The original dataset developed by the MA is a subset of the aridity zone map created by CRU/UEA, UNEP/GRID for the World Atlas of Desertification. Aridity zones were derived from an Aridity Index (AI) which is calculated as the ratio of precipitation to potential evapotranspiration. Zones included in this dataset are: hyperarid, arid, semiarid, and dry subhumid. These are in accordance with the United Nations Convention to Combat Desertification (UNCCD) definition of Drylands.

In the first instance, we compared the map published by Miles et al (2006) to show global distribution of tropical dry forests with the dryland map produced by Sörensen (2007). The dry forest areas were almost entirely included within the areas originally mapped as drylands. The only exception were some seasonally dry forests of continental southeast Asia, but these are climatically relatively moist, being mostly characterised by P/PET ratios well above 0.65 and having few other dryland features.

In a second approach we reviewed the WWF ecoregions dataset. Sörensen (2007) had identified a number of additional ecoregions for possible inclusion. Based on expert knowledge, this resulted in the inclusion of the 13 additional ecoregions most likely to include dryland features and/or dry and subhumid tropical forests. (Table 1).