---
title: "Factors affecting evapotranspiration"
subject: "Engineering Hydrology"
chapter: 3
key: "semester_5/engineering_hydrology:factors-affecting-et"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/factors-affecting-et
site: BCT Engineering
---

# Factors affecting evapotranspiration

Engineering Hydrology · Chapter 3: Hydrological Losses

- Asked in: 2066 Magh (old course), 4 marks
- Video: [Basics of Evapotranspiration - Evaporation - GATE Hydrology](https://www.youtube.com/watch?v=FH5WriQoawM) — Ekeeda GATE & ESE
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/factors-affecting-et
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3

## Question

Write down the factors affecting evapotranspiration.

## Short answer

Evapotranspiration is governed by meteorological factors such as solar radiation, temperature, humidity and wind. Plant/crop factors, soil-moisture availability, soil properties and management practices like irrigation and mulching also affect it.

## Answer

**Evapotranspiration (ET)** is the combined loss of water by evaporation from soil and water surfaces and transpiration from plants. Its rate depends on four groups of factors.

#### 1. Meteorological (climatic) factors
- **Solar radiation:** supplies the latent heat of vaporisation; the most important factor.
- **Temperature:** higher temperature raises the saturation vapour pressure and ET.
- **Humidity:** low humidity increases the vapour pressure deficit $(e_s - e_a)$ and therefore ET.
- **Wind speed:** removes the moist air layer near the surface, so ET increases with wind (up to a limit).
- **Atmospheric pressure and sunshine hours (day length)** also have an effect.

#### 2. Plant factors
- Type of crop or vegetation, its leaf area, root depth and stomatal behaviour.
- Stage of growth and crop cover density (ET is small at sowing and maximum at full cover).
- Colour and roughness of the canopy (albedo).

#### 3. Soil and water factors
- **Soil moisture availability:** when moisture falls below the wilting point, actual ET drops far below potential ET.
- Soil type, texture and water-holding capacity; depth of the water table.
- Quality of water (salinity lowers ET).

#### 4. Management factors
- Method and frequency of irrigation, tillage, mulching and planting density.
- Presence of wind breaks and shading.

In design, ET is expressed as potential ET (PET) for a well-watered, fully covering crop; actual ET (AET) is then PET reduced for the soil-moisture and crop-stage effects.

## Check yourself

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

1. The most important meteorological factor is
   A. Wind only
   B. Solar radiation
   C. Colour
   D. Pressure
   Answer: B. Radiation supplies latent heat. (Key point: Main factor)

2. Low humidity
   A. No effect
   B. Stops wind
   C. Increases vapour pressure deficit and ET
   D. Decreases ET
   Answer: C. Larger (es - ea) drives more ET. (Key point: Humidity)

3. When moisture falls below wilting point
   A. PET is zero
   B. No change
   C. AET drops far below PET
   D. AET exceeds PET
   Answer: C. Water-limited. (Key point: Soil moisture)

4. ET is max at
   A. Sowing
   B. Full crop cover
   C. Dormancy
   D. Harvest
   Answer: B. Cover density raises ET. (Key point: Plant factor)

## Related in Engineering Hydrology

- [Catchment and factors affecting runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acatchment-runoff-factors) — Engineering Hydrology, chapter 4
- [Rating curve and factors affecting runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-runoff-factors) — Engineering Hydrology, chapter 4
- [Rainfall-runoff relationship, factors affecting runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arainfall-runoff-relation-factors) — Engineering Hydrology, chapter 4
- [Hydrological losses statement, factors affecting evaporation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Alosses-not-actual-evap-factors) — Engineering Hydrology, chapter 3
- [Factors affecting flood hydrograph, baseflow separation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aflood-hydrograph-factors-baseflow) — Engineering Hydrology, chapter 5

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