---
title: "Irrigation interval for clay loam soil"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:interval-clay-loam-fc25"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-clay-loam-fc25
site: BCT Engineering
---

# Irrigation interval for clay loam soil

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2068 Chaitra (old course), 6 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-clay-loam-fc25
- 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 soil (clay loam of 1.5 g/cc dry density, Field capacity = 25% and PWP = 13%) in order to ensure efficient irrigation of the crop for which daily consumption use of water is 10 mm. Take 75 cm of root zone depth.

## Short answer

Available moisture = (25–13)/100 × 1.5 × 750 = 135 mm. Interval = 135 / 10 = 13.5 days. Irrigate every 13 days.

## Answer

**Given:** FC = 25%, PWP = 13%, $\gamma_d = 1.5$ g/cc, root zone depth $D_r = 75$ cm $= 750$ mm, $ET_c = 10$ mm/day. 

**Step 1: Available moisture depth**

$$
\begin{aligned}
d &= \frac{FC-PWP}{100}\times\frac{\gamma_d}{\gamma_w}\times D_r\\
&= \frac{25-13}{100}\times 1.5\times 750 = 135.00\ \text{mm}
\end{aligned}
$$

**Step 2: Irrigation interval**

$$
I = \frac{d}{ET_c} = \frac{135.00}{10} = 13.50\ \text{days}
$$

**Answer: irrigate after about 13.5 days, i.e. every 13 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. 270 mm
   B. 90 mm
   C. 225 mm
   D. 135 mm
   Answer: D. 0.12 x 1.5 x 750. (Key point: Available moisture)

2. Interval = 
   A. 13.5 days
   B. 27 days
   C. 6.75 days
   D. 12 days
   Answer: A. 135/10. (Key point: Interval)

3. Practical interval is
   A. 14 days
   B. 13 days
   C. 15 days
   D. 12 days
   Answer: B. Round down. (Key point: Rounding)

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