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Agricultural Erosion Indicated by ¹³⁷Cs Redistribution: II. Estimates of Erosion Rates¹

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.

¹ Contribution from the Oregon Agric. Exp. Stn., Oregon State Univ., Corvallis, OR 97331, Technical Paper no. 5,753, and from the Environmental Sciences Division, Oak Ridge National Lab., Oak Ridge, TN 37830, Publication no. 1,785.

² Formerly Graduate Research Assistant, Dep. of Soil Sci., Oregon State Univ., now Assistant Professor, Soil Sci. Dep., Univ. of Florida, Gainesville, FL 32611; Assistant Professor, Dep. of Soil Sci., Oregon State Univ.; and Manager, ORNL Low Level Waste Management Program, Environmental Sciences Division, Oak Ridge National Lab.

Received for publication April 27, 1981. Accepted for publication July 27, 1981.

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