Environmental Risk Assessment and Management from a Landscape Perspective (Kapustka/Environmental Risk) || Predicting Climate Change Risks to Riparian Ecosystems in Arid Watersheds: The Upper San Pedro as a Case Study
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10PREDICTING CLIMATE CHANGE
RISKS TO RIPARIAN ECOSYSTEMSIN ARID WATERSHEDS: THE UPPER
SAN PEDRO AS A CASE STUDYdasdaaas Hector Galbraith, Mark D. Dixon, Juliet C. Stromberg,
and Jeff T. Price
Riparian areas function as keystone elements of the landscape, having a functionalimportance that far exceeds their proportional area. Ecosystem services provided byriparian areas include their roles as buffers controlling lateral movements of pollutantsor sediments between aquatic and terrestrial environments, corridors for facilitatinglongitudinal movement of organisms or materials across the landscape, and highly pro-ductive habitats that are often hotspots for biodiversity (Naiman et al. 1993, Naimanand Decamps 1997). In arid regions of the southwestern United States, riparian habitatsare particularly important for sustaining regional biodiversity, with a large proportionof species dependent on riparian systems (Patten 1998). Southwestern riparian sys-tems have an important influence on continental diversity of neotropical migrant birds,providing critical migratory corridors and stopover habitats through an otherwise aridregion (Skagen et al. 1998). Finally, as ecotones between terrestrial and aquatic ecosys-tems, riparian zones may be highly sensitive indicators or integrators of environmentalchange in the watersheds within which they occur (DeCamps 1993).
Watersheds and riparian systems of semi-arid to arid regions, such as the south-western United States, should be particularly sensitive to environmental changes thatinfluence hydrologic processes. Water is a limiting resource in the Southwest, bothfor natural ecosystems and for humans, presenting an important challenge for balanc-ing economic development and the conservation of riparian and aquatic ecosystems.
Environmental Risk and Management from a Landscape Perspective, edited by Kapustka and LandisCopyright 2010 John Wiley & Sons, Inc.
188 PREDICTING CLIMATE CHANGE RISKS TO RIPARIAN ECOSYSTEMS IN ARID WATERSHEDS
The majority of riparian and river systems in the desert Southwest have already beendegraded by a variety of anthropogenic stressors, including flow diversions and dams,groundwater depletion, land use change, urbanization, and overgrazing by livestock(Tellman et al. 1997, Patten 1998). Climate change is another stressor on these alreadystressed riparian systems. Climate change may directly affect the riparian ecosystemsor may interact with other stressors in complex ways, potentially exacerbating orameliorating their effects.
ECOLOGICAL IMPORTANCE OF THE UPPER SAN PEDRO RIVER
The source of the San Pedro River is near the town of Cananea in the State of Sonora,Mexico, from where it flows 240 km north through Arizona, to its confluence with theGila River near Winkelman, Arizona (Arias Rojo et al. 1999). The total drainage areais about 1900 km2 at the international border and about 12,000 km2 at its confluencewith the Gila River (Stromberg 1998). For most of its course, the San Pedro is a low-elevation, low-gradient (0.0020.005 m/km) alluvial stream, with elevation rangingfrom 1300 m at the Mexican border to 586 m at the confluence of the Gila River, adistance of 198 km (Huckleberry 1996). The section of the San Pedro that is within theUnited States (Fig. 10.1) is one of the few low-elevation rivers of its size in the desertSouthwest that contains significant reaches of perennial flow and is not regulated bydams.
Throughout its course, the San Pedro flows through an ecological matrix com-posed mainly of desert or semi-desert, with arid grasslands, Chihuahuan desert, andmesquite scrub being the most prevalent community types. Within this drier matrix,perennial or nearly perennial flows and shallow groundwater support lush riparianvegetation communities along the upper San Pedro (Fig. 10.2), including region-ally threatened or rare vegetation types like Fremont cottonwoodGooddings willow(Populus fremontiiSalix gooddingii ) gallery forest, riverine marsh or cienega, velvetmesquite (Prosopis velutina) woodland (bosque), and sacaton (Sporobolus wrightii )grassland. Maintenance of the high vegetative diversity and the cottonwoodwillowforests, in particular, are viewed as critical for sustaining the high avian biodiversityof the riparian corridor.
The presence of this riparian corridor within an arid landscape matrix providescritical habitat for migratory and breeding birds, as well as a high diversity of mam-mals, reptiles, amphibians, butterflies, and other animals (Arias Rojo et al. 1999). Forits size, the area has one of the highest vertebrate diversities found anywhere in theUnited States. Almost 390 bird species have been recorded there, of which 250 aremigrants that winter in Central or South America and depend on the San Pedro as astaging post on their journeys to and from their breeding areas in the United Statesand Canada. Between one and four million songbirds use the area as a migratory cor-ridor each year. Without a riparian northsouth habitat corridor provided by the SanPedro, these migratory journeys might not be possible. The San Pedro also provides(a) breeding habitats for some Central American bird species that reach their north-ernmost outposts in the area and (b) a wintering habitat for species breeding farthernorth.
ECOLOGICAL IMPORTANCE OF THE UPPER SAN PEDRO RIVER 189
San Pedro River
San Pedro Riparian NationalConservation Area...................
0 10 20 30 miles
0 10 20 30 40 50 km
Upper San PedroRiver Basin
Figure 10.1. Map showing the location of the San Pedro riparian area.
The Nature Conservancy has recognized the high ecological value of the SanPedro through its designation as one of the Last Great Places. Approximately 58,000ha (Steinitz et al. 2003) are protected along a 50-km reach of the river within the SanPedro Riparian Conservation Area (SPRNCA), administered by the US Departmentof the Interior Bureau of Land Management (BLM).
The high biodiversity of the San Pedro River riparian habitats provide a varietyof important ecosystem services. For example, the SPRNCA is nationally and inter-nationally recognized for its birdwatching opportunities, one of the fastest growingrecreational activities in the United States. Within the United States, Arizona is a
190 PREDICTING CLIMATE CHANGE RISKS TO RIPARIAN ECOSYSTEMS IN ARID WATERSHEDS
Figure 10.2. A typical view of the San Pedro Riparian National Conservation Area showingthe north-south running ribbon of lush hydric vegetation embedded in a more desert-like
major birding destination with most birdwatching occurring in the southeastern por-tion of the state, including the SRNCA. By 1997, the annual number of visitors tothe SPRNCA had grown to an estimated 100,000 visitors, a high proportion of thembeing birders and nature viewers.
Previous studies suggest that the San Pedro riparian ecosystem is already beingimpacted adversely by human use of the underlying groundwater aquifers. Ground-water withdrawals for human consumption jeopardize the riparian habitat by low-ering regional and potentially local floodplain water tables. The amount of waterpumped from the aquifer supporting the San Pedro riparian ecosystem has increasedby an order of magnitude in the last 5060 years. From small amounts of pump-ing (200,000 m3/day over the period 19761985, a level of intensity thatcontinued at least into 1997 (Goode and Maddock 2000), and probably continuestoday.
In this chapter we report the results of modeling studies performed on the ripar-ian ecosystem along the upper San Pedro River in southern Arizona and project howriparian ecosystem structure and function might be affected by changes in climate.Specifically, we model the potential effects of several plausible climate change sce-narios on the structure and composition of the riparian vegetation community, and weproject how such change may affect the systems ability to continue supporting a highdiversity bird community.
MODELING APPROACHES 191
We used two modeling approaches in this study. The first exercise was to modelclimate change scenarios. The second involved linking these scenarios to hydrologic,geomorphic, and vegetation responses.
Climate Change Scenarios
We used a 52-year daily time series of historic weather data (19512002) from theNational Weather Service station at Tombstone (station ID 028619, 31.7 latitude,110.05 longitude) to create four climate scenarios for the period 20032102. Thedaily 52-year record was cycled through twice to generate the 100-year scenario, withsimulation years 2003 and 2054 initialized with the adjusted 1951 daily data. Historictemperature trends from 1951 to 2003 were not removed from the time series. Useof daily historic data preserved the important seasonal and year-to-year patterns ofclimatic variation (e.g., influences of ENSO and PDO) that characterize the climateof the Southwest. All of the climate scenarios were transient, beginning with thesame conditions in 2003 and progressively diverging over the 100-year simulationperiod. We chose to use transient, rather than fixed, climate change scenarios to morerealistically represent gradual, cumulative changes in climate over the next century.
The scenarios were chosen to represent a reasonable set of potential climatetrajectories, given the range of projections for the region derived from climate modelsfor the southwestern United States regional assessment (SRAG 2000). The scenariosare as follows:
1. No Climate Change: (19512002 daily temperature and precipitationrepeated)
2. Warm: progressive temperature warming over 100 years, with a 4C increasein maximum daily temperature and a 6C increase in minimum daily temper-ature by 2102
3. Warm Dry: progressive temperature warming as in #2 and a progressivedecline in winter (nonmonsoonal: October 1 to May 31) daily precipitation of50% by 2102
4. Warm Wet: progressive temperature warming as above, with a progressiveincrease in winter daily precipitation of 50% by 2102.
Changes in temperature and precipitation, relative to historic values, were appliedlinearly over the 100-year period. Hence, for the warming scenarios (scenarios 24),daily minimum temperatures were increased 0.06C per year and daily maximumtemperatures 0.04C per year over 20032102. Similarly, for the precipitation increase(scenario 4), winter precipitation was increased by 0.5% per year from 20032102.Changes in precipitation were applied only to days in the historic record that hadmeasurable precipitation. Hence, precipitation totals for individual days were adjustedupward or downward, without changing the frequency of rain events. For the wetter
192 PREDICTING CLIMATE CHANGE RISKS TO RIPARIAN ECOSYSTEMS IN ARID WATERSHEDS
scenario, this effectively increased the magnitude of extreme events, which is oneexpectation of climatic change (Easterling et al. 2000, Houghton et al. 2001).
Changes in temperature were similar to those projected for the region by the Cana-dian Climate Centre (CCC), Hadley 2, and NCAR Regional models (SRAG 2000). Weassumed that daily minimum temperatures would increase more than maximum dailytemperatures, consistent with IPCC projections and recent observations (Houghtonet al. 2001). Our consideration of both wetter and drier scenarios reflected differencesamong climate models in projected winter precipitation changes for the Southwest(SRAG 2000). The CCC and Hadley 2 models suggest a strong increase (perhaps adoubling or more) in winter precipitation, while the NCAR regional model suggests adecrease. None of the models projected a significant change in summer (monsoonal)precipitation, so historic daily values for June 1 to September 30 were retained in thescenarios.
Hydrologic, Geomorphic, and Vegetation Modeling
Following is an overview of these modeling efforts at three study sites. Each studysite was evaluated to examine the hydrologic, geomorphic, and vegetation responses,which in turn were used to assess vulnerability of bird species and ultimately theresponse to overall bird diversity.
Overview. In addition to the direct physiological effects of temperatureincreases on vegetation, we assumed that important effects of climate changeon riparian ecosystems would be mediated by influences on river flow regimes,disturbance (e.g., fire, flood), and geomorphic dynamics (river channel migration).Changes in alluvial groundwater levels would also have an important influence onriparian vegetation. Although we did not simulate groundwater dynamics in responsesto climate change or regional groundwater pumping, we did model vegetationdynamics across sites that spanned a gradient in groundwater depth and surface flowintermittency. Comparisons of vegetation dynamics on these sites may yield insightsto the potential interactive effects of climate and groundwater change.
Study Sites. The three sites chosen for simulation runs all occur within theSPRNCA and roughly correspond to the three classes of the Riparian Condition Indexdeveloped by Stromberg et al. (2006). The perennial flow type (Kolbe site) can becharacterized as a hydrologically gaining reach, in which shallow groundwater sup-ports baseflow throughout the year; while the intermittent flow types represent losingreaches in which baseflows cease during seasons in which the groundwater level fallsbelow the river thalweg. Wet intermittent reaches (Palominas site) are those that havesurface flow during the majority of months during a normal year (but are not peren-nial), while dry intermittent reaches (Contention site) flow less frequently, typicallyless than 60% of the time.
Hydrology. We simulated the potential effects of the four climate scenarios ondaily stream flow at the Charleston USGS gage, using a watershed runoff model,the Soil Water Assessment Tool (SWAT; Arnold et al. 1994) within the Automated
MODELING APPROACHES 193
Geospatial Watershed Assessment Tool (AGWA) interface (Hernandez et al. 2000,2003; Kepner et al. 2004) in ArcView. These simulated stream flows were then usedas inputs to the geomorphic and vegetation models (see below). We derived climaticinputs for runoff model from five weather stations (including Tombstone) distributedthroughout the upper San Pedro basin, and we adjusted daily precipitation valuesto reflect the four climate scenarios. Temperature data were derived only from theTombstone station. Other inputs to the SWAT model included soils, basin topography,and land cover datasets from the upper San Pedro basin. Hydrologic parameter settingswere based on previous calibration of the model to historic annual stream flow valuesin the upper San Pedro basin (Hernandez et al. 2000, 2003).
Geomorphology. We modeled the potential effects of stream flow changes onriver channel migration using the program MEANDER (Larsen and Greco 200...