PriestleyTaylor

The Priestley–Taylor equation is a simplified empirical method for estimating evapotranspiration (ET) from net radiation, developed by H. A. Priestley and R. J. Taylor in 1972. It provides a practical alternative to more data-demanding methods when atmospheric conditions are favorable and the vegetation is well watered.

The equation is ET as a function of net radiation: ET = α × (Δ/(Δ + γ)) × Rn, where Δ is the slope of the saturation vapor pressure curve at air temperature, γ is the psychrometric constant, and Rn is the net radiation reaching the surface. The term Δ/(Δ + γ) represents the atmospheric demand for moisture, while α is an empirical coefficient that scales the result to observed ET. In well-watered crop conditions, α is commonly taken as about 1.26, though it can vary with canopy characteristics and moisture status.

The Priestley–Taylor approach assumes minimal aerodynamic and surface resistance, so it is best applied to surfaces with largely unobstructed exchange with the atmosphere and little water stress. It requires data to compute Δ and γ (typically from temperature, humidity, and pressure) and measurements or estimates of net radiation. Because of its simplifications, the method may be less accurate under drought stress, sparse or tall canopies, or heterogeneous landscapes, where calibration or a more comprehensive method like Penman–Monteith is preferred.

Historically, the Priestley–Taylor model has been adopted in agronomy, hydrology, and remote sensing as a practical ET estimator, especially when data on wind speed and vapor pressure deficit are limited. See also evapotranspiration, Penman–Monteith equation, and net radiation models.