Evaluation of the MERIS terrestrial chlorophyll index

The medium resolution imaging spectrometer (MERIS), one of the payloads on Envisat, has fine spectral resolution, moderate spatial resolution and a three day repeat cycle. This makes MERIS a potentially valuable sensor for the measurement and monitoring of terrestrial environments at regional to global scales. The red edge, which results from an abrupt change in reflectance in red and near-infrared wavelength has a location that is related directly to the chlorophyll content of vegetation. A new index called the MERIS terrestrial chlorophyll index (MTCI) uses data in three red/NIR wavebands centred at 681.25 nm, 705 nm and 753.75 nm (bands 8, 9 and 10 in the MERIS standard band setting). The MTCI is easy to calculate and can be automated. Preliminary indirect evaluation using model, field and MERIS data suggested its sensitivity, to notably high values of chlorophyll content and its limited sensitivity to spatial resolution or atmospheric effects. As a result this index is now a standard level-2 product of the European Space Agency. For direct MTCI evaluation two different approaches were used. First, the MTCI/chlorophyll content relationship were determined using a chlorophyll content surrogate for sites in southern Vietnam and second, the MTCI/chlorophyll relationship was determined using actual chlorophyll content for sites in the New Forest, UK and for plots in the greenhouse. Forests in southern Vietnam were contaminated heavily with Agent Orange during the Vietnam War. The contamination level was so high that it led to a long term decrease in chlorophyll content within forests that have long since regained full canopy cover. In this approach the amount of Agent Orange dropped on to the forest between 1965 and 1971 was used as a surrogate (inverse) for contemporary chlorophyll content and was related to current MTCI at selected forest sites. The resulting relationship was negative. Further per pixel investigation of the MTCI/Agent Orange concentration relationship is under way for large forest regions. In the second approach MTCI was related directly to chlorophyll content at two scales and the initial resulting relationships were positive. Further plans involve the evaluation of the MTCI at local, regional and eventually global scales