Although these studies are useful, and have provided general overviews of the groundwater situation at various scales, there are still factors that limit their use. For example, the coarseness of scale (e.g. 1:40 000 000) makes it almost unusable at subnational level. In addition, often the studies lack a systematic accuracy assessment that tests their validity, while others considered too few factors, e.g. only rainfall and geology in the case of MacDonald et al. (2001).