• The hydrological cycle is the system that describes the distribution and movement of water between the earth and its atmosphere. The model involves the continual circulation of water between the oceans, the atmosphere, vegetation, and land.
  • The hydrological cycle is the circulation of water within the earth’s hydrological in different forms i.e. the liquid, solid and gaseous phases.
  • The hydrological cycle also called the water cycle is the earth’s water circulatory system. The hydrologic cycle is a continuous process. The total water supply of the earth, except the deep groundwater, is in constant circulation from the earth to the atmosphere to the atmosphere and back to the earth.


  • There are six processes at work in the hydrological cycle:
  1. Evaporation
  2. Transpiration
  3. Condensation
  4. Precipitation
  5. Infiltration
  6. Run-Off
  • Evaporation Solar energy powers the cycle. Heat energy from the sun causes evaporation from the water surface. (rivers, lakes, and oceans)
  • Evaporation is the change of state in a substance from a liquid to a gas. In meteorology, the substance we are concerned about the most is water.
  • Evaporation- Vaporization of liquid to gas (unsaturated),
  • Sublimation-o When evaporation occurs directly from solid to gas. (Ex. to gas)
  • Water boils at 100 degrees C, but it actually begins to evaporate at 0 degrees C; it just occurs extremely slowly.
  • As the temperature increases, the rate of evaporation also increases.
  • The amount of evaporation depends on the temperature, and it also depends on the amount of water there is to evaporate. For example, there is not much evaporation occurring in the Sahara Desert, but why? Although it can get extremely hot in the Sahara, it’s just sand- there’s just not much water to be evaporated.
  • Water is continuously evaporating from the surface of the earth, literally pumping more and more water vapor into the atmosphere.
  • Evaporation on both sea and the land surfaces where the sun is the most important factor determining the rate of evaporation.
  • In the water cycle, evaporation occurs when sunlight warms the surface of the water. The heat from the sun makes the water molecules move faster and faster until they move so fast they escape as a gas. Once evaporated, a molecule of water vapor spends about ten days in the air.
  • As water vapor rises higher in the atmosphere, it begins to cool back down. When it s cool enough, the water vapor condenses and returns to liquid water. These water droplets eventually gather to form clouds and precipitation.

Image by Freepik
Factors Affecting Evaporation
  1. Temperature
  2. Water Availability
  3. Relative Humidity
  4. Wind
  5. Solar Radiation
  6. Barometric pressure & Altitude (low-pressure high evaporation)
  7. Dissolved Solids, Composition of water, Turbidites
  8. Depth of water
  9. Area, type, and slope of evaporating surface
  10. color of water
  11. Velocity of water
  12. Waves at the water surface

(Evaporation is inversely proportional to salinity, at the same temperature oceanic water evaporates approx. 5% less than fresh water.)

(Evaporation accounts for 90 percent of the moisture in the Earth’s atmosphere; the other 10 percent is due to plant transpiration. )

  • Transpiration is the process where plants absorb water through the roots and then give off water vapor through the pores called stomata. Stomata are small openings found on the underside of leaves.
  • Transpiration from plants. Transpiration is essentially the evaporation of water from plant leaves.
  • Of the transpired water passing through a plant only 1% is used in the growth process of the plant. The remaining 99% is passed into the atmosphere.
  • Evaporation, transpiration, and sublimation, plus volcanic emissions, account for almost all the water vapor in the atmosphere that isn’t inserted through human activities. While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the hydrological cycle, transpiration is also significant. For example, a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day.
Image by brgfx on Freepik

Factors Affecting Transpiration


  • Leaf area
  • Stomata frequency
  • Structure and facing of leaf


  • Light
  • Wing
  • Temperature
  • Humidity
  • Atmospheric Pressure
  • Water Supply
  • Condensation is the process by which water vapor changes it’s a physical state from a vapor, most commonly. to a liquid.
  • Water vapor condenses onto small airborne particles to form dew, for, or clouds.
  • Condensation is brought about by cooling the air or by increasing the amount of vapor in the air to its saturation point.
  • Condensation is the process whereby water appears on the side of a cold drink can or bottle.
  • Condensation, in Hydrological Cycle, refers to water being cooled at a high altitude where the temperature is low enough for the formation of a cloud.
  • Condensation- Change of the physical state of matter from gas to liquid. Condensation is the process where water vapor becomes liquid.
Water cycle process on Earth – Scientific illustration
  • Precipitation is the result when the tiny condensation particles growing too large. Water is 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.
  • Precipitation is used to denote all forms of water that reach the earth from the atmosphere as Rainfall, Snowfall, Hail, Glaze, Dew, etc.
  • Most of the precipitation is in the form of rainfall or snowfall.
  • Precipitation- Product of condensation (all liquid and solid water particles) that falls under gravity.
  • Precipitation occurs, either as rain or snow depending on altitude.
  • There are three types of precipitation:
  1. Convective
  2. Orographic
  3. Cyclonic


  • Rain – (>0.5 mm diameter) – drops of liquid water
  • Drizzle – (<0.5 mm diameter)
  • Snow – (ice fall)- gas to solid (flake)
  • Hail – (ice fall) – round shape (>5 mm)
  • Sleet – water becomes ice (freeze) during fall usually smaller than hailstorms
  • Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so the water condenses and “precipitates”. Precipitation is a major component of the water cycle and is responsible for depositing the fresh water on the planet. Approximately 505,000 cubic kilometers. 398,000 cubic kilometers of it over the oceans and 107,000 cubic kilometers over land.
  • Precipitation forms in the clouds when water vapor condenses into bigger and bigger droplets of water. When the drops are heavy enough, they fall to the Earth. If a cloud is colder, like it would be at higher altitudes, the water droplets may freeze to form ice. These ice crystals then fall to the Earth as snow, hail, or rain, depending on the temperature within the cloud and at the Earth’s surface. An average raindrop has a mass equivalent to about one million cloud droplets.
  • A part of precipitation is intercepted by the trees and other obstructions and this part is an interception.
  • The remaining part of the precipitation that reaches the ground and is used for feeling the small depressions in the ground is known as depression storage.

Factors Affecting precipitation:

  1. Height
  2. Presence of mountain
  3. Closeness to a water body
  4. Vegetation
  5. Urbanization
  6. Prevalent wind direction
  7. Latitude
Image by Freepik

  • The flow of water from the ground surface into the ground. Once infiltration, the water becomes soil moisture or groundwater.
  • Infiltrates downwards through the soil and rocks where it is returned to the oceans through groundwater flow.
  • Infiltration is the downward entry of water into the soil.
  • A part of infiltrated water is held by capillarity at or near the ground surface and is ultimately evaporated from the surface.
  • Another portion is used by vegetation and returned to the atmosphere as Transpiration.
  • Some part of Infiltrated water moves through the unsaturated zone and meets the stream which is known as Interflow.
  • Some portion of the Infiltrated water percolates deep into the ground.

  • Runoff occurs when there is excessive precipitation and the ground is saturated and cannot absorb any more water.
  • Rivers and lakes are the results of runoff. There is some evaporation from runoff into the atmosphere but for the most part water in rivers and lakes return to the ocean.
  • The portion of precipitation that appears in surface streams is called runoff.
  • Runoff may consist of component contributions from such sources as surface runoff and subsurface runoff. or groundwater runoff.
  • Surface runoff travels over the ground surface and through surface channels to leave a catchment area called a drainage basin or watershed and the portion of the surface runoff that flows over the land surface towards the stream channels is called overland flow and the total runoff confined in the stream channels is called the streamflow.

Factors Affecting Runoff

  1. Storm Characteristics
  • Type or nature of storm and season
  • Intensity
  • Duration
  • Areal extent (Distribution)
  • Frequency
  • Antecedent precipitation
  • The direction of storm movement

2. Meteorological Characteristics

  • Temperature
  • Humidity
  • Wind velocity
  • Pressure variation

3. Basin Characteristics

  • Size
  • Shape
  • Slope
  • Altitude (elevation)
  • Topography
  • Geology (type of soil)
  • Land use/ vegetation
  • Orientation
  • Type of drainage net
  • Proximity to ocean and mountain
  • Ranges

4. Storage Characteristics

  • Depressions
  • Pools and ponds/ lakes
  • Stream
  • Channels
  • Check dams (in gullies)
  • Upstream reservoir/ or tanks
  • Flood plains, swamps
  • Groundwater storage in previous

Discover more from Geographic Book

Subscribe now to keep reading and get access to the full archive.

Continue Reading

Scroll to Top