SOIL-EROSION AND DEPOSITION EVIDENCE IN A SMALL WATERSHED USING FALLOUT CESIUM-137

Soil measurements of Cs-137, 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 Cs-137 activity in soil profiles on eroded and depositional landscape positions, and (ii) relate soil erosion and deposition calculated Cs-137 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 Cs-137 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 Cs-137 measurements in undisturbed adjacent woodland. Erosion calculated by Cs-137 activity losses from baseline values were compared with the USLE estimates on slope transects. Mean total Cs-137 activities on baseline, eroded, slightly eroded, and depositional sites were 4860, 1804, 2807, and 5367 Bq m-2, respectively. Peak Cs-137 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 Cs-137 and USLE methods for a slope transect with slight erosion (28 and 26 Mg ha-1 yr-1). On eroded side-slopes, Cs-137 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 Cs-137 activity in depositional soil profiles.