Lots of factors to consider, Bruce. If all possibilities of leaks have been ruled out, then evaporation is the likely cause. General evaporation rates will vary from day to day depending on many factors: air temperature, wind, sunny or overcast, total exposed water surface area. In the case of many water features, total aquatic plantings will factor in.
In my area, the average pan evaporation rate per day is .24 inches. This equates to about 0.15 gallons of water loss per square foot or 1 gallon of water loss per 6.67 square feet of water surface area WITHOUT ANY PLANTS.
Considering that plants are in active growth this time of year, their water demand will be high.
My personal pond, which is heavily planted, commonly loses 1" - 2" per day this time of year.
Keep in mind that water loss in a small feature will be more noticeable than in a larger one.
If the rate of loss is fairly consistent, it is most likely a leak; if it varies as ambient weather conditions change, it is evaporation.
"Everybody is ignorant, only on different subjects." - Will Rogers