18.7: The Hydrologic Cycle - Biology

18.7: The Hydrologic Cycle - Biology

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Learning Outcomes

  • Discuss the hydrologic cycle and why it is essential for all life on Earth

Water contains hydrogen and oxygen, which is essential to all living processes. The hydrosphere is the area of the Earth where water movement and storage occurs: as liquid water on the surface and beneath the surface or frozen (rivers, lakes, oceans, groundwater, polar ice caps, and glaciers), and as water vapor in the atmosphere.

Water is the basis of all living processes. The human body is more than 1/2 water and human cells are more than 70 percent water. Thus, most land animals need a supply of fresh water to survive. However, when examining the stores of water on Earth, 97.5 percent of it is non-potable salt water (Figure 1). Of the remaining water, 99 percent is locked underground as water or as ice. Thus, less than 1 percent of fresh water is easily accessible from lakes and rivers. Many living things, such as plants, animals, and fungi, are dependent on the small amount of fresh surface water supply, a lack of which can have massive effects on ecosystem dynamics. Humans, of course, have developed technologies to increase water availability, such as digging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, the supply of fresh water is still a major issue in modern times.

Water cycling is extremely important to ecosystem dynamics. Water has a major influence on climate and, thus, on the environments of ecosystems, some located on distant parts of the Earth. Most of the water on Earth is stored for long periods in the oceans, underground, and as ice. Figure 2 illustrates the average time that an individual water molecule may spend in the Earth’s major water reservoirs. Residence time is a measure of the average time an individual water molecule stays in a particular reservoir. A large amount of the Earth’s water is locked in place in these reservoirs as ice, beneath the ground, and in the ocean, and, thus, is unavailable for short-term cycling (only surface water can evaporate).

There are various processes that occur during the cycling of water, shown in Figure 3. These processes include the following:

  • evaporation/sublimation
  • condensation/precipitation
  • subsurface water flow
  • surface runoff/snowmelt
  • streamflow

The water cycle is driven by the sun’s energy as it warms the oceans and other surface waters. This leads to the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor) of frozen water, which deposits large amounts of water vapor into the atmosphere. Over time, this water vapor condenses into clouds as liquid or frozen droplets and is eventually followed by precipitation (rain or snow), which returns water to the Earth’s surface. Rain eventually permeates into the ground, where it may evaporate again if it is near the surface, flow beneath the surface, or be stored for long periods. More easily observed is surface runoff: the flow of fresh water either from rain or melting ice. Runoff can then make its way through streams and lakes to the oceans or flow directly to the oceans themselves.

Rain and surface runoff are major ways in which minerals, including carbon, nitrogen, phosphorus, and sulfur, are cycled from land to water. The environmental effects of runoff will be discussed later as these cycles are described.

Biogeochemical Cycles: Hydrologic, Gaseous and Sedimentary Cycles

Nearly 30 to 40 elements are required for proper growth and development of living organisms.

Most important of these are C, H, O, P, K, N, S, Ca, Fe, Mg, B, Zn, CI, Mo, Co, I and F. These materials flow from abiotic to biotic components and back to the non-living component again in a more or less cyclic manner.

This is known as the biogeochemical cycle or inorganic-organic cycle. The flow of these elements through the ecosystem must be cyclic, with matter being consistently reused. Because the flow involves not only the living organisms but also a series of chemical reactions in the abiotic environments, these cycles are called biogeochemical cycles.

There are three types of biogeochemical cycles:

(1) Hydrologic cycle or water cycle,

1. Hydrologic or Water Cycle:

Interchange of water between atmosphere, land and sea and between living organisms and their environment is accomplished through water cycle. Water cycle or hydrologic cycle involves evaporation, transpiration, cloud formation and precipitation. Water of atmosphere reaches the earth surface through precipitation and from the earth surface it reaches the atmosphere through evaporation and transpiration. The amount of water available for evaporation is determined by the amount supplied by precipitation and condensation. Between rainfall input and evaporation output there lies a precarious water balance (Fig. 3.16).

2. Gaseous Cycles:

Oxygen cycle:

Oxygen is found in free state in atmosphere and in dissolved state in water It is liberated as by-product of photosynthesis and is utilized in respiration by the plants and animals. When the living organisms respire, CO2 is liberated which is utilized by green plants as an essential raw material for carbohydrate synthesis. In this way, a simple yet vital O cycle IS maintained in the ecosystem (Fig. 3.17).

Carbon cycle:

Carbon is the basic constituent of all organic compounds. Since energy transfer occurs in the consumption and storage of carbohydrates and fats, carbon moves to the ecosystem with flow of energy The source of nearly all carbon found in the living organisms is CO2 which is found in free state in atmosphere and in dissolved state in the water on the earth. Green plants (producers) use CO2 through photosynthesis in the presence of sunlight and carbohydrate is formed. Later on, complex fats and polysaccharides are formed in plants which are utilized by animals.

Flesh eating animals (carnivores) feed on herbivores and the carbon compounds are again digested and converted into the other forms. Carbon is released to the atmosphere directly as CO2 in respiration of both plants and animals.

Bacteria and fungi attack the dead remains of plants and animals. They degrade the complex organic compounds into simple substances which are then available for other cycles. Part of the organic carbon is incorporated into the earth’s crust as coal, gas, petroleum, limestone and coral reef Carbon from such deposits may be liberated after a long period of time (Fig. 3.18).

Nitrogen cycle:

Of all the elements which plants absorb from the soil, nitrogen is the most important for plant growth. This is required in greatest quantity. Nitrogen is required for the synthesis of amino acid, proteins, enzymes, chlorophylls, nucleic acids, etc.

Green plants obtain nitrogen from the soil solution in the form of ammonium, nitrate and nitrite ions and the main source of all these nitrogen compounds is the atmospheric nitrogen. The atmospheric nitrogen is not directly available to the organisms with the exception of some prokaryotes like blue green algae and nitrogen fixing bacteria.

Nitrogen cycle consists of the following steps:

Conversion of free nitrogen of atmosphere into the biologically acceptable form or nitrogenous compounds is referred to as nitrogen fixation.

This process is of two types:

(a) Physicochemical or non-biological nitrogen fixation

(b) Biological nitrogen fixation.

In physicochemical process of nitrogen fixation, atmospheric nitrogen combines with oxygen (as ozone) during lightning or electrical discharges in the clouds and produces different nitrogen oxides:

The nitrogen oxides get dissolved in rain water and on reaching earth surface they react with mineral compounds to form nitrates and other nitrogenous compounds:

Biological nitrogen fixation is carried out by certain prokaryotes. Some blue-green algae fix significant amounts of nitrogen in the oceans, lakes and soils. Symbiotic bacteria (Rhizobium) inhabiting the root nodules of legumes (Fig. 3.19) and also the species of alder, buck brush and a number of other non-leguminous genera and symbiotic blue-green algae (species of Nostoc Anabaena, etc.) found in free state or in the thalli of Anthoceros, Salvenia, Azolla, coralloid roots of Cycas fix atmospheric nitrogen. The relation is mutualistic because the microbes use energy from the plants to fix nitrogen that is made available to the host plants and other plants of the community.

Certain free living nitrogen fixing bacteria, such as Azotobacter, Clostridium Beijermckia. Derxia. Rhodospirillium also fix free nitrogen of atmosphere in the soil. Frankia an actmomycetous fungus found in the roots of Alnus, Percia, Casuarina, etc. also fixes nitrogen. Nitrogen fixing organisms combine the gaseous nitrogen of atmosphere with hydrogen obtained from respiratory pathway to form ammonia which then reacts with organic acids to form aminoacids. Biological nitrogen fixation is the major source of fixed nitrogen upto 140—700 mg/m 2 /year as against 35 mg/m 2 /year by electrical discharge and photochemical fixation.

2. Nitrogen assimilation:

Inorganic nitrogen in the form of nitrates, nitrites and ammonia is absorbed by the green plants and converted into nitrogenous organic compounds. Nitrates are first converted into ammonia which combines with organic acids to form amino acids. Amino acids are used in the synthesis of proteins, enzymes, chlorophylls, nucleic acids, etc. Animals derive their nitrogen requirement from the plant proteins. Plant proteins are not directly utilized by the animals. They are first broken down into amino acids during digestion and then the amino acids are absorbed and manipulated into animal proteins, nucleic acids, etc.

The dead organic remains of plants and animals and excreta of animals are acted upon by a number of microorganisms especially actinomycetes and bacilli (Bacillus ramosus, B. vulgaris, B. mesenterilus). These organisms utilize organic compounds in their metabolism and release ammonia.

Certain bacteria, such as Nitrosomonas, Nitrococcus, Nitrosogloea and Nitrospira in oceans and soils convert ammonia into nitrites and then nitrites into nitrates. These bacteria primarily use the energy of dead organic matter in their metabolism.

Conversion of nitrites to nitrates is brought about by several microbes like Penicillium species, Nitrobacter, Nitrocystis etc. Nitrocystis oceanus is the common marine autotroph which performs nitrification for obtaining energy.

Some nitrates are also made available through weathering of nitrate containing rocks.

Ammonia and nitrates are converted into free nitrogen by certain microbes. This process is referred to as de-nitrification. Thiobacillus denitrificans, Micrococcus de-nitrificans, Pseudomonas aeruginosa are the common examples of denitrifying bacteria.

Nitrates of the soil are washed down to the sea or leached deep into the earth along with percolating water. Nitrates thus lost from the soil surface are locked up in the rocks. This is sedimentation of nitrogen. Nitrogen of rock is released only when the rocks are exposed and weathered.

Thus a large part of nitrogen is fixed up and stored in plants, animals, and microbes. Nitrogen leaves the living system in the same amount it is taken in from the atmosphere and the input and outflow of nitrogen are balanced in the ecosystem. The overall nitrogen cycle in nature v presented in Fig. 3.20.

3. Sedimentary Cycles:

Mineral elements required by living organisms are obtained initially from inorganic sources. Available forms occur as salts dissolved in soil water.

Mineral cycles essentially consist of two phases:

(i) The salt solution phase, and

Mineral salts come directly from earth crust by weathering. Soluble salts then enter the water cycle. By movement of water minerals move from the soil to streams, lakes and ultimately to sea where they remain permanently. Other salts return to the earth’s crust through sedimentation. They become incorporated into sediments or rock beds and after weathering of rocks they again enter the cycle.

Plants and some animals take minerals in the form of mineral solution from their habitats. After the death of living organisms the nutrients return to the soil and water through the action of decomposers (bacteria and fungi) and transformers. Green plants at one end and decomposers at the other play very important role in circulation of nutrients.

Phosphorus cycle:

Plants and animals obtain phosphorus from the environment. Phosphorus is a component of nucleic acids, ADP, ATP, NADP, phospholipids etc. It occurs in the soil as rock phosphate, apatite or calcium phosphate, fluorapatite [Ca10Fe2 (PO4)6], iron phosphate or aluminium phosphate. Soils derived from the rock beds rich in phosphates are rich in phosphorus.

Phosphorus occurs in the soil in five forms P1 (stable organic), P2 (labile organic), P3 (labile inorganic), P4 (soluble) and P5 (mineral form) and of these forms, P3 and P4 are in equilibrium and entry of phosphorus in the green plants is considered to occur via labile inorganic pool.

The dissolved phosphorus is absorbed by plants and converted into organic form. From plants it travels to various trophic levels in the form of organic phosphates. When the plants and animals die the decomposers attack them and liberate phosphorus to the environment. Thus, this process proceeds in cyclic way. A general picture of the phosphorus cycling is presented in Fig. 3.21.

Phosphorus along with many other mineral elements reaches the oceans and settles down as sediment. A good proportion of phosphorus leaches down to deep layers of soil. In this way, major proportion of phosphate becomes lost to this cycle by physical processes, such as sedimentation and leaching. Biological processes such as formation of teeth and bones also keep phosphorus locked up for some time.

Sulphur Cycle:

Sulphur cycle links soil, water and air. Sulphur occurs in the soil and rocks as sulphides (FeS, ZnS, etc.) and crystalline sulphates. In the atmosphere sulphur occurs in the form of SO2 and H2S. SO2 gas is formed during combustion of fossil fuels or as a result of decomposition. H2S or hydrogen sulphide gas is released to the atmosphere from water logged soils, continental shelf, lakes and springs. The organic and inorganic sulphur and SO2 are formed through oxidation of H2S in the atmosphere.

A small amount of sulphur occurs in dissolved state in rain water and through rains it reaches earth surface. Except a few organisms which need organic form of sulphur as amino acids and cysteine, most of the organisms take sulphur as inorganic sulphates. Most of the biologically incorporated sulphur is produced in the soil from aerobic breakdown of proteins by bacteria and fungi. Under an aerobic condition, however, sulphur may be reduced directly to sulphides, including H2S.

Green and purple photosynthetic bacteria use hydrogen of H2S as the oxygen acceptor in reducing carbon dioxide. Green bacteria are able to oxidise sulphide to elemental sulphur whereas the purple bacteria can carry oxidation to sulphate stage. In the ecosystems, sulphur is transferred from autotrophs to animals, then to decomposers and finally it returns to environment through the decay of dead organic remains (Fig. 3.22).

Sedimentary aspect of sulphur cycling involves precipitation of sulphur in presence of iron under anaerobic conditions. Sulphides of iron, copper, zinc, cadmium, cobalt are insoluble in neutral and alkaline water and consequently sulphur is bound to limit the amount of these elements. Thus, sulphur cycle affords an excellent example of interaction and complex biochemical regulation between the different mineral cycles.

The study of biogeochemical cycles in the ecosystem makes it clear that the abiotic components of ecosystem are transformed into biotic structures through metabolic processes and locked up in the biomass for some time depending upon the turnover rate. In lower plants with soft tissues the turnover rate is quicker than in higher plants and animals. The materials held up in the biomass are released to the environment by decomposing activities.

The nutrient cycle is not a close circuit within an ecosystem. The nutrients are continuously being imported as well as carried out of the ecosystem. Appreciable quantities of plant nutrients are brought to ecosystem by rain and snow. Small quantity of nutrients is carried to the forest by rains. The gain nutrients to the ecosystem from precipitation, extraneous material and mineral weathering is offset by losses.

Water draining away from forest carries with it more mineral matter than supplied through precipitation. Considerable quantities of nutrients in the forest are locked up in the trees and the humus layer. When trees and vegetation are removed sufficient amounts of nutrient are removed. Intensive forestry and agriculture on some soils may reduce the nutrient reserves to such an extent that soils become unfertile. Ecosystem can remain productive only It the nutrients withdrawn are balanced by an inflow or replacement.

Freshwater Ecology and Hydrology

Rivers and streams rank as some of the most imperiled ecosystems on Earth. They have been heavily impacted by transportation, agricultural and forest practices, energy production, waste disposal and recreation. Due to both high extinction rates of freshwater species and projected influences of climate change on the hydrologic cycle, it is crucial to understand both how freshwater ecosystems function and how stream ecosystems can be restored. This program will cover freshwater ecology and hydrologic concepts to understand rivers from a landscape perspective and to understand how streams, lakes, wetlands and groundwater interact with terrestrial ecosystems. We will investigate the impacts of local geology, land-use practices (logging, urbanization, agriculture), and how terrestrial disturbances (forest fires, landslides, insect outbreaks) influence water quantity and quality.

This program will cover freshwater ecology in streams, rivers, lakes and wetlands. A major focus will be on research methods in both the field and the lab. Topics covered will include: water chemistry, ecosystem processes, aquatic insect identification, trophic dynamics, ecological interactions, organic matter and nutrient dynamics, current threats to freshwater ecosystems and ecological restoration. The course will focus on current research in ecosystem ecology, community ecology, ecological genetics, and terrestrial-aquatic interactions.

This program will cover components of the hydrologic cycle, including precipitation, evaporation & transpiration, infiltration, runoff, the role of groundwater and stream flow. Erosion, sediment transport, deposition and stream channel morphology will also be examined. These topics will be considered through the lens of climate change and the direct relationship between hydrology and freshwater ecology.

Numerical and spatial data analysis will be emphasized. Students will be expected to collect and analyze data associated with group research projects. Students will learn how to calculate descriptive statistics, understand probability and probability distribution functions, perform inferential statistics and more advanced statistical methods using various statistical software packages. Geographic Information Systems (GIS) will be used to analyze and display spatial data. Students will be introduced to ArcGIS Pro and the basics of remote sensing.

Seminar readings will focus on human-freshwater interactions and regionally important freshwater topics in the Pacific Northwest. Field trips will be undertaken regardless of weather conditions to local freshwater environments. This program will include extensive work in the field, the lab, and the computer lab.

The program will be a blend of synchronous and asynchronous activities. To be successful in this program, students should have a laptop (mac or PC) with reliable internet connection and a working microphone and camera. If students find themselves unable to participate in any synchronous video conference meetings due to technology, living situations, care-giving obligations, economic disruption, health risk, or illness, they can work with faculty to pursue alternate options to earn related credit.


To take this program, you need to have a foundation in biology, chemistry, and mathematics at the college-level. That foundation requires that you have taken two quarters of college-level General Biology with labs, two quarters of college-level General Chemistry with labs, and two quarters of pre-Calculus.

The Hydrologic Cycle (With Diagram) | Irrigation

This article provides a note on hydrologic cycle with the help of a diagram.

Streams transport water to the sea. From the surface of the oceans, streams and lakes (or reservoirs) water is evaporated into the atmosphere. In addition, water is also evaporated into the atmosphere through plants and trees which take water from their root-zone soil. Furthermore, moisture in the upper layers of ground also evaporates into the atmosphere.

The evaporated water (from the earth’s surface) goes into the atmosphere and, under favourable condi­tions, comes back to the earth’s surface in the form of precipitation. Thus, the same water has been transferred time and again from the atmosphere to the land and then to sea and back to the atmosphere. This cycle of water amongst earth, ocean and atmospheric systems is known as the hydrologic cycle (Fig. 2.2)

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Hydrological cycle

hydrological cycle (water cycle) The circulation of water between the atmosphere, land, and oceans on the earth. Water evaporates from the oceans and other water bodies on earth to form water vapour in the atmosphere. This may condense to form clouds and be returned to the earth's surface as precipitation (e.g. rainfall, hail, and snow). Some of this precipitation is returned to the atmosphere directly through evaporation or transpiration by plants some flows off the land surface as overland flow, eventually to be returned to the oceans via rivers and some infiltrates the ground to flow underground forming groundwater storage.

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The Water Cycle for Adults and Advanced Students

Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.

Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.

The Natural Water Cycle for Adults and Advanced Students

The Water Cycle for Adults and Advanced Students

The links below will give you an in-depth explanation of the components of the natural water cycle, as well as to allow you to view and download water-cycle diagrams and illustrations (in over 60 languages!). These links are aimed more at high school students and adults, but we have an extensive set of water-cycle information aimed at younger students and kids, too.

Note: Our information only covers the natural water cycle, which does not take human activities into account. In today's world, humans have a major impact on many components of the water cycle.

Water and Forested Ecosystems

Ecologists consider water to be the defining part in an ecosystem, including the forest ecosystem. Water shapes the physical landscape through erosion

Forests, in turn, are vital to the water cycle and to water quality. In essence, the forest acts like a giant sponge, filtering and recycling water. Approximately 80 percent of U.S. fresh-water resources are estimated to originate in forests, which cover one-third of the U.S. land area.

Tree leaves intercept water from rain, snow, and fog the leaves also release water back to the atmosphere by evapotranspiration . Tree roots extract water from the soil while helping hold the soil in place. Forested land reduces the surface impact of falling rain through interception and delay of water reaching the surface. Forestland also decreases the amount and velocity of storm runoff over the land surface. This in turn increases the amount of water that soaks into the ground, a portion of which can ultimately recharge underlying aquifers . Conversely, water from hydraulically connected surficial aquifers may enter streams and wetlands , helping to maintain their water levels during dry periods.

Forests and the Hydrologic Cycle.

The surface water in a stream, lake, or wetland is most commonly precipitation that has run off the land or flowed through topsoils to subsequently enter the waterbody. If a surficial aquifer is present and hydraulically connected to a surface-water body, the aquifer can sustain surface flow by releasing water to it.

In general, a heavy rainfall causes a temporary and relatively rapid increase in streamflow due to surface runoff. This increased flow is followed by a relatively slow decline back to baseflow, which is the amount of streamflow derived largely or entirely from groundwater. During long dry spells, streams with a baseflow component will keep flowing, whereas streams relying totally on precipitation will cease flowing.

Generally speaking, a natural, expansive forest environment can enhance and sustain relationships in the water cycle because there are less human modifications to interfere with its components. A forested watershed helps moderate storm flows by increasing infiltration and reducing overland runoff. Further, a forest helps sustain streamflow by reducing evaporation (e.g., owing to slightly lower temperatures in shaded areas). Forests can help increase recharge to aquifers by allowing more precipitation to infiltrate the soil, as opposed to rapidly running off the land to a downslope area.

Riparian Areas.

The riparian zone is broadly defined as the area between a body of water and the upland parts of the landscape that are rarely flooded except under the most extreme conditions. But the term also can refer more specifically to the immediate streamside area.

Riparian areas represent less than 10 percent of most forest ecosystems, yet these areas often are the most productive portions. Compared to upland regions, riparian areas have more water available the vegetation is more robust the soils are deeper the timber often is of higher quality and the waterbodies have more shade. The riparian zone also may include wetlands bordering streams and lakes. This combination of factors makes riparian areas among the most heavily used portions of a forest. Riparian and wetland areas provide abundant and reliable forage for wildlife, as well as transportation corridors. They also may receive heavy human use for recreation.

Riparian zones also are attractive destinations for logging and for livestock grazing as a result, riparian areas in forests are sometimes heavily damaged, especially in the forests of the arid American Southwest. Fortunately, riparian areas respond well to good management practices.

Aquatic Biodiversity.

Forest lands and waters are vitally important in maintaining biodiversity and providing habitat for fish and wildlife, including threatened or endangered aquatic species. In the United States, over one-third of national forest lands are critical for maintaining aquatic biodiversity and protection of listed species.

For aquatic species, watersheds provide the basic unit of any conservation strategy. Many watersheds also contain isolated habitats with unique characteristics producing a high potential for rare species. Some species occur only near a single spring or in a single stream within a given watershed. Lands set aside to protect these unique habitats also benefit the entire watershed and its ecosystem.

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Precipitation and the Water Cycle

The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds. When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail. all forms of precipitation, the superhighway moving water from the sky to the Earth's surface.

Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.

Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff

Precipitation and the Water Cycle

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

How do raindrops form?

A localized heavy summer rainstorm in Colorado, USA.

Credit: Howard Perlman, USGS

The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud you can see some parts disappearing (evaporating) while other parts are growing (condensation). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds.

For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Water droplets may grow as a result of additional condensation of water vapor when the particles collide. If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop. A more efficient mechanism (known as the Bergeron-Findeisen process) for producing a precipitation-sized drop is through a process which leads to the rapid growth of ice crystals at the expense of the water vapor present in a cloud. These crystals may fall as snow, or melt and fall as rain.

How much water falls during a storm

You might be surprised at the number of gallons of water that fall from the sky in even a small but intense storm. One inch of rain falling on just a single acre results in 27,154 gallons of water on the landscape. If you'd like to know how much water falls during a storm, use our Interactive Rainfall Calculator (English units or Metric units) to find out - you just enter an area size and rainfall amount and see how many gallons of water reach the ground.

What do raindrops look like?

Let me introduce myself - I'm Drippy, the (un)official USGS water-science icon! It's obvious that I'm a raindrop, right? After all, all of you know that raindrops are shaped, well . like me. As proof, you've probably seen me on television, in magazines, and in artists' representations. Truth is, I'm actually shaped more like a drip falling from a water faucet than a raindrop. The common raindrop is actually shaped more like a hamburger bun!

As Alistair Frasier explains on his web page, Bad Rain, small raindrops, those with a radius of less than 1 millimeter (mm), are spherical, like a round ball. As droplets collide and grow in size, the bottom of the drop begins to be affected by the resistance of the air it is falling through. The bottom of the drop starts to flatten out until at about 2-3 mm in diameter the bottom is quite flat with an indention in the middle - much like a hamburger bun. Raindrops don't stop growing at 3 millimeters, though, and when they reach about 4-5 mm, things really fall apart. At this size, the indentation in the bottom greatly expands forming something like a parachute. The parachute doesn't last long, though, and the large drop breaks up into smaller drops.

Precipitation rates vary geographically and over time

Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40-50 inches (102-127 centimeters (cm)) per year. Summer thunderstorms may deliver an inch or more of rain on one suburb while leaving another area dry a few miles away. But, the rain amount that Georgia gets in one month is often more than Las Vegas, Nevada observes all year. The world's record for average-annual rainfall belongs to Mt. Waialeale, Hawaii, where it averages about 450 inches (1,140 cm) per year. A remarkable 642 inches (1,630 cm) was reported there during one twelve-month period (that's almost 2 inches (5 cm) every day!). Is this the world record for the most rain in a year? No, that was recorded at Cherrapunji, India, where it rained 905 inches (2,300 cm) in 1861. Contrast those excessive precipitation amounts to Arica, Chile, where no rain fell for 14 years, and in Bagdad, California, where precipitation was absent for 767 consecutive days from October 1912 to November 1914.

The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts?

Generalized map of global precipitation.

Credit: Earth Forum, Houston Museum of Natural Science

On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters).

Precipitation size and speed

Have you ever watched a raindrop hit the ground during a large rainstorm and wondered how big the drop is and how fast it is falling? Or maybe you've wondered how small fog particles are and how they manage to float in the air. The table below shows the size, velocity of fall, and the density of particles (number of drops per square foot/square meter of air) for various types of precipitation, from fog to a cloudburst.


Water is the basic element of nature. It covers 70% of the earth’s surface. It provides life, eases out heat, drains harmful substances and mediates many day-to-day works. Water needs to be replenished, purified and circulated again and again so that it can perform its functions. Nature does this job through a process called the water cycle. Also known as hydrologic cycle, the water cycle is a phenomenon where water moves through the three phases (gas, liquid and solid) over the four spheres (atmosphere, lithosphere, hydrosphere and biosphere) and completes a full cycle. The water cycle has many effects: it regulates the temperature of the surroundings. It changes weather and creates rain. It helps in conversion of rocks to soil. It circulates important minerals through the spheres. It also creates the many geographical features present on earth like the ice caps of mountains, icebergs, the rivers and the valleys, lakes, and more. Hence it is quite important to understand and learn the processes of the water cycle. The full cycle forms an endless loop, but let's start the whole process at the ocean. Since that is where about 96% of total water exists on Earth.

Step 1: Evaporation

The water cycle begins with evaporation. It is a process where water at the surface turns into water vapors. Water absorbs heat energy from the sun and turns into vapors. Water bodies like the oceans, the seas, the lakes and the river bodies are the main source of evaporation. Through evaporation, water moves from hydrosphere to atmosphere. As water evaporates it reduces the temperature of the bodies.

Step 2: Condensation

As water vaporizes into water vapor, it rises up in the atmosphere. At high altitudes the water vapors changes into very tiny particles of ice /water droplets because of low temperature. This process is called condensation. These particles come close together and form clouds and fogs in the sky.

Step 3: Sublimation

Apart from evaporation, sublimation also contributes to water vapors in the air. Sublimation is a process where ice directly converts into water vapors without converting into liquid water. This phenomenon accelerates when the temperature is low or pressure is high. The main sources of water from sublimation are the ice sheets of the North Pole and the South Pole and the ice caps on the mountains. Sublimation is a rather slower process than evaporation.

Step 4: Precipitation

The clouds (condensed water vapors) then pour down as precipitation due to wind or temperature change. This occurs because the water droplets combine to make bigger droplets. Also when the air cannot hold any more water, it precipitates. At high altitudes the temperature is low and hence the droplets lose their heat energy. These water droplets fall down as rain. If the temperature is very low (below 0 degrees), then the water droplets would fall as snow. In addition, water could also precipices in the form of drizzle, sleet and hail. Hence water enters lithosphere.

Step 5: Transpiration

As water precipitates, some of it is absorbed by the soil. This water enters into the process of transpiration. Transpiration is a process similar to evaporation where liquid water is turned into water vapor by the plants. The roots of the plants absorb the water and push it toward leaves where it is used for photosynthesis. The extra water is moved out of leaves through stomata (very tiny openings on leaves) as water vapor. Thus water enters the biosphere and exits into gaseous phase.

Step 6: Runoff

As the water pours down (in whatever form), it leads to runoff. Runoff is the process where water runs over the surface of earth. When the snow melts into water it also leads to runoff. As water runs over the ground it displaces the top soil with it and moves the minerals along with the stream. This runoff combines to form channels, rivers and ends up into lakes, seas and oceans. Here the water enters hydrosphere.

Step 7: Infiltration

Some of the water that precipitates does not runoff into the rivers and is absorbed by the plants or gets evaporated. It moves deep into the soil. This is called infiltration. The water seeps down and increases the level of ground water table. It is called pure water and is drinkable. The infiltration is measured as inches of water-soaked by the soil per hour.

Look below for more information in understanding the phenomenon of the water cycle.

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