watervulnerability

Grand Mesa, Uncompahgre and Gunnison National Forest Watershed Vulnerability Assessment, Rocky
Mountain Region (R2)
onset of snowpack. The largest annual increase in temperatures is predicted for the Uncompahgre Plateau,
followed in order by Grand Mesa, San Juans, West Elk, Upper Taylor, and Cochetopa.
An aridity index was used to forecast where water availability may be most affected. By determining the
ratio of precipitation to potential evapotranspiration, we identified, in a very simplistic way, those
locations where water surpluses or deficits are most likely to occur. A reduction in precipitation with an
increase in potential evapotranspiration will reduce soil moisture, fuel moisture, groundwater recharge,
and availability of water to contribute to sustained stream flow. An aridity index of 1.0 means
precipitation meets the demand of potential evapotranspiration. An aridity index of less than 1.0 means
potential evapotranspiration exceeds precipitation and plants are under water stress. An aridity index
greater than 1.0 means precipitation exceeds potential evapotranspiration and there is available water in
the system. We compared the change in the seasonal aridity index for the MIROC_3.2 model to the
historic trend (Figure 18).
Aridity Index
Aridity Index
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
70
60
50
40
30
20
10
0
Historic
MIROCPrediction
Historic
MIROCPrediction
Spring
Winter
Figure 18. Seasonal Aridity Indices by Geographic Area
The MIROC_3.2 model predictions indicate a significant change in aridity indices throughout the year,
but once again, spring appears to be the season that may be most affected by climate change. Historically,
only the Uncompahgre Plateau has had an aridity index below 1.0 in the spring. Predictions from the
MIROC_3.2 model indicate the Cochetopa and West Elk geographic areas may also become waterstressed
in the spring. All geographic areas have had and will continue to have aridity indices below 1.0 in
the summer. Water availability has not generally been a limiting factor in the fall for any of the
geographic areas, but the aridity index is expected to drop to less than 1.0 for the three driest geographic
87 Assessing the Vulnerability of Watersheds to Climate Change
Aridity Index
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Aridity Index
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Historic
MIROCPrediction
Historic
MIROCPrediction
Summer
Fall