Water also readily infiltrates into loose gravels or other coarse sediments. However, water infiltration is much less effective in hard, unbroken clays. Vegetation may also increase infiltration. The leaves and limbs of trees, in particular, may interrupt the fall of intense rain and allow the water to drain down the sides of the vegetation and slowly percolate into the subsurface. Because vegetation is important in reducing runoff, the deforestation of the highlands of Kenya resulted in a four-fold increase in water runoff.
Grasslands are less effective in promoting infiltration than trees.
However, grasslands are obviously much more effective in encouraging infiltration than urban asphalt or concrete. Water from snow melts and storms infiltrates downward by gravity into the subsurface through cracks and connected pore spaces in soils, rocks and sediments. In saturated materials, all of the pore spaces and other openings in the rocks, sediments or soils are filled with water. The pore spaces and other openings in unsaturated materials contain both air and water. Underground water will be defined as any water under the Earth's surface, whether it's in the saturated or unsaturated zone.