Evaporation is technically one form of "leak." Rates of evaporation depend on several factors. I live in central Florida where my two waterfall-connected 600 gal ponds can develop temperatures in the high 80s (F), which accelerates evaporation to the point where I can lose about an inch of water a day in my 28" deep koi ponds. Relative humidity is another factor. Evaporation is accelerated by low humidity (not a problem here). Surface area is another factor. Smaller, deeper ponds allow less evaporation than wider, shallower ponds. Atomization of water, such as occurs with a waterfall feature, spitter or fountain, creates increased surface area at the interface between water droplets and air and accelerates evaporation. Low barometric pressure increases evaporation rates. Finally, as one reader suggested, wind is a significant contributing factor to evaporation loss.
Ways to retard evaporation include providing pond-side shade plants to keep the sun off the water surface, or utilizing space-occupying semi-submerged plants right in the pond, such as water lilies or water lettuce. Landscaping with dense hedges pond-side can help reduce wind across the pond surface as well.
You can determine how much your waterfall contributes to evaporation by turning off the waterfall for a day and comparing water loss rates. Waterfalls can also lose water in varying amounts at the edges of the feature where splash can go right out of the pond. As you implied, running a waterfall 24/7 can make even a small amount of splash cause a measurable drop in water level over time.
Depending upon the season, I can pretty much predict my evaporation rates. Right now, with the "cool" season upon us, rainfall has been just about offsetting evaporation loss. I don't have access to an irrigation well and for a year had to rely upon city tap water to refill the ponds, and therefore had to pay close attention to chloramine levels so as not to kill my fish (scary to think that the water we're expected to drink will kill fish). So I invested in a rainwater collection system and so far have six connected 55-gallon barrels of additive-free water to top off the ponds.
Good luck with your pond! It looks great.