---
title: "Soil-moisture-irrigation relationship"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:soil-moisture-irrigation-relationship"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soil-moisture-irrigation-relationship
site: BCT Engineering
---

# Soil-moisture-irrigation relationship

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2078 Poush, 3 marks; 2072 Asoj, 3 marks (asked 2 times)
- Video: [Soil - Moisture Relationship | Irrigation Engineering | Irrigation depth,Irrigation interval](https://www.youtube.com/watch?v=pNjYV5scQOA) — Sudip Dhakal
- 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/soil-moisture-irrigation-relationship
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Explain the soil-moisture-irrigation relationship.

## Short answer

Soil acts as a water reservoir; irrigation refills to field capacity when moisture falls to the readily available limit. Available moisture = FC – PWP; net irrigation depth = (FC – present moisture)% × (γd/γw) × root depth; interval = depth / ETc.

## Answer

Soil acts as a reservoir for the water used by crops, and irrigation refills it. The relationship is explained by the soil-moisture constants.

```text
 Moisture
 content
  Saturation  ---- gravity water drains ----
  FC          ---- upper limit (irrigate up to here)
                |  available moisture
  RAM limit   ---- (allowable depletion) ----
  PWP         ---- plant wilts; lower limit
  Hygroscopic ---- not available
```

- **Saturation:** all pores are filled. The excess (gravity) water drains in 1-3 days.
- **Field capacity (FC):** the moisture held after free drainage; the upper limit of useful soil moisture.
- **Permanent wilting point (PWP):** the moisture at which plants wilt permanently; the lower limit.
- **Available moisture** = FC - PWP. **Readily available moisture (RAM)** is the portion (usually 50-75%) that the crop can take without stress.

#### Link with irrigation
1. Irrigation is given when moisture falls to the RAM limit, not to PWP.
2. Net depth of water per irrigation = the depth needed to bring the root zone back to FC:
$$
d = \frac{(FC - \theta)}{100}\times \frac{\gamma_d}{\gamma_w}\times D_r
$$
where $\theta$ is the moisture content (%) at the time of irrigation and $D_r$ is the root zone depth.
3. Frequency (interval) of irrigation = $d/ET_c$.
4. Water applied beyond FC is lost as deep percolation and may cause waterlogging; under-irrigation causes stress and yield loss.

Thus the soil moisture characteristics decide how much water to apply and how often.

## Check yourself

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

1. Available moisture equals:
   A. Saturation - FC
   B. PWP - hygroscopic
   C. FC - PWP
   D. FC + PWP
   Answer: C. Between field capacity and wilting point. (Key point: Available moisture)

2. Irrigation is given when moisture falls to:
   A. PWP
   B. Saturation
   C. Hygroscopic
   D. The RAM limit, not PWP
   Answer: D. Avoid crop stress. (Key point: When to irrigate)

3. Over-irrigation beyond FC causes:
   A. Deep percolation and waterlogging
   B. Higher yield
   C. Wilting
   D. Nothing
   Answer: A. Excess is lost. (Key point: Over-irrigation)

4. Irrigation interval is:
   A. d x ETc
   B. d / ETc
   C. ETc / d
   D. d + ETc
   Answer: B. Net depth divided by daily use. (Key point: Frequency)

## Related in Irrigation and Drainage Engineering

- [Frequency of irrigation from soil moisture](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afrequency-of-irrigation-soil-moisture) — Irrigation and Drainage Engineering, chapter 2
- [Soil moisture tension and related terms](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-tension-terms) — Irrigation and Drainage Engineering, chapter 2
- [Forms of soil moisture with diagram](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-diagram) — Irrigation and Drainage Engineering, chapter 2
- [Soil moisture forms; factors affecting CWR](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-cwr-factors) — Irrigation and Drainage Engineering, chapter 2
- [Irrigation interval for clay loam soil](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-clay-loam-fc25) — Irrigation and Drainage Engineering, chapter 2
- [Field capacity, available moisture definitions](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adefine-fc-am-ram-deficiency) — Irrigation and Drainage Engineering, chapter 2

## Related in other subjects

- [Volume-moisture relation; sludge disposal](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avolume-moisture-sludge-disposal) — Sanitary Engineering, chapter 8
- [Volume-moisture relation; specific gravity](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-specific-gravity-95) — Sanitary Engineering, chapter 8
- [Causes of moisture variation in subgrade](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Amoisture-causes) — Transportation Engineering I, chapter 4, asked 13×