Deriving spatial and temporal context for point measurements of suspended-sediment concentration using remote-sensing imagery in the Mekong Delta

摘要

Rivers are a major supplier of sediment to the open oceans, but observations of sediment dynamics are limited in downstream reaches where significant amounts of sediment can be trapped. Point observations of riverine properties are the usual method for estimating processes and building predictive models. Although point observations provide extensive data at one location for a short duration, they do not easily provide larger spatial or temporal context. Remote-sensing has the capability to provide this context if imagery values can be translated into riverine properties. In this paper, we present a demonstration of using a combined data set of remote sensing imagery and in situ observations to derive spatial maps of suspended-sediment concentration (SSC) in the Mekong Delta as conditions change from high-flow (Sep 2014) to low-flow (Mar 2015), and to estimate a decadal change in SSC from 1987 to 2016. Using a test set of coincident in situ observations of SSC with multi-spectral remote-sensing imagery, we develop a six parameter empirical model using ratios of reflectivity that generates a root-mean-squared error (RMSE) for the test set of 0.038 g/l. Using these models, we generate large-scale spatial maps of SSC and extract the spatial changes in SSC within the Dinh An and Tran De distributary channels as conditions change from high-to low-flow. We determine the SSC patterns within this region that correspond to high-, intermediate-and low-flow conditions and show that these patterns exist during Sep-Oct (high-flow), Nov-Jan (intermediate-flow), and Feb-Mar (low-flow). We show that during high-flow the SSC values throughout the region are increased, and there is a gradual increase in SSC from the upstream channel regions of the study area to the offshore plumes consistent with the intrusion of a salt wedge. We also show that during low flow there is an unique increase in SSC in the upstream channel regions, consistent with this being the region of opposing river and tidal-inflow currents. The decadal analysis indicates a decrease in SSC from 1987 to 2016 throughout the Mekong Delta of 1% per year starting in 2001.