淡水从降水和熔化冰中渗入地面。  Infiltrated water that moves through soil and rock prior to meeting the groundwater table is considered unsaturated flow, evident in varying degrees of soil moisture.  The groundwater table is defined by the first depth at which all soil and rock void spaces are completely filled with water, at atmospheric pressure.  Water below the groundwater table behaves as saturated flow, where the soil and rock voids are completely filled with water, and pressure increases with greater depth.  Groundwater will move from areas of greater to lesser hydraulic pressure along flow nets which can be predicted by understanding the types of soils present, and determining the elevation of the groundwater table at specific site locations.  Flow nets will often transition groundwater back to the earth’s surface by connecting into lakes, rivers and springs.  Groundwater is the source of well water for millions of people, and represents about one fourth of the world’s total freshwater supply.