The Energy-Budget Method for Evaporation

The energy-budget method evaporation by conservation of energy: the energy available at the water surface is net radiation, and it must go somewhere — heating the air, heating the water, or evaporating water. Whatever is left after the first two is the evaporation term.

The equation

Here $\lambda E$ is the latent-heat flux (the energy consumed by evaporation), $R_n$ is net radiation, $H$ is the sensible-heat flux that warms the air, and $G$ is the change in heat stored in the water. Dividing $\lambda E$ by the latent heat of vaporisation $\lambda$ converts energy to a depth of water.

Inputs & data needed

Net radiation, a way to estimate sensible heat (often via the Bowen ratio), and the change in stored heat — which on a deep reservoir requires repeated water-temperature profiles. The stored-heat term is small for shallow ponds but dominant seasonally for deep lakes.

Worked example

Take a day with $R_n = 15$, $H = 3$ and $G = 2\ \text{MJ/m}^2/\text{day}$:

With $\lambda \approx 2.45\ \text{MJ/kg}$ (and water density $1000\ \text{kg/m}^3$, so $1\ \text{mm} \approx 2.45\ \text{MJ/m}^2$):

Accuracy & when to use

When every term is measured well, the energy budget is among the most rigorous methods — it is often used to calibrate simpler ones. Its weakness is data hunger, especially the stored-heat term $G$ on deep water. If you lack radiation data, fall back to aerodynamic mass-transfer or Hargreaves-Samani; for a combined approach, see Penman-Monteith.