---
title: "Steps of crop water requirement calculation"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:crop-water-requirement-steps"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/crop-water-requirement-steps
site: BCT Engineering
---

# Steps of crop water requirement calculation

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2063 Baisakh (old course), 5 marks; 2062 Kartik (old course), 4 marks (asked 2 times)
- Video: [CROP WATER REQUIREMENT { CALCULATION } By. Mr. Sameer Mohapatro](https://www.youtube.com/watch?v=58YKyOuh0dg) — Agriculture with Sameer 
- 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-steps
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Write down the steps for crop water requirement calculation.

## Short answer

Compute reference evapotranspiration (ET0), multiply by crop coefficient Kc for ETc, add percolation/leaching needs, subtract effective rainfall and soil moisture contribution to obtain net irrigation requirement. Convert to gross requirement using efficiency; the peak demand period governs canal design.

## Answer

**Crop water requirement (CWR)** is the total water needed by a crop from sowing to harvest, equal to the consumptive use plus losses and special needs, supplied by rainfall, soil moisture and irrigation.

#### Steps of calculation
1. Collect climatic data (temperature, humidity, sunshine, wind, rainfall) and the crop calendar (sowing, growth stages, harvest).
2. Compute reference evapotranspiration $ET_o$ by Penman, Blaney-Criddle, Hargreaves, or pan evaporation method ($ET_o = K_p E_{pan}$).
3. Choose the crop coefficient $K_c$ for each growth stage from FAO-24/56 tables.
4. Compute crop evapotranspiration:
$$
ET_c = K_c\times ET_o
$$
5. Add other needs: percolation and seepage (rice), land preparation, nursery, leaching requirement, and water for planting.
6. Compute effective rainfall $P_e$ (75% dependable rainfall).
7. Compute net irrigation requirement:
$$
NIR = ET_c + \text{other needs} - P_e - \text{soil moisture contribution}
$$
8. Compute gross irrigation requirement using efficiency:
$$
GIR = \frac{NIR}{\eta_{a}\,\eta_c}
$$
9. Convert depth to discharge: $Q = \dfrac{GIR\times A}{T}$ or express as l/s/ha.
10. Find the peak demand period which governs the design capacity of canal and structures.

## Check yourself

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

1. How is crop evapotranspiration ET_c obtained?
   A. ET_c = Kc x ET_o, with Kc chosen for each growth stage
   B. ET_c = ET_o / Kc
   C. ET_c = ET_o - effective rainfall
   D. ET_c = Kp x Kc
   Answer: A. ET_c is the reference ET_o multiplied by the stage-wise crop coefficient Kc. (Key point: Crop evapotranspiration)

2. Which expression gives the net irrigation requirement NIR?
   A. ET_c + effective rainfall + soil moisture
   B. ET_c + other needs - effective rainfall - soil moisture contribution
   C. ET_c / (efficiency) - rainfall
   D. ET_o - ET_c + other needs
   Answer: B. Rainfall and soil moisture supply part of the demand, so they are deducted from ET_c plus special needs. (Key point: Net irrigation requirement)

3. How is the gross irrigation requirement found from NIR?
   A. Multiply NIR by the efficiencies
   B. Add the efficiency losses to ET_o
   C. Divide NIR by the product of application and conveyance efficiencies
   D. Divide NIR by crop coefficient
   Answer: C. Losses in conveyance and application mean more water must be released: GIR = NIR/(eta_a x eta_c). (Key point: Gross irrigation requirement)

4. Which period governs the design capacity of canals and structures?
   A. The average demand over the whole season
   B. The sowing period only
   C. The month with highest rainfall
   D. The peak demand period
   Answer: D. Canals must carry the maximum demand, so the peak period is used for design. (Key point: Design governing demand)

## Related in Irrigation and Drainage Engineering

- [Crop water requirement and its factors](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acrop-water-requirement-factors) — Irrigation and Drainage Engineering, chapter 2
- [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×
- [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

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