Estimating pool evaporation

The Penman Evaporation Model

The Penman equation (1948) combines energy balance and aerodynamic transport approaches to estimate evaporation from open water surfaces. For pool applications, we use the modified form: E = (Δ·Rn + γ·f(u)·(es − ea)) / (Δ + γ) Where Δ is the slope of the saturation vapour pressure curve, Rn is net radiation, γ is the psychrometric constant, f(u) is the wind function, and (es - ea) is the vapour pressure deficit, which is the difference that drives evaporation.

• Saturation vapour pressure calculated from the Magnus-Tetens formula
• Wind function calibrated for small water bodies (pool-scale fetch)
• Net radiation accounts for shortwave gain and longwave emission
• Psychrometric constant adjusted for Melbourne's mean atmospheric pressure

Heated Pool Correction

Pools maintained above ambient temperature evaporate significantly faster than natural water bodies. The water surface temperature elevates the saturation vapour pressure exponentially (Clausius-Clapeyron relationship), dramatically increasing the vapour pressure deficit. A pool at 28°C in 15°C ambient air can evaporate 3 to 5× faster than an unheated pool at equilibrium temperature. Our model uses the actual maintained water temperature rather than assuming thermal equilibrium with the environment.

Wind & Microclimate Effects

Wind speed is the primary aerodynamic driver of evaporation. Moving air removes the saturated boundary layer above the water surface, maintaining the vapour pressure gradient that drives mass transfer. Our model accounts for the sheltering effect of fences, buildings, and vegetation that reduce effective wind speed at pool level. Users specify their exposure category, and the model applies appropriate wind reduction factors based on empirical shelter coefficients.

Pool Cover Suppression

Pool covers suppress evaporation by physically blocking vapour transport from the water surface. Different cover types provide varying levels of suppression: • Solid covers: 95 to 99% evaporation reduction • Bubble/solar covers: 85 to 95% reduction (small air gaps at edges) • Liquid solar blankets: 30 to 50% reduction (molecular monolayer disrupted by wind) • Automated slatted covers: 90 to 95% reduction when deployed The model estimates how much water a cover may save over a year.

Seasonal Melbourne Profile

Melbourne's climate produces strongly seasonal evaporation patterns. Summer combines high temperatures, low humidity, and extended daylight hours to produce peak evaporation rates (8 to 12 mm/day for heated pools). Winter rates drop to 2 to 4 mm/day due to reduced vapour pressure deficit and shorter days. Our model uses Bureau of Meteorology climate normals for Melbourne to generate month-by-month predictions without requiring users to input complex meteorological data.