Evapotranspirairon Flashcards by Abbie Pilkington

What are the three types of evapotranspiration

Open water evaporation
Wet canopy evaporation
Transpiration

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What are the different ways of assessing evapotranspiration

Evaporation pan

Pennman method

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Example of evaporation pan

Symons tank (Uk)

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Problem with evaporation tank

Smaller heat storage.

Higher advective transfers.

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How to solve the problems with evaporation pans

Correction by a pan coefficient I

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What is the penman method

3 factors control evaporation.
Net radiation, saturation vapour pressure, wind speed.
E = aRn + b(Es-e)u

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What is net radiation

Balance of energy of what’s coming from sun and out from earth or water surface Rn

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What is saturation deficit (Es-e)

How wet atmosphere is.

Saturation vapour pressure (holding capacity of the air) take maximum vapour pressure of air

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What is wind speed (u)

Wind blows away wet air exchanging for dry air increasing wind speed turbulence

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How to measure Es and e in pennman equation

Dry and wet bulb temp

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How to measure u in pennman equation

Anemoter

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How to measure Rn

Net radiometer

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What equipment is used to measure all the components of penmans equation

Automatic weather station (AWS)

Has wind vane, anemometer, wet and dry bulbs, net radiometer

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Local weathering station

LECA hazelrigg automatic weather station

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What is the problem with pan evaporation rates

Used saturation deficit, wind speed and net radiation to estimate open water evaporation at Hazelrigg. Says rate is 2mm. Before evaporation pan correction, it says about 3mm a day which is over estimation and needs to be corrected by taking 1 off.

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What is wet canopy evaporation measured by

Canopy water balance - what hits it and what gets beneath

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What is Pgross

Rainfall reaching canopy. Throughfall + stemfall is Pnet.

Pnet + Iloss

What are the controlling factors of wet canopy evaporation

Rainfall intensity

Vegetation type

What is rainfall intensity

How fast it’s raining and size of drops. Short period of big raindrops.

What is the difference in vegetation type

In tropical rainforests you can lose between 20-10% but in Uk grassland it’s less than 10% bc grass is hard to measure.

How much precipitation do conifers intercept

20-40%

Why do we have largest rates of wet canopy evaporation from UK conifers

Because it rains at low intensity for a long time and is usually windy

equarion to measure wet canopy evaporation

Iloss = PPT above canopy - throughfall (Pgross) nd stemflow (Pnet)

How to measure wet canopy evaporation

Canopy rain gauge but complicated.
Mast that goes to top of tree with a funnel to see what’s received by canopy.
Underneath canopy are volumetric rain gauges.

How many throughfall plots in Borneo and why

500 bc it’s enormougjly variable through canopy

How to collect stem flow

Collect from stem and put in a huge rain gauge tipping bucket

How does transpiration work

Plants need co2 through stomata and in opening them leak water. So take water up through root and out through stomata.

What reduces transpiration

Guard cell shuts off stomata and will try to take up water through soil which reduces transpiration

What is half the water in a catchment lost by

Transpiration as opposed to going to a river

What are he controls of transpiration

Stomata loss and root absorption

How to measure transpiration

Porometer (leaf scale) Sapflow sensor (tree scale) Lysimeter water balance (few plant scale) Long term catchment water balance (regional scale)

How does porometer work

Clamp leaf in jaw and water evaporates from leaf into chamber then measure humidity in chamber

How does a sapflow sensor work

Using cooling effect of moving water past two sensors to estimate speed of sap going up tree. Generally the amount going up is equal to amount going out the top

How does lysimeter work

Trying to work out transpirational losses from grassland. Modified water balance equation

What is the equation for lysimeter water balance

T = Pnet - Q +/- ^S | Drainage (out of lysimeter) +/- soil moisture

How to measure soil moisture

Neutron probe to measure change in storage

How to measure long term catchment water balance

The same equation as lysimeter water balance. Across catchment if we integrate the rain gauges and measure discharge coming out it leaves evaporation and change in storage

Problem with assuming everything out of catchment is going into the river or sky

Basin leakage through deep seepage and channel underflow so you grossly inflate evaporation estimate. so really it’s E+P-Q plus or minus error

What is done to reduce errors in evaporation measurements

Modified pennman method - above canopy measurements.

How to use modified penman method

Use meterolgical variables above the forest

What is the impact of rainforest logging

We cut off evaporation and there is less WCE and transpiration so big decrease. Small reduction in precipitation but not by much. Large increase in Q.

What is one exception of precipitation being reduced in logged forests

Cloud forests as they strip water out of clouds. Very small amount of them but in Hawaii cloud forest 40% of import is water being stripped from clouds.