hydrology, study of water and its properties, including its distribution and movement in and through the land areas of the earth. The hydrologic cycle consists of the passage of water from the oceans into the atmosphere by evaporation and transpiration (or evapotranspiration), onto the lands, over and under the lands as runoff and infiltration, and back to the oceans. Hydrology is principally concerned with the part of the cycle after the precipitation of water onto the land and before its return to the oceans; thus meteorology and oceanography are closely related to hydrology. Hydrologists study the cycle by measuring such variables as the amount and intensity of precipitation, the amount of water stored as snow or in glaciers, the advance and retreat of glaciers, the rate of flow in streams, and the soil-water balance. Hydrology also includes the study of the amount and flow of groundwater. Though the flow of water cannot be seen under the surface, hydrologists can deduce the flow by understanding the characteristics, including permeability, of the soil and bedrock; how water behaves near other sources of water, such as rivers and oceans; and fluid flow models based on water movements on the earth's surface. Hydrology is also important to the study of water pollution, especially of groundwater and other potable water supplies. Knowledge of hydrology is extensively used to determine the movement and extent of contamination from landfills, mine runoff, and other potentially contaminated sites to surface and subsurface water. See water supply.