---
title: "AET versus PET, field measurement of ET"
subject: "Engineering Hydrology"
chapter: 3
key: "semester_5/engineering_hydrology:aet-pet-measurement"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/aet-pet-measurement
site: BCT Engineering
---

# AET versus PET, field measurement of ET

Engineering Hydrology · Chapter 3: Hydrological Losses

- Asked in: 2081 Bhadra, 2+2 marks
- Video: [26. Evapotranspiration ||Water Resource Engineering](https://www.youtube.com/watch?v=0ma7gCCxkzo) — NITIANS ACADEMY
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/aet-pet-measurement
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3

## Question

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

## 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.

## Answer

#### 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.

```text
   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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which statement correctly relates AET and PET?
   A. AET and PET are controlled only by soil
   B. AET is always greater than PET
   C. PET depends on soil moisture
   D. AET equals or is less than PET, and equals PET when soil moisture is at or above field capacity
   Answer: D. PET assumes unlimited water; AET is limited by soil moisture and falls below PET as soil approaches wilting point. (Key point: AET vs PET)

2. What mainly controls PET?
   A. Irrigation schedule
   B. Climate only
   C. Soil moisture only
   D. Crop type and soil
   Answer: B. PET is for a short green crop with unlimited water, so climate is the controlling factor. (Key point: PET control)

3. What is a lysimeter?
   A. A flow measuring weir
   B. A watertight tank of undisturbed soil with crop, buried level with the field, used to measure ET
   C. A rain gauge
   D. A pan for open water evaporation
   Answer: B. The lysimeter holds soil and crop in field conditions so that water inputs and drainage can be measured. (Key point: Lysimeter)

4. Which equation gives ET from lysimeter data?
   A. ET = P + I - D +/- change in storage
   B. ET = P x I / D
   C. ET = P - I + D
   D. ET = D - P
   Answer: A. ET = P + I - D +/- delta S, using precipitation, irrigation, drainage and soil moisture change. (Key point: Water balance equation)

## Related in Engineering Hydrology

- [PET versus AET, Penman's method](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Apet-vs-aet-penman) — Engineering Hydrology, chapter 3
- [Actual versus potential evapotranspiration](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aactual-vs-potential-et) — Engineering Hydrology, chapter 3

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