The calculation below explains how much water loss to expect from evaporation:
1/2% (Shaded, Little Wind) - 1% (Full Sun & Wind) of the Gallons Per Hour (GPH) being pumped / Day
Evaporation can easily cause 1’’ or more of water level drop per week! Here’s an example:
3,000 GPH Pump after head loss has approx. 2,500 GPH of actual output
2,500 x .005 = 12.5 gallons on the low end
2,500 x .01 = 25 gallons upper end
*This is just the average evaporation rate. The rate will fluctuate throughout a season based on relative humidity, temperature, wind direction and velocity, sunniness, etc.
To turn these numbers into an actual level of water loss on a pond you need to calculate the total square footage of the pond or pondless waterfall.
11’x16’ pond = 176 sq. ft.
But it’s not a rectangle and it has a rock perimeter, to compensate for this multiply by .8 or .75 for irregular shapes
176 x .8 = 140 sq. ft.
140 x .62(gal/sq. ft. x 1” deep) = 86.8 gallons for the top 1” of water
Back to the evaporation numbers; let's use 1/2% saying it’s shaded and protected from wind
86.8 divided by 12.5 = 6.9 days before the pond loses 1” of water
If it’s full sun and wind - 1% evaporation:
86.8 divided by 25 = 3.5 days for 1” of water loss