The hydrological cycle

The hydrological cycle

1. water is essential to life

2. Hydrology is the study of origins and fate of the water

3. It describes the movement of water in all forms above, in and on the Earths surface.

   • Driven by solar power there are inputs, outputs, storage and series of flows.

4. Input = precipitation

   • Includes all the different ways moisture from the atmosphere can be transferred.
     • E.g. rain, snow, hail, sleet and dew.
     • generally the greater the intensity the shorter the duration. Some may cover a small area e.g. convectional rainfall or some a larger area i.e. those associated with a warm front.

5. Evapotranspiration

   • The combination of the physical process - evaporation and the biological process - transpiration
     • Evaporation affected by daylight hours, wind speed, humidity and temp.
     • Transpiration affected by type and amount of vegetation, length of the growing season, time of year and available moisture.
   • can be described in two different ways: potential and actual
     • Potential evapotranspiration Ep is the amount of water that could be lost in an area E.g. in deserts Ep is high as the amount that can be lost is greater than what is available.
     • Actual evapotranspiration Ea = The amount of water available to be lost.

6. Interception

   • rainfall that lands and settles on trees (before it reaches the ground it is known as interception storage)
     • If it is a short rainfall, it may never reach the ground as it may be evaporated before
     • E.g. Woodlands 30% intercepted, helping to limit soil erosion.
     • If rainfall persists it may reach the ground via throughfall or stemflow
     • undergrowth may creates potential for secondary interception.

7. Surface storage and surface runoff (overland runoff)

   • When water can not penetrate the surface below it may just sit on top e.g. puddles. Eventually it may flow away over the surface known as surface runoff.

8. Infiltration

   • water being taken downwards into the soil.
   • Rate depends on: the amount of water already in the soil and soil structure and porosity, nature of the surface e.g. ploughed and the type and amount of vegetation cover.

9. Throughflow

   • The lateral movement of water within the soil

10. Capillary action

    • During dry periods, the process by which water is drawn up to the surface.

11. Percolation

    • constant vertical movement of water through unsaturated soil where water fills the pore spaces between the soil particles.
      • Eventually the water may build to fill all the pore spaces forming a new zone of saturation, the upper layer of which forms the water table.

12. Groundwater flow/baseflow

    • The lateral movement of water below the water table.

13. Channel flow

    • water that enters the river channel and then flows out of the drainage basin.
      • Inputs include rain that falls directly in, surface runoff, throughflow and baseflow.

14. The water balance

    • Drainage basin equilibrium
      • Inputs are equal to outputs.
      • P = Q + E + Change in storage
        • P = precipitation measured by a rain gauge Q = runoff measured as river discharge E = evapotranspiration which is hard to measure.
          • If E > P there may be a soil moisture deficit E.g. may occur in Europe in Summer where there is a mild climate and hardly any precipitation.
            • During Autumn the climate is wetter and cooler allowing for soil moisture recharge until the field capacity is reached.
            • Winter, P >E there may be a water surplus.
    • This water budget is usually displayed graphically.