Aerodynamic Mass-Transfer (Dalton) Method

The aerodynamic mass-transfer method is a Dalton-type law: evaporation is proportional to the vapour-pressure deficit between the water surface and the air, multiplied by a wind function. It is the form behind the illustrative estimator on this site.

The equation

$e_s$ is the saturation vapour pressure at the water-surface temperature, $e_a$ is the actual vapour pressure of the air, and $f(u)$ is an empirical wind function — larger wind, larger $f(u)$, faster evaporation. The classic calibration comes from the Lake Hefner studies (Harbeck, 1962).

Inputs & data needed

Wind speed at a known height, air temperature and relative humidity (to get $e_a$), and the water-surface temperature (to get $e_s$). No radiation data is required, which is why it suits wind-driven sites where radiation instruments are absent.

Worked example (illustrative)

Using the simplified wind function $f(u) = 1 + 0.45\,u$ from this site’s estimator, with water/air near $25\,^\circ\text{C}$ so $e_s \approx 3.17\ \text{kPa}$, relative humidity $40\%$ so $e_a = 0.40 \times 3.17 = 1.27\ \text{kPa}$, and wind $u = 3\ \text{m/s}$:

The coefficient here is nominal — real wind functions are site-calibrated against observed loss, so treat this as illustrative.

Accuracy & when to use

Mass-transfer is robust on windy, well-mixed ponds and the natural choice for industrial reservoirs. Its accuracy hinges on the wind function: a generic $f(u)$ can be off by tens of percent until calibrated. Where radiation dominates and wind is light, prefer Priestley-Taylor; for a combined treatment, see Penman-Monteith.