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Gallatin National Forest Watershed Vulnerability Assessment, Northern Region (R1) WATERSHED VULNERABILITY ASSESSMENT PROCESS Figure 5. The Gallatin National Forest Watershed Vulnerability Assessment Model. The assessment consists of several different types of information (added and removed as necessary) to identify vulnerable watersheds. Geophysical/Sensitivity Characterization The first step of the WVA process, the geophysical/sensitivity characterization, was the most timeconsuming. As interdisciplinary team (fish biologist, hydrologist, and soil scientist), identified the dominant physical processes and features of the watershed that affect ecosystem function and condition. Identifying which watersheds are the most geophysically reactive can indicate how much a watershed responds to disturbances such as floods, drought, intense precipitation, and fires. The datasets determined to be most important for the watershed characterization were soils, geology, hydrology, terrain, and groundwater. The initial run of this analysis utilized pre-existing datasets (often outdated and of lower resolution and accuracy). These datasets include the GNF Soil Survey (slope classes, surficial geology, and shallow groundwater) and datasets derived from the National Hydrography Dataset, National Elevation Dataset, and R1 VMap (water yield, high flows, and low flows). After this initial run, the team met again to evaluate the results and determine which watershed characteristics were most important. The second run of the analysis included newer datasets developed for the analysis. The state surficial geology layer from MBMG was reclassified into broad rock class categories to identify sensitive geologies. A compound index of slope and aspect from 10m digital elevation models (USGS) was derived to identify sensitive terrain areas. The original hydrology metrics were omitted in the second run due to their strong correlation with the terrain analysis (see Hydrology section below). Each variable was quantified by subwatershed and given a rating of 1, 2, or 3, based on specific threshold values identified by literature and professional judgment. All scores were added together by subwatershed. Higher scores indicate higher sensitivity to disturbance. Geology Sensitivity 36 Assessing the Vulnerability of Watersheds to Climate Change
Gallatin National Forest Watershed Vulnerability Assessment, Northern Region (R1) The surficial geology layer covering the GNF was reclassified, based on a relative assessment of soil erosion/sediment delivery and rapid runoff potential from different bedrock types. Three classes were created (low, moderate, and high) to identify geology sensitivity by each HUC-6 subwatershed. Terrain Sensitivity A mathematical equation was used to explain the empirical relationship between slope, aspect, and elevation. The results of this analysis have been extrapolated beyond Forest boundaries to allow characterization of entire subwatersheds, however characterizations are truly only valid within Forest boundaries. The equation is developed for montane areas and will need to be recalibrated for use on flatter areas (outside of GNF boundaries). Three classes were created (low, moderate, and high) to identify terrain sensitivity for each HUC-6 subwatershed. A future iteration of this analysis will expand this terrain analysis beyond Forest boundaries to increase the accuracy. Geophysical Characterization The geology sensitivity and terrain sensitivity datasets were combined and reclassified with more weight given to the terrain dataset (Figure 6). Hydrology Groundwater is expected to play an important role in buffering the impacts of changing flows and stream temperatures, however currently there is no accurate and comprehensive dataset for groundwater. This information will be included in the model as better and more reliable methods of identifying groundwater data are determined. The first run of the WVA analysis developed hydrology metrics for water yield, high discharge, and low flows. Each metric was categorized into high, moderate, and low categories. The water yield sensitivity map compares reasonably well with the newly developed terrain sensitivity dataset. The main hydrology variable, water yield, appears to be accurately characterized, and is heavily influenced by the elevation variable. The aspect and slope steepness terrain variable further refines the elevation variable, accounting for less water yield on 150- to 210-degree aspect slopes and faster runoff on steep (35% + ) slopes. The hydrologic factors determined to be the most influential in watershed sensitivity to climate change are best represented by the terrain sensitivity analysis and, therefore, no hydrology metrics were included in the second run of the WVA. 37 Assessing the Vulnerability of Watersheds to Climate Change
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White River National Forest Watersh
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Coconino National Forest Watershed
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Sawtooth National Forest Watershed
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Umatilla National Forest Watershed
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Ouachita National Forest Watershed
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Chugach National Forest Watershed V
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