Hydrology

INTRODUCTION
Hydrology, science that deals with the waters of Earth—their properties, behavior, and distribution. Hydrologists, as scientists in this field are called, study the occurrence, distribution, and circulation of Earth’s waters as well as their chemical and physical properties and their interaction with the environment and living things. The research that these scientists do is important in developing, managing, and controlling water resources. Key applications of hydrology include controlling floods, cleaning up water pollution, generating hydroelectric power, and planning recreational uses of rivers, lakes, and other waters.
The science of hydrology grew out of the desire to know why the oceans do not rise even though the world’s rivers continually empty great volumes of water into them. Once people came to realize that water could change its state from a liquid to a vapor (gas) through the application of heat, it became clear that the heat of the Sun on the ocean surfaces continually converts water to vapor. Meanwhile, the science of meteorology was lifting the veil of superstition from the causes of changes in weather. People knew that rain came from clouds and learned that clouds were made up of minute particles of water or ice. Ultimately, scientists identified the cloud particles with the invisible water vapor in the atmosphere. Hydrologists soon mapped out the series of movements of water above, on, and below the surface of Earth that is known as the water cycle or the hydrologic cycle. In this endless cycle, water is stored temporarily in the ground; in oceans, lakes, and rivers; and in ice caps and glaciers. It evaporates (see Evaporation) from Earth’s surface, condenses (see Condensation) in clouds, falls back to the surface as precipitation (rain, snow, or hail), and eventually either runs into the sea or reevaporates into the atmosphere.
Essentially, the sea is the source of all water found on the land. When seawater is heated enough by the Sun, it evaporates, or turns into water vapor. The water vapor rises into the air. The air into which it rises circulates about the planet because of differences in heating at the poles and the tropics, differences in atmospheric pressure, and the rotation of Earth. Within the circulating air, parcels (small volumes) of warm, moist air rise and cool until the air can no longer hold its moisture as vapor. At this point the vapor condenses, turning into tiny droplets of liquid water. The minute water droplets are suspended in the air and appear as clouds. Eventually, tiny droplets coalesce into bigger drops around a solid core, such as a particle of ice or dust. When these drops attain weight sufficient to overcome the resistance of the air, they fall to the surface as precipitation (rain, snow, or hail).

PRECIPITATION
Hydrologists measure precipitation. They also study the different types of precipitation, how and why precipitation occurs, and the distribution of precipitation over time and area.

Measurement
Hydrologists use an instrument called an automatic recording rain gauge to measure precipitation. The instrument weighs and charts rainwater as it falls, keeping a record on a paper strip. The record shows the time, duration, and intensity of rainstorms. Hydrologists may also use nonrecording gauges, clear tubes with markings in millimeters or inches on them. Light, intermittent snowfall is measured by the same instruments as rainfall. In areas where snow often accumulates to great depths, gauges in the form of collecting tanks store the snow, sometimes over a whole season. To measure the depth of liquid water to which the snowfall is equal, the snow is mixed with a salt brine to melt it as it enters the gauge. Snowfall is also measured with a snow tube, which is inserted into an accumulation of fallen snow to get a sample that is weighed to determine the depth of water equivalent.

Types of Precipitation
The intensity and amount of precipitation depend on the moisture content of the air and the rate and amount of cooling in the air. Hydrologists recognize two general types of precipitation, which are identified by the kind of storm with which it is associated. The first kind of storm is the extensive cyclonic storm—that is, an area of low atmospheric pressure surrounded by a wind system blowing in a circular direction (see Cyclone). The second kind of storm is the less extensive thunderstorm.
The precipitation associated with cyclonic storms may be frontal or nonfrontal. Frontal precipitation results when warm, moist air is lifted over cold, dry air. Nonfrontal precipitation comes when warm, moist air mixes with and then rises above cold, dry air as both types of air flow into a low-pressure area. Thunderstorm precipitation occurs when warm, moist air at low levels is carried aloft rapidly by strong currents of rising air. Thunderstorm precipitation may occur during a cyclonic storm, and both types may be intensified as the air passes over a mountain or some other pronounced topographic feature.

Distribution of Rainfall over Time
Rainstorms of the cyclonic type may continue for days as moderate or light rainfall. Such rainfalls are a boon to farmers because much of the rainwater soaks into the ground, where it supports plant growth. However, a cyclonic storm may produce heavy rainfall at times. When rain falls so rapidly that most of it runs off the ground surface directly into streams, it may wash away topsoil and damage crops. Even worse, the streams may not be able to store and discharge the volume of water delivered to them in so limited a time. As a result, they overflow their banks and damaging floods ensue.

Distribution of Rainfall over Area
A flood is usually the immediate consequence of a storm. In the United States, hydrologists have studied the records of several thousand great storms. Their studies reveal that the average depth of rainfall is highest at the center of a storm and decreases with distance from the center. For mountainous areas, hydrologists have drawn isohyets, or lines on a map that connect places that receive the same amount of rainfall during a storm. They have found that the isohyetal pattern of a storm in a mountainous area is influenced by the elevation, aspect (exposure to the storm), and orientation (compass direction) of the more rugged features of the landscape.

Snowfall
When water vapor condenses at temperatures much below 0° C (32° F), ice crystals form. Under certain temperature conditions these crystals cling together and fall to the ground as snowflakes. The density of freshly fallen snow varies considerably, but the generally accepted average water equivalent is that 25 cm (10 in) of snow equals 2.5 cm (1 in) of rain. In areas with adequate rainfall, snow may make a substantial contribution to a flood if it melts immediately before or at the same time as a heavy rain falls. In high, arid, mountainous areas, snow in the mountains is often the economic lifeblood, providing water for drinking and irrigation. The snow cover on mountains begins to accumulate at an elevation of about 2,100 m (6,900 ft) throughout the fall and winter months, and the depths increase to 6 m (20 ft) or more at 3,000 m (9,800 ft) and above.

EVAPORATION
Conversion of water from liquid to vapor is an important step in the never-ending hydrologic cycle. This process, called evaporation, occurs almost continuously from all water surfaces and moist soil. It is even done by plants, which absorb water from the soil and, in a process known as transpiration, give off water vapor through their leaves. Hydrologists usually measure evaporation by indirect means. They maintain records of evaporation from special pans of specified size that are exposed to the elements. The data so obtained must be adjusted for each individual case to estimate the evaporation from a particular surface.
Under ideal conditions, evaporation from a lake can be determined by measuring all the liquid water entering, leaving, and stored in the lake and by assuming how much water must always remain in the lake to maintain a balance. This method is usually unsatisfactory because other losses of water, such as seepage, cannot be measured accurately. A similar approach, known as the energy budget, involves an accounting of the heat energy entering, leaving, and stored in the lake. This method is more reliable, especially because tremendous quantities of heat energy are required to evaporate water.
Transpiration from land areas fully covered with lush green vegetation having an ample water supply is about equivalent to the evaporation from an adjacent lake. If the water taken from the soil through transpiration is not replaced by precipitation or irrigation, the soil begins to dry and the transpiration rate diminishes. In such a case, the plants eventually wilt because of an inadequate water supply.

RUNOFF
If rain falls or snow melts faster than it can enter the ground, surface runoff occurs. Small depressions in the ground surface first fill, then overflow and merge into small streams, known as rills or rivulets. These streams continue to converge and enlarge until the channels reach the dimensions of creeks and rivers, at which point runoff can be measured. Hydrologists maintain thousands of gauging stations worldwide to measure stream flow.
Hydrologists study how water that falls to the surface ends up in streams. Streams are fed in two ways: from rainwater or snowmelt that runs off the ground surface and from water that enters through the sides and bottom of the stream channel. Some of the water that enters through the sides or bottom of the channel is water that has fallen during a storm, soaked into the soil to a shallow depth, and flowed sideways to the channel. The rest of this water is groundwater, or water that exists permanently below the surface of the land. Groundwater feeds streams whose channels are deep enough to intercept the surface of the groundwater. At times of no precipitation, the groundwater sustains the streams.