Comparing landscape-scale estimation of soil erosion in the palouse using Cs-137 and RUSLE

ABSTRACT:

Soil loss from farmed fields is most often estimated using the Universal Soil Loss Equation (USLE) or the Revised Universal Soil Loss Equation (RUSLE), but few methods exist to test the accuracy of these estimates. Our objectives were to use the Cesium-137 isotopic tracer technique to assess the spatial patterns of net soil movement across a farmed watershed and to compare the results with estimates from RUSLE. The Cesium tracer technique was applied for the first time to a 46 ha (114 ac) closed watershed (outlet blocked) in the Palo use region of northern Idaho. A basic sample grid of 50 × 75 m (164 × 246 ft) that also had clusters of additional points 25 m (82 ft) apart supported both transect-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 2G-year period following Cesium fallout (1963–1989), erosion averaged -11.6t ha⁻¹ yr⁻¹ (-5.2t ac⁻¹ yr⁻¹)from erosional areas of the watershed, and deposition averaged 18.6t ha⁻¹ yr⁻¹ (8.3 t ac⁻¹ yr⁻¹) onto depositional areas of the watershed. Extreme values of average erosion and deposition were -58 t ha⁻¹ yr⁻¹(-25.9 t ac⁻¹ yr⁻¹) and 277 t ha⁻¹ yr⁻¹ (123.5 t ac⁻¹ yr⁻¹), respectively. Contour maps of net soil movement developed using geostatistical techniques show that convex ridge tops and mid-slope knobs have undergone the most severe erosion; concave areas in valley bottoms and on slopes received deposition; and mid slopes generally had small amounts of either erosion or deposition, suggesting that they are zones of soil transport. The average erosion rate estimated by RUSLE for the entire watershed was -31.4 t ha⁻¹ yr⁻¹ (-14 t ac yr⁻¹), compared to a rate estimated by Cs-137 of -11.6 t ha⁻¹ yr⁻¹ (-5.2 t ac⁻¹ yr⁻¹) for erosional areas (about 60 percent of the watershed), suggesting that RUSLE may have overestimated water erosion on this farmed watershed.