How to calculate evaporation

There is no single formula for open-water evaporation — there is a family of methods, each trading data requirements for accuracy. This page explains the main ones, when to use them, and how confident you can be in the answer.

Mist drifting over a still lake at sunrise with forested hills reflected in the water — Open-water evaporation is driven by temperature, humidity, wind, radiation and surface area — the inputs these methods quantify. Illustrative

1. Pan evaporation

The most direct field method: measure water lost from a standardised open pan and scale it with a pan coefficient $K_p$ (typically ~0.7) to estimate lake-equivalent loss.

E_{\text{lake}} = K_p \times E_{\text{pan}}

Simple and cheap, but pans gain and lose heat faster than a large water body, so coefficients vary with siting and season.

2. Energy-budget method

Closes the surface energy balance: the energy available for evaporation is net radiation minus what heats the air and the water. Evaporation is then the latent-heat term:

\lambda E = R_n - H - G

where $R_n$ is net radiation, $H$ sensible heat, $G$ the change in stored heat, and $lambda$ the latent heat of vaporisation. Accurate but data-hungry.

3. Aerodynamic mass-transfer

A Dalton-type law: evaporation is proportional to the vapour-pressure deficit multiplied by a wind function $f(u)$ .

E = f(u)\,\big(e_s - e_a\big)

Robust for wind-exposed industrial ponds; rooted in the classic Lake Hefner work (Harbeck, 1962). The simplified estimator below uses this form.

4. Penman & Penman-Monteith (FAO-56)

The reference standard (Allen et al., 1998) combines the energy budget and the aerodynamic term into one expression:

ET = \frac{\Delta (R_n - G) + \rho_a c_p \dfrac{(e_s - e_a)}{r_a}}{\Delta + \gamma\left(1 + \dfrac{r_s}{r_a}\right)}

where $Delta$ is the slope of the saturation curve, $gamma$ the psychrometric constant, and $r_a, r_s$ the aerodynamic and surface resistances. For open water the surface resistance term is dropped and an open-water albedo is used; otherwise the vegetated form over-predicts.

5. Priestley-Taylor

A radiation-driven simplification of Penman for conditions where advection is small (calm, well-watered surfaces):

\lambda E = \alpha \, \frac{\Delta}{\Delta + \gamma}\,(R_n - G)

with $alpha approx 1.26$ . Excellent for calm open lakes.

6. Hargreaves-Samani

A temperature-only fallback when humidity, wind and radiation data are unavailable:

ET_0 = 0.0023 \,(T_{\text{mean}} + 17.8)\,(T_{\max} - T_{\min})^{0.5}\, R_a

where $R_a$ is extraterrestrial radiation (from latitude and day of year).

How accurate are these?

Under moderate conditions the methods typically agree with one another, and with observations, to within roughly ±10–20%. Divergence grows in extreme wind, strong advection, or when inputs are poorly measured. Treat any single number as an estimate with a band around it, and validate against pan data or observed drawdown where you can.

Try a quick estimate

The widget below applies the aerodynamic mass-transfer form to give a rough, illustrative number. It is deliberately simplified — use the AWTT calculator for authoritative, site-specific results.

Quick evaporation estimate

A simplified Dalton-type mass-transfer estimate for open water. Indicative only — see the note below and use the AWTT calculator for real numbers.

Illustrative

Evaporation rate

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mm/day

Volume lost

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m³/day

Volume lost

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US gallons/day

How this is calculated & its limits: a Dalton mass-transfer estimate, E = (1 + 0.45·u)·VPD mm/day, assuming water-surface temperature ≈ air temperature and ignoring solar radiation, salinity and altitude. Real methods typically agree to within ±10–20% under moderate conditions; this simplified form can differ more. For authoritative, site-specific results — use the full AWTT calculator ↗.

The on-site estimator above is for intuition only. AWTT's free evaporation calculator is the authoritative tool: it runs five methods — Penman-Monteith (FAO-56), aerodynamic mass-transfer, Priestley-Taylor, Hargreaves-Samani and an empirical mass-transfer model — against real-time weather, then applies per-product reduction factors and reports water savings, ROI and CO₂. We link to it rather than rebuilding it.

Frequently asked questions

Which evaporation method is most accurate?

Penman / Penman-Monteith (FAO-56) is the reference standard when you have full meteorological data. Priestley-Taylor is strong for calm, radiation-dominated lakes; aerodynamic mass-transfer is robust for windy industrial ponds; Hargreaves-Samani is a reasonable temperature-only fallback. Most methods agree within about ±10–20% under moderate conditions.

Can I just use a crop ET equation for my reservoir?

Not directly. Penman-Monteith reference ET is calibrated for a vegetated surface and over-predicts open-water loss unless the open-water albedo and resistance terms are substituted. Use an open-water-adapted form or a method built for water bodies.

Why do estimates differ from my measured drawdown?

Measured drawdown also includes seepage, inflow, outflow and rainfall. Isolate evaporation by accounting for those terms, and remember that any method carries ±10–20% uncertainty before measurement error is added.