Untitled Quiz

Evaporation

• Evaporation is the process of a liquid changing into a gas.
• Water changes to vapor through the absorption of heat.
• Evaporation is a fundamental component of the hydrological cycle.
• Key requirements for evaporation:
  • A source of energy to vaporize the liquid water (solar or wind)
  • A gradient of concentration between the evaporating surface and the surrounding air.
• Evaporation is a function of the differences in vapour pressure of the water and the vapour pressure of the air.
• Factors affecting evaporation:
  • Degree of saturation of the surface
  • Temperature of the surface and air
  • Humidity
  • Wind velocity
  • Vegetation cover

Transpiration

• Transpiration is the process by which water vapor leaves the living plant body and enters the atmosphere.
• Involves continuous flow of water from soil into a plant and out through stomata (leaves) to the atmosphere.
• Transpiration is essentially an evaporation process.
• Transpiration ratio represents the amount of water transpired to produce a unit weight of dry matter.
• Factors affecting transpiration:
  • Climate:
    • Light intensity
    • Atmospheric vapor pressure
    • Temperature
    • Wind
  • Soil:
    • Availability of water
  • Plant factors:
    • Extent and efficiency of the root system
    • Leaf Area
    • Leaf arrangement and structure
    • Stomatal behavior

Evapotranspiration

• Evapotranspiration (ET) is the quantity of water transpired by plants during growth or retained in plant tissue, plus the moisture evaporated from the surface of the soil and vegetation.
• Accounts for the movement of water from sources such as soil, canopy interception, and water bodies, to the air.
• Potential evapotranspiration (PET) is the theoretical amount of moisture that could be lost from the surface to the atmosphere if it were available.
• Effective evapotranspiration (EET) is the actual amount of water lost due to evapotranspiration from the soil along with actively growing plants or crops.
  • Depends on plant and soil characteristics, and upon the amount of available water in the soil.

Measuring Evapotranspiration

• Methods for measuring evapotranspiration:
  • Lysimeter experiment
  • Field experimental plots
  • Soil moisture depletion studies
  • Water balance/budget method
  • Eddy covariance
  • Using US-open pan evaporimeter
  • Energy balance

Lysimeter

• Lysimeter: a device where a volume of soil planted with vegetation is located in a container to isolate it hydrologically from the surrounding soil.
• Consists of a weighing device and a drainage system, which allow continuous measurement of excess water and draining below the root zone and plant water use, hence evapotranspiration.
• The amount of water lost by evapotranspiration is calculated by the difference between the weight of the lysimeter before and after precipitation input.

Direct Methods for Measuring Evapotranspiration

• Direct methods involve measuring fast fluctuations of vertical wind speed, which are correlated with fast fluctuations in atmospheric water vapor density.
• Directly estimate the transfer of water vapor (evapotranspiration) from the land (or canopy) surface to the atmosphere.

Formulas for Calculating Evapotranspiration

• Water balance method:
  ET = P – Q – S – ΔD where:

  • ΔS = watershed storage variation (mm): Send – Sbeginning
  • P = Precipitation (mm)
  • Q = Stream flow (mm)
  • ΔD = Seepage out – seepage in (mm)
  • ET = evaporation and transpiration (mm)

• Energy balance method:
  Rn - G - H = λET where:

  • Rn : Net surface radiation flux density (Wm-2)
  • G : Ground heat flux density (Wm-2)
  • H : Sensible heat flux density (Wm-2)
  • λET : Latent heat flux density (Wm-2)
  • λ : Latent heat of vaporization of water (Jkg-1)

Temperature and Evapotranspiration

• When a surface evaporates, it loses energy and cools itself.
• This cooling can be observed from space.
• Satellites can map the infrared heat radiated from Earth, differentiating cool surfaces from warm surfaces.