watervulnerability

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The initial run of this analysis utilized pre-existing datasets (often outdated and of lower resolution and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