Agricultural Erosion Indicated by 137Cs Redistribution: II. Estimates of Erosion Rates

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Abstract
Erosion rates were estimated for 6‐ and 12.9‐ha nested agricultural watersheds in the western Willamette Valley, Oreg., using 137Cs activity levels and two different approaches, termed “volumetric” and “gravimetric.” The volumetric approach involved calculation of the volume and mass of sediment currently residing in depositional zones, based on areal extent of the zone and depth of occurrence of 137Cs in the zone. This sediment was assumed to have come from the uplands (sideslopes plus ridgetops) by erosion during the period of fallout (since 1954). Reasonable ranges of watershed sediment delivery ratios and lengths of record were assumed. It was estimated by this technique that modern erosion rates in the uplands had been between 3 and 14 metric tons ha−1 yr−1.The gravimetric approach involved algebraic manipulation of areal concentrations of 137Cs activities in depositional and upland zones to obtain estimates of erosion loss of 137Cs from the uplands. Estimated 137Cs losses were converted to estimated soil losses, which ranged from 6 to 27 metric tons ha−1yr−1.Considering the imprecision of these markedly different means of estimating soil loss rates, the volumetric and gravimetric estimates agree reasonably well. Together, they provide good evidence that the rate of modern erosion at the study site has been within and very possibly above the range of erosion rates normally thought of as tolerable.

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