Monitoring of suspended sediment concentration in the Rhine River using Landsat (Google Earth Engine)

Abstract

The traditional methods for measuring water quality are time-consuming and do not give a spatial and temporal view of SSC needed for assessment of the water quality. The prediction of the suspended sediment using remote sensing through the main channel and tributaries of Rhine River can provide valuable information to assess the sources of the suspended sediment under different hydrology condition. The main objective of this study is to understand the suspended sediment concentration (SSC) characteristics in the Rhine River using Landsat satellite images. This study developed a method of quantifying SSC based on Landsat imagery and corresponding SSC data from Bundesanstalt für Gewässerkunde (German Federal Institute of Hydrology) from 1995 to 2016. The model is built using the ratio of logarithmic transformation of a red/green band and logarithmic transformation of SSC based on in-situ sampling measurements. The SSC model works well and shows satisfactory performance. Landsat satellites (Thematic Mapper (TM), Multi-Spectral Scanner (MSS), Enhanced Thematic Mapper (ETM), Operational Land Imager (OLI)) explained an acceptable result accuracy (R2: 0.32; error/ bias -0.02 mg/L; RMSE 12.23 mg/L; NSE 0.33). With the derived empirical regression, we produced Landsat-derived SSC maps for the 1995-2016 period. Analyses of images product suggest that the change of SSC shows a marked increase of about 6 mg/L at Upper Rhine, gradually increase of about 3 mg/L at Middle Rhine, and relative constant at Lower Rhine. Turning to seasonal variation, high SSC is mostly found in winter and low SSC is in summer. However, another result shows that the SSC tend to be high in June at almost all monitoring stations where the discharge is minimum. The major sediment source at Lobith was mostly regulated by the River Moselle in high discharge condition, which shows a clockwise loop. The major source of SSC at Lobith was mostly regulated by the Neckar River and Main River in moderate discharge condition since it is affected by exhaustion of the SSC in the channel and the travel time of the sediment supply from the tributaries. Another result in moderate discharge condition, counter-clockwise loop, indicates that the major sources of SSC were mainly from the Neckar River and Main River in moderate discharge condition. No hysteresis loop at low discharges shows the upstream tributaries input such as the Neckar River, the Main River, and the Moselle River contributed to almost equal amounts of SSC to the downstream. This type of hysteresis occurred relatively often during summer and SSC peak simultaneously, which suggest that Q–SSC relations are more controlled by the entrainment of bed material. The shape of the hysteresis loops is not only affected by exhaustion of the SSC in the channel, but also the travel time of sediment supply from the tributaries. The travel time for high discharge is one day, while for moderate and low discharge conditions is within 2-3 days.