NATURAL EROSION RATES AND THEIR PREDICTION IN THE IDAHO BATHOLITH

Natural rates of surface erosion on forested granitic soils in central Idaho were measured in 40 m(2) bordered erosion plots over a period o f four years. In addition, we measured a variety of site variables, so n properties, and summer rainstorm intensities in order to relate eros ion rates to site attributes. Median winter erosion rates are approxim ately twice summer period rates, however mean summer rates are nearly twice winter rates because of infrequent high erosion caused by summer rainstorms. Regression equation models and regression tree models wer e constructed to explore relationships between erosion and factors tha t control erosion rates. Ground cover is the single factor that has th e greatest influence on erosion rates during both summer and winter pe riods. Rainstorm intensity (erosivity index) strongly influences summe r erosion rates, even on soils with high ground cover percentages. Few summer storms were of sufficient duration and intensity to cause rill ing on the plots, and the data set was too small to elucidate differen ces in rib vs. interrill erosion. The regression tree models are relat ively less biased than the regression equations developed, and explain ed 70 and 84 percent of the variability in summer and winter erosion r ates, respectively.