---
title: "Actual versus potential evapotranspiration"
subject: "Engineering Hydrology"
chapter: 3
key: "semester_5/engineering_hydrology:actual-vs-potential-et"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/actual-vs-potential-et
site: BCT Engineering
---

# Actual versus potential evapotranspiration

Engineering Hydrology · Chapter 3: Hydrological Losses

- Asked in: 2075 Chaitra, 3 marks
- Video: [Evapotranspiration | Potential Evapotranspiration | Actual Evapotranspiration | Hydrology (CE)](https://www.youtube.com/watch?v=58JiuyeCBV4) — ONLINETYARI CLASSES
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/actual-vs-potential-et
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3

## Question

Differentiate actual and potential evapotranspirations.

## Short answer

PET is maximum evapotranspiration with unlimited water, controlled by climate. AET is actual loss limited by soil moisture, crop type, and stage; AET ≤ PET, equal only when soil moisture is adequate.

## Answer

| Point | Potential evapotranspiration (PET) | Actual evapotranspiration (AET) |
|---|---|---|
| Meaning | Evapotranspiration from a large area of short green crop fully covering the ground, with unlimited water supply | Actual amount of water lost by evaporation and transpiration under existing soil moisture |
| Water supply | Not limited | Limited by soil moisture |
| Controlled by | Climate (radiation, temperature, humidity, wind) | Climate plus soil moisture, crop type and stage |
| Value | Maximum possible rate | Equal to or less than PET |
| Relation | AET = PET when soil moisture is adequate | AET < PET in dry soil (AET = k × PET, k < 1) |
| Use | Crop water requirement, irrigation planning | Water balance, actual catchment losses |

## Check yourself

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

1. PET occurs when:
   A. Water supply is unlimited from short green crop
   B. Soil is dry
   C. No sun
   D. Crops are absent
   Answer: A. PET assumes unlimited water. (Key point: PET)

2. AET is:
   A. Always more than PET
   B. Equal to or less than PET
   C. Unrelated
   D. Independent of soil
   Answer: B. AET <= PET. (Key point: AET vs PET)

3. AET depends on:
   A. Climate only
   B. Climate plus soil moisture and crop
   C. Only wind
   D. Only latitude
   Answer: B. Soil moisture limits actual ET. (Key point: Control)

## 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
- [AET versus PET, field measurement of ET](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aaet-pet-measurement) — Engineering Hydrology, chapter 3
- [Double ring infiltrometer, phi-index versus W-index](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Adouble-ring-phi-vs-w-index) — Engineering Hydrology, chapter 3
- [Velocity-area versus slope-area methods](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Avelocity-area-vs-slope-area) — Engineering Hydrology, chapter 4
- [PET by Penman method, Kathmandu in March](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Apenman-kathmandu-march) — Engineering Hydrology, chapter 3, asked 2×
- [PET by Penman method, Kathmandu in February](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Apenman-kathmandu-february) — Engineering Hydrology, chapter 3, asked 2×

## Related in other subjects

- [Potential evapotranspiration by Penman method](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Apenman-pet-steps) — Irrigation and Drainage Engineering, chapter 2
- [Ideal versus actual vapour compression cycle](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aideal-vs-actual-vcr-cycle) — Applied Thermodynamics, chapter 3
- [Potential difference versus emf](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Apotential-difference-vs-emf) — Basic Electrical Engineering, chapter 1
- [Merits of functions; actual vs formal parameters](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Audf-merits-demerits-parameters) — Computer Programming, chapter 6