Identifying and estimating soil erosion and sedimentation in small karst watersheds using a composite fingerprint technique

Abstract

The highly heterogeneous surface environment and subsurface conduit network in karst areas make estimating soil erosion extremely complicated. Sediment fingerprint techniques have been successfully applied in soil erosion research in karst areas as a new way to more effectively collect information on soil loss. However, few sediment fingerprint factors, most of which are based on radioactive fallout tracers, such as 137Cs and/or 210Pb, have been used in existing studies. The soil loss rate estimated by these fingerprint factors varies greatly among different watersheds. To clearly identify the sedimentary sequence, to estimate the changes in soil erosion in recent years, and to relate the soil erosion process to rocky desertification, a composite sediment fingerprint technique is used in this study. Two typical watersheds with peak clusters and depressions and with different degrees of rock desertification were selected, and sediment cores were extracted from the depressions to measure changes in 137Cs, magnetic susceptibility, soil organic carbon (SOC) and sediment particle composition at different depths. The results indicated that the soil loss rates in the Liaojiawodang (LJWD) and Dajutang (DJT) watersheds showed a fluctuating decrease from 1954 to July 2018. The severe soil erosion in the periods of 1954–1963，1963–1970 and 1976–1986 was mainly caused by large-scale reclamation and deforestation. In addition, heavy rainfall in 1983 and 1991 might have played a significant role in the soil loss in the DJT watershed in 1978–1986 and in the LJWD watershed in 1986–2000, respectively. The construction of agricultural terraces several decades ago and the implementation of the Closing Mountains for Forest Cultivation, Grain for Green and Poverty Alleviation projects in recent years to improve the ecological environment have effectively alleviated the soil erosion in the studied watersheds. Over the past 60 years, soil loss in the LJWD and DJT watersheds decreased by approximately 89 % and 87 %, respectively. In karst areas, rocky desertification and soil erosion do not exhibit identical trends. Because of severe rocky desertification, the soil erosion in the LJWD watershed was significantly less than that in the DJT watershed. This study provides important information on detailed sediment dating and demonstrates the potential of using 137Cs, magnetic susceptibility, SOC and sediment particle composition to identify changes in erosion in a watershed.