Identify the ideal water level for your pool. Generally, the most efficient water level is one-third to halfway up the opening of the filter skimmer. If the water is too low, the filter will not work properly. If it is too high, the filter will be sluggish. You can probably estimate the proper level, but ask a pool cleaner if you need help.
Run a hose from an outdoor spigot into the pool when the water level drops. Fill it until the desired level is reached, and then remove the hose.
Refill the pool as necessary throughout the season to keep the water at its ideal level.
Check the chlorine and pH levels of the pool after filling it to make sure it is still properly balanced.