Surface Erosion Response to Climate Change in the Ouachita National Forest, USA

Surface erosion has been a major ongoing concern for the Ouachita National Forest. The Forest covers over 6,800 km2 of the Ouachita Mountains in west-central Arkansas and eastern Oklahoma. Predicted changes in precipitation due to anticipated climate change suggest that surface erosion risks may change in the future. To assess the overall risk of climate change to water resources on the Forest, we investigated how potential surface erosion risk changes within non-road areas under two different climate projections. The two climate projections were derived using the A1B emission scenario and the CISRO-Mk3.0 and MIROC3.2 global circulation models. The A1B emission scenario assumes an intermediate growth level in greenhouse gases and aerosols through the next century. Mean annual temperature and total annual rainfall values were predicted for 2012-2060. A USLE-type model developed specifically for forest management practices is used to estimate the erosion risk. Erosion risk is computed for all 6-level subwatersheds (typically 40-160 km2 in size) within the Forest. The rainfall erosivity (R) factor for each watershed was computed from annual rainfall using predictive models. The current erosion risk is rated based on 2011 conditions and R factors computed from historic climate data (1961-2000), while those for 2012-2060 were computed using data from the two climate projections. Mean R factors for two 10-yr periods (2026-2035 and 2051-2060) were computed for each subwatershed and used to compute the erosion risks for comparison against the current risk ratings. Soil erodibility, hillslope length, and slope steepness factors are all considered to not change over time. The relative proportions of different cover management practices (e.g., forest thinning) and the erosion contribution from roads and trails are also considered to remain constant over the analysis period. In general, the erosion risk appears to decrease over the next 50 yr in response to climate change. Both climate projections show that the Forest will experience increasing average temperatures and decreasing precipitation during this time. Within the declining rainfall trend, the CISRO-A1B projection predicts somewhat higher rainfall amounts and much greater rainfall variability than the MIROC-A1B. The CSIRO-A1B projections produce R factors that are 5-10 percent greater than current values for 2026-2035 and then decrease to 15-20 percent less than current by 2051-2060. Under this projection, the percent of high- and moderate-risk subwatersheds within the Forest will decrease from 43 and 21 percent currently to 32 and 18 percent, respectively, by 2051-2060. In contrast, the MIROC-A1B projections produce a steady and sharp decline in R factors of 30-35 percent by 2026-2035, and 40-45 percent by 2051-2060. Under the MIROC-A1B climate, the percent of high- and moderate-risk subwatersheds decrease to 12 and 15 percent, respectively, by 2051-2060. This assessment is a first approximation and planned improvements will incorporate an explicit assessment of climate change impacts on road and trail erosion, and an evaluation of how potential vegetation changes in response to hotter, drier climates might affect the cover management factor in the erosion model.