watervulnerability

Helena National Forest Watershed Vulnerability Assessment, Northern Region (R1)
Projected Climatic
Changes
Anticipated Watershed
Response
Warmer air temperatures • Warmer water
temperature in streams
Changes in precipitation
amounts and timing
Less snowfall, earlier
snowmelt, increased
snowpack density
Intensified storms,
greater extremes of
precipitation and wind
• Altered timing and
volume of runoff
• Altered erosion rates
• Higher winter flows
• Lower summer flows
• Earlier and smaller peak
flows in spring
• Greater likelihood of
flooding
• Increased erosion rates
and sediment yields
50 Assessing the Vulnerability of Watersheds to Climate Change
Potential Consequences to Watershed
Services
• Decrease in coldwater aquatic habitats
• Increases or decreases in availability of
water supplies
• Complex changes in water quality related
to flow and sediment changes
• Changes in the amounts, quality and
distribution of aquatic and riparian
habitats and biota
• Changes in aquatic and riparian habitats
• Increased damage to roads, campgrounds,
and other facilities
Table 1. Projected hydrologic changes relative to the HNF identified values. Adapted from Water, Climate Change,
and Forests GTR (Rieman and Isaak, 2010)
The models used to predict climate changes were developed by the Climate Impacts Group (CIG) at the
University of Washington. The Climate Impacts Group selected the A1B climate scenario to provide
projections most relevant for vulnerability assessment and scenario planning exercises. They then
modeled change (from time period 1916-2006 representing historic) and for two future time periods
representing the mid-21 st century (2030-2049) and late 21 st century (2070-2099), using the emissions
scenario A1B with the composite climate model. The composite model is an ensemble of climate models
that falls between those models that predict cooler and warmer climate scenarios. It includes 10 Global
Circulation Models that perform similarly well in the PNW / Columbia Basin, the Northern Rockies /
Upper Missouri Basin, and the Central Rockies / Upper Colorado Basin and this is what the Helena
National Forest chose to use to represent climate change in this analysis. Data was summarized at the
HUC-5 scale for the entire Forest (downloaded from ftp://ftp2.fs.fed.us/incoming/gis/PNF/WVA/ on
12/10/2010).
Predicted changes in selected hydrologic attributes were derived from the Variable Infiltration Capacity
(VIC) model. Parameters from VIC modeling were used to assess potential impacts to the selected forest
water resource values. We compared the HUC-5 scale CIG’s VIC outputs for the historic trend and
composite models for the following parameters (by resource value):
1. Bull trout – Average summer maximum air temperature
2. Cutthroat trout – Average summer maximum air temperature
3. Infrastructure – Snowpack vulnerability (defined as the ratio of April1 snow water equivalent and
October-March precipitation)