---
title: "Crop water requirement and its factors"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:crop-water-requirement-factors"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/crop-water-requirement-factors
site: BCT Engineering
---

# Crop water requirement and its factors

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2068 Chaitra (old course), 5 marks
- Video: [Irrigation Requirement of Crops - Water Requirement of Crops - Water Resource Engineering 1](https://www.youtube.com/watch?v=RPRK7hIVbHg) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/crop-water-requirement-factors
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

What do you mean by crop water requirement? Explain the factors affecting the crop water requirement.

## Short answer

Crop water requirement is the total water needed for full crop growth, including evapotranspiration and application losses. Factors are climate (temperature, humiditty, wind, sunshine), crop type and duration, soil texture, water table depth, effective rainfall, irrigation method and efficiency, tillage and mulching, salinity, and season.

## Answer

**Crop water requirement (CWR)** is the total quantity of water, regardless of its source, needed by a crop to grow to maturity. It includes consumptive use (evapotranspiration) plus losses during application (percolation, seepage, runoff) and special needs (puddling, leaching):

$$
CWR = ET_c + \text{losses} + \text{special needs}
$$

and the irrigation requirement is $CWR - P_e - $ soil moisture used.

#### Factors affecting CWR
1. **Climate:** higher temperature, more sunshine hours, stronger wind and lower humidity all increase evapotranspiration.
2. **Crop:** type, variety, leaf area, root depth and growth stage; rice and sugarcane have a high CWR.
3. **Crop period:** longer periods need more water.
4. **Soil:** texture, structure and infiltration; sandy soil has more deep percolation.
5. **Water table:** a shallow water table contributes capillary water.
6. **Rainfall:** its distribution and the effective rainfall.
7. **Method and efficiency of irrigation:** surface methods lose more water than drip or sprinkler.
8. **Agronomic practices:** tillage, mulching, planting date, spacing and weed control.
9. **Salinity of soil and water:** extra water is needed for leaching.
10. **Season and altitude** of the area.

## Check yourself

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

1. CWR is made up of
   A. ET_c only
   B. ET_c plus application losses and special needs
   C. Rainfall plus runoff
   D. Effective rainfall minus ET_c
   Answer: B. CWR = ET_c + losses + special needs. (Key point: CWR definition)

2. Irrigation requirement is obtained as
   A. CWR + effective rainfall
   B. CWR x efficiency
   C. CWR - effective rainfall - soil moisture used
   D. ET_c - CWR
   Answer: C. Rainfall and soil moisture meet part of CWR. (Key point: Irrigation requirement)

3. Why does sandy soil raise CWR?
   A. Higher salinity
   B. Smaller leaf area
   C. Higher capillary rise
   D. More deep percolation losses
   Answer: D. Coarse soil holds less and loses more water. (Key point: Soil effect)

4. Which method usually gives the lower water requirement?
   A. Drip or sprinkler
   B. Surface flooding
   C. Basin flooding
   D. All are equal
   Answer: A. Surface methods lose more water. (Key point: Method of irrigation)

## Related in Irrigation and Drainage Engineering

- [Steps of crop water requirement calculation](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acrop-water-requirement-steps) — Irrigation and Drainage Engineering, chapter 2, asked 2×
- [Soil moisture forms; factors affecting CWR](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-cwr-factors) — Irrigation and Drainage Engineering, chapter 2
- [Irrigation water requirement of rice](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Arice-irrigation-requirement-definition) — Irrigation and Drainage Engineering, chapter 2
- [Command area, crop rotation, necessity](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acommand-area-crop-rotation) — Irrigation and Drainage Engineering, chapter 1
- [Steps for rice irrigation requirement](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Arice-irrigation-requirement-steps) — Irrigation and Drainage Engineering, chapter 2, asked 4×

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