Agricultural erosion and sediment in the western Willamette Valley as indicated by redistribution of Cesium-137 Public Deposited

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  • Modern agricultural erosion was studied using ¹³⁷Cs from fallout as an indicator of erosion/deposition. The study area was a 285 ha agricultural watershed in the hilly western margin of the Willamette Valley, a high winter rainfall zone in which fall-planted crops are commonly grown. Objectives of the study were (i) to identify, based on areal concentration and depth distribution of ¹³⁷Cs activity, those parts of the landscape that had been most severely eroded over the period of fallout (since early 1954), (ii) to identify, based likewise on ¹³⁷Cs signature, those parts of the landscape that had been subject to net deposition over the same period, and (iii) to quantify erosion rates over the fallout period. Along each of eight transects representing a variety of elevations, slope aspects, and slope gradients, sample sites were selected in ridgetop, steep sideslope, and concave footslope areas. Also sampled were a single convex ridge shoulder, a high ridgetop, two fencerow-controlled alluvial fans, two sites on a floodplain, and two sites in a farm pond built in 1971. Replicate soil cores were collected within a small (< 2 m²) area at each of the 32 sampling sites. Cores were cut into depth increments of 7.5, 10, or 15 cm increments. Corresponding increments from replicates at each site were lumped. Samples were analyzed at Oak Ridge National Laboratory for ¹³⁷Cs activity. Patterns of depth distributions of ¹³⁷Cs activity suggested strongly that ¹³⁷Cs had been retained in surface soils on the watershed and therefore was a good tracer in erosion/sediment studies. Cesium-137 signatures of sideslope sites and those of ridgetop sites were, on the average, indistinguishable from each other. Depositional sites, in contrast with both sideslopes and ridgetops, tended to have overthickened ¹³⁷Cs profiles and high total contents of ¹³⁷Cs. Average total ¹³⁷Cs activity in depositional sites was about 12 pCi/ cm². In sideslopes and ridgetops it was about 8 pCi/cm². Concave footslope positions are important zones of sediment storage. Two of eight footslopes sampled had ¹³⁷Cs signatures that did not reflect deposition, suggesting the existence of a dynamic deposition/re-entrainment environment in footslopes. The two alluvial fans sampled had strongly depositional ¹³⁷Cs signatures, but contained much less total sediment than did upstream footslopes. The two floodplain sites had not experienced net detectable deposition over the fallout period, which was indicative of a diffuse, thin spreading of sediment in the wide floodplain zone. Sites in the eight year old pond were marginally to strongly depositional. Two different approaches, "volumetric" and "gravimetric," were used to estimate post-1954 erosion rates in two nested watersheds, 6 and 13 ha in size. 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. Erosion rate estimates by this technique ranged from 3 to 14 MT/ha/yr (1 to 6 T/a/yr). The "gravimetric" approach involved algebraic manipulation of measured areal concentrations of ¹³⁷Cs activity in depositional and non-depositional zones, to obtain estimates of the amount of depletion of fallout ¹³⁷Cs that had occurred in upland zones. Conversion of the ¹³⁷Cs loss to sediment loss yielded soil loss estimates ranging from 6 to 27 MT/ha/yr (3 to 12 T/a/yr). Imprecision in these estimates is great. But the estimates do indicate strongly that modern erosion rates, while not spectacular, have been great enough to warrant the use of appropriate conservation measures in hilly croplands of the region.
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