Calculates fallow land on grid cell level, based on physical cropland extend and harvested area output of LandInG data. The formula "fallow land are = max( physical cropland area - harvested cropland area, 0)" is used. Due to multiple cropping, harvested cropland area can be greater than non-fallow land area and even greater than physical cropland area. Thus, the results can only be considered a rough estimate of fallow land area.