Most of the world's potable water is stored as groundwater in the pores and fractures of underground rock, much of it at depths of less than 1,000 meters. A. Sedimentary rock may make poor aquifers because of tightly compacted sediment, which reduces porosity and permeability. B. Groundwater reservoirs are characterized by the porosity and permeability of the rock in which they lie, and these factors vary according to the type of rock. C. Porosity is a measure of the empty space within rock, while permeability measures the degree to which water can flow freely through rock. D. The vadose zone is typically dry because water does not stay in it, but instead percolates down to aquifers below or drains out through springs and streams. E. In arid regions, the water tables remain at a constant level far below the surface, preventing stream-beds from filling up even during wet years. F. Although the water table usually follows the contours of the land surface, its level may vary from year to year and may intersect the surface in places.