---
title: "Irrigation interval: FC 32%, PWP 13%"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:interval-fc32-pwp13-root130"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-fc32-pwp13-root130
site: BCT Engineering
---

# Irrigation interval: FC 32%, PWP 13%

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2078 Chaitra, 5 marks
- Video: [#12 numerical on irrigation engineering || readily available moisture || civil engineering ||](https://www.youtube.com/watch?v=edIHBOuCWXM) — Numerical's Civil
- 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-fc32-pwp13-root130
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Find the irrigation interval in which the field capacity is 32%, permanent wilting point is 13%, dry density of soil is 1.3 gm/cc, effective root zone depth is 130 m [as printed] and daily consumptive use of water for the given crop is 14 mm.

## Short answer

Assuming the printed depth 130 m is 130 cm, root depth = 1300 mm. Available moisture = (32–13)/100 × 1.3 × 1300 = 321.1 mm. Interval = 321.1 / 14 = 22.94 days; irrigate every 22 days.

## Answer

**Given:** FC = 32%, PWP = 13%, $\gamma_d = 1.3$ g/cc, root zone depth $D_r = 130$ cm $= 1300$ mm, $ET_c = 14$ mm/day. The depth "130 m" is a misprint and is taken as 130 cm.

**Step 1: Available moisture depth**

$$
\begin{aligned}
d &= \frac{FC-PWP}{100}\times\frac{\gamma_d}{\gamma_w}\times D_r\\
&= \frac{32-13}{100}\times 1.3\times 1300 = 321.10\ \text{mm}
\end{aligned}
$$

**Step 2: Irrigation interval**

$$
I = \frac{d}{ET_c} = \frac{321.10}{14} = 22.94\ \text{days}
$$

**Answer: irrigate after about 22.9 days, i.e. every 22 days** (rounded down so the crop does not suffer stress).

## Check yourself

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

1. The root depth '130 m' is taken as
   A. 130 m
   B. 130 cm = 1300 mm
   C. 13 mm
   D. 13 cm
   Answer: B. 130 m is unrealistic. (Key point: Misprint handling)

2. d = 
   A. 247 mm
   B. 417 mm
   C. 321.10 mm
   D. 19 mm
   Answer: C. 0.19 x 1.3 x 1300. (Key point: Available moisture)

3. The interval is
   A. 45 days
   B. 11 days
   C. 32 days
   D. 22.9 days, irrigate every 22 days
   Answer: D. 321.10/14, rounded down. (Key point: Interval)

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