Hydrologic cycle with sketch

The hydrologic cycle is the continuous circulation of water between the oceans, atmosphere and land, driven by solar energy and gravity. Water changes state and place, but the total amount on earth stays nearly constant.

   Sun's energy
      |
  Evaporation          Condensation -> Clouds
  + Transpiration  ^        |
      ^            |        v
 [Ocean]---------->|   Precipitation
      ^                     |
      |    Surface runoff   v
      +----- Rivers <--- [Land/Catchment]
      |                     |
      +--- Groundwater <----+  (infiltration,
           flow to sea          percolation)

Main processes

1. Evaporation and transpiration: Solar heat turns water from oceans, lakes and rivers into vapour. Plants release vapour through leaves (transpiration). Together they are called evapotranspiration.
2. Condensation: Rising vapour cools, condenses on dust nuclei and forms clouds.
3. Precipitation: Water returns to earth as rain, snow, hail or sleet. About 78% falls directly on oceans.
4. Interception and depression storage: Part of the rain is caught by leaves and small surface depressions and later evaporates.
5. Infiltration and percolation: Water enters the soil, and moves down to the water table to become groundwater.
6. Surface runoff: Rain beyond infiltration capacity flows over the ground into streams and rivers.
7. Groundwater flow (base flow): Water moves slowly through aquifers and feeds rivers, springs and the sea.
8. Snow and glacier melt: In Himalayan basins like Nepal's, snow stored in winter melts in summer and feeds rivers.

Importance

The cycle gives the fresh water used for drinking, irrigation and hydropower, and its study allows engineers to estimate the water available and the floods that may occur.