The Northeast region of Brazil is a drought prone region characterised by low rainfall (less than 800 mm of annual mean rainfall).
By Humberto Alves Barbosa (LAPIS)
Since 2011, semi-arid Northeast Brazil (NEB) has experienced a decline in rainfall, most notably during the rainy season from February to May. The extreme drought that has recently affected the region provides a unique opportunity to characterise the response of NEB’s vegetation activity to such events.
This assessment is carried out using the Meteosat SEVIRI-derived Normalized difference vegetation index (NDVI) images. NDVI= (R(NIR)-R(VIS)) / (R(NIR)+R(VIS)) where:R(VIS) = the reflectance in the 0.6 µm channel and R(NIR) = the reflectance in the 0.8 µm channel.
The response of vegetation activity to the 2018 drought is evaluated using seven-day NDVI average images from January to August. The NEB NDVI is a response variable to rainfall.
Figure 1: SEVIRI NDVI imagery from January to August 2018. Download animation (MP4, 2 MB)
The video (Figure 1) shows the Caatinga vegetation of semi-arid NEB undergoing drastic changes in NDVI values in response to the 2018 drought. Caatinga is a type of desert vegetation, and an ecoregion characterised by this vegetation in interior northeastern Brazil.
On the imagery green indicates non-drought areas, i.e. with high NDVI values, and red indicates severe drought areas, i.e. with low NDVI values.
The drought intensified in the semi-arid region of NEB, as indicated by an increase of red pixels, from January to August 2018. This intensity decreased somewhat between February and April, as indicated with a decrease of red pixels.
In this case study, the gradual evolution of vegetation stress (NDVI red pixels) is assumed to have been caused by drought. When vegetation stress is caused by another phenomenon than drought, such as human activity, the changes of observed NDVI vegetation values, thus drought values, are sharp, whereas in Figure 1 they can be seen to be gradual.
This visualisation was created using Meteosat-10 data disseminated via the EUMETCast service at LAPIS. It shows the spatial location of the study area, identifying the Northeast region of Brazil and its states (Alagoas; Bahia; Ceará; Maranhão; Paraíba; Pernambuco; Piauí; Rio Grande do Norte; e Sergipe) plus the state of Mina Gerais e Distrito Federal (Brasília).
The semi-arid region of NEB is highlighted by a blue-edged line. Vegetation appears on a scale from dark red (low vegetation) to dark green (lots of vegetation).
The use of Meteosat-10 data to monitor drought in NEB is effective, not only as a drought index, but also by the fact that remote sensing techniques offer the possibility to collect a lot of information in one go, covering a large geographic area more frequently, with fewer resources compared to ground-based observations.