Which statement correctly relates AET and PET?

AET equals or is less than PET, and equals PET when soil moisture is at or above field capacity

AET and PET are controlled only by soil

A watertight tank of undisturbed soil with crop, buried level with the field, used to measure ET

A pan for open water evaporation

Which equation gives ET from lysimeter data?

ET = P + I - D +/- change in storage

2081 Bhadra · 2+2 marks

AET versus PET, field measurement of ET

Short answer

AET is actual evapotranspiration under existing soil moisture, limited by water; PET is the maximum possible under unlimited water and full crop cover, controlled by climate. A field lysimeter—a buried tank with soil and crop—measures ET via water balance: ET = P + I – D – ΔS.

Differentiate AET and PET. Explain the method of measurement of evapotranspiration in field with sketch.

Difference between AET and PET

Point	PET	AET
Meaning	Evapotranspiration from a large, uniform, short green crop fully shading the ground with unlimited soil water	Actual evapotranspiration occurring under existing soil moisture and crop condition
Water supply	Unlimited	Limited and variable
Controlled by	Climate only	Climate, crop, soil moisture
Value	Maximum possible	Equal to or less than PET
Use	Crop water requirement, planning	Water balance, actual loss

$AET = PET$ when soil moisture is at or above field capacity, and $AET < PET$ when it falls toward the wilting point.

Field measurement of evapotranspiration: Lysimeter

A lysimeter (tank or evapotranspirometer) is a watertight tank, about 1 to 2 m deep and 1 to 3 m² in area, filled with undisturbed soil and planted with the crop. It is buried so that its surface is level with the surrounding field, and the crop condition is kept the same inside and outside.

   crop  crop  crop     field level
 ---|--------------|---
    |  soil + crop |  <- lysimeter tank
    |  ~~~~~~~~~~~ |
    |______________|
        |  drain
        v  measuring jar

Procedure: the water added (rainfall and irrigation) is measured, and the drainage collected from the bottom is measured. The change in soil moisture is found by weighing the tank (weighing lysimeter) or by soil moisture sampling.

ET = P + I - D - \Delta S

where $P$ = precipitation, $I$ = irrigation, $D$ = drainage and $\Delta S$ = increase in soil moisture storage over the period. Other field methods include soil-moisture depletion studies, field plots, the water balance of a field, and the inflow-outflow method for a watershed.

✓ Answer checked by Claude Sonnet 5.5

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