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Read this if you want to get a deeper understanding of evaporation. This is not essential for the quiz, but may help you master the subject and score high. Additionally, this is a good alternative to parties, meeting friends and any other “normal” activities in “normal” times. [just a little joke – cuz I’m bored at home]
Evaporation is a process that transfers energy from the Earth’s surface to the atmosphere.
Some evaporation will always occur as long as the air is not saturated (RH = 100%, or MV = MSV) with water vapor.
Solar radiation heats the land surface (or the surface of a body of water like an ocean, sea, lake or river).
Heat from the surface is transferred to water in the upper layer of the soil.
That causes the water to evaporate thereby transferring heat from the surface to the water.
The result is that the surface is cooled.
The heat in the water vapor is released later in the atmosphere when the vapor condenses into water droplets.
Balances of flows
The net flow into most reservoirs in a year is zero!
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Review of global water cycle – evapotranspiration (ET) highlighted red. Note how small ET over land is in comparison to that over the ocean. Despite that, land-surface ET is important for regional and local mass balance (ET flux out it is always present) and also for climate and weather (evaporation has a cooling effect on the air).
Earth’s energy balance: inputs
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Review of the energy balance covered earlier this semester (see lecture notes #2). ET, which corresponds to “Latent heat transfer” here, accounts for nearly a quarter (24% here, 23%-25% in other sources)) of the outgoing energy.
Latent heat
A change of state (or phase) is a change between solid, gas, and liquid.
Latent heat is the energy involved in changing the state of a substance.
Ice to vapor: absorb energy, cool environment (melt, evaporation, sublimation)
Vapor to ice: release energy, heat environment (freeze, condense, deposition)
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Review latent heat in lecture notes #2.
Sensible heat vs. latent heat
Sensible heat is what we feel from different temperatures; energy needed to raise T (or released to decrease T)
Latent heat is the energy needed to change state (ice to water, water to vapor)
Remember this number!
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This is copied from lecture notes #2. Note that latent heat of vaporization (evaporation) is much larger than that of fusion (melting). We will use the numerical value given in the box to convert measured latent heat to ET. Latent heat is one of the many measurements made using the eddy covariance tower method (see infographics on eddy covariance tower). Hint: Look at the units of latent heat – J/kg, where J stands for Joule, a unit of energy, and kg stands for kilogram of mass of water. Look at what Joule is; what are the components of this unit in terms of the basic dimensions: mass (M, units of kg), length (L, units of m), and time (T, units of s). For example, a unit of force called Newton has the following basic dimensions: M*L/T2, in units kg*m/s2. How is Joule expressed? This will be needed to convert measured latent heat to the amount of ET in mm/day (or m/yr etc). That conversion is part of Homework #5.
Potential evaporation
Potential evaporation (PE) is the amount of evaporation that would occur if a sufficient water source was available.
Actual evaporation is the net result of atmospheric demand for moisture from a surface (PE) and the ability of the surface to supply moisture.
Surface and air temperatures, insolation, and wind all affect this.
A dryland is a place where annual potential evaporation exceeds annual precipitation.
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Introduce potential evaporation. Potential means maximum evaporation possible from the surface of a water body under given weather conditions. Measured evaporation (or evapotranspiration) is always (usually?) lower than the potential evaporation.
Pan evaporation
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Pan evaporation is a simple instrument with which we measure evaporation from a body of water. It is a bucket (pan) with water exposed to the atmosphere. Water level is measured periodically and from the decreasing values evaporation amount is computed. Some instruments are read manually, some have sensors that can take automatic readings that can be combined with data loggers or with data transmission systems (such as cellular phones). Question 1: Is this a good method to estimate real evaporation from a body of water, for example a lake? Why or why not? Question 2: Would you expect pan evaporation to be lower, the same or higher than that from a lake?
Water path through a plant
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Transpiration is a second flux from the land surface out to the atmosphere. It is evaporation from surfaces of plants (usually leaves). But water is first taken by plant’s roots from the soil and transported upwards by plant’s capillary system (more on that in infographics “water in trees”). Together with evaporation from soil surface, transpiration forms the total flux out called evapotranspiration (ET).
Regional scale: the water-balance equation
Remember the steady-state equation:
(P + Gin) – (Q + ET + Gout) = 0
The total amount of water leaving the region is the runoff,
RO = Q + Gout
Usually the groundwater terms are negligible (Gin = 0 and
Gi << Q).
Hence another way to think of runoff is
RO = P – ET
Thus:
ET = P – RO
The importance of this is that regional precipitation and runoff are relatively easy to measure, but evapotranspiration is not.
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Switching gears – to water mass balance. It is an accounting system, like a bank account. There are flows in (like precipitation, P) and flows out (such as evapotranspiration, ET), and the end effect is change in storage (dS). In steady state change in storage is zero (dS=0). In transient state dS is either greater than zero (dS>0, surplus, accumulation) or smaller than zero (dS<0, deficit). On long time scales (a year or longer) dS=0 (steady state), but on shorter time scales it is usually not zero (not steady state). For example, the global mass balance that we discussed before is always zero. The assumption of dS=0 is useful in estimation of water fluxes that are difficult to measure; ET is difficult to measure and it must be estimated from other fluxes: precipitation (P) and surface runoff (RO), either of which is easy to measure (see infographics on “tipping bucket rain gauge” for measuring P and on “stream gauge” for measuring RO).
Measuring transpiration: methods
Measurement of liquid water loss.
Measurement of vapor flow in the atmosphere.
Remote sensing.
All these methods have a specific scale of measurement, so the measurements must be “scaled up” or “scaled down” depending on the application.
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ET can be measured directly by a number of methods. None are easy, inexpensive, reliable or made at the scale relevant to land surface water processes. The best is the eddy covariance method, but it is difficult and expensive.
Measurement of liquid water loss
All these methods depend on a basic water balance model that is averaged over a given amount of time, t.
P – (ET + VR + VL) = ∆S
ET = net evapotranspiration loss (mm of water) from soil surface.
P = net precipitation input (mm of water).
VR = net amount of water entering or leaving at the surface (runoff).
∆S = change in storage over time t.
VL = Leakage of water out.
If leakage and runoff are small, ET is approximately equal to the change in storage:
ET = P - ∆S
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This method uses the mass balance of flows and states (volume in a reservoir is called the “state” of the system) that are easy to measure. We can use the full mass balance equation (the top equation), or assume that some terms are negligibly small and use a reduced mass balance equation (the bottom equation). A simple example here is that of soil that is drying by ET. In absence of precipitation and leakage out, the mass balance would be ET = -dS; we would measure the amount of water in the soil and compute water loss per unit time (typically one day). That water loss would be attributed to ET.
Measuring ET in small plots
This is especially true of agricultural fields where engineered tanks aren’t too hard to set up.
Their biggest advantage is that they measure ET directly.
The plant(s) is allowed to grow in the tank and the amount of water used for irrigation is carefully measured, as is the overall weight of the system.
The most straightforward way to measure evapotranspiration from a single plant, or a few, is to use a lysimeter.
These are specialized growing tanks that can be set in fairly realistic field conditions.
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Lysimeter – a good way to measure ET directly at small scale (order of 1 m). Accurately accounts for all fluxes and storage, so the computed ET is highly reliable. However, the method has two drawbacks: (1) it is expensive, and (2) irrelevant to field scale (the computed ET has no meaning outside of the lysimeter).
Measuring ET in small plots
The disadvantages include the relatively small scale of the experiment: it’s still necessary to extrapolate up to a whole field.
It’s not easy to get a representative soil or vegetation sample of a natural system in the tank.
Also, it’s difficult to evaluate natural ecosystems, especially in hilly terrains.
And the equipment is expensive.
Evapotranspiration from one or a few plants can be pretty accurately measured in a lysimeter.
ET from soil moisture depletion
How it works. Hydrogen atoms are the right size to interact with neutrons. A neutron probe generates fast neutrons that can bounce into hydrogen atoms in water molecules. The hydrogen atoms slow down the neutrons.
The probe counts the number of slow neutrons that come back to it. The ratio of fast neutrons sent out to slow neutrons returned lets the probe count the number of hydrogen atoms encountered and thus the number of water molecules in the soil.
By sending out pulses of neutrons at 2 different times, scientists can calculate the difference between the number of water molecules detected with the 2 pulses. That difference is proportional to the evaporation rate.
Neutron probe
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Measuring change in the amount of water in soil – and from it computing ET. One method uses a source of fast neutrons and measures the resulting thermal neutrons (slower than fast neutrons). The fast neutrons are sent into soil where they bounce around and some are removed or changed to thermal neutrons. We measure those thermal neutrons. Their numbers are proportional to the number of water molecules in soil. From that we compute the water amount in the soil.
ET from soil moisture depletion
ET is estimated as the change in amount over a period of measurement.
One issue is inserting the probe without disturbing the plant canopy or soil.
Instruments that can make these kinds of measurements accurately also include:
• Time-domain reflectometers;
• Capacitance probes.
Neutron probe
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There are many other methods for measuring soil moisture in place. Most have the same shortcoming – small measurement volume that makes the measured values not representative of field scale. One novel method has overcome this problem – cosmic-ray soil moisture sensor measures at the field scale of hectometers, relevant to water processes at the land surface.
COSMOS : COsmic-ray Soil Moisture Observing System
The idea is basically the same as a neutron probe, except:
• Cosmic-ray neutrons are the source.
• The measurement is over a circular area of about 200 m in radius.
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The cosmic-ray soil moisture probe. Developed at U Arizona in the early 2000s, with the first paper published in 2008.
The cosmic-ray soil moisture probe. Developed at U Arizona in the early 2000s, with the first paper published in 2008. This is optional reading – read if you would like to learn about this cutting-edge science. [Or if you are bored – blame the virus]
The average wind near the land surface is parallel to the ground. However, turbulent eddies in the air can cause local upward and downward motions of the air.
Eddy covariance towers measure the difference between the water flux going up from the surface (w2 below) and going down (w1).
Then:
ET = w2 – w1
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We have already talked about the eddy covariance method – here is a quick recap. Also see the infographics on this subject.
Meteorological estimates: Eddy covariance
Eddy covariance towers measure the difference between the water flux going up from the surface (w2 below) and going down (w1).
Then:
ET = w2 – w1
These fluxes depend on the local vertical velocity of the air, it’s specific humidity, and the air’s density.
Measuring these things simultaneously is a major challenge, but good towers can yield estimates of ET that are accurate up to 5-10%.
The measurements are primarily local so they must be scaled up for regional analyses.
Larger scale measurements: LiDAR
LiDAR (Light Detection and Ranging) works like a radar, but uses light instead of radio waves. Water vapor LiDARs send out beams of light that bounce off water molecules in the atmosphere.
Kind of like a neutron probe (but on a much larger scale and in the atmosphere instead of the soil), the LiDAR counts how many photons return from which directions and distances. The return gives a picture of the amount of water vapor in the atmosphere.
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LiDAR is another method for measuring water vapor in the air. From variations in the amount of water vapor we can compute fluxes, and from those fluxes we can estimate ET. The method is expensive and not reliable for estimating ET (other fluxes have to be accounted for and they may or may not be easy to estimate).
Larger scale measurements: LiDAR
LiDAR uses reflected light to estimate the quantity of water vapor in a volume of atmosphere.
Water vapor LiDARs send out beams of light that bounce off water molecules in the atmosphere.
Kind of like a neutron probe (but on a much larger scale and in the atmosphere instead of the soil), the LiDAR counts how many photons return from which directions and distances. The return gives a picture of the amount of water vapor in the atmosphere.
Water path through a plant
Measuring transpiration in an individual plant
One way to estimate transpiration is to take a cutting from a plant and measure the short-term loss in a pipette as water streams through the cutting.
This is obviously not completely realistic since no roots are involved and it’s not possible to do this with a large plant like a tree.
For that it’s necessary to extrapolate the loss of water from one branch to a whole tree and that has many uncertainties.
Also it’s hard to reproduce the actual environmental conditions a plant undergoes in a lab.
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Direct measurement of ET from a single plant by measuring flow within the plant’s stem or branch. Gives a good estimate, but the scale is irrelevant to field scale at which water balances are considered (this scale is too small).
Measuring transpiration in an individual plant
One way to measure transpiration in the field is to put a bag around a branch (or a whole plant) and measure the flow of gases, including water vapor, in and out of the plant.
This is somewhat more realistic, but it disturbs the thermal (heat) balance of the plant.
The bags keeps air from blowing over the branch.
Also, the scale is still limited to a single branch (or plant at most).
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And yet another method. This is at small scale (so it is irrelevant) and also not reliable. Question: Can you think of the reason why this is unreliable? What is the effect of the plastic bag on the plant’s physiology?
