---
title: "Methods of estimating lake evaporation"
subject: "Engineering Hydrology"
chapter: 3
key: "semester_5/engineering_hydrology:lake-evaporation-methods"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/lake-evaporation-methods
site: BCT Engineering
---

# Methods of estimating lake evaporation

Engineering Hydrology · Chapter 3: Hydrological Losses

- Asked in: 2078 Kartik, 6 marks
- Video: [AVAILABLE METHODSPROCEDURES FOR ESTIMATING OPEN WATER EVAPORATION](https://www.youtube.com/watch?v=LGJg2pk-w7Q) — John Michael Alvarez
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/lake-evaporation-methods
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3

## Question

Discuss briefly the methods used to estimate evaporation from a lake.

## Short answer

Methods include evaporation pans with a pan coefficient; empirical formulas (Meyer, Rohwer) using wind and vapor deficit; water and energy budget; mass transfer; and the Penman combination method. Remote sensing is used for large lakes.

## Answer

Evaporation from a lake or reservoir is estimated by the following methods.

1. **Evaporation pans:** Standard pans (US Class A land pan, ISI pan, sunken Colorado pan) are placed near the lake and the fall of water level is measured. Lake evaporation $= C_p \times$ pan evaporation, where the pan coefficient $C_p$ is 0.6-0.8 (0.7 for Class A). Simple and widely used, but the pan coefficient is only an approximation.

2. **Empirical formulae:** Evaporation is related to vapour pressure deficit and wind speed.
   - Meyer's formula: $E_L = K_M (e_w - e_a)\left(1 + \dfrac{u_9}{16}\right)$ (mm/day).
   - Rohwer's formula: $E_L = 0.771(1.465 - 0.000732 P_a)(0.44 + 0.0733 u_0)(e_w - e_a)$.
   They are easy to use but need calibration for the local area.

3. **Water budget method:** Applies continuity to the lake:
   $E_L = P + V_{is} + V_{ig} - V_{os} - V_{og} - \Delta S - T_L$.
   It is simple in principle, but seepage and the other terms are hard to measure, so errors are large for short periods.

4. **Energy budget method:** Applies conservation of energy with Bowen's ratio: $E_L = \dfrac{Q_n - Q_\theta - Q_v}{\rho L(1+\beta)}$. Accurate but needs detailed instruments.

5. **Mass transfer (aerodynamic) method:** Uses theory of turbulent transfer of vapour: $E_L = B(e_w - e_a)$ with $B$ a wind-speed function.

6. **Penman (combination) method:** Combines energy balance and aerodynamic terms: $E = \dfrac{A H_n + E_a \gamma}{A+\gamma}$ with $r = 0.05$ for water. Gives good results from routine weather data.

7. **Analytical/remote sensing methods:** Used for large lakes and where ground data are scarce.

## Check yourself

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

1. Lake evaporation from pan data is:
   A. Cp x pan evaporation, Cp about 0.6-0.8
   B. Pan / Cp
   C. Pan + 0.7
   D. Pan x 1.5
   Answer: A. Lake evaporation = Cp x pan; 0.7 for Class A. (Key point: Pan coefficient)

2. Water budget estimates are poor for short periods because:
   A. Seepage and other terms are hard to measure
   B. It needs radiation
   C. It ignores precipitation
   D. It needs wind
   Answer: A. Errors in seepage and the other terms are large over short periods. (Key point: Water budget)

3. The energy budget method uses what to evaluate sensible heat?
   A. Rohwer's formula
   B. Bowen's ratio
   C. Pan coefficient
   D. Water budget
   Answer: B. Bowen's ratio relates Qh to Qe. (Key point: Energy budget)

4. Penman's method is a:
   A. Combination of energy balance and aerodynamic method
   B. Pure empirical wind formula
   C. Water budget
   D. Pan method
   Answer: A. It combines both and uses r = 0.05 for water. (Key point: Penman)

## Related in Engineering Hydrology

- [Energy budget method of lake evaporation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aenergy-budget-lake-evap) — Engineering Hydrology, chapter 3, asked 2×
- [Daily lake evaporation by Penman, 30 N in April](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Apenman-lake-30n-april) — Engineering Hydrology, chapter 3
- [Lake loss from monthly pan evaporation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Apan-evap-kathmandu-lake) — Engineering Hydrology, chapter 3
- [Reservoir evaporation by Meyer's formula](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ameyer-reservoir-evap-2km2) — Engineering Hydrology, chapter 3

## Related in other subjects

- [Purposes of estimating; estimate never actual cost](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Apurposes-estimate-never-actual) — Estimating and Costing, chapter 1
- [Estimating, purpose and data required](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aestimating-purpose-data-required) — Estimating and Costing, chapter 1, asked 6×
- [Complete estimate: why and when](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Acomplete-estimate-why-when) — Estimating and Costing, chapter 3
- [Approximate estimate: what and when](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aapproximate-estimate-when-needed) — Estimating and Costing, chapter 3, asked 6×