---
title: "Irrigation interval: FC 40%, PWP 20%"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:interval-fc40-pwp20-root90"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-fc40-pwp20-root90
site: BCT Engineering
---

# Irrigation interval: FC 40%, PWP 20%

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2070 Chaitra (old course), 4 marks; 2063 Asoj (old course), 4 marks (asked 2 times)
- Video: [Water Requirement of Crops | Root Zone Depth | Moisture content | PWP | FC |Two & Three Phase System](https://www.youtube.com/watch?v=jTVHCyTHyos) — Civil Engineering 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-fc40-pwp20-root90
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

The field capacity of soil is 40%, permanent wilting point is 20%. Density of soil is 1.2 gm/cc, effective root depth is 90 cm, ET crop is 10 mm/day. Calculate the irrigation interval if the readily available moisture is 75% of available soil moisture capacity.

## Short answer

Available moisture capacity = (0.40–0.20) × 1.2 × 900 = 216 mm; readily available moisture = 0.75 × 216 = 162 mm; interval = 162 / 10 = 16.2 days, so irrigate every 16 days.

## Answer

**Given:** FC = 40%, PWP = 20%, $\gamma_d = 1.2$ g/cc (so $\gamma_d/\gamma_w=1.2$), root depth $D_r = 90$ cm = 900 mm, $ET_c = 10$ mm/day, RAM = 75% of available moisture.

**Step 1: Available moisture capacity (AMC)**

$$
\begin{aligned}
AMC &= \frac{FC-PWP}{100}\times\frac{\gamma_d}{\gamma_w}\times D_r\\
&= 0.20\times 1.2\times 900 = 216\ \text{mm}
\end{aligned}
$$

**Step 2: Readily available moisture (RAM)**

$$
RAM = 0.75\times 216 = 162\ \text{mm}
$$

**Step 3: Irrigation interval**

$$
I = \frac{RAM}{ET_c} = \frac{162}{10} = 16.2\ \text{days}
$$

**Answer: irrigation interval = 16.2 days, say 16 days.** (Round down so the crop never goes beyond RAM.)

## Check yourself

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

1. With FC 40%, PWP 20%, density 1.2 and root depth 900 mm, what is AMC?
   A. 216 mm
   B. 180 mm
   C. 360 mm
   D. 162 mm
   Answer: A. AMC = 0.20 x 1.2 x 900 = 216 mm. (Key point: Available moisture capacity)

2. What is RAM when it is 75% of AMC = 216 mm?
   A. 54 mm
   B. 162 mm
   C. 216 mm
   D. 288 mm
   Answer: B. RAM = 0.75 x 216 = 162 mm. (Key point: Readily available moisture)

3. Which relation gives the irrigation interval?
   A. AMC multiplied by ET_c
   B. FC divided by ET_c
   C. RAM divided by ET_c
   D. ET_c divided by RAM
   Answer: C. Interval = depth usable (RAM) / daily use = 162/10 = 16.2 days. (Key point: Irrigation interval)

4. Why is 16.2 days rounded down to 16 days?
   A. To save canal water
   B. Because the soil dries faster after 16 days
   C. To make the interval an even number
   D. So the crop never goes beyond RAM into stress
   Answer: D. Irrigating slightly early keeps moisture within the readily available range. (Key point: Rounding the interval)

## Close variants set in other exams

- [Irrigation interval: FC 60%, PWP 25%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc60-pwp25-ram85) — 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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