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The Hydrologic Cycle

Many processes work together to keep Earth's water moving in a cycle. There are seven processes at work that make up the hydrologic cycle: evaporation, condensation, transportation, precipitation, infiltration (percolation), transpiration, and runoff. These occur simultaneously and, except for precipitation, continuously.

Hydrological Cycle
Figure 1

The hydrologic cycle is a conceptual model that describes the storage and movement of water between the biosphere, atmosphere, lithosphere, and the hydrosphere (see Figure 1). Water on this planet can be stored in any one of the following reservoirs: atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields, and groundwater.

Evaporation is the transfer of water from a liquid to a gas from the surface to the atmosphere.

Approximately 80 percent of all evaporation is from the oceans, with the remaining 20 percent coming from inland water. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world. For example, a typical hot and humid summer day in the Midwestern United States is caused by winds blowing tropical oceanic air northward from the Gulf of Mexico.

Condensation (the opposite of Evaporation)
Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools, and loses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets. The upward motions that generate clouds can be produced by convection in unstable air, convergence associated with cyclones, lifting of air by fronts, and lifting over elevated topography such as mountains.

In the hydrologic cycle, transport is the movement of water through the atmosphere, specifically from over the oceans to over land. Some of the Earth's moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets. Clouds are propelled from one place to another by either the jet stream, surface-based circulations like land and sea breezes, or other mechanisms. However, a typical 1-kilometer thick cloud contains only enough water for a millimeter of rainfall, whereas the amount of moisture in the atmosphere is usually 10 to 50 times greater.

Most water is transported in the form of water vapor, which is actually the third most abundant gas in the atmosphere. Water vapor may be invisible to us, but not to satellites, which are capable of collecting data about the moisture content of the atmosphere.

Precipitation is the primary mechanism for transporting water from the atmosphere to the surface of the Earth. There are several forms of precipitation, the most common of which for the United States is rain. Other forms of precipitation include hail, snow, sleet, and freezing rain. A well-developed extra-tropical cyclone could be responsible for the generation of any or all of these forms of precipitation.

Groundwater is all the water that has penetrated the Earth's surface and is found in one of two soil layers. The one nearest the surface is the "zone of aeration", where gaps between the soil are filled with both air and water. Below this layer is the "zone of saturation", where the gaps are filled with water. The water table is the boundary between these two layers. As the amount of groundwater water increases or decreases, the water table rises or falls accordingly. When the entire area below the ground is saturated, flooding occurs because all subsequent precipitation is forced to remain on the surface.

Transpiration is the transfer of water to the atmosphere by plants and vegetation from the leaves and stems of plants. Plants absorb water through their roots and this water can originate from deep in the soil. (For example, corn plants have roots that are 2.5 meters deep, while some desert plants have roots that extend 20 meters into the ground). Plants pump the water up from the soil to deliver nutrients to their leaves. This pumping is driven by the evaporation of water through small pores called "stomates," which are found on the undersides of leaves. Transpiration accounts for approximately 10 percent of all evaporating water.

Rivers, lakes, and streams transport water from land to the oceans. Runoff consists of precipitation that neither evaporates, transpires, nor penetrates the surface to become groundwater. Even the smallest streams are connected to larger rivers that carry billions of gallons of water into oceans worldwide. Too much rainfall can cause excess runoff, or flooding.

This refers to water that penetrates into the surface of soil. Infiltration is controlled by soil texture, soil structure, vegetation, and soil moisture status. High infiltration rates occur in dry soils, with infiltration slowing as the soil becomes wet. Coarse-textured soils with large well-connected pore spaces tend to have higher infiltration rates than fine textured soils. However, coarse-textured soils fill more quickly than fine-textured soils due to a smaller amount of total pore space in a unit volume of soil. Runoff is then generated quicker than one might have with a finer-textured soil.

Vegetation also affects infiltration. For instance, infiltration is higher for soils under forest vegetation than bare soils. Tree roots loosen and provide conduits through which water can enter the soil. Foliage and surface litter reduce the impact of falling rain, keeping soil passages from becoming sealed.

Often, removal of vegetation in a watershed results in a short-term increase in annual streamflow. This is caused by:
  • Reduced transpiration, resulting in greater soil moisture and water available to streamflow, and
  • Reduced interception, which means less water is captured in the canopy to evaporate back to the atmosphere.
Water is continually cycled by the processes talked about above. On average, water is renewed in rivers once every 16 days. Water in the atmosphere is completely replaced once every 8 days. Slower rates of replacement occur in large lakes, glaciers, ocean bodies, and groundwater. Replacement in these reservoirs can take from hundreds to thousands of years. Some of these resources (especially groundwater) are being used by humans at rates that far exceed their renewal times. This type of resource use is making this type of water effectively nonrenewable.

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