20.3. Methods of Measurement of Evapotranspiration

Evapotranspiration can be measured or estimated using:

20.3.1 Direct Methods

These methods involve actual field measurements of ET.

a) Lysimeter Method

• A lysimeter is a tank embedded in the ground filled with soil and crops identical to the surrounding field.
• Measures change in weight due to water loss.
• Water inputs (rainfall, irrigation) and outputs (drainage, ET) are carefully monitored.
• Advantages: High accuracy.
• Disadvantages: Expensive, labor-intensive, not suitable for large areas.

b) Field Experimental Plots

• Known quantity of water is applied.
• Change in soil moisture and plant growth is observed.
• ET is calculated as a water balance.
• Used mostly for calibration of models.

Direct methods of measuring evapotranspiration (ET) rely on actual field measurements. The lysimeter method involves using a special tank filled with soil and plants, mirroring the surrounding environment. This tank measures changes in weight as water is lost, allowing researchers to calculate ET based on inputs like rainfall and irrigation, and outputs such as drainage. Despite its high accuracy, this method can be costly and labor-intensive and is not practical for large areas. Field experimental plots are another direct method where a known amount of water is applied, and researchers monitor changes in soil moisture and plant growth to calculate ET as a water balance, typically used to calibrate models.

Imagine a gardener who wants to know exactly how much water their garden loses through evaporation and plant transpiration. They might set up a lysimeter filled with the same type of soil and plants as in their garden to accurately measure the water loss. Just like a bathroom scale shows the weight of a person, the lysimeter shows changes in weight due to lost water, letting the gardener understand how much irrigation they need.

20.3.2 Indirect Methods

These methods estimate ET using meteorological and empirical data.

a) Water Balance Method

• Based on the continuity equation: ET=P−R−D−ΔS Where:
• P = Precipitation
• R = Runoff
• D = Deep percolation
• ΔS = Change in soil water storage
• Effective for basin-scale studies.

b) Energy Balance Method

• Based on conservation of energy: R =G+H+LE n Where:
• R = Net radiation n
• G = Soil heat flux
• H = Sensible heat flux
• LE = Latent heat flux (related to ET)

c) Aerodynamic Method

• Uses principles of mass transfer.
• Based on wind speed and vapor pressure difference: ET=C⋅u⋅(e −e ) s a Where:
• u = wind speed
• e −e = vapor pressure deficit s a
• C = empirical constant

d) Combination Method (Penman Equation)

• Combines energy balance and aerodynamic methods.
• Penman equation: Δ(R −G)+γ⋅f (u)(e −e ) ET = n s a 0 Δ+γ Where:
• Δ = Slope of vapor pressure curve
• γ = Psychrometric constant
• f(u) = Wind function

Indirect methods for measuring evapotranspiration (ET) do not rely on direct measurements but rather on estimates derived from meteorological data. The Water Balance Method calculates ET based on precipitation, runoff, deep percolation, and changes in soil water storage, making it useful for larger basins. The Energy Balance Method applies the principle of energy conservation, linking net radiation absorbed by the surface to heat flux and ET. The Aerodynamic Method evaluates ET by measuring wind speed and vapor pressure differences, while the Combination Method (Penman Equation) integrates both energy and aerodynamic principles to yield more accurate ET estimates.

Think of a farmer who cannot measure water loss directly from every single crop in the field. Instead, they might use the Water Balance Method, where they account for rainfall and runoff to get a better idea of how much water is being used. It's like figuring out how much water is needed in a bucket when you know how much comes in through the tap (rain) and how much spills out (runoff) when the bucket is full.

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