Evaporation and Transpiration
Water reaching the surface of the ground can return to the atmosphere as vapor through the process of evaporation. Water in streams and lakes or held in soil near the surface may also evaporate. Water used by plants may return to the atmosphere as vapor through transpiration which occurs when water passes through the leaves of plants. Collectively known as evapotranspiration, both evaporation and transpiration occur in greatest amounts during periods of high temperatures and wind, dry air, and sunshine.
Water vapor is transported by winds and air currents through the atmosphere. When the air mass cools sufficiently, the water vapor condenses into clouds, and a portion falls to the ground as precipitation in the form of snow, rain, sleet, or hail.
Surface Runoff and the Watershed
That portion of water which does not infiltrate the soil but flows over the surface of the ground to a stream channel is called surface runoff. Water always takes the path of least resistance, flowing downhill from higher to lower elevations, eventually reaching a river or its tributaries. All of the land which eventually drains to a common lake or river is said to be in the same watershed. Watersheds are defined by topographic divides which separate surface flow between two water systems. Use of the watershed as a water management unit continues to gain support because all of the water-related issues which arise for a particular lake, river, or watershed can then be managed as a whole.
Where water infiltrates the ground, gravity pulls the water down through the pores until it reaches a depth in the ground where all of the spaces are filled with water. At this point, the soil or rock is said to be saturated, and the water level which results is called the water table. The water table is not always at the same depth below the land surface. During periods of high precipitation, the water table can rise. Conversely, during periods of low precipitation and high evapotranspiration, the water table falls. The area below the water table is called the saturated zone, and the water there is called groundwater. The area above the water table is the unsaturated zone.
Infiltration and Soils
As water reaches the land surface, it can seep downward through the pores between soil particles. A high porosity soil has the ability to hold large amounts of water due to the presence of many pore spaces. If the pores are well connected and allow water to flow easily, the soil is said to be permeable. The size and shape of clay particles along with the arrangement of the pores between these particles cause clay soils to resist infiltration. Sands and gravels allow more rapid infiltration due to porosity and high permeability.