Remote sensing and water quality

When you look across a lake, what colour do you expect to see? The common answer is usually blue – but what shade of blue? The image below shows some of the larger lakes in Te Waipounamu South Island – notice how the lakes have a range of colours. Lake colour tells us a lot about the lake’s surrounding landscape and water quality.

Rights: The University of Waikato Te Whare Wānanga o Waikato

Lake colours in Te Waipounamu

This NASA satellite image was taken on a mostly cloudless day. It shows some of the larger lakes and rivers in the South Island. The lake colours are influenced by the surrounding terrain, water depth, dissolved particles and water quality.

Lake colour

Lakes appear to be different colours because they absorb some colours of visible light and reflect others. The colours we see are those that are reflected. Lakes Wakatipu, Wānaka and Hāwea are very deep – up to 400 m – and the water is very clear. The water molecules absorb the longer visible wavelengths from the red end of the spectrum and reflect the shorter wavelengths to give the lakes a deep blue colour.

Lakes Pukaki, Tekapo and Ōhau are a different colour because they are fed by water coming from glaciers. The water carries glacial flour, extremely finely ground rock particles, into the lakes. The particles suspended in the water absorb different wavelengths, so the reflected colour is turquoise.

The colour of Lake Ellesmere/Te Waihora tells a different story. The lake has poor water quality and clarity. Algal growth is responsible for the greenish colour of the water. Chlorophyll in the algae absorbs red and blue light and reflects green light.

Atmospheric scattering

The image of the South Island lakes was taken by NASA’s Terra Earth observation satellite using true-colour imagery – the colours are similar to what humans would see from space. But what humans see on the ground and what the satellite sees aren’t the same. The satellite sensor looks down through the atmosphere, which is full of air molecules, water vapour and dust particles. These tiny bits absorb and scatter light, changing the intensity and colour of light received by the satellite.

In some cases, 90% of the data that the satellite sees actually comes from the atmosphere and not from the thing you are interested in. If you look at a mountain in the distance and it’s kind of hazy and milky, that’s what we need to get rid of in order to see what the mountain really looks like.

Dr Moritz Lehmann

Central Otago’s Xerra Earth Observation Institute is working on ways to correct this atmospheric scatter. Aotearoa New Zealand is a useful testing place because of its diverse environment – there are different types of lakes and atmospheric conditions throughout the motu. Xerra senior scientist Dr Moritz Lehmann and his team have collected data from almost 100 lakes. Many of Moritz’s measurements were timed with satellite passes, so while he collected data on the ground, the satellite was collecting data from up above. This information was carefully analysed and is used to calibrate and validate the satellite data. It provides valuable guidance for scientists across the world who use satellites for remote sensing.

Rights: The University of Waikato Te Whare Wānanga o Waikato

Atmospheric haze

Water vapour, dust particles and other substances reflect sunlight, creating an atmospheric haze that interferes with the ability to clearly see the small mountain in the distance.

Eye on Lakes – freshwater observations

Xerra is partnering with the University of Waikato and Cawthron Institute to use satellite images to track changes in lake colour in all of Aotearoa New Zealand’s 3,800 lakes. Lakes can change colour throughout the year as warmer weather encourages algal and cyanobacteria growth. Algae are the basis of most aquatic food webs, but they can become a problem when they form blooms. Cyanobacteria can also form free-floating blooms, and in addition to the usual water-quality impacts, many cyanobacteria species produce toxins that are harmful to people and animals.

Cyanobacteria are usually measured by collecting water samples that are analysed in a lab. This provides information about one spot in a lake at a particular time. The Eye on Lakes project is making it easier to identify if and when cyanobacteria are building up in lakes. As satellites such as Sentinel-2 pass over the lakes at the same time every few days, the data may enable authorities to create a more effective monitoring and alert system.

There is a lot of information that is needed to create and calibrate the algorithms the satellites use. Moritz takes measurements that the satellite would see – light reflected from chlorophyll (the green colour produced during photosynthesis) and suspended particles in the water. To get this data, he uses high-quality light sensors under the water and at the water’s surface.

The PACE mission is special because it provides a type of data that has not been available before. The Ocean Color Instrument is a hyper-spectral light sensor capable of taking images of 1 km pixel size over the entire world and in all the colours of the rainbow.

Dr Moritz Lehmann

PACE – marine observations

NASA has a new satellite mission called PACE – Plankton, Aerosol, Cloud, ocean Ecosystem – to assess ocean health by measuring the distribution of phytoplankton. PACE’s Ocean Color Instrument is a huge leap in technology compared to its first satellite sensor used in 1978. Moritz has been part of the PACE early-adopter programme, working with scientists around the globe to exchange ideas about how to use the data. Moritz hopes to use the data to observe harmful algal blooms in Aotearoa New Zealand’s coastal waters as well as the lakes.

Rights: NASA/GFSC

How PACE collects accurate data

This image depicts how the PACE satellite collects information. The data is validated using buoys and aircraft. The sensors are calibrated using the Sun, Moon and buoys as reference sources. These processes help to ensure accurate information is collected.

Rights: Crown copyright

Ministry of Business, Innovation and Employment and the New Zealand Space Agency

The New Zealand Space Agency, part of the Ministry of Business, Innovation and Employment, is the lead government agency for space policy, regulation and sector development.