---
title: "Design capacity of a canal"
subject: "Irrigation and Drainage Engineering"
chapter: 4
key: "semester_6/irrigation_and_drainage_engineering:canal-design-capacity-method"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/canal-design-capacity-method
site: BCT Engineering
---

# Design capacity of a canal

Irrigation and Drainage Engineering · Chapter 4: Design of Canals

- Asked in: 2072 Magh, 3 marks
- Video: [Lecture 8 - First Numerical Problem based on Design Capacity of Irrigation Canal](https://www.youtube.com/watch?v=W9nyPml3_zE) — K & A Academy
- 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/canal-design-capacity-method
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-4

## Question

How to determine the design capacity of a canal.

## Short answer

Capacity is based on culturable commanded area, cropping pattern, Kor depth and period to find crop water requirement. Peak seasonal demand plus conveyance losses (seepage, evaporation) and a margin determines design discharge.

## Answer

The design capacity (full supply discharge) of a canal is fixed from the water demand of the area it serves, plus the losses.

#### Steps

1. **Gross commanded area (GCA)** and **culturable commanded area (CCA)** of the canal (CCA = GCA minus unculturable land such as roads, villages, rocky areas).
2. **Cropping pattern and intensity of irrigation** for each crop (percentage of CCA).
3. **Water requirement of each crop**: field irrigation requirement from Kor depth $\Delta$ and Kor period $B$ (or duty). Discharge per hectare $= \dfrac{\Delta\ \text{(m)}\times 10^4}{B\ \text{(s)}}$ m³/s/ha.
4. **Crop discharge** $=$ area under the crop $\times$ discharge per ha. Crops of different seasons are not added; the **critical period (peak demand)** governs the canal capacity.
5. **Add losses**: seepage and evaporation from the canal (about 4-5% of discharge per km for unlined canals or by Punjab / Davis-Wilson formulas) and other operational losses (usually 10-20%).
6. **Design discharge** at the head of each reach = demand of the area below + conveyance losses below. A margin (about 10%) for future needs is generally added.

## Check yourself

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

1. Crops of different seasons are not added. What governs the canal capacity?
   A. The sum of all crop demands
   B. The critical (peak demand) period
   C. The average of all crops
   D. The crop with the smallest area
   Answer: B. Seasons do not overlap, so the peak (critical period) demand governs. (Key point: Governing demand)

2. How is discharge per hectare found from Kor depth and Kor period?
   A. Kor depth (m) x 10^4 / Kor period (s)
   B. Kor period / Kor depth
   C. Kor depth x Kor period x 10^4
   D. Kor depth / (10^4 x Kor period)
   Answer: A. q = Delta x 10^4 / B in m3/s per ha, with Delta in metres and B in seconds. (Key point: Discharge per hectare)

3. What is added to the demand of the area below to get design discharge at the head of a reach?
   A. Only a 10% future margin
   B. Evaporation only
   C. Conveyance losses below (seepage, evaporation, operational), plus a margin
   D. Nothing, losses are in the Kor depth
   Answer: C. Design discharge = demand below + losses below, with about 10% margin for future needs. (Key point: Adding losses)

## Related in Irrigation and Drainage Engineering

- [Lined canal capacity and irrigable area](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alined-canal-trapezoid-5m-bed-capacity) — Irrigation and Drainage Engineering, chapter 4, asked 2×
- [Canal head regulator design, Q 150](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-head-regulator-design-q150) — Irrigation and Drainage Engineering, chapter 7
- [Semi-theoretical approach in canal design](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asemi-theoretical-approach-canal) — Irrigation and Drainage Engineering, chapter 4
- [Canal design discharge at 0, 1, 2, 3 km](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-design-discharge-along-reach) — Irrigation and Drainage Engineering, chapter 4
- [Canal head regulator design, Q 200](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-head-regulator-design-q200) — Irrigation and Drainage Engineering, chapter 7
- [Canal fall design, Q 12, drop 1.5 m](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-fall-design-q12-drop15) — Irrigation and Drainage Engineering, chapter 7

## Related in other subjects

- [Best hydraulic rectangular canal design](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Abest-rectangular-canal-design) — Hydro Power, chapter 1
- [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2
- [Memory capacity from range and interfacing](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Amemory-range-capacity-interfacing) — Microprocessor, chapter 4
- [Compression capacity of double angle ISA 150x75x12](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Adouble-angle-compression-capacity) — Design of Steel and Timber Structure, chapter 6