Project "GlintMapper"

Resolving near-coastal remote sensing signal into contributions by bottom, water column, glint and the adjacency effect

Acronym: GlintMapper

ESA contract: 4000144012/24/I-DT-bgh

Duration: 01.05.2024 – 31.10.2025

Project leader: Martin Ligi

Social media: #ESA_GlintMapper

Project description:

All waterbodies are changing in the variable climate conditions. Monitoring of these changes over large areas is possible only by using remote sensing. However, remote sensing of lacustrine waterbodies is hampered by the nearby land as part of the signal measured above the waterbodies originates from the nearby land not from the water itself. This problem, called the adjacency effect, is detectable up to kilometers from the shore. In the case of majority of lakes on Earth it means that every water pixel is affected by the nearby land. Moreover, the signal measured near the shores may contain effects from the bottom (if water is optically shallow), signal from emerging vegetation, sun and sky glint. In the case of marine remote sensing these problematic areas are usually masked out in remote sensing studies as too complicated to resolve. This is not an option in lake remote sensing as just a few tens of lakes (out of 117 million) are large enough to contain pixels free from the adjacency and other coastal effects. Moreover, up to 99% of carbon is processed in the near-shore waters and never reaches the deep ocean carbon pool. Thus, many important processes take place in the near-coastal waters that are currently masked out from remote sensing imagery as too complex to resolve. This limits the use of remote sensing in environmental research and monitoring.

Our aim is to study the very nearshore waters in order to resolve the contribution of the adjacency effect, lake/sea bottom, sun and sky glint and the water column itself and develop algorithms for removing the adjacency effect and glint from Sentinel imagery. We have an unmanned surface vehicle equipped with radiometers, fluorometers, sonar, underwater and in-air video cameras. This package allows us to make high frequency reflectance measurements almost from the shore (from 20 cm water depth) to open parts of lakes and coastal waters and assess the contributions of bottom, water column, glint and the adjacency effect on the water reflectance. The contribution of adjacency effect in the remote sensing signal depends on the contrast between the (dark) water and (bright) land. Therefore, we will carry out experiments in lakes with different optical water properties and during different season (i.e. different brightness of land). For example, in dark CDOM-rich lakes most of the remote sensing signal in visible part of spectrum is due to glint and the adjacency effect while in shallow waters with bright bottom or in lakes with massive phytoplankton blooms the adjacency effect and glint may be parameters that have nearly negligible effect on the remote sensing signal.

In the end, we will propose a methodology how to resolve different components in the remote sensing signal measured near shores of lakes and seas. This will allow to make significant step forward in studying properties and processes in near-coastal waters and lakes that are currently not studied with remote sensing because the areas are flagged, or masked out completely, due to their optical complexity.