1. USE OF 18O TO ASSESS COMPETITION FOR WATER BETWEEN GRASSLAND AND SHRUBS IN THE SONORAN DESERT1
Jean-Pierre Brunel2 and D.G.William3
1 Paper presented at the 11th Annual Symposium of the Arizona Hydrological Society, Tucson, September 23-26, 1998
2 ORSTOM-IMADES, Reyes y Aguas Calientes Esq., Hermosillo, Mexico; tel./fax: 52 62 10 15 95; email: brunel@cideson.mx.
3 University of Arizona, School of Natural Renewable Resources, Tucson, AZ 85721, USA; tel: 520-621-7259; email: dgw@ag.arizona.edu.
Introduction
The delicate ecological balance of the semi-arid grassland of North America was greatly disturbed by the introduction of cattle more than one century ago. As a consequence there has been a decreasing of grass density and invasion by shrub species (Mesquites).
In the Upper San Pedro Basin straddling Sonora (Mexico) - Arizona (USA) where our research is currently carried only, grasslands decreased by 35% and woodlands increased by 50% between 1974 and 1987. Our study aims to understand the competition for water between grass and shrubs and will attempt to estimate potential for space-time extension of invasion of grassland by shrubs. For that, stable isotope of water, 18O is used to determine over several growing seasons the origins of water used by the vegetation.
This paper aims to present some preliminary results from the 1997 monsoon
season campaign (July - October).
Site description
The study area is near Cananea ( 30 58 N, 110 17 W, elevation 1500 m). It is a high plateau surrounded with mountain peaks culminating between 2500 and 3000 m. The mean annual rainfall is certainly spatially highly variable because of strong hydrographic gradients. At the altitude of the plateau (1400-1600 m), mean annual rainfall is respectively 349 mm (Naco) and 507 mm (Cananea) with 67% of annual precipitation falling between July and October. Our field site is a natural 1 km transect starting within a dense canopy mesquite shrub ( Prosopis sp.) area with bare soil (site B) to a degraded grassland ( Bouteloua sp. and Aristides sp.) with sparse trees (site S).
Site B was near a river bed occasionally flooded, site S was on the top of a plateau 30 to 40 m higher.
Methods
From July to October the following samplings were made: every rainfall at a near by site and Mesquite stems for further extraction of sap water. Ground water was sampled at the beginning and the end of the sampling period. Samples were analyzed for 18O composition using a mass spectrometer. 18O composition of the Mesquite sap water was then compared to rainfall and ground water 18O composition.
Preliminary results and discussion
Results from this first campaign showed:
- 18O composition of ground water was -9.4‰ and -8.6‰ indicating clearly a recharge from winter fall even if more than 60% of rainfall occurred during the monsoon.
- averaged weighted 18O composition for the monsoon rainfall is -3.1‰
- at site B,18O composition of sap water in trees (-7‰ to -9‰) showed that they have a permanent access to the water table ,
- at site S, this composition showed that trees use either ground water or rainfall or a mixture of both but the time series showed that as soon as substantial rainfall water is available they would use it.
At site B Mesquite are able to extend quickly and by building a dense canopy tend to prevent grass to develop. At site S there is a direct competition for water between grass and trees, competition being in favor of Mesquite with the access to water table even if deep.
Conclusion
These first results seem to show that just considering water availability there is high potential for spatial extension of the phenomenon.
The next field campaign should bring more information and the addition of micrometeorological instrumentation and sampling of water vapor for isotope. analysis on the sites would allow to partition transpiration from both the trees and the grass.
2. RÍO SAN PEDRO: WATER RESEARCH ISSUES IN THE MEXICAN WATERSHED1
(to be presented by H. Arias)
Bruce F. Goff2, Héctor M. Arias3, David C. Goodrich2, and Ghani Chehbouni4
1 Paper presented at the 11th Annual Symposium of the Arizona Hydrological Society, Tucson, September 23-26, 1998.
2 USDA Agricultural Research Service, 2000 E. Allen Road, Tucson, AZ 85719, USA; tel: 520-670-6380; fax: 520-670-5550; email: bgoff@tucson.ars.ag.gov and goodrich@tucson.ars.ag.gov
3 IMADES, Reyes y Aguascalientes, Esq., Col. San Benito, Hermosillo, C.P. 83190, Sonora, Mexico; tel: 52 (62) 15 9881 or 15 9864; fax: 52 (62) 14 6508; email: arias@cideson.mx
4 ORSTOM, Reyes y Aguascalientes, Esq., Col. San Benito, Hermosillo, C.P. 83190, Sonora, Mexico; tel: 52 (62) 10 1595; fax: 52 (62) 14 6508; email: Ghani@cideson.mx
The San Pedro River of southeastern Arizona, USA and northeastern Sonora, Mexico is an important ecological and economic resource for both countries. In recent decades, accelerated human development in the watershed and consequent increases in groundwater use have created a water deficit that threatens the survival of the riparian ecosystem. Although much is known about the US side of the watershed, expert reports and comments by the public indicate a relative lack of knowledge about the hydrology and water-related resources on the Mexican side. In some cases, the data do not exist; in other cases, the data that exist are not readily available due to institutional or cultural (e.g., communication) constraints. One ongoing effort, the SALSA global change research program (see description by Goodrich et al., this issue), seeks to fill this knowledge gap through a coordinated, multi-national study of the factors affecting the water balance and ecological complexity of the upper San Pedro watershed, on both sides of the border.
The río San Pedro is formed by the contributions of several small tributary streams which originate in the surrounding mountain ranges and the grass and shrub covered uplands. Despite the ephemeral nature of many of these streams, certain reaches along the larger tributaries, such as "Los Fresnos," and the río San Pedro itself are considered perennial or intermittent and support significant stands of riparian vegetation. At the upper end of the watershed, the Cananea copper mine maintains an extensive network of groundwater wells to support its operations and for municipal and rural water supply. Ranching occurs throughout the río San Pedro watershed and farming takes place along the floodplains of major tributaries. Each of these activities influences the basin water balance in some way, and each is, in some way, in competition with the others over the limited water resource.
Two federal agencies share responsibility for monitoring water resources in the río San Pedro watershed, the Comisión Nacional del Agua (The National Water Commission) and the Comisión Internacional de Límites y Aguas (International Water Boundary Commission). However, the main source for quantitative information about the flow in the río San Pedro is the USGS gaging station located just across the border at Palominas, Arizona. But because stream flow at this station may also be affected by groundwater use on the US side, measured trends do not necessarily reflect changes in the water use on the Mexico side. Much of the effort to monitor water and other water-related resources in the watershed has been undertaken by the state agency, Instituto del Medio Ambiente y Desarrollo Sustentable del Estado de Sonora (IMADES), along with several institutions and non-governmental agencies. They have been assisted in these efforts by various US and Arizona agencies, including the EPA, USGS, National Park Service, USDA, Arizona Game and Fish Department, among others.
As part of the SALSA program, several studies have been undertaken to increase our understanding of the hydrology of the río San Pedro. These include the use of satellite imagery to map and classify vegetation cover on the watershed, an intensive field campaign in 1998 to evaluate the evapotranspiration component of the water balance, the measurement of surface flows from grassland and shrubland sub-watersheds, and the development of a comprehensive watershed model. However, more must be done to understand the effect of various landuse and groundwater withdrawal activities on surface water-groundwater interactions. There is a critical need to determine the actual geometry of the aquifer or aquifers underlying the basin. This is needed to evaluate the effect on the river of current and prospective groundwater pumping by the mine. SALSA, in cooperation with other agencies working in the area, will provide a framework for answering questions about the hydrology of the río San Pedro, and integrate this into a comprehensive understanding of the water balance and ecological complexity of the entire Upper San Pedro Basin.
3. PANEL DISCUSSION: WATER RESEARCH NEEDS FOR THE UPPER SAN PEDRO BASIN 1
Bruce F. Goff2 (Moderator)
1 Paper presented at the 11th Annual Symposium of the Arizona Hydrological Society, Tucson, September 23-26, 1998.
2 USDA Agricultural Research Service, 2000 E. Allen Road, Tucson, AZ 85719, USA; tel: 520-670-6380; fax: 520-670-5550; email: bgoff@tucson.ars.ag.gov
The Upper San Pedro Basin is a broad, high-desert valley sustaining a small stream and located in southeastern Arizona, USA, and northeastern Sonora, Mexico. The valley is home to a major Army installation in the US and a large copper mine in Mexico. Although significantly altered over the past century by human activities, the valley still supports a great diversity of plant and animal species-some endangered with extinction-and is widely recognized as a regionally and globally important ecosystem.
In the past few decades, local residents, resource management agencies, and environmental organizations have become increasingly concerned about the over-allocation of the basin's ground and surface water resources, and the potential loss of critical riparian habitat caused by lowering water table levels. Government agencies have taken a variety of practical measures to try to mitigate the impact of groundwater used by the municipalities, Fort Huachuca, and the Cananea mine, but many critics view these as insufficient. A recent report by the tri-national Commission on Environmental Cooperation has prompted the community to take a closer look at the problem and to try to find more effective means of protecting water supplies and associated riparian resources.
This panel brings together several stakeholders involved in the Upper San Pedro Basin issue, including members of local resource and environmental groups, and federal, state, and government agencies. The panelists will briefly discuss their institutional and personal perspectives of the problem, and then propose ways in which the science community can help local residents and management agencies obtain the information needed to better manage water resources in the basin. After the panel discusses a set of questions posed by the moderator, the audience will be invited to direct questions to the panel for discussion.
4. The San Pedro River and the Semi-Arid Land-Surface-Atmosphere (SALSA) Program1
D. C. Goodrich2, A. Chehbouni3, B. Goff2 , B. MacNish4, T. Maddock4, S. Moran5, D.G. Williams4, C. Watts12, L.H. Hipps6, D.I. Cooper7, Y.H. Kerr8, H. Arias12, G. Boulet3, J.P. Brunel13, H. DeBruin11, G. Dedieu8, W.E. Eichinger10, B. Jones15, W. Kepner15, J-P. Lhomme3, B. Monteny3, Y. Nouvellon9, D. Pool14, J. Qi5, S.M. Schaeffer4, J. Schieldge13, R. Scott4, K.A. Snyder4, S. Sorooshian4, M.P.L. Whitaker4
1 Paper presented at the 11th Annual Sym. of the Arizona Hydrological Soc., Tucson, AZ, Sept. 23-26, 1998.
2 USDA-ARS, 2000 E. Allen Rd., Tucson, AZ 85719, Ph. (520)670-6380x175,
Fax (520)670-5550; goodrich@tucson.ars.ag.gov; corresponding author;
3 ORSTOM/IMADES, Hermosillo, Sonora, Mexico
4 University of Arizona, Tucson, AZ
5 USDA-ARS Water Conservation Laboratory, Phoenix, Arizona, USA
6 Utah State Univ., Logan, UT, USA
7 Los Alamos Nat. Lab., Los Alamos, NM, USA
8 CESBIO, Toulouse, France
9 CIRAD, Montpellier, France
10 Univ. of Iowa, Iowa City, IA USA
11 AUW, Wageningen, The Netherlands
12 IMADES, Hermosillo, Sonora, Mexico
13 Jet Propulsion Lab., Pasadena, CA, USA
14 USGS-WRD, Tucson, AZ, USA
15 US-EPA, Las Vegas, NV, USA
INTRODUCTION
The primary objective of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program is to understand, model and predict the consequences of natural and human-induced change on the basin-wide water balance and ecological diversity of semiarid regions over a wide range of time scales. SALSA is a long-term program whose current research and integrated measurement efforts are focused on the San Pedro River basin which originates in Sonora, Mexico and flows north into Arizona. This presentation provides an overview of the 1997 SALSA Program elements and data collection activities.
It also serves as an introduction to several more detailed SALSA presentations and posters being made at this meeting.
1997 SALSA SCIENTIFIC OBJECTIVES
1997 SALSA activities are part of a longer term (3 to 10 year effort) to address the primary objective developed from initial program goals and an international workshop (Wallace, 1995). The 1997 SALSA objectives, focused on priorities established at the workshop and within logistical and monetary constraints are:
1) To improve the diagnosis of surface fluxes used in atmospheric models with grid spacings of several kilometers and compare remote and in-situ observations with real-time model runs;
2) To initiate the development and validation of a coupled soil-vegetation-atmosphere transfer (SVAT) and vegetation growth model for semi-arid regions that will assimilate remotely sensed data with several years of observed data;
3) To conduct in-situ and remote measurements to: a) quantify and develop models for groundwater, surface water, and evapotranspiration interactions on a seasonal basis; b) identify plant water sources; and c) identify plant function and atmospheric controls on a semi-arid riparian system consisting of mesquite, sacaton, and cottonwood/willow vegetation communities.
4) To develop and validate aggregation schemes with data over very highly heterogeneous surfaces; and,
5) To develop a multi-scale system of landscape pattern indicators using remotely sensed data to estimate current status, trend and changes in ecological condition.
Other factors involved in the selection of the above objectives include the desire to maintain and initiate long term observations and multi-disciplinary research in the San Pedro Basin, and to assist in addressing a number of pressing socio-economic concerns in the region. Long-term monitoring and research is essential to capture a range of both seasonal and interannual climatic variations and their associated impacts on basin water resources and ecology.
Approaches and methodology to address the objectives outlined above will be presented as well as initial results.
References
Wallace, J., 1995: Multidisciplinary Program Studies Land-Atmosphere Interactions in Semi-Arid Regions, EOS, Transactions, American Geophysical Union, 76(46), 465,469.
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 Nat. Research Initiative Grant Program, Electrical Power Research Institute, Arizona Dept. of Water Resources, U.S. Environmental Protection Agency -Office of Research and Development, CONACYT, ORSTOM, the French Remote Sensing Program (PNTS), 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, NSF, 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 and to USDA-ARS Weslaco. 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. A LANDSCAPE APPROACH FOR EVALUATING ECOLOGICAL RISK IN A SEMI-ARID ENVIRONMENT1
William G. Kepner2, Christopher J. Watts3, Curt M. Edmonds2, Hector Arias3
1 Poster presented at the 11th Annual Symposium of the Arizona Hydrological Society, Tucson, AZ, September 23-26, 1998.
2 U.S. Environmental Protection Agency, Office of Research and Development, Las Vegas, Nevada 89193-3478 (kepner.william@epamail.epa.gov, edmonds.curtis@epamail.epa.gov)
3 Instituto del Medio Ambiente y el Desarrollo Sustentable del Estado de Sonora, Hermosillo, Sonora 83190 (watts@cideson.mx, arias@cideson.mx)
The U.S. Environmental Protection Agency and the Instituto del Medio Ambiente y el Desarrollo Sustentable del Estado de Sonora have initiated a regional approach to assess ecological risk relative to exposure to environmental stressors under the interagency SALSA project. The Semi-Arid Land-Surface-Atmosphere (SALSA) project is a collaborative research effort comprised of international scientists committed to the study of land degradation in the Upper San Pedro Watershed using advanced space-based technologies. A key component of the SALSA framework enlists landscape ecology as a theoretical basis from which to assess cumulative exposure to stress at multiple spatial and temporal scales. This project has focused its research into developing a system of landscape composition and pattern indicators which can be used to estimate current status, trend, and changes in ecological and hydrological condition. Specifically, it is designed to determine ecosystem vulnerability relative to large-scale natural or man-induced disturbances using a system of landscape pattern metrics derived from remote sensing, spatial statistics, and geographic information systems technology. The project utilizes a strategy which incorporates triplicate Landsat Multi-spectral Scanner imagery from the early 1970s, mid-1980s, and early 1990s to generate land cover data (at 60m resolution) and establish 20-year trends in condition for selected geographic areas.
This process has been tested in a small community-based watershed in southeast Arizona (US) and northeast Sonora (Mexico) using 10-class land cover digital maps generated from Landsat-MSS imagery. Land cover (including urban) has been classified at the Formation level from the Brown, Lowe, and Pase Biotic Communities Classification System for North America.
Preliminary results indicate that extensive grassland and desertscrub areas with high connectivity are the most vulnerable biome type to encroachment of woody shrubs (mesquite). During the period of study, grasslands and desertscrub decreased by 18% and 21%, respectively, on a relative basis. Xerophytic cover types, i.e. mesquite woodland, increased in overall extent by 413%. In 1992, grasslands remained the most extensive land cover type within the study area, however, human-dominated activity has lead to precipitous increases in anthropogenic cover during the last 18 years. Barren (hardrock mining), agriculture, and urban have increased in extent by 84%, 139%, and 285%, respectively.
The change detection information can be further quantified as pixel gain/lost tables which demonstrate how land cover converted at the pixel scale of resolution during the 19-year period. In this example, the native desertscrub and grassland was predominantly lost to mesquite woodland (74.6% and 67.9%, respectively), urban (13.3% and 17.5%, respectively), agriculture (5.3% and 9.5%, respectively), and barren (3.1% and 2.2%, respectively). Collectively, 98% of the mesquite woodland gain was derived from desertscrub and grassland.
Grasslands and desertscrub not only decreased in extent during the study period but also became more fragmented, i.e. the number of grassland and desertscrub patches increased by 13% and 47%, respectively, and the average grassland and desertscrub patch sizes decreased 28% and 46%, respectively. These two dominant cover types not only dramatically changed in patch size metrics but they each became less connected over time. In stark contrast, mesquite woodland increased in connectivity by 23% and the average patch size and number of patches increased 51% and 239%, respectively.
The present exploratory study was limited to the application of some simple landscape pattern metrics to demonstrate change detection methodology and landscape indicator development to assess watershed vulnerability in a semi-arid environment. The landscape analysis can be enhanced to accommodate more complex diagnostic statistics and can be applied to more detailed land cover maps with finer grain and greater classification detail (such as those derived from SPOT or fine-scale aerial photography). The process demonstrates a simple procedure to characterize biological systems that are at potential risk from exposure to one or more stressors. This approach provides an important new tool in which to evaluate land management and ecological change over time.
6. A STATISTICAL ANALYSIS OF LOW FLOWS ON THE SAN PEDRO RIVER, ARIZONA1
Richard Koehler2 and George Ball3
1 Paper presented at the 11th Annual Symposium of the Arizona Hydrological Society, Tucson, September 23-26, 1998.
2 Cochise County Highway and Floodplain Department, 1415 W. Melody Lane, Bisbee, AZ 85603, USA; tel: 520-432-9674, fax: 520-432-9645; email: hydro@theriver.com
3 School of Renewable Natural Resources, Advanced Resource Technology Group, University of Arizona Tucson, AZ 85721, USA; tel: 520-621-7255; fax: 520-626-7401; email: gball@nexus.srnr.arizona.edu
A statistical analysis of season low flow characteristics for the San Pedro River was completed for data from the USGS gaging station at Charleston, Arizona. The 7-day annual and winter (December through February) low flows were used as an approximation of baseflow using data from 1935 to 1996.
Multivariate regression shows that for the 7-day low annual series that La Niña episodes, El Niño episodes and previous winter 7-day low flow values are statistically significant beyond the 95% confidence interval. The regression slope indicates an annual decline of 0.04 ± 0.01 cubic-feet per second (cfs)/year in the annual series. The adjusted r2 of the regression was 0.66 with an RMS of 0.65. The winter 7-day low flow series shows no significant trend over time but does have more variability than the annual series.
Additionally, daily values were used in an analysis of spring decreases and fall increases to the baseflow. Results show both spring and fall values ranged from 0.1 to 0.2 cfs/day with no detectable trend over time.
7. GROUNDWATER-SURFACE WATER INTERACTIONS IN THE UPPER SAN PEDRO BASIN, Robert D. Mac Nish2, Thomas Maddock, III2, and Martha P. L. Whitaker
Poster presented at the 11th Annual Symposium of the Arizona Hydrological
Society, Tucson, AZ, September 23-26, 1998.
Dept. of Hydrology & Water Resources, University of Arizona, Tucson,
AZ, 85721; macnish (621-3041), maddock (621-7115), mplw (621-7115), (all)@hwr.arizona.edu
The Lewis Springs Research Site is located on the north side of Highway 90 where the highway crosses the San Pedro River near Sierra Vista, Arizona. Twenty nine piezometers ranging in diameter from 6 mm to 100 mm, and in depth from 3 m to 9 m are located in three transects perpendicular to the river with each transect having a two or three well cluster on each side close to the river, and another two or three well cluster about 100 meters from the river on each side. The twelve clusters form a 200 meter wide rectangular area that extends 425 meters parallel to the river. All but one cluster contain two or three piezometers open to the floodplain aquifer over 150 mm intervals immediately below the water table, and over similar intervals at depths of approximately 3 and 6 meters below the water table. In the central or Middle transect, the clusters on the west side of the river also have piezometers open to the regional aquifer over a 1.5 meter interval at depths of approximately 60 meters. Staff gages and/or stilling tubes were installed at five points along the stream, with the upstream, middle, and downstream installations closely coincident with the piezometer transects.
Over the course of 32 to 48 hour long periods in March, April, June, and August, groundwater levels and stream stages were measured at hourly intervals at all piezometers and staff gage/stilling tubes. Concurrently during these synoptic studies, dye was injected into the stream about 200 meters upstream of the upstream transect, and samples of the stream-dye mixture were collected hourly at each stream stage installation. Other data collected in this effort included streamflow measurements by current meter during all synoptic periods. A flume was installed at the upstream site in June, with data continually recorded at the flume by a bubble gage system. Bubble gages were also installed at some stream stage sites in June, and in all stream stage sites in August. A few meters north of the upstream transect, arrays of water content reflectometers, tensiometers, thermistors, and neutran probe access tubes were installed on opposite sides of the stream to monitor changes in soil moisture in space and time, as the fluctuations in streamflow and plant transpiration acted as stressors on the vadose zone.
In March, over the 19th and 20th, water levels in the piezometers dropped approximately 0.006 m over the course of the synoptic period, but displayed no pattern of diurnal fluctuations. The stream stage dropped about 0.004 m over the same time period, and did show a slight diurnal range of about 0.002 m, with the streamflow at about 210 l/s. The vegetation was still largely dormant at this time with only a few trees displaying emerging buds.
A month later, in the synoptic period that started April 19th and ran through the 21st, the trees were well leafed out, and ground water levels in the piezometers showed a distinct diurnal fluctuation of about 0.010 m, and declined at a rate of about 0.007 m per day. Stream stage during the April synoptic showed a diurnal fluctuation of about 0.016m, and over the entire synoptic period dropped approximately 0.012 m. Streamflow was about 140 l/s in April.
By June 7th, when the third synoptic study began, streamflow had dropped to about 40 l/s. The stage data for June has not yet been fully processed, and the diurnal variations, and decline over the 48 hour period have not yet been determined. The trees were in full leaf, and the water levels in the floodplain aquifer showed a diurnal fluctuation of 0.025 m, and dropped about 0.002 m per day during the synoptic period.
The August synoptic period began on August 11th, and ran through the 13th. The trees were still in full leaf, but unlike the other synoptic period, "monsoon" precipitation had fallen, providing moisture for the plants. Streamflow had diminished to about 2 l/s, and stream stage showed a diurnal range of about 0.006 m and dropped over the course of the synoptic at a rate of about 0.003 m/day. Ground water levels in the floodplain aquifer showed a diurnal fluctuation of 0.012 m, and dropped at a rate of about 0.002 m/day.
Horizontal water level gradients in the floodplain aquifer are primarily northward parallel to the river, but do show some convergence of flow toward the river. The river-ward gradients did not change appreciably over the summer, but the down valley gradients diminished over 15% just between April and June.
Vertical gradients in the floodplain aquifer showed consistently upward components of flow in the floodplain aquifer in March, but as the riparian vegetation leafed out and began transpiring water, the vertical gradients changed from upward to downward and back to upward in some piezometer clusters over the course of a single day.
Concurrent measurements of sap flow rates in the trees of the riparian corridor are providing estimates of transpiration rates for individual trees during the synoptics, and when this information has been aggregated to develop areal estimates of transpiration fluxes, the variations observed in the vertical gradients may be analyzed in light of a "known" stress.
The dye injection equipment did not inject dye into the stream at a constant rate over any of the synoptic periods, and the determination of the variation in injection rate over time has been a daunting task, but is nearing a successful conclusion. As this information is developed, we will be able to determine the incremental increases in flow between the stream stage installations and be able to correlate that with the observed vertical and horizontal gradients in the adjacent floodplain aquifer to get definitive estimates of aquifer/stream bed hydraulic parameters.
Data from the soil moisture equipment likewise awaits complete analysis to complement the data from the piezometers, stream stage sensors, the dye dilution gaging, and the transpiration study in developing a quantitative understanding of the functioning of hydrologic processes in a riparian corridor.
8. EVAPOTRANSPIRATION FROM THE RIPARIAN CORRIDOR, UPPER SAN PEDRO RIVER1
J. Qi2, R. Marsett3, D. C. Goodrich4, M. S. Moran5, R. Scott6, S. Schaeffer7, A. Chehbouni8, B. F. Goff4
1 Paper presented at the 11th Annual Symposium of the Arizona Hydrology Society, Tucson, AZ, September 23-26, 1998.
2 Dept. of Geography, Michigan State University, East Lansing, MI 48824, Email: qi@pilot.msu.edu
3 USDA-ARS Southwest Watershed Research Center, Tucson, AZ 85719, Tel. (520) 670-6380
4 USDA-ARS Southwest Watershed Research Center, Tucson, AZ 85719, Tel. (520) 670-6380
5 USDA-ARS Southwest Watershed Research Center, Tucson, AZ 85719, Tel. (520) 670-6380
6 Hydrology and Water Resources Dept., University of Arizona, Tucson, AZ 85721
7 School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721
8 ORSTOM, Hermosillo, Mexico
Introduction
Water in the southwest US has been and remains an issue of paramount importance. Its quality and usage have been the source of political and legal activity for over 100 years (Carey Act, 1894). In Arizona, the focal point of has been on the water issues in the San Pedro Riparian corridor. Increased water pumping from near by cities such as Sierra Vista, therefore, reduced recharge rate of the river, has raised serious concerns about the ecological impacts on the corridor. In order to gain scientific understanding of these issues, the SALSA Program was initiated at the USDA-ARS Southwest Watershed Research Center in 1997 (Goodrich et al, this proceedings). One of the research objectives was to estimate the evaporative water losses from riparian vegetation compared with runoff and ground water recharges. This paper briefly describes how remote sensing technology can be used in conjunction with in situ measurement to spatial and temporal evaporative water losses from the corridor.
Experimental Description and Methodology
During an intensive field campaign in August 1997 remote sensing images
from satellite and multi-altitude aircraft sensors were acquired at spatial
resolutions ranging from 0.63m to 30m. Ground in situ measurements of surface
temperatures, evaporative water losses, sensible heat fluxes, and the available
incoming solar radiation, along with other meteorological variables, were
made at three sites: one being dominated with cottonwood trees, one with
mesquite trees, and another one with Sacaton grasses.
The evapotranspiration (ET) or evaporative water losses to the atmosphere
was estimated based on the basic water balance equation:
ET = Rn - G - H (1)
where Rn is net radiation (w/m2), G is soil heat flux, and H is sensible heat flux. Equation (1) can be modified (Jackson et al., 1977) as a function of the temperature difference between the surface and the air above: dT:
ET = A - B (dT) (2)
where A = Rn - G, which is sometimes termed the available energy. Parameters A and B can be determined empirically using ground data, and the dT can be calculated with surface temperature from remote sensing images and the air temperature from meteorological stations.
Preliminary Results
Figure 1 is the ET relationship with the differential temperature dT for Sacaton grass. The relationship indicates that as the surface temperature is warmer than the air temperature, there is more water loss. This is contradictory to the theory, which indicates that when the surface temperature is cooler than the air temperature (driving force), the greater water losses to the atmosphere.
In Figure 2, the data over cottonwood and mesquite was hourly averaged and plotted against the dT. It agrees with the theory. The different trends of these three types of vegetation indicate that the use of eq. (2) remains further investigation.
REFERENCES
Carey Act, 1894, Federal government ceded land to the state for irrigation.
Jackson R. D. R. J. Reginato, S. B. idso, 1977, Wheat canopy temperature: A practical tool for evaluating water requirements, Water Resourc. Res., 13, 651-656.
9. LANDSCAPE CHANGE IN THE UPPER SAN PEDRO WATERSHED1
Christopher Watts2, William Kepner3, Curtis Edmonds3 and Hector Arias2
1 Paper presented at the 11th Annual Symposium of the Arizona Hydrological Society, Tucson, AZ, September 23-26, 1998.
2 Instituto del Medio Ambiente y el Desarrollo Sustentable del Estado de Sonora, Hermosillo, Sonora 83190 (watts@cideson.mx)
3 U.S. Environmental Protection Agency, Office of Research and Development, Las Vegas, Nevada 89193-3478 (kepner.william@epamail.epa.gov)
The U.S. Environmental Protection Agency (EPA) and the Instituto del Medio Ambiente y el Desarrollo Sustentable del Estado de Sonora (IMADES) have initiated a regional approach to assess ecological risk relative to exposure to environmental stressors under the interagency Semi-Arid Land-Surface-Atmosphere (SALSA) program, which is a collaborative research effort comprised of international scientists committed to the study of land degradation processes in semi-arid areas using advanced space-based technologies. The main study area was a small international watershed in southeast Arizona (US) and northeast Sonora (Mexico), the Upper San Pedro Watershed which is part of the Lower Colorado river system. A key component of the SALSA framework enlists landscape ecology as a theoretical basis from which to assess cumulative exposure to stress at multiple spatial and temporal scales.
This project has focused its research into developing a system of landscape composition and pattern indicators which can be used to estimate current status, trend, and changes in ecological and hydrological condition. Specifically, it is designed to determine ecosystem vulnerability relative to large-scale natural or man-induced disturbances using a system of landscape pattern metrics derived from remote sensing, spatial statistics, and geographic information systems technology. The project utilizes the data base from the North American Landscape Characterization project which incorporates triplicate Landsat Multi-Spectral Scanner (MSS) imagery from the early 1970s, mid 1980s, and early 1990s, which have been remapped and projected to UTM coordinates with 60m pixels to generate land cover data and establish 20-year trends in condition for selected geographic areas.
Very little quantitative information on a regional or watershed scale
is available about the status of land degradation for rangelands such as
those that occur in the San Pedro watershed, yet these represent the most
extensive land use type in the world's drylands. Nearly 85% of the North
American rangelands are estimated to be in degraded condition; more than
any other continent in the world. Landscape composition, connectivity,
and patch sizes and number were used to evaluate ecosystem resilience and
changes in land cover extent over a 19-year period in the San Pedro River
basin. Land cover was derived from Landsat MSS images (June 73 and August
1992) and classified into the Brown, Lowe and Pase (BLP) Biotic Communities
Classification System for North America (1979). All vegetation has been
classed to the Formation (Biome) level of the BLP system and an urban cover
class has been included. The principal cover classes for the region are
grassland, desertscrub and woodland. However there are important differences
across the international border. In the US, the three classes have similar
relative areas, whereas in Mexico, the relative area of the grassland is
much greater and that of desertscrub much smaller.
Preliminary results about changes in land cover for the study period indicate
that extensive grassland and desertscrub areas with high connectivity are
the most vulnerable ecosystems to fragmentation due to encroachment of
woody xerophytic shrubs and trees associated with the mesquite woodland.
In the study period, grasslands and desertscrub not only decreased in extent
but also became more fragmented. That is, the number of grassland and desertscrub
patches increased and their average patch sizes decreased. In stark contrast,
the mesquite woodland patches increased in size, number and connectivity.
These changes have significant impact for the hydrology of the region,
since the energy and water balance characteristics for these cover types
are significantly different. These differences are currently being studied
both sides of the border in the context of SALSA.
Primary anthropogenic stressors in the project area include urbanization and livestock grazing, however they differ in their magnitude and distribution throughout the watershed. The preliminary results of this project are presented to 1) illustrate the change detection strategy and 2) demonstrate the potential application of the approach for developing a regional or watershed program for systematic assessment of ecological condition, especially in regard to land degradation in arid and semi-arid regions of the United States and Mexico.
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