---
title: "Irrigation interval: FC 27%, PWP 14%"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:interval-fc27-pwp14-depth75"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-fc27-pwp14-depth75
site: BCT Engineering
---

# Irrigation interval: FC 27%, PWP 14%

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2079 Asoj, 5 marks
- 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/interval-fc27-pwp14-depth75
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

After how many days will you supply water to a field in order to ensure sufficient irrigation of a given crop, if the field capacity of the soil is 27%, the permanent wilting point is 14%, the density of soil is 1.5 gm/cm³, the effective depth of root zone is 75 cm, and daily consumptive use of water for the given crop is 12 mm.

## Short answer

Available moisture = (27–14)/100 × 1.5 × 750 = 146.25 mm. Irrigation interval = 146.25 / 12 = 12.19 days. Water should be supplied every 12 days.

## Answer

**Given:** FC = 27%, PWP = 14%, $\gamma_d = 1.5$ g/cm$^3$, $D_r = 75$ cm = 750 mm, $ET_c = 12$ mm/day. The full available moisture is taken as usable (no RAM fraction is given).

$$
\begin{aligned}
AM &= \frac{27-14}{100}\times 1.5\times 750 = 146.25\ \text{mm}\\
I &= \frac{AM}{ET_c} = \frac{146.25}{12} = 12.19\ \text{days}
\end{aligned}
$$

**Answer: water should be supplied after about 12.2 days, i.e. every 12 days.** (Round down for safety.)

## Check yourself

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

1. AM for FC 27, PWP 14, density 1.5, 750 mm is
   A. 202.5 mm
   B. 97.5 mm
   C. 146.25 mm
   D. 121.9 mm
   Answer: C. 0.13 x 1.5 x 750. (Key point: Available moisture)

2. Since no RAM fraction is given, we
   A. use 50%
   B. use FC only
   C. ignore PWP
   D. use full available moisture
   Answer: D. The answer takes AM as usable. (Key point: No RAM fraction)

3. The interval is
   A. about 12 days
   B. about 24 days
   C. about 6 days
   D. about 18 days
   Answer: A. 146.25/12 = 12.19; round down. (Key point: Interval)

## Close variants set in other exams

- [Irrigation interval: FC 28%, PWP 13%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc28-pwp13-root70) — Irrigation and Drainage Engineering, chapter 2, asked 2×

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