Dr Abel Ramoelo
Remote sensing specialists are standing by to capitalise on the launch of the European Space Agency’s (ESA) Sentinel-2 Earth observation satellite in 2013, as that will, for the first time, enable them to develop a nitrogen map for southern Africa.
Sentinel-2 is one of five Sentinel missions that ESA is developing for Europe’s global monitoring for environment and security programme, and will provide full and open access to its data.
First in line is the CSIR’s Dr Abel Ramoelo, who recently completed a PhD at the University of Twente in the Netherlands, entitled ‘Savanna grass quality: remote sensing estimation from local to regional scale’.
With this work he developed remote sensing methods to improve estimates of grass quality over large areas. He explains, “The quality of grass is determined by the concentrations of nitrogen and phosphorous in the grass foliage. Foliar nitrogen concentration is known to relate to the protein content. Protein is a major nutrient requirement for herbivores (animals that only eat plants). Foliar phosphorous, on the other hand, is crucial for reproduction and lactating animals.”
Understanding how these elements are distributed is important for the effective management of wildlife and livestock, and will help ecologists, farmers and resource managers to understand the feeding patterns, distribution and densities of herbivores in protected and communal areas. When combined with remote sensing measurement and analysis, managers can achieve a landscape-wide view of the nutrient distribution and limitation of an area.
Developing a remote sensing method for estimating grass quality
For his PhD, and in anticipation of what Sentinel-2 will have to offer, Dr Ramoelo had to go back to the basics of biochemistry, starting in the CSIR’s greenhouse where he sowed grass seeds into 90 pots.
He explains, “Light from the sun required for photosynthesis by plants is measured in a red region of the spectrum, which is influenced by chlorophyll absorption, and reflection on the green region of the spectrum, which we observe as the ‘green’ leaves and blades. At different stages of growth, plants will have different levels of ‘green’, each stage with its own unique footprint when measured with a spectrometer.”
A spectrometer is an instrument used to measure the interaction between the electromagnetic energy (light) reflected by any kind of object, like plants. Just like every person has a unique thumb print, every element – such as nitrogen – absorbs light at specific spectral regions.
Over a period of nearly three months he measured the nitrogen content of the growing grass in the greenhouse with a spectrometer. When the grass reached maturity, it was dried and sent for chemical analysis in a laboratory to retrieve leaf nitrogen and phosphorus, which will be used to develop prediction models in combination with remote sensing data using various modelling programmes.
At the same time, he also had to develop a method to work around the problem of water in the leaves and the atmosphere, which ‘masks’ the actual nitrogen content of the leaves when using remote sensing techniques. In a process called ‘spectral transformation’ he factored out the effect of water, and was then able to model his results and test it out in the field, using data acquired by the spectrometer.
For the next part of his thesis Dr Ramoelo worked on eight different experimental sites and over 49 plots of 900 m2 each in an area that stretched from the Kruger National Park and an adjacent private game reserve, to the rangelands in the surrounding communal area – covering an area of about 35 000 hectares. During one of these field trips he nearly lost all the data stored on his laptop when a buffalo charged them. Retreating at first, he then ran back to retrieve the laptop, just in time.
Lastly, he had to integrate remote sensing with environmental variables such as temperature and rainfall into the model before he could declare the remote sensing method ready to tackle the data that will become available after the launch of Sentinel-2.
Currently, a nitrogen map for the Kruger National Park, Sabi Sands and surrounding communal areas is available. However, derived using the red edge band from the RapidEye spaceborne sensor at a cost of around R60 000 for a scene of about 5000 km², it is simply not feasible to do this for the entire country.
The map shows the distribution of leaf (top) and canopy (bottom) nitrogen stretching from the Kruger National Park, Sabi Sands and surrounding communal lands in the Bushbuckridge area (Extracted from a published paper: Ramoelo A, Skidmore K. A, Cho M.A., Schlerf M, Mathieu R, Heitkonig I.M.A. (2012) Regional estimation of savanna grass nitrogen using the red edge band of the spaceborne RapidEye sensor, International Journal of Applied Earth Observation and Geoinformation, 19, pp: 151-162)
Sentinel-2 – a dream come true
According to the ESA website, Sentinel-2 will carry a single multi-spectral instrument with 13 spectral channels in the visible, red edge, near infrared and shortwave infrared spectral range. With a resolution of 10m per pixel, and wide swath of 290 km, researchers will be able to estimate nitrogen for a wider area or even at country level. A ‘swath’ is the area covered by one scene or picture as collected by Sentinel-2.
Add to that a repeat cycle of only five days and full and open access to the data, it will now be possible and cost-effective to map the nitrogen content of grasslands and forests over southern Africa, explains Dr Ramoelo.
“Traditional satellites such as LandSat – which have been around for 39 years – do not offer special spectral bands in the red and infrared range and also not at a resolution that is useful to us. We have been able to produce some maps with commercial satellites like RapidEye, but the data are prohibitively expensive and thus not sustainable in the long term.”
Furthermore, while it takes Landsat 18 to 42 months to obtain imagery of the entire country, Sentinel-2 will require less than a year because of its more frequent coverage.
“And with Sentinel-2, we will have full and open access to data,” he emphasises, literally a remote sensing scientist’s dream come true.
A mockup of the high-resolution images that the future Sentinel-2 mission will deliver, with a swath of 290 km and a resolution of 10 m per pixel. Using 82 observations from the German RapidEye satellites, the image covers the border area of northern Switzerland, southern Germany and eastern France, and includes a small portion of Austria and Lichtenstein. Sentinel-2, envisaged for launch in 2013, is one of five Sentinel missions that ESA is developing for Europe’s Global Monitoring for Environment and Security (GMES) programme. Credit: RapidEye
For further reading or sources consulted http://www.esa.int/esaLP/SEMPCKQWJ1H_LPgmes_0.html
Model of Sentinel 2
Dr Abel Ramoelo