Most water loss, not including "Splash Out", from pools is from evaporation. The amount of evaporation depends on the pool water temperature, air temperature, humidity, and the wind speed at the pool surface. The higher the pool temperature and wind speed and the lower the humidity, the greater the evaporation rate. A typical medium-sized pool will lose anywhere from 0.5" to 1.5" per week of water or 0.5 to 5% of the pool volume. This evaporation is also the largest source of energy loss and pool chemicals use. The water loss can be reduced substantially by up to 50% or more by the use of a pool cover as published by US Department of Energy.
With water being a valuable resource, pools are increasingly being looked upon as a luxury and nonessential. As a result, some municipalities are now restricting the filling of pools from local municipality reservoirs. Other municipalities offer rebates up to $200 per pool cover in an effort to reduce water consumption by the pool owner due to water evaporation. Check with your local municipalities for rebates.
Reference: U.S. Department of Energy