Evapotranspiration

13 important questions on Evapotranspiration

Latent heat flux or LE

Used in meteorology to indicate energy required for liquid-vapour phase transition

What is the clothes-line parallel?

  • Low evaporation due to low temperatures and high relative humidity (clothes are really close to each other on the line, faced to each other)
  • High evaporation due to available energy, wind (transport) and low humidity (clothes are hanging next to each other with enough space between)

What is the land surface energy balance?


Rnet=lambda*ET + H + G
linked by the latent heat of vaporization (lambda) J/kg
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What measures eddy covariance?

Measures whole ecosystem ET, but expensive to operate. Data requires processing and key assumption often violated (due to turbulence)

The pan evaporation paradox

In warming climate, evaporation should increase. Decreasing trends in pan evaporation were at odds with expectation. Trends reflect downward trend in global radiation (Dimming).

What is the effect of dimming/brightening on ET?

Incoming solar radiation has shown considerable decadal variation. In humid european climate, ET trends follow trends in radiation.

Why is the increase in global ET slowed down since 1980's?

Has slowed down due to increasing water stress in semi-arid regions.

Available soil water and ET: saturation

All pores are full of water. Gravitational water is lost.

Why can the actual ET differ? (Actual ET: Penman-Monteith)

Actual ET can differ from potential due to lack of soil moisture.

Soil moisture limitation: drying stages. What happens during stage 1 drying?


Sufficient soil moisture is available in the soil for Evapotranspiration to occur at the potential rate.
  • ET is thus independent of the soil moisture conditions.

Soil moisture limitation: drying stages. What happens during stage II drying?

The phase during which evapotranspiration becomes self-limiting and strong function of soil moisture.

What happens with ET during long rainless periods and which drying stage is this?

Stage II drying when long rainless periods occur. ET decreases exponentially.

According to Penman-Monteith what are the consequences when ET reduces?

Leads to drier air, higher VPD and higher potential ET.

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