---
title: "Irrigation interval: FC 28%, PWP 13%"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:interval-fc28-pwp13-root70"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-fc28-pwp13-root70
site: BCT Engineering
---

# Irrigation interval: FC 28%, PWP 13%

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2077 Chaitra, 5 marks
- Video: [After how many days will you supply water to the soil | Frequency | Soil Moisture Numerical Problem](https://www.youtube.com/watch?v=nsM4OqiI51E) — SBK 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/interval-fc28-pwp13-root70
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

After how many days will supply water to soil in order to ensure sufficient irrigation of the given crop if, (i) Field capacity of the soil = 28%, (ii) Permanent wilting point = 13%, (iii) Dry density of soil = 1.3 gm/cc, (iv) Effective depth of root zone = 70 cm, (v) Daily consumptive use of water for the given crop = 12 mm.

## Short answer

Available moisture = (28–13)/100 × 1.3 × 700 = 136.5 mm. Interval = 136.5 / 12 = 11.38 days; irrigate every 11 days.

## Answer

**Given:** FC = 28%, PWP = 13%, $\gamma_d = 1.3$ g/cc, root zone depth $D_r = 70$ cm $= 700$ mm, $ET_c = 12$ mm/day. 

**Step 1: Available moisture depth**

$$
\begin{aligned}
d &= \frac{FC-PWP}{100}\times\frac{\gamma_d}{\gamma_w}\times D_r\\
&= \frac{28-13}{100}\times 1.3\times 700 = 136.50\ \text{mm}
\end{aligned}
$$

**Step 2: Irrigation interval**

$$
I = \frac{d}{ET_c} = \frac{136.50}{12} = 11.38\ \text{days}
$$

**Answer: irrigate after about 11.4 days, i.e. every 11 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. d = 
   A. 136.5 mm
   B. 273 mm
   C. 91 mm
   D. 196 mm
   Answer: A. 0.15 x 1.3 x 700. (Key point: Available moisture)

2. Interval = 
   A. 22.8 days
   B. 11.4 days
   C. 5.7 days
   D. 15 days
   Answer: B. 136.5/12. (Key point: Interval)

3. Rounded interval is
   A. 12 days
   B. 13 days
   C. 11 days
   D. 10 days
   Answer: C. Round down for safety. (Key point: Rounding)

## Close variants set in other exams

- [Irrigation interval: FC 27%, PWP 14%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc27-pwp14-depth75) — Irrigation and Drainage Engineering, chapter 2, asked 2×

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