The Color of Water from Space: A Case Study for Italian Lakes from Sentinel-2

Abstract

Lakes are inestimable renewable natural resources that are under significant pressure by human activities. Monitoring lakes regularly is necessary to understand their dynamics and the drivers of these dynamics to support effective management. Remote sensing by satellite sensors offers a significant opportunity to increase the spatiotemporal coverage of environmental monitoring programs for inland waters. Lake color is a water quality attribute that can be remotely sensed and is independent of the sensor specifications and water type. In this study we used the Multispectral Imager (MSI) on two Sentinel-2 satellites to determine the color of water of 170 Italian lakes during two periods in 2017. Overall, most of the lakes appeared blue in spring and green-yellow in late summer, and in particular, we confirm a blue-water status of the largest lakes in the subalpine ecoregion. The color and its seasonality are consistent with characteristics determined by geomorphology and primary drivers of water quality. This suggests that information about the color of the lakes can contribute to synoptic assessments of the trophic status of lakes. Further ongoing research efforts are focused to extend the mapping over multiple years.

An Evaluation of Citizen Science Smartphone Apps for Inland Water Quality Assessment

“…In a ground-breaking article, Wang et al [28] demonstrated that the worldwide monitoring of the eutrophic state of inland waters with the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument is possible, based on the hue and the FUI. Recently, quantification of the color characteristics of inland waters by satellite-derived color has been applied at the national level for New Zealand [29] and Italy [30]. In 2014, the EyeOnWater (EoW) and HydroColor (HC) smartphone applications (Apps) became available to measure the color of water.…”

Citizen Scientists Contribute to Real-Time Monitoring of Lake Water Quality Using 3D Printed Mini Secchi Disks

“…Sensors on satellites and in-situ sampling devices are used for estimation of water clarity and colour. Multispectral Imager (MSI) on two Sentinel-2 satellites used to determine seasonal variation in the colour of water of 170 Italian lakes in 2017 showed that while 13 lakes moved from blue to yellow, indicating reduction in water clarity, another 16 lakes showed transitions in the opposite direction, from green to blue or from yellow to green, suggesting improved water clarity from spring to late summer (Giardino et al, 2019). Li et al (2016) based on the study of the largest 10 lakes in China in 2000-2012 are of the view that the relations of FU index with water clarity and trophic state are not accurate, as the FU index reflects both Secchi depth and chl-a, which are important input parameters in trophic state assessment models.…”

The Color of Rivers

“…λ d is often binned into water color categories using the Forel‐Ule scale (Pitarch et al., 2019; Wang et al., 2018). However, recent work used λ d without binning to examine patterns and trends in water color across thousands of lakes in New Zealand and Italy (Giardino et al., 2019; Lehmann et al., 2018).…”

“…It is intuitive since it is based on human perception of color, and people often discern water's suitability for consumption, recreation, and aesthetic value based on water color (Smith & Davies‐Colley, 1992; West et al., 2016). Color is broadly applicable because it does not require knowledge of the inherent optical properties of water and can be directly measured by any optical imager with bands in the visible spectrum (Giardino et al., 2019; Van der Woerd & Wernand, 2018). Lastly, water color is one of the oldest measurements of water quality (Forel, 1895; Hutchinson, 1957) and is widely used in community science campaigns (https://www.eyeonwater.org/).…”