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Two approaches to biomass mapping of shrublands across sub-humid and arid transition zones are integrated. The first generalizes relationships between biomass and precipitation from sites in the Mediterranean Basin, California, Namibia and Mongolia. The second represents existing Normalized Difference Vegetation Index (NDVI)-based models for biomass estimation on a regional scale. A new modified NDVI-based model is presented that uses relative rain availability as the ratio between the mean annual precipitation and the threshold rain level representing the transition from herbaceous growth to shrub dominance. While the data accounts for the actual vegetation cover, the relative rain parameter accounts for the potential biomass production. Implementation of the modified relative rain model with Landsat imagery of climatic gradients (the east-west gradient between the Judean Mountains and the Judean desert and the north-south gradient between the Judean Mountains and the Negev Desert) yielded realistic estimates of biomass in areas of high human disturbance to the natural ecosystems. These results support the possibility that the modified model can be used to map biomass across wide Mediterranean and desert-fringe ecosystems.