Hydrologic cycle and water balance equation

Hydrologic cycle

The hydrologic cycle is the endless movement of water from the oceans to the atmosphere, to the land and back to the oceans by evaporation, condensation, precipitation, runoff, infiltration and groundwater flow. The sun supplies the energy and gravity moves the water.

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

• Evaporation/transpiration puts vapour in the air.
• Condensation forms clouds, and precipitation brings water to earth.
• Part of the precipitation is intercepted, part infiltrates, and the rest becomes surface runoff that reaches streams and the sea.
• Infiltrated water feeds groundwater, which flows slowly to rivers and oceans.

Water balance equation

By the law of conservation of mass, for any basin and time interval:

$$\text{Inflow} - \text{Outflow} = \Delta S$$ $$P - R - G - E - T = \Delta S$$

Here $P$ is precipitation, $R$ surface runoff, $G$ net groundwater outflow, $E$ evaporation, $T$ transpiration and $\Delta S$ the change in storage. Over a long term $\Delta S \approx 0$, so $P = R + G + ET$.

For the global balance, the land receives more precipitation than it evaporates, and the extra flows to the oceans as runoff. The equation is used to find unknown components, to size reservoirs and to assess the water yield of a basin.