watervulnerability

Grand Mesa, Uncompahgre and Gunnison National Forest Watershed Vulnerability Assessment, Rocky
Mountain Region (R2)
revision, but is currently available, and has been added to the suite of factors evaluated for this sensitivity.
The list of factors evaluated for erosion or sediment production sensitivity include those listed below.
1. Erosion Risk Rating – percent of severe and very severe erosion risk classes by subwatershed.
This was derived from Kw factor (from soil survey data) and prevailing slope. The Kw factor is
an indication of susceptibility of a soil to sheet and rill erosion by water, based on soil
composition, structure, and permeability. The erosion risk rating was considered to be a stressor.
2. Runoff potential – percent of subwatershed in Hydrologic Group D. Runoff potential is
determined by soil infiltration capacity after prolonged wetting, permeability, depth to water
table, and depth to restrictive or impervious layer. Soils with the highest potential for runoff are
identified as Hydrologic Group D in soil survey data. Runoff potential was considered to be a
stressor.
3. Rainfall Intensity Factor – weighted average for each subwatershed. The rainfall intensity factor
was derived from the Revised Universal Soil Loss Equation (RUSLE) R factor from PRISM data
(obtained from Oregon State University). When other factors remain constant, soil loss is directly
proportional to a rainfall factor related to the total quantity and intensity of rainfall. The RUSLE
R factor is the average annual product of kinetic energy and maximum 30-minute rainfall
intensity. The rainfall factor was considered to be a stressor. Based on the prediction that storm
intensity is likely to increase, this factor is expected to increase in the future.
4. Stream density – total miles of perennial and intermittent streams per square miles of
subwatershed. This factor characterizes the degree of dissection and network transport capacity
for both runoff and sediment. The higher the stream density, the larger the amount of sediment
that may be moved through a subwatershed. Stream density was considered to be a stressor.
5. Hydrologic Response Channels – percent of total stream network that is a response channel,
compared to the total perennial and intermittent stream network in a subwatershed. Response
channels are streams of third order or higher, with a gradient less than or equal to 1.5%,
containing alluvial channel material, and classified as a Rosgen stream type of C, D or E.
Response channels could be considered either buffers or stressors, depending on the situation.
Response channels would be buffers in the situation where sediment is deposited in these areas
and prevented from moving downstream. Response channels could also be added stressors
because of the sediment loads they may retain, which under intense storms with high runoff could
be released to impact downstream locations.
6. Mass wasting potential – percent of a subwatershed with high mass wasting potential. Areas
with mass wasting potential include areas with identified geological instability and areas with
potential for mass wasting based on presence of vulnerable sedimentary geology and slopes
greater than 50 percent. This factor was considered a stressor.
Values for each of the individual factors listed above were calculated and then standardized for each
factor (as described above for the values). The overall erosion or sediment potential sensitivity ranking
was determined by adding the individual factor standardized ratings together for each subwatershed. The
resulting Erosion Sensitivity Rankings were classified into quartiles. The top 25% were classified 3
(high), middle 50% were classified 2 (moderate), and lowest 25% were classified 1 (low). Figure 21
shows the resulting Erosion Sensitivity Ranking.
92 Assessing the Vulnerability of Watersheds to Climate Change