---
title: "Field capacity, available moisture definitions"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:define-fc-am-ram-deficiency"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/define-fc-am-ram-deficiency
site: BCT Engineering
---

# Field capacity, available moisture definitions

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2079 Asoj, 2 marks
- Video: [Field capacity and wilting point](https://www.youtube.com/watch?v=9BYxTrX_ynU) — EarthScienceMovies
- 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-fc-am-ram-deficiency
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Define the terms field capacity, available moisture, readily available moisture and soil water deficiency.

## Short answer

Field capacity (FC) is moisture after drainage (≈1/3 atm). Available moisture AM = FC – PWP. Readily available moisture RAM is the 50–75% easily extracted fraction. Soil water deficiency is the depth needed to restore root zone to FC.

## Answer

- **Field capacity (FC):** the amount of water held in the soil after the excess gravity water has drained away (usually 1-3 days after irrigation or rain); soil moisture tension is about 1/3 atm. It is the upper limit for irrigation.
- **Available moisture (AM):** the water held between field capacity and permanent wilting point, which plants can use: $AM = FC - PWP$.
- **Readily available moisture (RAM):** the fraction of available moisture (about 50-75%) which crops extract easily without moisture stress.
- **Soil water deficiency (moisture deficiency):** the amount of water required to bring the soil moisture of the root zone back to field capacity at any time; $= FC - \text{present moisture}$ (expressed as depth, mm). It is the net depth to be applied in irrigation.

## Check yourself

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

1. Field capacity is the water held in soil
   A. at 15 atm tension
   B. when all pores are full
   C. in the chemical structure of minerals
   D. after excess gravity water has drained, about 1/3 atm tension
   Answer: D. FC is the upper limit of irrigation, reached 1-3 days after wetting. (Key point: Field capacity)

2. Available moisture equals
   A. FC - PWP
   B. FC + PWP
   C. Saturation - FC
   D. PWP - oven dry
   Answer: A. AM is the water between FC and PWP. (Key point: Available moisture)

3. Soil water deficiency is
   A. PWP minus present moisture
   B. FC minus present moisture, the net depth to be applied
   C. RAM x ET_c
   D. Saturation minus FC
   Answer: B. It is the water needed to bring the root zone back to FC. (Key point: Soil water deficiency)

## Related in Irrigation and Drainage Engineering

- [Readily available moisture, PWP, interval](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Armc-pwp-irrigation-interval-terms) — 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
- [Field capacity from applied water data](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afield-capacity-from-applied-water) — 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
- [Storage capacity, depth and water at outlet](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Astorage-capacity-wheat-field-efficiency) — 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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