GEOSPHERE & HYDROSPHERE Hydrology

Some definitions
  • Evaporation
  • conversion of a liquid to a vapour
  • Transpiration
  • that part of evaporation which enters the atmosphere through plants
  • Total Evaporation
  • the combined processes of evaporation and transpiration
  • this has been referred to as evapotranspiration(an ugly word)
  • Potential evaporation (PE)
  • evaporation that would occur from an extensive well-watered surface
  • Actual evaporation (AE)
  • evaporation that actually occurs, given water availability

Factors affecting evaporation

 Several factors affect the rate of evaporation from surfaces:
1.Energy availability.
2.Humidity.
3.Rate of turbulent diffusion.
4.Water availability.
5.Vegetation characteristics.

Energy availability




Solar Energy: the relationship between mean monthly evaporation and solar radiation appears to be closer than the relationship to temperature.


Albedo affects the reflectance of solar energy available:                

High albedo= high reflectance
Low albedo= low reflectance

Humidity

  • Actual VapourPressure (e)
  • The density of water vapourwithin the air.
  • Saturation VapourPressure (es)
  • The limit to the density of water vapourwhich may be contained in the air, at temperature t.
  • Relative humidity
  • e/ es
  • Saturation Deficit or Vapour Pressure Deficit (VPD)
  • es-e, or the extra water that can be held in the air at temperature t.
  • Evaporation is greatest in warm and dry atmospheric conditions, and least in cold and humid conditions.
  • In warm and dry conditions:

        the Vapour Pressure Deficit(es-e) is large, which promotes evaporation.

  • In cold and humid conditions:
                 the Vapour Pressure Deficit is low, which limits evaporation.


Turbulent diffusion

  • The VapourPressure Deficit soon reaches zero in calm conditions, as evaporation occurs.
  • This would have the effect of stopping evaporation, so a mixing of air by turbulenceis required.
  • Air movement is needed to remove the lowest moist layers in contact with the water surface and to mix them with the upper drier layers.


  • The stronger the wind, the greater will be the turbulence, so there will be more convection and more evaporation.
  • There is some relationship between evaporation and wind speed, but a critical speed may be reached where no additional evaporation takes place.


Water availability


Evaporation from free water bodies
  • Water availability is nota problem. Evaporation is mainly controlled by the drivers of energy, humidity and windspeed
  • Additional energy may come from heat stored in the water.         
Evaporation from bare soil
  • Water availability is a problem. Evaporation is limited by amount of water available
  • Sensitivity to lack of water varies with soil type
  • Additional energy may come from heat stored in the soil

Evaporation from free water bodies

In addition to meteorological factors, the physical characteristics of a lake, pond or reservoir also influence the rates of evaporation from the surface:

1.Salinity of the Water Body.
2.Depth of the Water Body.
3.Size (Area) of the Water Body Surface.

Salinity of the Water Body:

  • evaporation decreases by about 1% for each 1% increase in salinity. This is due to the reduced vapourpressure of saline water.

Water Depth: 

  • has an effect on the seasonal distribution of evaporation.
  • Shallow water bodies have a similar seasonal temperature regime to the air temperature above them
  • maximum rates of evaporation occurs during the summer; minimum rates in winter.
  • Deep lakes have a greater capacity for heat storage and many exhibit thermal stratification which also influences evaporation.
  • Deep lakes may slowly release stored heat during the autumn and winter months.
In deep lakes, water temperatures tend to be:
  • cooler than the air above the lake in summer
  • warmer than air temperatures in the winter.
The Vapour Pressure Deficit (es-e) is calculated from:
  • The saturation vapour pressure (es) at the temperature of the water surface
  • The actual vapour pressure (e) of the overlying air.
It follows that the VPD is higher in winter than summer, leading to greater evaporation.
Turbulent diffusion is also greater in winter
  • In summer, the warmer overlying air will tend to be cooled by the water body, which would tend to stabilise it.

Evaporation from soils

Meteorological factors apply.
Also depends on the water availability at the evaporating surface (evaporation opportunity), influenced by:
1.Soil Moisture Content.
2.Soil Capillary Characteristics.
3.Soil Colour and Temperature.


Soil Moisture Content: surface layers particularly important.
  • Water vapour movement from the subsoil to the surface less important.
  • A rough soil surface comprises a larger evaporating surface than an identical area of water: the evaporation opportunity may be >100%.
  • Soil evaporation increases if the soil surface is re-wetted by frequent and intermittent showers than from a soil surface which is thoroughly soaked by a single storm with the same quantity of rain.

Soil Capillary Characteristics: where rainfall is infrequent, evaporation opportunity can depend on the capillary rise of moisture from depth.
  • In finer-textured soils: capillary movement is effective over larger vertical distances than for coarse-textured soils, but it is slower.
Capillary activity becomes important to evaporation where shallow groundwater exists:
  • Soil evaporation is maximum when the water table is at the surface.
  • Below about 1m depth, any further drop in the height of the water table leads only to a very slight change in the rate of evaporation.
  • This critical depth (c.1m) is influenced by the capillary characteristics of the soil
  • depends on the texture and grain size of the soil.
  • deeper for fine-textured soils than for coarse-textured soils.

Soil Colourand Temperature:

  • soil colouraffects albedo: darker soils tend to absorb more heat
  • soil temperature: warmer soils may have higher rates of evaporation as they have more energy available.

Evaporation from vegetation

Transpiration:





  • Loss of water from the stomata (and to a lesser extent from leaf cuticles and lenticels in the stems).
  • Main role of transpiration is to bring a stream of water and dissolved nutrients from the root hairs to the stem vessels (xylem) and up to the leaves.