Validation @ Roodeplaat Dam

Field validation

This past week, we had a successful field campaign at Roodeplaat Dam near Pretoria. The aim was to collect key data for validation of the Sentinel-3 Ocean and Land Colour Instrument, to feed back to the Sentinel-3 Validation Team during the instrument commissioning phase.

Roodplaat represents the most extreme case when it comes to inland waters: it is very small allowing only a few pixels to be extracted, and is almost always hypertrophic with cyanobacteria blooms present. Thus it is a good test site to ensure that we can reliably derive information from small inland water using OLCI.

We collected, simultaneous to the overpass of S3 and S2, water leaving reflectance measurements, atmospheric aerosol optical thickness estimates, and downwelling irradiance measurements using two ASDs and another irradiance meter and Microtops instruments. Accompanying me on the field work was Kenilwe, a masters student at the university of Cape Town, Derek Griffith from the CSIR, an atmospheric scientist and his student. The staff from the Department of Water and Sanitation at the nearby Resource Quality Services, provided excellent support, with Solomon skippering the boat, and the labs doing chl-a and TSS measurements.

The weather was on fine form with clear, albeit dusty, conditions, and only a few small clouds appearing just before the S2 overpass around 10:30 am; but having achieved two clear S3 images things went as well as they could have possibly gone.

There was plenty of phytoplankton in the water with the usual offender being Microcystis with some brown algae below the surface producing some nice fluorescence signals.

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Autonomous platform

Hein Swart and his CSIR team successfully deployed Gizmo, an autonomous buoy platform designed by CSIR’s Dr. Stewart Bernard.

Gizmo has two trios radiometers at different depths measuring the upwelling radiance, a fluorometer measuring chlorophyll fluorescence, pressure and temperature sensors, a downwelling irradiance sensor, and an anemometer and tilt and roll sensors. It sends the data via telemetry taking a sample every 15 minutes and is solar and battery powered.

The aim is to collect 3 months of continuous reflectance measurements that can be used for Sentinel-3 validation.

Funding for the work is gratefully provided through the WRC S3VAL and EONEMP projects, DWS in-kind contributions, and the CSIR.