Water is a natural resource renewed through the physical processes of the hydrologic or water cycle. Through the action of solar energy, water is evaporated from the surfaces of oceans, lakes, and rivers and from land surfaces, returning to the atmosphere in the form of water vapor. Water is also returned through plants, which use it to satisfy their physiological needs and send it back in the form of transpiration. The whole process of transfer of water vapor from the Earth’s surface to the atmosphere is called evapotranspiration. Once present in the atmosphere, it may precipitate in the form of rain, snow, dew, or frost. When it reaches a surface, it may run off the surface or infiltrate soil layers. Due to topographical conditions, surface runoff converges to valley regions, giving rise to rivers and lakes, which drain to ever larger bodies of water until reaching the ocean. The infiltrated water may flow to deeper soil layers, emerging in the form of springs, or percolate to even deeper layers, reaching underground aquifers. When an aquifer is in direct contact with the surface, it is said to be non-confined, and the water is stored in what is called the water table, which is acted on by atmospheric pressure. When there is a geological formation which separates the water storage zone from the soil surface, the aquifer is said to be confined and it is subject to a pressure greater than atmospheric pressure. The water stored in either of these aquifers may emerge in the form of base flow, due to the topographic gradient, feeding rivers, lakes, and other bodies of water. Indeed, this base flow is responsible for regularization of river flow during dry periods.
