COMPARING LANDSCAPE-SCALE ESTIMATION OF SOIL-EROSION IN THE PALOUSE USING CS-137 AND RUSLE

Soil loss from farmed fields is most often estimated using the Univers al Soil Loss Equation (USLE) or the Revised Universal Soil Loss Equati on (RUSLE), but few methods exist to test the accuracy of these estima tes. Our objectives were to use the Cesium-13 7 isotopic tracer techni que to assess the spatial patterns of net soil movement across farmed watershed and to compare the results with estimates from RUSLE. The Ce sium tracer technique was applied for the first time to a 46 ba (114 a c) closed watershed (outlet blocked) in the Palouse region of northern Idaho. A basic sample grid of 50 x 75 m (164 x 246 ft) that also bad clusters of additional-points 25 m (82 ft) apart supported both transe ct-based and geostatistical analysis. Net soil loss from the watershed was zero because of the blocked outlet, allowing a mass balance to be calculated between erosion and deposition. Over the 26-year period fo llowing Cesium fallout (1963-1989), erosion averaged -11.6 t ha-1 yr1 (-5.2 t ac-1 yr1) from erosional areas of the watershed, and depositio n averaged 18.6 t ha-1 yr1 (8.3 t ac-1 yr1) onto depositional areas of the watershed. Extreme values of average erosion and deposition were -58 t ba-1 yr1 (-25.9 t ac-1 yr1) and 277 t ha-1 yr1 (123.5 t ac-1 yr1 ), respectively. Contour maps of net soil movement developed using geo statistical techniques show that convex ridge tops and mid-slope knobs have undergone the most severe erosion; concave areas in valley botto ms and on slopes received deposition; and mid slopes generally bad sma ll amounts of either erosion or deposition, suggesting that they are z ones of soil transport. The average erosion rate estimated by RUSLE fo r the entire watershed was -31.4 t ha-1 yr1 (-14 t ac yr1), compared t o a rate estimated by Cs-137 of -11.6 t bal yr1 (-5.2 t ac yr1) for er osional areas (about 60 percent of the watershed), suggesting that RUS LE may have overestimated water erosion on this farmed watershed.