Scenario Analysis for the San [Pedro River:
analyzing Hydrological Consequences of
a Future Environment
William G. Kepner1*, Darius J. Semmens1, Scott D. Bassett2, David A. Mouat2, arid David C. Goodrich3
1U.S. Environmental Protection Agency, Office of Research and Development, P.O. Box 93478, Las Vegas, Nevada 89193 USA
2Desert Research Institute, Division of Earth and Ecosystem Sciences, 2215 Raggio Parkway, Reno, NV 89512 USA
3USDA Agricultural Research Service, Southwest Watershed Research Center, 2000 E.Allen Road, Tucson, AZ 85719 USA
("author for correspondence, phone: 702-798-2193, fax: 702-798-2208, e-mail: kepner.wiliiam@epa.gov)
1. Introduction
EPA's Office of Research and Devefopment estabfished a goal within the 2001 Strategic Plan "To anticipate future environmental issues." Studies of future management and policy options based on differ-
ent assumptions provide a mechanism to examine possible outcomes and especially their likely benefits and consequences. The San Pedro River in Arizona and Sonora, Mexico (Figure 1), is an area that
has undergone rapid changes in land use and cover, and subsequently is facing keen environmental crises related to water resources. It is the location of a number of studies that have dealt with change
analysis, watershed condition, and most recently, alternative futures analysis. In the present study, previously defined future scenarios, in the form of land-use/'land-cover grids, were examined relative to
their impact on surface-water conditions. These hydrological outputs were estimated for the baseline year of 2000 and predicted 20 years in the future as a demonstration of how new geographic informa-
tion system (GlS)-based hydrologic modeling tools can be used to evaluate the spatial impacts of urban growth patterns on surface-water hydrology.
2. Materials and Methods
In alternative futures analysis, potential impacts from a number of wide-ranging scenarios are compared to current
conditions of a region in terms of a set of processes that are modeled in a GIS. Alternative future landscape analysis
involves 1) describing the patterns and significant human and natural processes affecting a geographic area of con-
cern; 2) constr ucting GIS models to simulate these processes and patterns; 3) creating changes in the landscape by
forecasting and by design; and 4) evaluating how the changes affect pattern and process using models. In the present
study, primary source data were developed for three land-cover/land-use grids representing alternative futures for the
San Pedro River Basin in the year 2020. The year 2000 was used as baseline condition and a set of land-cover/land-
use maps were developed for the year 2020 based on current land management and projected census growth. For the
purpose of this study, the 2020 maps were selected for three scenarios which reflected important contradictions in
desired future policy based on stakeholder input. The scenarios are listed in Table 1 and basically reflect changes in
population within the watershed, patterns of growth, and development practices and constraints.
CONSTRAINED
Assumes lower population (78,500 inhabitants) than
presently forecast for 2020. Development is concentrated
in mostly existing developed areas (i.e., 90% urban).
Removes all irrigated agriculture within the river basin.
PLANS
Assumes population increase as forecast for 2020 (95,000
inhabitants). Development is in mostly existing developed
areas (i.e., 80% urban and 15% suburban). Removes
irrigated agriculture within a 1-mile buffer zone of the
river.
OPEN
Assumes population increase is more than the current
2020 forecast (111,500 inhabitants). Most constraints on
land development are removed. Development occurs
mostly into rural areas (60%) and less in existing urban
areas (15%). Irrigated agriculture remains unchanged from
current policy except for prohibiting new expansion near
the river.
Table 1. Scenarios for future urbanization of the
Upper San Pedro River Basin in the year 2020.
The watershed was discretized with a contributing source area of 9,200 ha, producing 67 sub-watershed elements
(Figures 2-4) via the Automated Geospatial Watershed Assessment (AGWA) tool. AGWA is a GIS-based interface tool for watershed modeling and assessment that
has been developed jointly by the U.S. EPA Office of Research and Development and the USDA Agricultural Research Service. AGWA combines hydrologic process
models in an intuitive interface for performing multi-scale change assessment and is provided at no cost via the Internet as a modular, open-source suite of programs
(www.tucson.ars.ag.gov/agwa/ or www.epa.gov/nerlesdl/land-sci/agwa/). The same simulation was performed using each of the three 2020 land-cover scenarios to develop parameter
inputs. Average annual outputs from the three alternative futures were then differenced from the baseline values to compute percent change in average daily values over the 20-year period
with a focus on the relative magnitude and spatial distribution of the computed changes.
Figure 1.
Location of the
Upper San Pedro
River Basin, Arizona/Sonora.
3. Results
Surface runoff, percolation, and sediment yield were simulated within
AGWA for the three 2020 scenarios. Results from the simulation runs
are given in Table 2 and Figures 2,3, and 4. The figures show the relative
departure from the 2000 baseline year and illustrate the spatial
variability of changes to the surface-water hydrology. In general, the
simulation results indicate that land-cover changes associated with future
development will significantly alter the hydrologic response of the water-
shed. Changes are primarily associated with increasing urbanization and
the associated replacement of vegetated surfaces with impervious ones.
In the case of surface runoff, the simulations show average increases
over the 20-year period commensurate with increases in urbanization.
Although most sub-watershed elements exhibited an increase in runoff,
other areas showed improvement or decreasing runoff (Figure 2).
¦ In &»dmnl VMS 2006 - 2O?0
Sediment yield and erosion are directly related to runoff
volume and velocity. The percent change in sediment yield
simulated also displayed a high degree of spatial variability
across the basin and between the three scenarios (Figure 3).
Percolation is a hydrologic measure of the water volume that is
able to infiltrate into the soil past the root zone to recharge the
shallow and/or deep water aquifers. Figure 4 displays the
simulated change in percolation for the three development
scenarios. Although the model predicts some improvement in
the watershed
headwaters where
human inhabitation
is most dispersed,
overall percolation
is expected to de-
crease in all options
as urban impervious
surfaces are expand-
ed, especially under
the Open Scenario
(Table 2).
Figure 3. Percent change in sediment yield, 2000 - 2020,
Upper San Pedro River Basin, Arizona/Sonora.
PtrtwK CtlflflQI In Stictec# Runoff. IOM - 20»
a
B
Or
Figure 2. Percent change in surface runoff, 2000 - 2020,
Upper San Pedro River Basin, Arizona/Sonora.
Simulated Percent Relative Change 2000 - 2020
Baseline 2000
Constrained 2020
Plans 2020
Open 2020
Surface runoff
(m3/day)
186,538
4.3
3.7
6.9
Percolation
(m3/day)
42,760
-2.7
-3.0
-4.6
Sediment yield
(t/day)
1,042
4.4
3.7
7.0
Table 2. Simulated average daily surface runoff, percolation, and
sediment yield for the 2000 baseline conditions and predicted relative
change for each of the three development scenarios.
• in P»reota&on ?9dtl - 30JD
; v-
"n
I
¦
i V"
•5*'
*
' -v.v.
Figure 4. Percent change in percolation, 2000 - 2020,
Upper San Pedro River Basin, Arizona/Sonora.
4. Summary and Conclusions
The hydrologic responses resulting from three development scenarios
for the Upper San Pedro River Basin wer e evaluated using AGWA, a
GIS tool developed to integrate landscape information with hydrologi-
cal process models to assess watershed impacts. With this type of as-
sessment, it is possible to rapidly evaluate likely changes in surface run-
off throughout a basin, as well as the cumulative downstream change
as widely distributed tributary impacts are felt in the main channel. In
general, under a future urbanizing environment, the model simulation
results appear to indicate that important impacts to the watershed hy-
drology can be expected. The most notable changes are likely to be in-
creases in the amount of runoff, sediment discharge, and a loss of
surface-water access to the groundwater table.
For the purpose of this study, negative impacts are considered to be any
increase in surface runoff, sediment yield, and/or declines in ground-
water percolation. The impacts are summarized graphically by percent
change relative to the 2000 reference condition for each of the alterna-
tive futures using sub-watersheds as the comparative unit. The hydros
logic modeling results indicate that negative impacts are likely under all
three of the future scenarios as a result of predicted urbanization; how-
ever, there is remarkable variation in their specific hydrologic responses,
particularly between the Constrained and Open Scenarios. In general,
the Open Scenario has the greatest negative impact on surface water
hydrology and results in greater simulated surface runoff and sediment
yield than the other options, especially in the downstr eam reaches near
Benson, Arizona. Additionally, percolation and thus groundwater re-
charge is most reduced under this option. This has particularly impor-
tant bearing on the San Pedro which is the only unimpounded river in
Arizona and where all municipal and most agricultural is water is
derived from groundwater sources.
The present study endeavors to demonstrate the general potential of in-
tegrating spatial data and distributed modeling in environmental man-
agement. The combination of both landscape analysis with hydrological
modeling can be widely applied on a variety of landscapes, watersheds,
and regions and provides an important tool to assess vulnerability. The
use of scenarios thus allows stakeholders and decision-makers to assess
the relative impacts of several alternative sets of options and thus pro-
vides an important tool to help make better informed choices for an
improved future.
For more information see:
Kepner, W.G., D.J. Semmens, S.D. Bassett, D.A. Mouat, and D.C. Goodrich. 2004.
"Scenario Analysis for the San Pedro River, Analyzing Hydrological
Consequences of a Future Environment." Journal of Environmental
Monitoring and Assessment, 94: 115-127. Kluwer Academic Publishers
("http: www.epa.gov lierlesdl/land-sci pdf scenario spedro.pdf).
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