Evapotranspirairon Flashcards
What are the three types of evapotranspiration
Open water evaporation
Wet canopy evaporation
Transpiration
What are the different ways of assessing evapotranspiration
Evaporation pan
Pennman method
Example of evaporation pan
Symons tank (Uk)
Problem with evaporation tank
Smaller heat storage.
Higher advective transfers.
How to solve the problems with evaporation pans
Correction by a pan coefficient I
What is the penman method
3 factors control evaporation.
Net radiation, saturation vapour pressure, wind speed.
E = aRn + b(Es-e)u
What is net radiation
Balance of energy of what’s coming from sun and out from earth or water surface Rn
What is saturation deficit (Es-e)
How wet atmosphere is.
Saturation vapour pressure (holding capacity of the air) take maximum vapour pressure of air
What is wind speed (u)
Wind blows away wet air exchanging for dry air increasing wind speed turbulence
How to measure Es and e in pennman equation
Dry and wet bulb temp
How to measure u in pennman equation
Anemoter
How to measure Rn
Net radiometer
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
Local weathering station
LECA hazelrigg automatic weather station
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.
What is wet canopy evaporation measured by
Canopy water balance - what hits it and what gets beneath
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.
