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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. Water that falls to the ground as precipitation follows many paths on its way back to the atmosphere. The water may be intercepted and taken up by plants; it may be stored in small depressions or lakes; it can infiltrate the soil; or it can flow over the surface to a nearby stream channel. The sun may cause the water to evaporate directly back into the atmosphere, or the force of gravity may pull it down through the pores of the soil to be stored for years as slowly moving groundwater. Some of the water flowing through the ground returns to the surface to supply water to springs, lakes, and rivers.

For example, in Michigan, most of the water entering rivers and streams eventually flows to the Great Lakes. Nearly all water in the Great Lakes flows to the St. Lawrence River and eventually to the Atlantic Ocean. Water on the ground surface, in streams or in lakes can return to the atmosphere as vapor through the process of evaporation. 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.

The Role of Soils.

As water reaches the land surface, it can seep downward through pores between soil particles. Soil is made up of tightly packed particles of many shapes and sizes. 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 permeable. The size and shape of clay particles along with the arrangement of the pores between these particles result in clay soils being relatively impermeable and resistant to infiltration. Sands and gravels allow more rapid infiltration due to their high permeability. The initial water content of the soil is also important. In general, water infiltrates drier soils more quickly than wet soils. The intensity of a storm, or the length of time during which precipitation occurs, can also influence infiltration. If rain or snowmelt reaches the soil surface faster than it can seep through the pores, then the water pools at the surface, and may run downhill to the nearest stream channel. This limitation on the soil's capacity to allow infiltration is one of the reasons why short, high intensity storms produce more flooding than do lighter rains over a longer period of time.

Surface Runoff and Watersheds.

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The 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 considered to be in the same watershed. Watersheds are defined by topographic divides which separate surface flow between two water systems. Land use activities in a watershed can affect the water quality of surface water as contaminants are carried by runoff and of groundwater, especially through infiltration of pollutants. Understanding the factors which influence the rate and direction of surface water and groundwater flow helps to determine where good water supplies exist and how contaminants migrate.

How groundwater functions.

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 becomes 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 in the saturated zone is called groundwater. The area above the water table is the unsaturated zone. Groundwater is found in aquifers which consist of soil or rock in the saturated zone that can yield significant amounts of water. In an unconfined aquifer the top of the aquifer is defined by the water table.
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