The Hydrologic Cycle

Water Movement

From a raging stream during spring snowmelt, to a gentle summer rain, to the slow movement of water through the ground, water is in constant motion. The movement and endless recycling of water between the atmosphere, the land surface, and underground is called the hydrologic cycle. This movement, driven by the energy of the sun and the force of gravity, supplies the water needed to support life. Understanding the hydrologic cycle is basic to understanding all water and is a key to the proper management of water resources.

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. 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

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.

Groundwater

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. Confined aquifers are bound on the top by impermeable material, such as clay. Water in a confined aquifer is normally under pressure and can cause the water level in a well to rise above the water table. If the water rises above the ground surface it is designated a flowing artesian well. A perched water table occurs when water is held up by a low permeability material and is separated from a second water table below by an unsaturated zone. In the saturated zone, groundwater flows through the pores of the soil or rock both laterally and vertically.

Water moving from an aquifer and entering a stream or lake is called groundwater discharge, whereas any water entering an aquifer is called recharge. In Michigan, groundwater typically discharges from aquifers to replenish rivers, lakes, or wetlands. An aquifer may receive recharge from these sources, an overlying aquifer, or more commonly from precipitation followed by infiltration. The recharge zone is that area, either at the surface or below the ground, which provides water to an aquifer and may encompass most of the watershed.

Glacial and Bedrock Geology

The ease with which water moves through the ground is influenced by the glacial and bedrock geology of an area. In Michigan, glaciers covered much of the land surface and left behind till, outwash, and lacustrine (lake) deposits. Till is a mixture of soil and rock ranging in size from clay particles to boulder-size rocks. Tills generally have low permeability due to the presence of clay. Outwash consists primarily of highly permeable sand and gravel that allow groundwater to flow easily. Lacustrine deposits can be clay, silt, or sand, and their permeability depends on the sediment type.

The type of bedrock formations under glacial deposits also influences groundwater movement. Sandstone can transmit water if the pores between grains are connected, giving the rock a high permeability. Limestones fractured with many connecting cracks can also transmit water easily. Fine grained rocks such as shale and slate generally have a low permeability.