watervulnerability

Ouachita National Forest Watershed Vulnerability Assessment, Southern Region (R8)
WATER RESOURCES
The ACE model is a disturbance model that uses changes in sediment to compare various management
scenarios and determine the effect on aquatic biota. Model inputs include the following.
• Watershed layer
• Current land use (grid)
• Ecoregion (section level)
• Ownership (forest service or other)
• Slope class (derived from dems)
• Roads and trails (ownership, maintenance level and surface)
• Recreation use (motorized recreation use)
For terrestrial sediment yields; land use, ecoregion, slope class, and recreation use were summarized by
30 meter grids. An erosion coefficient (pounds/acre/year) was determined for each grid combination and
the grids were accumulated for each subwatershed. Sediment was determined using Roehl (1962). Roads
and trails were identified by ownership, maintenance level, ecoregion, and recreation use level. A
sediment coefficient (tons/mile/year) was determined from Water Erosion Prediction Project (WEPP,
1999) surveys for each road or trail combination. The roads and trails were clipped by subwatershed and
summarized by total miles of each combination.
EXPOSURE (CLIMATIC CHANGES)
Predictive Models Used
The forest ACE model was used to establish current condition and potential current condition (assuming
fully funded and implemented road and trail maintenance). The ACE model calculates general land uses
and linear events (roads and trails) separately.
From the TNC climate wizard, changes in precipitation and temperature were captured by month from the
composite climate change models. The changes in climate were used to modify the climate generator in
WEPP. Roads and trails coefficients were reanalyzed in WEPP Road to determine changes in sediment
production from roads and road use levels. Because of the time consuming nature of recalculating
individual climates, a proportional relationship for road and trail sediment increases was used.
The Universal Soil Loss Equation (USLE) (Dissmeyer and Foster, 1984) was used for terrestrial
coefficients. The R factor was modified using information from Phillips (1993). The new R value for the
climate change scenarios was used in the USLE equation. Results were proportionally distributed for
terrestrial coefficients.
Storm intensity was determined for roads and trail by reducing the number of days of precipitation in the
climate generator model. In theory, this should force the generator to predict more intense storms. The
value used was half of the percent change in precipitation volumes (personal communication, Bill Elliot).
Anticipated Climate Change
229 Assessing the Vulnerability of Watersheds to Climate Change