Hydrologic Cycle

Water circulates on Earth through a system called the hydrologic cycle. This water cycle functions through vegetation, in the atmosphere, on land and below the ground, and in lakes, rivers, and oceans. The sun and the force of gravity provide energy to drive the cycle from groundwater and surface water to atmospheric moisture, which then returns to the land and oceans as precipitation.

Evaporation, Condensation, and Precipitation

The unending circulation of water on Earth is called the hydrologic cycle. This system is driven by the heat energy received from the sun. Gravity pulls water that falls on the surface back to the oceans to be recycled once again. The total amount of water on Earth is an estimated 1.36 billion cubic kilometers. Most of this vast amount of water, some 97.2 percent, is found in the oceans. Another 2.15 percent is contained in the Greenland and Antarctic ice caps. The remaining 0.65 percent is divided among rivers (0.0001 percent), freshwater and saline lakes (0.017 percent), groundwater (0.61 percent), soil moisture (0.005 percent), the atmosphere (0.001 percent), and the biosphere and groundwater below four thousand meters (0.0169 percent). While these percentages seem small, the total volume of water contained in each of these reservoirs is immense.

A description of the hydrologic cycle must begin with the oceans, as most of Earth’s water is located there. Each year, about 320,000 cubic kilometers of water evaporate from the world’s oceans. Evaporation is the process whereby a liquid changes to a gas. Adding energy in the form of heat to the water causes the water molecules to become increasingly active and to move more rapidly, weakening the chemical and physical forces that bind them together. As the temperature of the water increases, water molecules tend to move from the ocean’s surface into the overlying air. Factors that influence the rate of evaporation from free water surfaces are solar radiation, temperature, humidity, and wind velocity. It is estimated that an additional 60,000 cubic kilometers of water enter the atmosphere from rivers, streams, and lakes or through transpiration by plants every year. Thus, a total of about 380,000 cubic kilometers of water are evapotranspired on Earth every year.

The amount of water vapor that can be present in the air depends largely on the temperature. More vapor can be present at higher temperatures. As vapor-laden air is lifted and cooled at higher altitudes, the vapor condenses to form droplets of water. Condensation is aided by small dust and salt particles or nuclei in the atmosphere. As droplets collide and coalesce, clouds begin to form and precipitation can begin. Wind may transport moisture-laden air long distances, and most precipitation events are the result of three causal factors: frontal precipitation, or the lifting of an air mass over a moving weather front; convectional precipitation related to the uneven heating of Earth’s surface, causing warm air currents to rise and cool; and orographic precipitation, resulting from a moving air mass being forced to move upward over a mountain range, cooling the air as it rises.

Each year, about 284,000 cubic kilometers of precipitation fall on the world’s oceans. This water has completed its cycle and is ready to begin a new cycle. Approximately 96,000 cubic kilometers of precipitation fall on the land surface each year. This precipitation follows a number of different pathways in the hydrologic cycle. It is estimated that some 60,000 cubic kilometers evaporate from the surface of lakes or streams or transpire directly back into the atmosphere. The remainder—about 36,000 cubic kilometers—is intercepted by human structures or vegetation, is infiltrated into the soil or bedrock, or becomes surface runoff.

Interception, Runoff, and Infiltration

Although the amount of water intercepted by and evaporated from human structures—the surfaces of buildings and other artificial surfaces—may approach 100 percent, much urban water is collected in storm sewers or drains that lead to a surface drainage system or that spread water over the land surface to infiltrate the subsoil. Interception loss from vegetation is dependent on interception capacity (the ability of the vegetation to collect and retain falling precipitation), wind speed (the higher the wind speed, the greater the rate of evaporation), and rainfall duration (the interception loss will decrease with the duration of rainfall, as the vegetative canopy will become saturated with water after a period of time). Broad-leaved forests may intercept 15 to 25 percent of annual precipitation, and a bluegrass lawn may intercept 15 to 20 percent of precipitation during a growing season.

When the duration and intensity of the rainfall are greater than the soil’s ability to absorb it, the excess water begins to run off, a process termed overland flow. Overland flow will begin only if the precipitation rate exceeds the infiltration capacity of the soil. Infiltration is the process whereby water sinks between soil particles directly into the soil surface or into fractures of rocks. It is dependent on the characteristics of the soil or rock type and the nature of the vegetative cover. Sandy soils have infiltration rates of 3.6 to 3.8 centimeters per hour, and clay rock soils average 2.0 to 2.3 centimeters per hour. Nonporous rock would have an infiltration rate of zero, and all precipitation would become runoff. The presence of vegetation impedes surface runoff and increases the potential for infiltration to occur.

Water infiltrating into the soil or bedrock encounters two forces: capillary force and gravitational force. A capillary force is the tendency of water in the subsurface to adhere to the surface of soil or sediment particles, which may actually draw the water upward against the downward pull of gravity. Capillary forces are responsible for the soil moisture found a few inches below the land surface.

Completion of the Cycle

Growing plants are continuously extracting soil moisture and passing it into the atmosphere through a process called transpiration. Soil moisture is drawn into the plant rootlet because of osmotic pressure. The water moves through the plant to the leaves, where it is passed into the atmosphere through specialized leaf openings called stomata. The plant uses less than 1 percent of the soil moisture in its metabolism, with the remainder being used for temperature regulation and the transport of nutrients and metabolites between the roots and leaves. Thus, transpiration is responsible for most water-vapor loss from the land in the hydrologic cycle. An oak tree, for example, may transpire 151,200 liters per year.

The water that continues to move downward under the force of gravity through the pores, cracks, and fissures of rocks or sediments will eventually enter a zone of water saturation. This source of underground water is called an aquifer—a rock or soil layer that is sufficiently porous and permeable to hold and transport water. The top of an aquifer, or saturated zone, is the water table. This water is slowly moving toward a point where it is normally discharged into a lake, spring, or stream. Groundwater that feeds and maintains the flow of a stream is called base flow. Base flow is typical of so-called spring-fed streams, and it enables such streams to continue to flow during droughts and through cold winter months. Groundwater may flow directly into the oceans along coastlines.

When the infiltration capacity of the soil surface is exceeded, overland flow can begin as broad, thin sheets of water, no more than a few millimeters thick, called sheet flow. After flowing a short distance, the sheets break up into threads of current that flow in tiny channels called rills. The rills can coalesce into gullies and, finally, into streams and rivers. While evaporation losses do occur from the stream surface, much of the stream’s water is returned to the oceans in this way, completing the hydrologic cycle.

Scientists are interested in how long it takes water to move through the hydrologic cycle. The term “residence time” refers to how long a molecule of water would remain in the various components of the hydrologic cycle. The average length of time that a water molecule would stay in the atmosphere is about one week; in a river, about two weeks; and in a lake, ten years. It would take four thousand years for all the water molecules in the oceans to be recycled just once. Groundwater may require anywhere from a few weeks to thousands of years to move through the cycle. This time period may appear extremely long to humans, yet it suggests that over the course of geologic time, every water molecule on Earth has been recycled millions of times.

Study of the Hydrologic Cycle

Scientists have developed a vast array of mathematical equations and data-collecting instruments with which to quantify the complexities of the hydrologic cycle. The geographic, secular, and seasonal variations in temperature, precipitation, evapotranspiration, solar radiation, vegetative cover, and soil and bedrock type, among other factors, must be evaluated to understand the local, regional, or global hydrologic cycle.

Precipitation, an extremely variable phenomenon, must be accurately measured to determine its input into the hydrologic cycle. The United States has more than thirteen thousand precipitation stations equipped with rain gauges placed strategically to compensate for wind and splash losses. Techniques have been developed to determine the average depth of precipitation falling on a given area or drainage basin. The effective uniform depth method uses a rain-gauge network of uniform density to determine the arithmetic mean for rainfall in the area. The isohyetal and polygonal methods are used to determine the arithmetic mean for an area or basin with a nonuniform distribution of rain gauges. The amount of water in a snowpack is estimated by snow surveys. The depth and water content of the snowpack are measured and the extent of the snow cover is mapped using satellite photography.

The amount of precipitation lost by interception can be measured and evaluated. Interception is determined by the type of vegetation, the amount of evaporation that occurs during the storm, and the length of the storm. Most often, interception is determined by measuring the amount above the vegetative canopy and the amount received at the surface. The difference is deemed to be the amount lost to interception.

The volume of water flowing by a given point at a given time in an open stream channel, measured in cubic meters per second (CMS), is called discharge. Discharge is determined by measuring the velocity of water in the stream channel with a current meter. The Price AA meter and the Price pygmy meter meet the specifications of the US Geological Survey and have historically been the current meters most frequently used by the USGS. The cross-sectional area of the stream channel is determined at a specific point and multiplied by the stream velocity to determine discharge. Automated stream-gauging stations are located on most streams to supply data for various hydrologic investigations.

The National Weather Service maintains about five hundred stations using class A land pans to measure free-water evaporation. These pans are 122 centimeters in diameter and 25.4 centimeters deep, and they are made of unpainted galvanized metal. Water depths of 17 to 20 centimeters are maintained. The wind velocity is also determined. Errors may result from splashing by raindrops or from birds. Because the metal pan will also heat and cool more rapidly than will a natural reservoir, a pan coefficient must be employed to compensate for this phenomenon. A lake-evaporation nomograph is employed to determine daily lake evaporation. The mean daily temperature, wind velocity in kilometers per day, solar radiation, and mean daily dew point are the variables used to determine daily lake evaporation.

The amount of evapotranspiration can be measured using a lysimeter, which is a large container holding soil and living plants. The lysimeter is set outside, and the initial soil moisture is determined. All precipitation or irrigation is measured accurately. Changes in the soil-moisture storage determine the amount of evapotranspiration.

All of these techniques are used to determine the water budget for different geographic areas. The information gathered is then used to estimate the total water budget of the hydrologic cycle.

In the twenty-first century, climate change and human interventions were severely interpreting several aspects of the hydrologic cycle, altering many of Earth's systems. Scientists and researchers worked to combat these changes and protect Earth’s water cycle.

Principal Terms

base flow: that part of a stream’s discharge derived from groundwater and interflow seeping into the stream, representing the normal amount of water in that system

capillary force: a phenomenon in which water moves through tiny pores in rock, soil, and other materials, driven by intermolecular attraction between the water and the porous materials

evaporation: the process by which substances, especially water, change from a liquid into a vapor; when a substance changes directly from solid to gas without an intermediate liquid stage, the process is called sublimation

infiltration: the movement of water into and through the soil

interception: the process by which precipitation is captured on the surfaces of vegetation before it reaches the land surface

overland flow: the flow of water over the land surface caused by direct precipitation

precipitation: atmospheric water in the form of rain, hail, mist, sleet, or snow that falls to the earth’s surface

runoff: the total amount of water flowing into a stream, including overland flow, return flow, interflow, and base flow

soil moisture: the water contained in the unsaturated zone above the water table

transpiration: the process by which plants give off water vapor through their leaves

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