power Research Institute,
Arizona Department of Water Resources, U.S. Environmental Protection Agency
-Office of Research and Development, CONACYT, ORSTOM, the French Remote
Sensing Program (PNTS) via the VEGETATION Project and the ERS2/ATSR2 Project,
and US Bureau of Land Management is gratefully acknowledged. Assistance
was also provided in part by the NASA/EOS grant NAGW2425, EPA STAR Graduate
Student Fellowship Program, National Science Foundation, US Geological Survey,
US Department of Energy contract W-7405-ENG-36, California Institute of
Technology-Jet Propulsion Laboratory (NASA, EOS/ASTER), WAU (Wag. Agricultural
University, Netherlands), Cochise County Highway and Flood Control Dept.,
and Ft. Huachuca; this support is also gratefully acknowledged. Special
thanks are extended to the ARS staff located in Tombstone, Arizona for their
diligent efforts and to USDA-ARS Weslaco for pilot and aircraft support.
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. |
5. REFERENCES
Arizona Department of Water Resources. 1991 Nov. Hydrographic survey
report for the San Pedro River watershed. Volume 1: general assessment,
in re the general adjudication of the Gila River system and source. Phoenix,
AZ: ADWR. Filed with the Court, November 20, 1991.
Shuttleworth, W. J., 1992. Evaporation. In Handbook of Hydrology, Chapter
4, Ed. by D.R. Maidment, Pub. by McGraw-Hill, Inc., New York, N.Y., p. 4.18. |
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