---
title: "Frequency of irrigation from soil moisture"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:frequency-of-irrigation-soil-moisture"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/frequency-of-irrigation-soil-moisture
site: BCT Engineering
---

# Frequency of irrigation from soil moisture

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2078 Baisakh, 1+2 marks
- Video: [Depth & Frequency of Irrigation | Irrigation Engineering | Available Moisture | Consumptive Use](https://www.youtube.com/watch?v=cw2gkeVc72U) — Civil Engineering Concept
- 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/frequency-of-irrigation-soil-moisture
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Define the frequency of irrigation and how do you calculate the frequency of irrigation on the basis of soil moisture?

## Short answer

Frequency of irrigation is the interval (days) between successive irrigations. It is calculated by available moisture depth = (FC – PWP)/100 × dry density/water density × root depth, then readily available moisture = p × available moisture, and frequency = readily available moisture / daily consumptive use (ETc).

## Answer

**Frequency of irrigation** is the number of days between two successive irrigations of a crop during its growing season (also called irrigation interval).

**Calculation on the basis of soil moisture**

1. Find the available moisture: $AM = \dfrac{FC-PWP}{100}\times\dfrac{\gamma_d}{\gamma_w}\times D_r$ (mm).
2. Find the readily available moisture (allowable depletion): $RAM = p\times AM$, with $p$ = 0.5-0.75.
3. Divide by the daily consumptive use $ET_c$:

$$
f = \frac{RAM}{ET_c}\ \text{(days)}
$$

Alternatively, $f=\dfrac{(FC-\theta_{min})}{100}\dfrac{\gamma_d}{\gamma_w}\dfrac{D_r}{ET_c}$ if a minimum allowed moisture $\theta_{min}$ is given.

## Check yourself

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

1. Frequency of irrigation is
   A. depth per irrigation
   B. area per cumec
   C. crop period
   D. number of days between successive irrigations
   Answer: D. Also called irrigation interval. (Key point: Frequency definition)

2. Frequency based on soil moisture is
   A. RAM / ET_c
   B. AM x ET_c
   C. FC / ET_c
   D. ET_c / AM
   Answer: A. Usable depth over daily use. (Key point: Method)

3. RAM = p x AM, where p is
   A. 1.5-2
   B. 0.5-0.75
   C. 0.05-0.1
   D. exactly 1
   Answer: B. Allowable depletion fraction. (Key point: RAM fraction)

## Related in Irrigation and Drainage Engineering

- [Soil-moisture-irrigation relationship](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-irrigation-relationship) — Irrigation and Drainage Engineering, chapter 2, asked 2×
- [Frequency of irrigation; area irrigable](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afrequency-and-area-irrigated-16-cumec) — Irrigation and Drainage Engineering, chapter 2
- [Depth and frequency of irrigation](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adepth-frequency-fc80-pwp30) — Irrigation and Drainage Engineering, chapter 2
- [Soil moisture tension and related terms](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-tension-terms) — Irrigation and Drainage Engineering, chapter 2
- [Forms of soil moisture with diagram](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-diagram) — Irrigation and Drainage Engineering, chapter 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

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