---
title: "Duty, delta, base period, crop period"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:define-duty-delta-base-crop-period"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/define-duty-delta-base-crop-period
site: BCT Engineering
---

# Duty, delta, base period, crop period

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2077 Chaitra, 4 marks
- Video: [WHAT IS DELTA, DUTY, BASE PERIOD, KOR DEPTH, AND CROP PERIOD IN IRRIGATION](https://www.youtube.com/watch?v=MLyIJZMpArs) — 𝐁𝐥𝐨𝐨𝐦 𝐰𝐢𝐭𝐡 𝐀𝐧𝐚𝐦𝐢𝐤𝐚
- 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/define-duty-delta-base-crop-period
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Define: Duty, Delta, Base period and Crop period.

## Short answer

Duty: area (ha) irrigated by 1 cumec during the base period. Delta: total depth of water required (m). Base period: days from first to last irrigation. Crop period: total days from sowing to harvest.

## Answer

- **Duty ($D$):** the area of crop (in ha) irrigated by one cumec of water supplied continuously for the base period of the crop. Unit: ha/cumec. It decreases from the field to the canal head because of losses.
- **Delta ($\Delta$):** the total depth of water required by a crop during its base period (in m or cm), if it were spread over the field. Related by $\Delta = 8.64B/D$ (m).
- **Base period ($B$):** the time between the first watering of a crop at sowing (or nursery) and its last watering before maturity/harvest, in days.
- **Crop period:** the total time from sowing to harvesting of the crop (days). It is a little longer than the base period, as it includes the time before the first watering and after the last watering.

## Check yourself

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

1. Duty is
   A. depth of water per crop
   B. area irrigated per cumec supplied continuously for the base period
   C. time between sowing and harvest
   D. water lost per km of canal
   Answer: B. Unit ha/cumec. (Key point: Duty)

2. Delta is
   A. area per cumec
   B. days of watering
   C. total depth of water a crop requires during its base period
   D. canal discharge
   Answer: C. Delta = 8.64 B / D. (Key point: Delta)

3. Base period is the time between
   A. sowing and harvesting
   B. two irrigations
   C. rain start and end
   D. first watering and last watering before maturity
   Answer: D. It is slightly shorter than crop period. (Key point: Base period)

4. Crop period compared with base period is
   A. slightly longer, from sowing to harvest
   B. shorter
   C. identical always
   D. unrelated to sowing
   Answer: A. It includes time before first and after last watering. (Key point: Crop period)

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