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climate (Casola et al. 2005). Only the Sawtooth NF applied a tool useful in describing exposure relative to increased peak flows and infrastructure. This analysis used the VIC-generated “Winter 95” metric. Winter 95 represents the number of days during winter that are among the highest 5% of flows for the year. Winter was defined as Dec. 1 – Feb. 28. Changes in Winter 95 were determined by comparing the increase in the number of days with the highest 5% flows between current and predicted conditions (2040 and 2080). Subwatersheds with less than a 0.5-day increase were considered low risk, those with 0.5- to 2-day increases were considered moderate risk, and subwatersheds with increases greater than 2 days were considered high risk. Results of this analysis for 2040 are depicted in Figure 9. At first glance, the difference in metrics and the level of detail might suggest pilots took very different paths in their characterization of exposure. In fact, all the pilots Important Considerations in Evaluating Exposure • Quantify trends in available, relevant, historical climate datasets. Local and regional data that display significant changes demonstrate the likelihood that changes will continue into the future. Observed changes and trends in ecosystem traits, such as ice duration, species phenology, and species composition, are of great value. • Use best available climate change projections. Focus on near-term time frames. • Identify the effects of a changing climate on watershed processes to inform iterations of exposure data acquisition and assessment. • Identify hydrologic processes important to the identified resource value. • Determine how projected changes in hydrologic processes might affect each resource value. • Quantify the relative magnitude of differences in effects, including spatial and temporal variability. • Include disturbance regimes in the analysis and quantify disturbance-related effects. • Document critical data gaps, rationale and assumptions for inferences, references for data sources, and confidence associated with assessment outputs. 14 | ASSESSING THE VULNERABILITY OF WATERSHEDS TO CLIMATE CHANGE took very similar approaches. All used review of historic data to display the trend in local climatic conditions. All pilot Forests also first looked at projected temperature and precipitation changes. Sometimes this information had been compiled for states, sometimes for river basins or larger geographical areas. The commonality is that the analysis was broad-scale. Next, pilots considered what impacts the climatic changes would have on hydrologic processes, and then how the hydrologic changes would impact water resources. Differences in pilot outputs resulted from decisions made at this point, primarily influencing the level of detail used to characterize the hydrologic changes. Perhaps the most valuable lesson learned by pilots in assessing exposure was that you don’t have to become a climate scientist to do a climate change vulnerability assessment. EVALUATE WATERSHED SENSITIVITY Models are tools for thinkers, not crutches for the thoughtless. —Michael Soule The goal of assessing sensitivity was to place areas (subwatersheds) into categories based on how they would respond to the expected climate-induced changes to hydrologic processes. The sensitivity of watersheds to any change is partially a function of parent geology, soils, typical climate, topography, and vegetation. Human influences also affect watershed resilience, depending on the extent and location of managementrelated activities. The Forest Service often evaluates watershed condition; watershed specialists routinely describe watershed condition in NEPA analyses, and many National Forests have watershed or aquatic species restoration plans in place that weigh heavily on assessments of watershed condition. Several pilots were able to take advantage of existing watershed condition ratings and apply them to their WVA. This included use of the Blue Mountains Forest Plan revision watershed condition (Umatilla NF) and “Matrix of Pathways and Indicators” determination of watershed condition factors in conjunction with Endangered Species Act compliance for several salmonid species (Helena, Gallatin and Sawtooth NFs). Sensitivity indicators were selected that most influenced the hydrologic process and water resource value in question. Some indicators tend to dampen effects
Intrinsic Anthropogenic Geology Road Density Soil Types Road-Stream Proximity Risk of Mass Wasting Road Crossings Groundwater-Baseflow Range Condition Slope Water Diverted Aspect Vegetation Condition Table 5. Attributes most commonly used in assessing sensitivity by the pilot Forests (buffers) and others amplify effects (stressors). For example, road density may amplify peak flow response and the potential for flood damage to vulnerable infrastructure near streams. In contrast, investment in road improvements such as disconnection of road surfaces from streams would tend to buffer effects. Attributes selected by pilots to characterize sensitivity included both intrinsic factors and anthropogenic or management-related factors (Table 5). In some cases, pilots termed the “natural” factors as sensitivity, and the anthropogenic factors as risks, combining the two types of indicators to derive a measure of sensitivity. Most pilots included both types of indicators (intrinsic and anthropogenic), though two pilots (Chequamegon- Nicolet and Ouachita NFs) employed only intrinsic Uncompahgre Grand Mesa West Elk San Juans Legend Forest Boundary Geographic Areas Erosion Sensitivity Rankings Low Moderate High Upper Taylor Cochetopa Figure 10. Erosion Sensitivity Rating from the GMUG NFs WVA. This rating derived from subwatershed characterizations of runoff potential, rainfall intensity, stream density, density of response channels, and mass wasting potential. 15 | ASSESSING THE VULNERABILITY OF WATERSHEDS TO CLIMATE CHANGE Figure 11. Influence of Hydrologic Soil Group (HSG) on groundwater Recharge (2046-2065 minus 1971-1990). HSG was one of two sensitivity attributes applied to the groundwa ter resource issue in the Chequamegon-Nicolet NFs W VA. factors, and the Coconino NF selected only factors that related to management activities. A sample output (relative erosion sensitivity of subwatersheds on the GMUG NFs) is shown in Figure 10. Soil hydrologic groups used to classify watershed sensitivity on the Chequamegon-Nicolet NFs are shown in Figure 11. The Chugach NF assessment used many of the same sensitivity attributes as the other pilot Forests, but the approach differed in that the analysis consisted of comparing two watersheds with significant management activity (primarily undeveloped watersheds or those in wilderness were not included). Ratings from National Watershed Condition Classification were used in the sensitivity evaluation by the Coconino and Gallatin NFs. The Coconino NF was completing the Condition Classification at the same time the WVA was underway, and staff realized the utility that data developed would have in both efforts. The Coconino NF used few intrinsic factors to characterize watershed Aquatic Biological Life Form Presence Native Species Exotic and or Invasive Species (Riparian) Vegetation Condition Terrestrial Physical Density Road Maintenance Proximity to Water Mass Wasting Soil Productivity Soil Erosion Soil Contamination Terrestrial Biological Fire Condition Class Wildfire Effects Loss of Forest Cover (Rangeland) Vegetation Condition (Riparian) Invasive Condition (Forest) Insects and Disease Figure 12. Watershed Condition Factors from the USFS Watershed Condition Classification. Aquatic-Physical attributes are not included. Factors shown in red are those used as sensitivity indicators in the Coconino NF WVA.
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