Evaporation is almost exclusively a daytime process. (E.T. Linacre, Agric. & Forestry Metoer, ​64:237-256).

The more exposed surface area, the faster water evaporates.
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So what does this all mean?

A 12ft x 20ft pond in the shade could lose from 60-350 gallons of water each month The same pond in full sun could lose from 500-1,000 gallons each month in the hot summer time. If there is wind multiply theses numbers by a factor of 4. So on a hot, windy July day you could see your pond drop over an inch each day and for the month you could replace the entire pond with fresh water.

To reduce the surface area exposed to wind and sun (thus reducing evaporation) use water lilies, water hyacinth, water lettuce or other surface growing plants. This will also help cut down on the string algae as well.
Use plants along the water's edge to cut down on reflectivity. 
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If you can, build your pond where it gets 6-8 hours of morning sun compared to afternoon or evening sun. The plants will wilt less and the pond won't get as warm or evaporate as fast.

POND CONSTRUCTION

Much has been said comparing evaporation of ponds to watering grass -- everything from ponds use less water to ponds use much more water than watering grass. After talking with the state extension service and those in the know, here is what we have discovered:

Water evaporates less from grass as compared to a pond with no plants when you compare an event, however we water our grass too long and too frequently; so we use more water than a pond loses to evaporation. Plants that cover 70% of the pond surface dramatically reduce evaporation.
 
Evaporation increases significantly in July and August and drops significantly November through March because of temperature, however, the winter months are less humid than the summer so there is still evaporation in the colder months of the year. 

Wind plays a major role in evaporation. A study in Gainesville Florida looked at evaporation rates of sprinklers and evaporation climbed from 2.1% at 0 mph to 8% at 15 mph.

In our arid state, the humidity is so low that there is a lot of evaporation until it reaches 74% in the air above the pond water surface, then it slows (that is commonly after the sun goes down).

In a study by Dale and Scheeringa (Agricultural Meteorology 18:463-474) it was shown that dryness of the ground surrounding water increases evaporation rates. Dry bare ground has higher reflectivity than a pond that has plants around it (rocks reflect even more heat than bare soil).