A swimming pool was created as a static water retention apparatus combined with a closed look pipe and filtration system. In theory, that means that as soon as you put water in it, it must remain there. Actually, there are more factors at work here than that.
Evaporation alone will account for at least some water loss, especially if you’re swimming pool products is subject to heavy crosswinds and you don't employ the use of some sort of cover system. This report serves to inform the means above and beyond evaporation, which all of us know and anticipate to a degree.
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Before going past evaporation though it is simple to gauge the water loss differential between evaporation and water loss through any other way efficiently. With a bucket or watertight and translucent container, then sit container onto a measure or improvised secure platform so you can fill the bucket into the exact same water level as the water that surrounds it.
Use pool water from the bucket to match the temperature and chemical content of the water for optimum outcomes. Also, avoid small opening or mouth containers which won't experience the identical crosswind effects your pool may encounter.
Simply track the water level as it evaporates from the pool in addition to the bucket. Any discrepancy between the two water levels is what you may consider being your outside water reduction.
Now, moving on to water reduction over and beyond evaporation you want to bear in mind that all swimming pools leak a bit. The vast majority of swimming pools I'm called out to see have escapes additional to that of evaporation.