Three things happen to rain that falls on the ground : it evaporates, it runs off the surface and it infiltrates into the soil. The water that runs off the surface and that which is absorbed by the soil constitute the part that is available for the development of water resources for various uses. The amount of evaporation depends on the meteorological elements of temperature, amount of sunshine, wind speed and relative humidity and is practically beyond human control for large open surfaces. Conversion of natural vegetation areas into built-up ones considerably increases surface run-off and reduces infiltration into the soil and, hence, subsurface and underground water recharge.