---
title: "Irrigation interval: FC 60%, PWP 25%"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:interval-fc60-pwp25-ram85"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-fc60-pwp25-ram85
site: BCT Engineering
---

# Irrigation interval: FC 60%, PWP 25%

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2069 Bhadra, 3 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-fc60-pwp25-ram85
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

The field capacity of soil is 60%, permanent wilting point is 25%, Density of soil is 1.2 gm/cc, effective root depth is 120 cm, ET crop is 9 mm/day. Calculate the irrigation interval if the readily available moisture is 85% of available soil moisture capacity.

## Short answer

Available moisture capacity is 504 mm; 85% RAM gives 428.4 mm net, so irrigation interval is 47.6 days (about 47 days).

## Answer

**Given:** FC = 60%, PWP = 25%, $\gamma_d = 1.2$, $D_r = 120$ cm $= 1200$ mm, $ET_c = 9$ mm/day, RAM = 85% of available moisture.

$$
\begin{aligned}
AMC &= \frac{60-25}{100}\times 1.2\times 1200 = 504\ \text{mm}\\
RAM &= 0.85\times 504 = 428.40\ \text{mm}\\
I &= \frac{RAM}{ET_c} = \frac{428.40}{9} = 47.60\ \text{days}
\end{aligned}
$$

**Answer: irrigation interval = 47.6 days, say 47 days.**

## Check yourself

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

1. AMC = 
   A. 420 mm
   B. 428.4 mm
   C. 504 mm
   D. 600 mm
   Answer: C. 0.35 x 1.2 x 1200. (Key point: AMC)

2. RAM = 
   A. 504 mm
   B. 75.6 mm
   C. 360 mm
   D. 428.4 mm
   Answer: D. 0.85 x 504. (Key point: RAM)

3. Interval = 
   A. 47.6 days, say 47
   B. 56 days
   C. 42.8 days
   D. 4.76 days
   Answer: A. 428.4/9. (Key point: Interval)

## Close variants set in other exams

- [Irrigation interval: FC 40%, PWP 20%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc40-pwp20-root90) — Irrigation and Drainage Engineering, chapter 2, asked 4×
- [Irrigation interval with AMC, RAM graph](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-ram75-graph) — Irrigation and Drainage Engineering, chapter 2, asked 4×

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