Rates and patterns of tillage and water erosion on terraces and contour strips: evidence from caesium-137 measurements

Despite the widespread use of contour-strips and terraces for soil conservation little is known concerning the impact of such measures on rates and patterns of tillage erosion and tillage translocation. Caesium-137 (137Cs) measurements would appear to offer a rapid means of assembling data relevant to the evaluation of tillage erosion and tillage translocation. However, use of 137Cs data in this way requires an approach to the analysis of 137Cs data which accounts for both tillage erosion and tillage translocation. The latter is particularly significant on short slopes. A method of analysing 137Cs data has been developed, that employs a linear, multistore, mass-balance model of soil and 137Cs redistribution to permit estimation of the contributions to erosion from both water and tillage erosion. This method is outlined and results from its application are discussed using case studies from Yanting, in Sichuan Province, China; Ha Sofonia, in Lesotho; and, Chinamora, in Zimbabwe. These case studies confirm the viability of the approach and provide valuable evidence for the importance of tillage erosion and translocation on terraces and contour-strips subject to cultivation by animal traction. Gross rates of tillage erosion were found to be of comparable or greater magnitude than gross rates of water erosion on the fields examined. It is, therefore, suggested that any evaluation of on-site impacts of erosion must take account of tillage erosion. Relationships between annual soil fluxes due to tillage and slope tangent were found to be very similar to the relationships established for a single pass of equivalent tillage equipment with mechanical traction. Furthermore, because tillage by animal traction necessitates downslope turning of the soil on every occasion, net downslope fluxes of soil associated with animal traction may exceed the levels associated with tillage by mechanised traction, in which the soil is typically turned in opposing directions on successive occasions.