The use of caesium-137 to estimate agricultural erosion on steep slopes ina tropical watershed

The estimation of erosion and sediment delivery rates in tropical mountain watersheds is difficult and most of the methods widely used for estimating soil erosion over large areas have serious limitations. The Cs-137 approach has potential for quantifying soil erosion because it can provide retrospe ctive estimates of long-term (since 1963) net sediment redistribution relat ively quickly. Despite its great potential, Cs-137 has not yet been used in an extensive, reconnaissance level survey of erosion in complex tropical m ountain environments. The objective of this study was to examine the applic ability of the Cs-137 method to estimate erosion on steep tropical agricult ural lands (23 to 80% slopes) in the Nizao watershed, a humid, tropical mou ntain area of the Dominican Republic. In this study we (i) examine the vari ation of Cs-137 in ten reference sites - eight coffee groves and two forest ed sites - and (ii) estimate erosion from 14 cultivated fields. The soil pool of Cs-137 ranged from to 150 to 192 mBq cm(-2) on reference s ites with minimal erosion. Variability among reference sites was less than expected for such complex mountain terrain. The variability within coffee a nd forested reference (average CV = 28%) sites was similar to the variabili ty found on grassland and forested reference sites in the temperate zone. The estimated annual soil loss from 14 sampled fields ranged from 6 to 61 t ha(-1) year(-1) with an overall mean of 26 t ha(-1) year(-1). Overall, the soil erosion estimates found using the Cs-137 method were much lower than those often assumed for such steep tropical hillsides. These erosion estima tes account for soil loss since 1963 only and it seems likely that soil los ses may have been much higher in earlier decades immediately after initial forest clearing earlier in the 20th century. Copyright (C) 2000 John Wiley & Sons, Ltd.