Rill erosion in natural and disturbed forests: 2. Modeling Approaches

As forest management scenarios become more complex, the ability to more accurately predict erosion from those scenarios becomes more important. In this second part of a two-part study we report model parameters based on 66 simulated runoff experiments in two disturbed forests in the northwestern U. S. The 5 disturbance classes were natural, 10-month old and 2-week old low soil burn severity, high soil burn severity, and logging skid trails. In these environments the erosion rates were clearly detachment limited, and the rill erodibility parameters calculated from four hydraulic variables increased by orders of magnitude. The soil shear stress based erodibility parameter, K-r, was 1.5 x 10(-6) s m(-1)in the natural plots, 2.0 x 10(-4) s m(-1) in the high soil burn severity plots, and 1.7 x 10(-3) s m(-1) in the skid trail plots; K-r values for the low soil burn severity plots had negative sign. The erodibility value for the skid trail plots fell within ranges reported for tilled agricultural fields and also for forest roads. The K-r values decreased as erosion occurred in the plots and therefore should not be a constant parameter. The stream power produced the largest R-2 value (0.41) when hydraulic predictors and the sediment flux were log-transformed, but none of the four hydraulic variables (soil shear stress, stream power, unit stream power, and unit length shear force) explained much of the variability in sediment flux rates across the five levels of disturbance when evaluated in the linear form of the erosion models under consideration.