Forest canopy disturbance on Kwamtili Estate
Kwamtili Estate blog
Kwamtili Estate in Tanzania's East Usumbara Mountains has the Kwamgumi National Forest along its southern border. About 70% of the 1150 hectare estate was once covered by dense forest. This area is now reduced to about 50% and is centered on Kwachawa Mountain which lies at the centre of the estate's southern boundary with the national forest (see description). The monitoring of illegal logging and agricultural activities is essential in order to preserve and protect the forest.
A relatively straightforward approach is to monitor disturbance of the forest canopy using the ESA Sentinel-2 satellite's MultiSpectral Instrument Level-2A product that provides Bottom Of Atmosphere reflectance images. A Normalised Burn Ration (NBR) vegetation index is calculated using data for Sentinel-2 Band 8a and Band 12 at two different times. This index is given by
- NBR = (Band 8a -Band 12)/(Band 8a -Band 12)
Before calculating the difference between NBR values at two different point in time the indexes are first self-referenced by subtracting the median of the pixels in the direct neighborhood of each pixel from each NBR pixel value. This well-established procedure focuses on locally abrupt, small-scale changes in the NBR that are associated with forest canopy disturbance such as logging. The procedure amounts to using a moving kernel window. Ideally the window should be circular in order to weight different pixel patterns equally. The difference in the self-referenced NBR index at the two different points in time gives deltaNBR . Negative deltaNBR values are set to zero using Band Maths in QGIS of the form:
- ("deltaNBR@1" >=0) * "deltaNBR@1"
where deltaNBR values are stored in Band 1.
Good quality, cloud free-Sentinel-2 images for northern Tantania are relatively rare so we tend to focus on annual disturbances. We discuss there the latest deltaNBR estimates which are for the 12-month period between mid-June 2020 and mid-June 2021.
Sentinel 2A MSIL2A products were downlaoded from the Copernicus OpenAccess Hub. Using SNAP, ESA's Sentinel toolbox, images for 24 June 2021 and 29 June 2020 were pan-sharpened and subset using the SNAP Sentinel-2 super-resolution plugin and the NBR indices calculated. The indices were then exported from SNAP as geotifs and each was median filtered using the WhiteBoxTools HighPass Median Filter as described in the WhiteBoxTools User Manual. This filter uses a square window where the x,y dimensions of the neighbourhood or filter should be given in odd, positive numbers (we used x = 7 and y = 7). QGIS was then used to calculate delataNBR using the filtered (self-referenced) NBR indices.
The canopy disturbance map for Kwamtili Estate for mid-2020 to mid-2021 indicates that deforestation continues in areas of illegal settlement in the relatively flat area between Kwachawa Mountain and the Muzi River which contitutes the estate's western boundary.
Significantly, disturbance has taken place deep in the forest away from cultivated areas, notably in the adjacent Kwamgumi Forest Reserve. The location of the reserve's probably illegal disturbance is possibly related to the fact that the estate's network of tracks and paths provides access to the reserve. The Tanzania Forest Service has been informed.
Regarding the map, it should be noted that deltaNBR was estimated using images sharpened to 10m. Also given is a 4.7m resolution image layer for mid-June 2021 supplied by Planet as part of the NICFI (Norway's International Climate and Forests Initiative) Imagery Programme. Monthly NICFI images have also been used to pan-sharpen NDVI (normalised difference vegetation index) indices that are also calculated for Kwamtili Estate using Sentinel-2 MSI imagery.
November 11, 2021