- 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
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.