Transcript How much water will be available in the upper

How much water will be available in the upper Colorado River Basin
under projected climatic changes?
Iris T. Stewart1, Darren L. Ficklin1, Edwin P. Maurer2
1Department of Environmental Studies and Sciences, Santa Clara University, Santa Clara, CA 95053
2Civil Engineering Department, Santa Clara University, Santa Clara, CA 95053
Abstract
Results
The upper Colorado River Basin (UCRB), is a principal
source of water for urban and agricultural demands in
the arid and semi-arid Southwestern United States.
Projected climatic changes in the basin suggest some
precipitation increases in the very highest regions, and
no changes or declines for the middle and low
elevations, while air surface temperatures are
expected to increase by about 5 ºC by the end of the
century. We model the effects of these climatic
changes on all hydrologic components (snowmelt, ET,
surface runoff, subsurface runoff, groundwater, and
streamflow) on the subbasin scale to project future
changes in water resources using SWAT. We find that
spring and summer season flows are likely decrease
substantially by the end of the century, %. Many
subbasins will turn from semi-arid to arid conditions by
the 2080’s. The role of the individual hydrologic
components and the impact on the availability of water
resources for urban and agricultural use in the region
are quantified.
For the UCRB (Fig. 1), the ensemble of GCM models projects increases in temperature by 4-6 °C, precipitation projections vary in
direction, magnitude, and spatial pattern, but drier conditions are more likely, especially in the San Juan and Colorado River
watersheds (Fig. 2). The SWAT model yielded satisfactory calibration and validation results (NS > 0.7, R^2 > 0.75). Our modeling
results suggest statistically significant changes in streamflow towards earlier runoff (Fig. 3) and a general decrease in annual
average streamflow (23% at Lee’s Ferry for the 2080’s). Projections range from a 44% decrease (25% percentile) to a 15%
increase (75% percentile). Streamflow changes propagate from headwaters to Lee’s Ferry outflow (Fig 3). Spring streamflow, an
important contribution to annual flow, is likely to decrease by 20 – 90% (Fig. 4). Snowmelt is likely to decrease throughout the mid
and low elevations of the upper Colorado River Basin. In the San Juan and Colorado Basins, snowmelt is likely to be reduced by
more than 80% by the 2080’s (Fig. 5). The lower elevation areas where the largest decreases in spring streamflow and snowmelt
are expected, are the most likely to stay or become arid landscapes, while the highest elevations are likely to remain mostly humid
(Fig. 6). With expected climatic changes, the snowmelt, streamflow, soil water, and surface flows are expected to decrease and
arrive earlier while ET is expected to increase.
Fig. 6 Projected changes in the aridity index under the
A2 scenario by the 2050’s and 2080’s.
Fig. 7
Hydrologic
flow
components.
Fig. 2: Average (over 16 GCMs) projected temperature
and precipitation changes under the A2 scenario (20th,
50th, and 80Th percentiles.
Conclusions
Fig. 3: The propagation of streamflow timing changes
from the headwaters to the Lee Ferry outflow.
Although there is a large range of projected future
precipitation patterns, the model ensemble in general
suggests less precipitation for large areas of the
UCRB. Projected decreases in streamflow from
changes in hydrologic components (surface runoff,
subsurface
flow,
soil
water
storage,
and
evapotranspiration) at the subbasin-level due to
climate change are likely to have serious impacts on
water resources in the UCRB, with the greatest
changes expected at lower elevations. The expected
changes in the volume and seasonality of streamflow
and other hydrologic components have potential
consequences for land cover and land use. A
decreased availability of soil water in arid landscapes
is likely to lead to a decrease in vegetative cover, and
increased soil exposure, soil erosion, and soil loss.
Due to increases in aridity in many low-elevation
regions within the CRB, many areas that are currently
under rain-fed agricultural production will likely have to
transition to irrigated agriculture, drought-tolerant
crops, or other uses.
Fig. 5: Projected changes in spring nowmelt under the A2
scenario by the 2050’s and 2080’s.
Acknowledgements
Fig. 1: Watersheds, elevation, and calibration sites in
the upper Colorado River Basin.
Objective and Methods:
The goal is to quantify the effects of climate
change in the UCRB on all hydrologic flow
components and water availability on the subbasin
scale
A calibrated and validated SWAT model of the UCRB
using 46 unimpaired sites and driven by downscaled
output from 16 GCMs was used to investigate the
effects of climate change under the A2 emission
scenario. All flow components (snowmelt, streamflow,
ET, surface runoff, subsurface runoff, groundwater
flow) were extracted and analyzed.
Fig. 4: Projected changes in spring streamflow under
the A2 scenario by the 2050’s and 2080’s. The 25th,
50th, and 75th percentile over the 16 GCMs are shown.
We gratefully acknowledge financial support for this work from
the U.S. Environmental Protection Agency through EPA STAR
Grant No. RD-83419101-0.