[L4] Evaporation

Earth surface energy balance

𝑅_𝑛 = 𝜆𝐸 + 𝐶 + 𝐺 + 𝑉 + 𝑃_𝑆 in Wm^-2

𝜆𝐸 → latent heat

• energy spent evaporating water

𝐶 → sensible heat

• warming the air

𝐺 → ground heat

• energy exchange with the ground

𝑉 → storage

• change in energy stored in the canopy or water body

𝑃𝑆 → energy consumed by photosynthesis (small)

Net radiation

$$R_n = R_I - R_\alpha - R_o$$

This is a radiation budget at the surface:

• R_I = incoming radiation

• R_α = part that is reflected straight bac = α*R_I (α is albedo)

• R_o = outgoing longwave radiation the surface emits

Bowen ratio

$$\beta=\frac{C}{\lambda E}$$

ratio of sensible to latent heat → how a surface partitions its energy

Energy balance through a day

• 𝐸 lags 𝑅𝑛 by about an hour

• 𝐺 changes sign at dusk

What does the Bowen ratio dictate?

How much energy becomes vapour.

Evaporation from open water surfaces

Magnitude E_o influenced by:

Meteorological factors

• solar radiation

• relative humidity

• wind speed

Physical factors

• salinity (sea water evaporates about 2–3% less than fresh)

• water depth

• size of the water surface

Evaporation from soils

Magnitude E_o influenced by:

Meteorological factors

• solar radiation

• relative humidity

• wind speed

Physical factors

• soil moisture content

• capillary characteristics (capillary rise and water movement)

• soil colour

• temperature

Finding E_o using energy budget

Close the energy balance and solve for 𝜆𝐸.

• Physically exact

• Needs measurements few stations make

Finding E_o using mass transfer (Dalton)

𝜆𝐸_𝑜 = 𝑓(𝑢) (𝑒_𝑠 − 𝑒_𝑎)

wind function x saturation deficit

Finding E_o using a combination of both

The role of plants

• Plants modulate water flows, particularly in precipitation and evaporation events

Two main impacts of vegetation on evaporation:

• interception of precipitation (part of which may evaporate)

• transpiration of water through leaves

Interception by vegetation components

interception loss = precipitation − throughfall − stemflow

𝐼 = 𝑃 − 𝑇_𝑓 − 𝑆_f

Factors Affecting Interception Loss

• interception storage capacity of the vegetation cover

• type and morphology of the vegetation cover

• velocity of evaporation (meteorological factors)

• duration and intensity of the precipitation event

• precipitation event frequency

Can interception have a positive effect on water balance?

Yes in specific circumstances.

• Tropical mountain forests (“cloud forests”) intercept fog and mist → contributing up to 20% of river discharge (“fog drip”)

Evapotranspiration

E_t

• plants also enhance evaporation through root water uptake and transpiration

• factors that determine direct evaporation and transpiration are so different that combined process = evapotranspiration (𝐸_𝑡)

Limits of evapotranspiration

(1) available water

(2) available energy

𝐸_𝑝 used to separate these limiting factors.

Potential, E_p → occurs when enough water is available

Actual, E_t → (sometimes Ea) may be lower, due to water stress

Reference, E_t,0 → of a sufficiently watered reference crop, typically grass

$\displaylines{Et\ll Ep}$ actual never exceeds potential

• open water evaporation E_o is limited by energy & wind only

Transpiration by plants process

• Plants have stomata to take up CO2 for photosynthesis & lose water through the same pores (and, to a lesser extent, the epidermis)

• Stomata = very efficient at diffusing water → a sunflower leaf can evaporate at ½ the rate of an open water surface, despite a maximum stomatal area of 1% of the leaf

What does transpiration by plants depend on?

• number, size and distribution of stomata

• thickness and permeability of the epidermis

• area of internal surfaces exposed to intracellular spaces

• arrangement of the vascular tissue

• solar radiation, stomata close at night → reduces transpiration

What does the Penman–Monteith method calculate?

Reference for evapotranspiration, Eₜ,₀, in mm/day, for a sufficiently watered reference grass crop. (formulas are given, there are different formulas that can be used)

What does the Penman–Monteith method combine?

The energy available for evaporation, atmospheric conditions controlling water-vapour transfer, and the physiology of the reference crop.

Why are crop coefficients used?

They allow reference evapotranspiration to be adapted to different vegetation without separately modelling each plant’s physiology.

How is potential crop evapotranspiration calculated?

Eₚ = K꜀Eₜ,₀, where K꜀ is the dimensionless crop coefficient.

How is actual crop evapotranspiration calculated?

Eₜ = KₛK꜀Eₜ,₀ = KₛEₚ, where Kₛ is the dimensionless water-stress coefficient.

What is the difference between reference, potential and actual evapotranspiration?

Reference ET applies to well-watered reference grass. Potential ET applies to the particular crop without water stress. Actual ET accounts for the water available to that crop.

Can actual evapotranspiration exceed potential evapotranspiration?

No. Eₜ ≤ Eₚ. They are equal when there is no water stress.

What does K꜀ > 1 mean?

The crop’s potential evapotranspiration is greater than that of the reference grass under the same weather conditions.

What affects the crop coefficient, K꜀?

Crop type, growth stage and climate. It commonly rises during development, remains high at mid-season, then falls during the late stage.

How does climate affect mid-season K꜀?

For many crops, K꜀ is higher under arid, windy conditions than under humid, calm conditions.

What do K꜀,ᵢₙᵢ, K꜀,ₘᵢ𝒹 and K꜀,ₑₙ𝒹 mean?

Crop coefficients for the initial, mid-season and end-of-season growth stages.

When is Kₛ equal to 1?

While the root zone still contains readily available water, so the crop experiences no water stress.

What happens to Kₛ as soil water is depleted?

Once readily available water is exhausted, Kₛ decreases linearly, reaching zero at the wilting point in the lecture’s model.

How is stemflow measured?

Water flowing down a plant’s stem is collected and measured using a collection system connected to a flow meter or rain gauge.

How is throughfall measured, and why are several gauges needed?

Rain gauges are placed beneath the canopy. Canopy variability causes uneven throughfall, so ideally more than 10 gauges are used for a representative sample.

What is a lysimeter?

A container holding a known volume of soil and vegetation that is weighed frequently to determine changes in water storage.

What water-balance equation is used for the lecture’s lysimeter?

Eₜ = P − D − ΔS, where P is precipitation, D is drainage and ΔS is the change in water storage, expressed as equivalent water depths over the same period.

What must be monitored carefully during a lysimeter experiment?

Precipitation, drainage and other causes of mass change, such as vegetation growth, so they are not mistaken for evapotranspiration.

Why is evapotranspiration usually calculated from weather-station data?

Direct measurements are complex and local. Weather measurements combined with empirical crop characteristics provide a practical estimate.

How does an evaporation pan work?

Water-level changes in a tank are measured daily. Pans typically overestimate evaporation by 20–40%, so a pan coefficient is applied.

How does eddy covariance measure evaporation?

Fast sensors measure water vapour and vertical wind speed. Their covariance gives the vertical water-vapour flux.

How can evaporation be estimated as a water-balance residual?

E = P − dᵣ − R − ΔS: precipitation minus drainage, runoff and the increase in storage.

What is the main weakness of estimating evaporation as a residual?

Errors in every other water-balance term are passed into the evaporation estimate.