The five main factors which contribute to outdoor pool water evaporation are,
1. Pool surface area
2. The temperature of the water
3. The temperature of the air
Seeing as Air and Water temperatures are two main factors contributing to pool evaporation, water evaporation at night is far less than during the day due to the reduced difference between the pool surface temperature and night air temperature, i.e. less Temperature Gradient (Thermal Diffusion) at night, less water evaporation
If the pump is off during the day the surface temperature of the static pool water will increase due to solar heating which will in turn also reduce the temperature gradient, hence less evaporation
If the pump runs during the day it mixes the warm surface pool water with colder pool water thereby maintaining a higher temperature gradient which will result in a higher water evaporation rate.
For those interested in the mathematics and theory of Thermal Diffusion (evaporation) see link below,http://www.engineeringtoolbox.com/evapo ... d_690.html
A substantial amount of water can also be saved if the pool filter is NOT back-washed frequently, pool filters are also more efficient if they are allowed to accumulate some dirt. The most precise method of determining whether the pool filter requires a back-wash is via a pressure indicator which indicates the pressure drop between filter inlet and outlet pressure.
Link for clarification on "Filtration" and "Purification",http://en.wikipedia.org/wiki/Water_purification
A Sand pool filter will only filter out particles down to a certain diameter, typically, 50 microns when the sand is clean and 20-25 microns when the filters sand is dirty. Bacteria (0.3 microns), viruses (0.005 microns) and fungus are much smaller than 20-25 microns hence why chemicals, principally chlorine, are then required to purify the water further, especially if the folks who use the pool pee in the pool.
Link for reference to Pool filter information,http://www.backyardcitypools.com/faqs/f ... lter-5.htm
Link for reference to particle diameters,http://www.engineeringtoolbox.com/parti ... d_934.html