Estimate water evaporation by temperature and wind.
Evaporation is driven by the vapour pressure deficit — the gap between how much moisture the air could hold at its temperature and how much it actually holds — and accelerated by wind, which removes the saturated layer sitting above the surface. Saturation vapour pressure comes from the Tetens equation, and multiplying the resulting depth by the surface area converts millimetres into litres, which is what a pool or pond owner actually replaces.
Evaporation from vapour pressure deficit
Saturation kPa = 0.6108 x exp(17.27 T / (T + 237.3)); deficit = saturation x (1 - RH/100); mm per day = (0.35 + 0.061 x wind m/s) x deficit x 7.2
Because wind continuously replaces the humid layer above the water with drier air, keeping the vapour pressure deficit high. Wind often matters more than a few degrees of temperature.
The difference between the moisture the air could hold when saturated and what it currently holds. It is the actual driving force behind evaporation and transpiration.
Typically 3-8 mm a day in warm weather, which on a 32 square metre pool is roughly 100-250 litres daily. Covers cut that by the large majority.