Sediment, soil organic carbon and runoff delivery at various spatial scales

Abstract: Water erosion is a very dynamic process with direct and indirect consequences on ecosystem functioning. While the processes of sediment (SED) and soil organic carbon (SOC) detachment and transport are well recognized, it is however difficult to assess and interpret their impact on SED and SOC detachment, transport and sedimentation within a given landscape. In this study of a tropical area of the Mekong Basin, our main objective was to evaluate sediment (SED), soil organic carbon (SOC) and runoff (R) delivery from various spatial scales from 1m² to 10km² and to interpret these results in term of erosion processes operating within the landscape. Deliveries from nested scales of microplots (1×1m²; n=12 installed at different topographic positions and soils of a hillslope), plots (1×2.5m²; n=8), hillslope (60×100m²) and catchment (30, 60, and 1000ha) were assessed during an entire rainy season. To improve understanding of soil erosion at landscape level, delivery of mobilized water, sediment and SOC from one surface area to the following one in the nested experimental design were confronted to environmental information on rainfall characteristics (rainfall intensity, I; maximum 6-min rainfall intensity, Imax; rainfall amount, R; rainfall depth Dur; cumulative yearly rainfall prior to the event, Cum), slope gradient (S), soil thickness (T) and soil crusting (Crust), antecedent soil water content (SWC), and depth to the water table (DWT). These data were finally compared to extensive mapping of the thickness and the type of the soils in an attempt to validate these results and to evaluate the longer term consequences of erosion processes on soil distribution. The mean sediment delivery from 1×1m² plots was 899gm−2 y−1 with standard error (SE) of 26gm−2 y−1. The SED delivery decreased to 275±63gm−2 y−1 on 1×2.5m2 plots and to 4.3gm−2 y−1 at the hillslope level but then increased to 16.6gm−2 y−1 at the basin level. The slight decrease in SED delivery flux from 899gm−1 y−1 on 1m long plots to 688±157gm−1 y−1 on 2.5m2 long plots and to 468gm−1 y−1 on the 100m long hillslope revealed that SED detachment and transport in slopes is mainly controlled by splash. The ratio of plot to microplot deliveries for R increased significantly as Crust increased (r=0.91) but decreased with increasing Cov (r=−0.88) while the ratio for SOC correlated the most with S (r=0.97) and Cov (r=−0.58). The R delivery ratio from hillslope to plot and from river to hillslope increased as soils get wetter and the water table rose while higher ratios for SED and SOC occurred at longer event duration and larger rainfall depth amount and at larger yearly antecedent rainfall in the case of the within catchment delivery. The large accumulations of SED and SOC at the lower parts of hillslopes confirms the observed erosion dynamics longer-term, (i.e., removal and transport of SED and SOC mainly by splash because of high infiltration occurring in slopes) thus suggesting a potential long-term sequestration of the SOC deposited in the lower parts of hillslopes while deposition in the river network appeared ephemeral. [Copyright &y& Elsevier]