Evapotranspiration and Soil Moisture

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Evaporation: Conversion of liquid water into water vapour.


1. Humidity of air must be less than that of the surface, so that a vapour pressure deficit exists. 

  • Vapour pressure deficit: if air is dry, there is a steep vapour pressure gradient between a wet surface and the air, so evaporation is rapid.

2. An energy input to convert liquid into water vapour (2480kJ are needed to convert one litre of water at 10 degrees C). 

  • Energy inputs: solar radiation, sensible heat (temperature), wind: turbulence disperses saturated air.

Other controls on evaporation: Surface moisture availabilty, vegetation cover (shading, increased RH, leaf litter shields surface, reduction of nearsurface wind). 

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Evapotranspiration and Soil Moisture

Transpiration: Water loss from plants to the atmosphere. 

Occurs when stomata on leaves open in sunlight, exposing moisture to the atmosphere. 

Favoured by dry air, strong sunlight, strong winds and high temperatures.

Effect of soil moisture on transpiration:

  • Reduction in soil moisture may reduce transpiration
  • Limited reduction in transpiration until wilting point
  • Differences may reflect difference in soil texture.
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Potential Evapotranspiration

Potential Evapotranspiration

Potential evapotranspiration (PE or Ep) is evapotranspiration that would occur if surface water supply were unlimited. 

Actual evapotranspiration (AE or Ea) is limited by surface moisture availability

PE is often calculated by the Thornthwaite method: PE = 1.6(10T/I)a

  • T = MMT (degrees C)
  • I = annual heat index
  • a = empirical constant

PE is used to calculate soil moisture budget:

  • In Autumn, as P > PE, the soil absorbs excess moisture, giving storage recharge.
  • During Winter: P > PE
  • During Summer: P <PE - seasonal soil moisture deficit
  • During Summer there is storage withdrawal (=AE-P) and soil moisture shortage (=PE-AE)
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Soil Hydrology

Hydrological Zones:

  • Upper unsaturated zone of soil moisture
  • Lower saturated groundwater zone
  • The two zones may be separated by a capillary fringe, which tends to be thickest for fine-grained (clay) soils.

Soil Moisture in the Unsaturated Zones

Water movement mainly vertical

  • Infiltration and percolation: downwards during rainfall
  • Capillary rise: upwards due to evapotranspiration
  • Infiltration rate (f): max rate at which soil can absorb water

For a given rainfall intensity (i):

  • 1. f > i: all incoming precipitation infiltrates soil
  • 2. f < i: excess water is lost by overland flow
  • f is not constant during a single storm due to swelling of clays, filling of voids and surface 'crusting' due to rainsplash.
  • f depends on: 1) Soil texture and structure 2) Vegetation cover (roots).  
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Antecedent Moisture Conditions

After a rainstorm, moisture content of the unsaturated zone stabilises as field capacity.

  • Typically 30% by volume for clay soil
  • 5-10% for sandy soil
  • Capillary rise: as surface of soil dries, surface tension of water in soil pores increases, drawing water upwards from the capillary fringe, and thus tapping groundwater.
  • Vital for plant life
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