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Deploying spectral probe systems in the Cathedral Peak LTER platform

By Kent Lawrence, Technician, SAEON Grasslands-Forests-Wetlands Node
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Impacts of climate variability and extreme events on ecosystems and human activities have become a major issue. The impacts on water resources are of primary importance but requires a great deal of research to fully understand the processes involved.

In South Africa, understanding hydrological responses to climate change is an ongoing focus area in the SAEON Cathedral Peak LTER (long-term ecological research) platform.

Another major issue for the 21st century is the two-way interaction between climate and the global carbon cycle. On the one hand, the effects of greenhouse gases on the Earth’s climate are well-known, and widely investigated worldwide (IPCC 2013, 2014). On the other hand, there are still major disagreements about the environmental impacts of climate variability, especially intra-seasonal spells and extremes, specifically on carbon fluxes.

Thus, assessing the impact of a changing climate on soil carbon fluxes (GHG, dissolved organic carbon [DOC]/particulate organic carbon [POC] leaching) is a key issue. Moreover, carbon fluxes are driven not only by average climate conditions, but also by abrupt changes in rainfall and runoff, and by the frequency and duration of droughts followed by wet periods.

Investigating climate/land-use/carbon flux relationships with an extreme event lens requires continuous in situ measurements, which should also consider soil history and management at the catchment scale. The SAEON Cathedral Peak LTER platform provides the perfect opportunity to undertake such work. 

International collaboration

The SAEON Grasslands-Forests-Wetlands Node is collaborating with the Université Bourgogne Franche-Comté (UBFC) (led by Olivier Mathieu, Philippe Amiotte Suchet and Mathieu Thevenot), who are supported by the I-SITE IMVULA project (led by B. Pohl), and the French National Center for Scientific Research (CNRS) through the EC2CO EvoC project.

The IMVULA project (which researches the predictability of extreme relevant intra-seasonal descriptors of South African rainfall across scales), aims to characterise intra-seasonal wet and dry spells over South Africa and their properties (including intensity and persistence), considering both the climatology during the austral summer season and the variability across temporal scales. Another objective of this project is to assess the impacts of weather extremes and associated climate variability on carbon fluxes in two catchments of the Cathedral Peak SAEON LTER platform.

To achieve this, one component of the project focuses on monitoring water quality and carbon export, which will be undertaken for at least three years, at high frequency (every 15 minutes), using conductivity and spectral probes at the outlet of catchments six and nine. The spectral probes for this project have been supplied by the French team, adding a significant boost to the SAEON carbon-monitoring network. 

The acquired data (Electric Conductivity, Turbidity, Temperature, Total Suspended Solids, Nitrate, Total Organic Carbon, Dissolved Organic Carbon and UV Absorbance) will allow the SAEON and French scientists to establish relationships between climatic events, hydrology, as well as quality and quantity of organic carbon export. 

Scan Spectrolyser and Condulyser

The Scan Spectrolyser is one of the most advanced fully submersible spectrophotometers available, with the ability to measure across the entire UV-Vis spectrum and deriving a number of parameters from analysis of those spectra.

Substances contained in the water that are measured weaken a light beam that moves through this water. The light beam is emitted by a lamp, and after contact with the water its intensity is measured by a detector over a range of wavelengths.

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Dry dock formed in weir six, allowing the team to proceed with casting the concrete base and pole.

Concrete base formed in weir six and allowed to cure properly before continuing with the installation.

Each molecule of a dissolved substance absorbs radiation at a certain and known wavelength. The concentration of substances contained determines the size of the absorption of the sample – the higher the concentration of a certain substance, the more it will weaken the light beam.

The operating electronics contained in this part of the probe are responsible for controlling the entire measuring process and all the various processing steps required to edit and check the measuring signal and to calculate fingerprints and parameter values. Fitted on the spectrolyser is a submersible rucksack auto brush which operates before every measurement, removing any debris within the optical pathway.

In addition to the Spectrolyser, the Condulyser is a probe designed for the continuous monitoring of the conductivity in water. This value indicates the capability of the water to transmit electrical current, which is highly correlated to the temperature. As a result, the sensor also measures the temperature of the water and corrects the measured conductivity accordingly. 

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SAEON technician Kent Lawrence (right) inspecting the concrete base before continuing with the installation in weir nine.                       

SAEON technicians Kent and Siphiwe Mfeka (second from left) continuing with the installation in weir nine after the concrete base had cured.

Complicated logistics

The installations of the spectral probe systems required a lot of pre planning as there were many factors to consider when it came to designing the housing, structure and positioning of the systems. Factors such as security, accessibility of the probes, length of the probes’ cables and disturbances to streamflow measurements were taken into account.

Considering all the above and more, the SAEON team had the idea to mount the probes on stand-alone structures within the weirs. This was achieved while the weirs were drained, by casting concrete blocks on level bedrock at the bottom of the weir ponds to act as a secure base to which aluminium tubing were inserted as the upright stands for the probe housings.

The probes were positioned to eliminate obstructions and disturbances to the streamflow measurements, being situated away from the weir plates. Installing the probe at this location, and not close to any accessible infrastructure, should serve to prevent theft and vandalism, as the probes are surrounded by water and hence inaccessible.

The positioning of the structures inside the weirs was dependent on the length of the probe cables (15 m long), so the final positioning of the structures from the loggers (inside the stilling hut) was set as far as possible towards the centre of the weir ponds as the cables would allow. Casting concrete bases meant that the concrete had to dry and cure properly before the probes could be installed and the weirs allowed to fill up, which resulted in an additional two days to the installations.

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Olivier Mathieu, Mathieu Thevenot and Kent standing next to the completed installation at weir nine. The weir has begun to fill up for final testing.

Final outcome at weir nine – the spectral probe system has been deployed and collecting reliable data successfully.                                

What is considered a great security measure, poses a problem too when access is required to the probes for maintenance and repairs as the system is inaccessible when the weirs are full. Fortunately, an inflatable kayak is on hand and ideal for floating across the weir ponds.

The kayak will be used to inspect the probes every two to three months for this purpose. It is expected that although there will be slight disturbances to the streamflow measurements during this activity on the water, the data will be flagged and corrected with staff gauge readings taken before and after accessing the probes. 

Installation

The installations of the spectral probe systems coincided with the annual weir-cleaning operations during the low-flow period in winter. In preparation for these installations, the weirs had to be drained and cleaned prior to any work being started.

On 18 June 2019, weir-cleaning operations began to prepare weirs six and nine for this massive task. A 15-person labour team was hired to assist during these operations.

The French team had only a few days on site, working with SAEON technicians, to do final system setups and testing before returning home. With time being of the essence, we had just over six days to get both weirs cleaned, probes installed, and weirs fully recovered in order to conduct the final testing of the systems once the probes had been completely submerged.

Weir six was particularly challenging as there was no bedrock above water when the weir was drained to form a concrete foundation, meaning that the problem the team faced was that a concrete block could not be set in the slightest depth of water.

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Final outcome at weir six – the weir has filled up and final testing was successful.

Spectrolyser and Condulyser assembled and ready for deployment.     

Outcomes

A simple solution was found, which turned out to be not that simple when isolated so far from any major town. It entailed forming a dry dock by barricading a section of the weir using sediment-filled sacks to divert the incoming water to the available outlet, allowing the concrete block to be set and cured properly on a dry surface. Each concrete block required a full day after casting to dry and harden before any installations on the poles could be attempted.

Node coordinator Susan Janse van Rensburg commented: “This highlights the innovation and persistent dedication of the SAEON technical team to achieve fantastic results under challenging conditions; a job well done indeed.” 

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Dataloggers fixed within lockable ammo cases and secured to the wall inside the weir huts.

The French team doing final checks and tweaks on the probe as the water starts to rise in weir nine.

The entire process – from the planning and designing to the engineering, installation and testing of the spectral probe systems – turned out to be a massive success. Both systems were functioning optimally from the outset, without any troubleshooting required post installation. The data emanating from this investment by the French team on the SAEON site will also benefit South African student Rowena Harrison’s PhD project, undertaken in collaboration with the French team and SAEON.

Highlighting the value of investing in technical support for long-term observation research platforms, node coordinator Susan Janse van Rensburg noted, “The SAEON Cathedral Peak LTER platform is gaining a reputation for integrated science and it is encouraging to see that our strategy of providing a solid basis for well-run long-term observation is now yielding co-investment from collaborator research groups.

“For South African researchers these spectral instruments would be prohibitively expensive, but by providing the platform with our technical support, SAEON, our collaborators and our South African students will benefit. I am very proud of what our two technicians, Kent Lawrence and Siphiwe Mfeka, are enabling on the instrument platform.” 

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Weir six complete and collecting reliable data successfully.

Weir nine complete and collecting reliable data successfully.

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