Priestley-Taylor: The Radiation-Driven Estimate

Priestley-Taylor is Penman-Monteith with the explicit aerodynamic term removed and replaced by a single empirical coefficient. It works well where radiation, not wind, drives evaporation — calm, well-watered surfaces such as open lakes.

The equation

The available energy $R_n - G$ is scaled by the dimensionless ratio $\Delta/(\Delta+\gamma)$ — the share of energy that goes to evaporation under equilibrium — and by the coefficient $\alpha \approx 1.26$, which restores the aerodynamic contribution the equation otherwise omits.

Inputs & data needed

Net radiation, the stored-heat change (negligible for shallow ponds), and air temperature to evaluate $\Delta$ and $\gamma$. No wind or humidity input is needed, which is its appeal over the full combination equation.

Worked example

At $20\,^\circ\text{C}$, $\Delta \approx 0.145\ \text{kPa/}^\circ\text{C}$ and $\gamma \approx 0.066\ \text{kPa/}^\circ\text{C}$, so:

With available energy $R_n - G = 12\ \text{MJ/m}^2/\text{day}$ and $\alpha = 1.26$:

Dividing by $\lambda \approx 2.45\ \text{MJ/kg}$ gives $E \approx 4.2\ \text{mm/day}$.

Accuracy & when to use

On calm, radiation-dominated open water, Priestley-Taylor is accurate and economical. It under-predicts when wind and advection are strong — in those conditions move to aerodynamic mass-transfer or the full Penman-Monteith equation. See the overview for the trade-offs.