FIELD-EVALUATION OF METHODS TO ESTIMATE SOIL-EROSION

The objective of this study was to evaluate alternative methods of estimating the postsettlement soil erosion from hillslopes in a small watershed in Bureau County, northwestern Illinois. Methods used include: (i) estimation of the soil erosion using total organic C balance or enrichment ratio, (ii) selected soil property comparisons between the cultivated, sloping, and eroded soils of the watershed and uncultivated, sloping, and uneroded reference soils of the same taxa occurring in the area cemeteries, and (iii) Universal Soil Loss Equation (USLE). Previously measured sediment remaining in the sediment basin and calculated sediment delivery were used as the bases for evaluating the effectiveness of proposed methods to estimate soil erosion from the surrounding hillslopes. Differences in thickness of the A horizon consistently underestimated soil erosion by 64% relative to the estimates made by the total sediment remaining in basin and delivered to the stream. This suggests erosion classes which are based on erosion of topsoil underestimate the extent of erosion from both the topsoil and subsoil, particularly where soils have been severely truncated. Organic C redistribution was highly correlated to sediment delivery method and appear to be related directly to the quantity of soil eroded and could provide an alternative to A horizon thickness differences as a basis for quantifying soil erosion. The soil erosion estimated by the USLE was 80% of the soil erosion estimated by the sediment delivery method. This difference can be accounted for by the fact that the sediment delivery method assumed all the sediment came from the sideslopes and USLE is not designed to include erosion resulting from concentrated overland flow (gully erosion). The comparison between cultivated and uncultivated sites of the depth to carbonates and the lowest part of the Bt horizon containing 27% clay between eroded and uneroded phases of Tama soils each produced an estimate of soil erosion within 17% of the soil erosion estimated hy sediment delivery method and could be used to estimate soil erosion.