---
title: "Crop coefficient curve for rice"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:crop-coefficient-curve-rice"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/crop-coefficient-curve-rice
site: BCT Engineering
---

# Crop coefficient curve for rice

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2066 Bhadra (old course), 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/crop-coefficient-curve-rice
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Draw the crop coefficient curve for rice crop.

## Short answer

The crop coefficient Kc for rice is about 1.05 in the initial stage (20-30 days), rises linearly to about 1.20 during mid-season (40-50 days), then falls to about 0.90 in the late stage (20-30 days). It is used in ETc = Kc × ETo.

## Answer

The crop coefficient curve shows how $K_c$ ($ET_c/ET_o$) changes over the growing period. For rice (FAO), $K_c$ is about 1.05 at the initial stage, rises to about 1.20 in the mid-season and falls to about 0.9 at the late stage.

```text
 Kc
1.2|              ______________
   |            /               \
1.1|          /                   \
1.0|  ______/                       \
0.9|                                  \__
   +-----+-----------+-------------+----+--> days
    Initial  Crop      Mid-season   Late
    (20-30d) development (40-50d)  (20-30d)
```

- **Initial stage:** low ground cover; $K_c$ about 1.05 (standing water in a nursery/field makes evaporation high).
- **Development stage:** $K_c$ rises linearly as the canopy develops.
- **Mid-season:** $K_c$ at the maximum, about 1.2 (full cover, flowering).
- **Late-season:** $K_c$ falls to about 0.9 (maturity and drying).

The values are typical (FAO Irrigation and Drainage Paper 24/56) and are used in $ET_c=K_c\,ET_o$.

## Check yourself

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

1. When is Kc highest for rice?
   A. Mid-season, about 1.2
   B. Initial stage
   C. Late stage
   D. Nursery only
   Answer: A. Full cover and flowering give maximum ET. (Key point: Peak Kc)

2. Kc is the ratio
   A. ET_o / ET_c
   B. ET_c / ET_o
   C. ET_c x ET_o
   D. P_e / ET_c
   Answer: B. It shifts reference ET to crop ET. (Key point: Kc meaning)

3. In the late season Kc
   A. rises to 1.5
   B. stays 1.2
   C. falls to about 0.9
   D. is zero
   Answer: C. Maturity and drying reduce ET. (Key point: Late stage)

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