Studying Soil Erosion and Sedimentation in Small Island Watersheds: Adaptive Techniques for a Changing Climate System

Abstract

<b>Abstract.</b> The U.S. Virgin Islands consist of three islands of 364 km² in total land area. Because of their size, there has been little ecohydrological study of watersheds, sedimentation, and soil erosion in the Virgin Islands, with a few notable exceptions (e.g., Ramos-Scharrón et al., 2014). In particular, the study of sediment transport from watersheds to coastal zones via the ephemeral stream network of the islands has been impeded because of major logistical challenges in sampling and monitoring these systems (Gardner et al., 2008). In addition, the Virgin Islands have been experiencing increasingly dry conditions, attributed partially to climate change (Herrera et al., 2018). These challenges combine to make the goals of adequately characterizing hydrological sediment flux very difficult in Virgin Islands watersheds. There is, however, a significant need for this kind of study, since observations of marine scientists confirm both the presence and deleterious impact of terrigenous sediments in the coastal zone (Ennis et al., 2016; Smith et al., 2008). Our objective was to develop and deploy a series of adaptive monitoring techniques suited to local conditions in two Virgin Islands watersheds that can successfully monitor hydrological conditions and collect sediment samples without real-time intervention. The primary challenge, the ephemeral nature of the stream network, necessitated the adaptation to local conditions of both passive and active automatic samplers and monitoring equipment. Based on the work of Diehl (2008) and Phillips (2000), suspended sediment samplers were constructed and deployed in dry streambeds subject to flashy water flow events, and semi-perennial pools in rock-confined upper reaches of the watersheds, respectively (Figure 1). <fig><graphic xlink:href=23073_files/23073-00.jpg id=E8FFC00A-C334-4C83-8D60-B9A6008E9CEF></graphic></fig> These samplers, combined with automatic water level loggers and weather stations with alert systems to allow for real-time measurements of flow (when possible), and grab samples, have been effective in producing a monitoring dataset of suspended sediment samples, hydrographs, and precipitation logs to characterize the hydrological and sedimentological situation in Virgin Islands watersheds, even when subjected to unexpected, sudden rain events after extended dry periods. As described by Ramos-Scharrón et al. (2014), land-based sediment traps were also deployed for comparison. Our results demonstrate the successful use of this kind of monitoring system, which can subsequently be expanded to other islands or similar watersheds, or continued for a longer period to provide crucial hydrological and erosion data for further studies on soil erosion and downstream mitigation in the U.S. Virgin Islands.