Agricultural Drought Indices - US Department of Agriculture

Similar results were obtained, as can be seen by comparing the different indices represented at
the following graphics:
Figure 4. Indices obtained from the real response to the wavelengths determined by the selected MERIS
bands for the Spanish model and MODIS bands used by the University of Nebraska
The NDDI drought index uses the NDVI and NDWI as indices of vegetation and moisture content,
and is designed for natural areas densely vegetated. Its adaptation to the Spanish territory as the
Spanish drought index model (NDDIE), in which the human being has an intensive influence on the
natural surface, makes it necessary to leave the low vegetated areas apart from the analysis. To
that effect, a mask containing the pixels with positive NDVI and NDWI values is generated after the
calculation of both indices, so that the NDDIE is only obtained for the less populated areas.
The changes applied to the NDDIE involve giving greater weight to the water and vegetation
indices, for the reasons exposed above and because of the huge difference between the weather
conditions of both areas of study. This means that the order of the elements in the NDDIE
equation must be reversed. Contrary to what happens with the NDDI, the NDDIE increases as the
vegetation dryness increases, as can be appreciated in the reversed slope the NDDIE shows in
the graphics obtained. The resulting equation of the index is:
Figure 5. Drought index adapted to the Spanish case (NDDIE)
To facilitate the interpretation of the indices, the original values, ranging from -1 to 1, have been
reclassified on a positive scale with values of 0 - 100. The following are some examples of the
results.
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