Soil Erosion and Deposition Evidence in a Small Watershed Using Fallout Cesium-137
- J. M. Soileau ,
- B. F. Hajek and
- J. T. Touchton
Soil measurements of 137Cs, a fallout component of nuclear testing, were made within a 3.8-ha research watershed in the Limestone Valley of northern Alabama. Objectives of the study were to: (i) compare 137Cs activity in soil profiles on eroded and depositional landscape positions, and (ii) relate soil erosion and deposition calculated by 137Cs methods to erosion estimated by the Universal Soil Loss Equation (USLE) and to sediment discharge measured at the watershed outlet. Triplicate soil profiles were analyzed for 137Cs activity to a 46-cm depth in cultivated sites representing slightly eroded ridgetop, eroded side-slope, and depositional downslope positions. These data were compared with baseline 137Cs measurements in undisturbed adjacent woodland. Erosion calculated by 137Cs activity losses from baseline values were compared with the USLE estimates on slope transects. Mean total 137Cs activities on baseline, eroded, slightly eroded, and depositional sites were 4860, 1804, 2807, and 5367 Bq m−2, respectively. Peak 137Cs activity and depth to zero activity were much greater in depositional than in eroded and slightly eroded sites. Estimated mean annual (1954–1987) soil-erosion rates were similar between 137Cs and USLE methods for a slope transect with slight erosion (28 and 26 Mg ha−1 yr−1). On eroded side-slopes, 137Cs estimates of erosion were 27 to 80% higher than the USLE estimates. This may be due to rill erosion unaccounted for by the USLE. The USLE estimated losses between 9 and 52 Mg ha−1 yr−1 for uplands during 1984 to 1988 cotton production. Sediment losses measured below the downslope depositional area of the small watershed (Gilbert Farm Research Watershed) were only 1 to 4 Mg ha−1 yr−1. This indirect evidence of extensive sediment deposition in a representative small watershed of karst terrain was supported by 137Cs activity in depositional soil profiles.
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