---
title: "Readily available moisture, PWP, interval"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:rmc-pwp-irrigation-interval-terms"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/rmc-pwp-irrigation-interval-terms
site: BCT Engineering
---

# Readily available moisture, PWP, interval

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2078 Chaitra, 2 marks
- Video: [Soil - Moisture Relationship | Irrigation Engineering | Irrigation depth,Irrigation interval](https://www.youtube.com/watch?v=pNjYV5scQOA) — Sudip Dhakal
- 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/rmc-pwp-irrigation-interval-terms
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Describe briefly the readily available moisture, permanent wilting point and irrigation interval.

## Short answer

Readily available moisture is the portion of available water (50–75%) easily extracted without stress. Permanent wilting point is the 15‑atm moisture content at which plants wilt permanently. Irrigation interval (days) = depth of RAM / daily ETc.

## Answer

- **Readily available moisture (RAM):** the part of the available moisture (FC - PWP) which the crop can extract easily without stress, usually 50-75% of the available moisture (the upper part of the available range).
- **Permanent wilting point (PWP):** the soil moisture content (as % of dry weight) at which plants wilt and cannot recover even if kept in a humid atmosphere; soil moisture tension is about 15 atm (1500 kPa).
- **Irrigation interval (frequency):** the number of days between two successive irrigations during the growing period. It is found as
$$
I = \frac{\text{RAM depth (mm)}}{ET_c\ (\text{mm/day})}
$$

## Check yourself

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

1. What is readily available moisture?
   A. The part of available moisture (usually 50-75%) the crop extracts without stress
   B. All water between saturation and FC
   C. Water held at 31 atm tension
   D. The water lost in canal seepage
   Answer: A. RAM is the easily extractable upper portion of FC-PWP. (Key point: Readily available moisture)

2. What happens at the permanent wilting point?
   A. Plants grow fastest, tension 1/3 atm
   B. Plants wilt and cannot recover, tension about 15 atm
   C. Soil is saturated
   D. Gravity water drains out
   Answer: B. PWP is the lower limit of available water at about 15 atm. (Key point: Permanent wilting point)

3. Irrigation interval is calculated as
   A. ET_c / RAM
   B. FC - PWP in %
   C. RAM depth (mm) / ET_c (mm/day)
   D. Root depth / ET_c
   Answer: C. Days between irrigations = usable depth divided by daily use. (Key point: Irrigation interval)

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