Water constituents and water depth retrieval from Sentinel-2A – a first evaluation in an oligotrophic lake

Abstract

Satellite remote sensing may assist in meeting the needs of lake monitoring. In this study, we aim to evaluate the potential of Sentinel-2 to assess and monitor water constituents and bottom characteristics of lakes at spatio-temporal synoptic scales. In a field campaign at Lake Starnberg, Germany, we collected validation data concurrently to a Sentinel-2A (S2-A) overpass. We compared the results of three different atmospheric corrections, i.e. Sen2Cor, ACOLITE and MIP, with in situ reflectance measurements, whereof MIP performed best (r = 0.987, RMSE = 0.002 sr-1). Using the bio-optical modelling tool WASI-2D, we retrieved absorption by coloured dissolved organic matter (aCDOM(440)), backscattering and concentration of suspended particulate matter (SPM) in optically deep water; water depths, bottom substrates and aCDOM(440) were modelled in optically shallow water. In deep water, spatial patterns of SPM and aCDOM(440) were plausibleshowed reasonable spatial patterns. Comparisons with in situ data (mean: 0.43 m-1) showed an underestimation of S2-A derived aCDOM(440) (mean: 0.14 m-1); S2-A backscattering of SPM was slightly higher than backscattering from in situ data (mean: : 0.019 m-1 vs. 0.027 m-1 vs. 0.019 m-1). Chlorophyll-a concentrations (~1 mg m-3) of the lake were too low for a retrieval. In shallow water, retrieved water depths exhibited a high correlation with echo sounding data (r = 0.95, residual standard deviation = 0.12 m) up to 2.5 m (Secchi disk depth: 4.2 m), though water depths were slightly underestimated (RMSE = 0.56 m). In deeper water, Sentinel-2A bands were incapable to of allowing a WASI-2D based separation of macrophytes and sediment which led to erroneous water depths. Overall, the results encourage further research into study lakes with varying optical properties and trophic states with Sentinel-2A.