---
title: "Types of evaporimeters with sketches"
subject: "Engineering Hydrology"
chapter: 3
key: "semester_5/engineering_hydrology:evaporimeter-types"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/evaporimeter-types
site: BCT Engineering
---

# Types of evaporimeters with sketches

Engineering Hydrology · Chapter 3: Hydrological Losses

- Asked in: 2082 Baisakh, 6 marks
- Video: [Pan Evaporation Method | Hydrology](https://www.youtube.com/watch?v=rj8lwGFd_gs) — Civil Engineer
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/evaporimeter-types
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3

## Question

Briefly describe the types of evaporimeters with neat sketches.

## Short answer

Common evaporimeters are the Class A land pan (120.7 cm diameter, 25 cm deep, coefficient about 0.70), the ISI pan (122 cm, 25.5 cm, coefficient 0.80), the Colorado sunken pan (92 cm square, coefficient 0.78) and the floating pan (90 cm square, coefficient 0.80). Lake evaporation is found by multiplying pan evaporation by the pan coefficient.

## Answer

An **evaporimeter (evaporation pan)** is an open container used to measure the evaporation from a free water surface. The depth of water lost per day gives the pan evaporation, which is converted to lake evaporation by a pan coefficient:

$$
E_{lake} = C_p \times E_{pan}
$$

#### 1. Class A Land Pan (US Weather Bureau)
- Circular, 120.7 cm diameter and 25 cm deep, made of unpainted galvanised iron.
- Placed on a wooden platform 15 cm above ground; water is kept 5 cm below the rim.
- Evaporation is found from the daily change in water level (hook gauge in a stilling well).
- $C_p = 0.70$ (range 0.60 to 0.80).

```text
      ____________  <- water 5 cm below rim
     |            |
     |   water    |  25 cm
     |____________|
   ====wooden frame====  15 cm
   ///////////////////  ground
          120.7 cm dia
```

#### 2. ISI (Bureau of Indian Standards) Pan
- A modified Class A pan: cylindrical, 122 cm diameter and 25.5 cm deep, made of 0.9 mm copper sheet, tinned inside and painted white outside.
- A wire-mesh cover is fitted to reduce interference by birds and animals.
- $C_p = 0.80$.

#### 3. Colorado Sunken Pan
- Square, 92 cm side and 46 cm deep, buried in the ground with the rim 10 cm above ground.
- The water temperature is close to that of the soil; $C_p = 0.78$.

#### 4. Floating Pan
- 90 cm square and 45 cm deep, floated on a raft in a lake; $C_p = 0.80$.
- It gives the best simulation of lake conditions, but is costly and hard to observe.

Pans overestimate evaporation because of the heat exchange through the pan walls, hence the coefficient.

## Check yourself

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

1. How is lake evaporation obtained from pan evaporation?
   A. E_lake = C_p x E_pan
   B. E_lake = E_pan + C_p
   C. E_lake = E_pan / C_p^2
   D. E_lake = E_pan - C_p
   Answer: A. Pan readings are multiplied by a pan coefficient C_p to give lake evaporation. (Key point: Pan coefficient)

2. Which description fits the Class A Land Pan?
   A. Cylindrical, 122 cm diameter, 61 cm deep, copper
   B. Floating on a raft, 90 cm square
   C. Square, 92 cm side, buried in ground, C_p = 0.78
   D. Circular, 120.7 cm diameter, 25 cm deep, raised 15 cm on a wooden platform, C_p about 0.70
   Answer: D. Class A is a 120.7 cm diameter, 25 cm deep galvanised iron pan on a wooden frame with C_p = 0.70. (Key point: Class A pan)

3. What is the main advantage and drawback of the floating pan?
   A. Easy to observe but affected by soil heat
   B. Cheap but inaccurate
   C. Best simulates lake conditions but is costly and hard to observe
   D. Needs no coefficient and is easy to install
   Answer: C. The floating pan on a raft gives the best lake simulation but is costly and difficult to observe. (Key point: Floating pan)

4. Why is a pan coefficient needed?
   A. Pans underestimate because of rain
   B. Lakes have no wind
   C. Pans overestimate evaporation due to heat exchange through the walls
   D. Pans are always buried
   Answer: C. Heat exchange through the pan walls makes pans overestimate lake evaporation, hence C_p < 1. (Key point: Why a coefficient is needed)