watervulnerability

Grand Mesa, Uncompahgre and Gunnison National Forest Watershed Vulnerability Assessment, Rocky
Mountain Region (R2)
Warmer fall, winter, and spring temperatures can result in more precipitation falling as rain instead of
snow, particularly at elevations less than 8,200 feet. Most of the GMUG NF is above 8,200 feet in
elevation, so the chance of rain-on-snow related flood events is judged to be relatively minor. (Only the
Uncompahgre geographic area has significant area at elevations below 8,200 feet.) Periods of freezing
weather will likely be shortened, especially on the Uncompahgre and Grand Mesa geographic areas, and
road and trail surfaces at lower elevations can remain saturated and subject to rutting for longer periods.
Warmer winter and spring temperatures will also result in earlier and more rapid snowmelt runoff, which
can result in flooding and increased sediment/debris flows. Dust-on-snow events have already been
documented to result in earlier and more rapid snowmelt runoff, with or without temperature increases
(Painter et al. 2010).
The greater risk to infrastructure values has to do with an increased severity in summer thunderstorm
events. Increased summer temperatures are likely to increase the potential energy associated with
convective storm development. These types of storms can result in very high-intensity rainfall events,
capable of localized flooding, and in certain geomorphic settings (i.e., those subwatersheds with high risk
for erosion or sediment production), triggering debris flows that are capable of great damage and risk to
life. While high intensity summer storms could potentially occur anywhere on the Forest, they historically
occur most frequently in the San Juans geographic area. Considering all this information, infrastructure
values are most vulnerable in the San Juans and Upper Taylor geographic areas.
Water Use Values Vulnerability
Water use values are vulnerable to predicted climate change impacts in several ways. Structures related to
water use values (dams, reservoirs, ponds, ditches, diversions) are most vulnerable to flooding and/or
sediment and debris flows, similar to infrastructure values. Water Use Values related to Erosion
Sensitivity × Activity Stressors are shown in Figure 28. The areas where erosion or sediment potential has
the highest risk of affecting water use values structures are highest in the Upper Taylor, San Juans, and
West Elk geographic areas. Because off-Forest water use data were not available for the Grand Mesa
geographic area, some additional subwatersheds on the Battlement and Sunnyside areas could actually
have higher risk rankings related to erosion sensitivity.
Water Use Values related to Runoff Response Sensitivity × Activity Stressors are shown in Figure 29.
The areas where runoff potential has the highest risk of affecting water-use-value-related structures are
mostly in the San Juans geographic area, with smaller groupings of subwatersheds in the remaining
geographic areas. Increasing peak flow and duration of high-stage events could result in storage and/or
diversion facilities being overtopped or washed away. Timing of runoff may also come at periods where
storage structures are full or are normally releasing water in preparation for later seasonal inputs.
Increased sediment loads that could result from flooding may fill in storage structures and diversions,
reducing the amount of water these facilities could hold or transport; this could potentially increase
maintenance costs to dredge, replace, or repair affected structures. Geographic areas where water use
related structures are most vulnerable are the San Juans and Upper Taylor.
102 Assessing the Vulnerability of Watersheds to Climate Change