Pan Evaporation & the Pan Coefficient

Pan evaporation is the most direct field method: you measure how much water disappears from a standardised open pan and scale it to lake-equivalent loss with a pan coefficient.

The equation

The pan coefficient $K_p$ corrects for the fact that a pan is not a lake — it is small, shallow, and heats and cools far faster than a large water body.

Inputs & data needed

You need daily water loss from a standardised pan (the US Class A pan is the common reference) and a pan coefficient suited to the pan’s siting, surroundings and season. Coefficients typically fall around 0.6–0.8, with ~0.7 a frequent default.

Worked example

Suppose a Class A pan loses $E_{\text{pan}} = 8\ \text{mm/day}$ and local guidance gives $K_p = 0.7$:

Over a 10-hectare reservoir surface, that is roughly $5.6\ \text{mm} \times 100{,}000\ \text{m}^2 = 560\ \text{m}^3$ of water lost per day.

Accuracy & when to use

Pans are cheap and intuitive, and excellent for tracking relative change. The weakness is the coefficient: because a pan exchanges heat through its walls and base, $K_p$ varies with wind, fetch, humidity and how the pan is sited (sunken vs. raised, vegetated vs. bare surroundings). For an absolute open-water figure, validate against a physically based method such as aerodynamic mass-transfer or Penman-Monteith, and see the calculation overview for how the methods compare.