When outside cold, the pool owner must use the pool cover to keep the debris fall into the pool. This cover also keeps leaves and other debris fall into the water in the months you cannot use the pool too. With that thought, you need to find a cover that will be able to meet the stretch of your pool and survive the elements through cooler months.
There are various types of pool cover for underground pools or on land. You know this type of pool because it's quite clear. The pool is underground or built, while the above land is set. One that is a pond is permanently installed, while what may not be lowered at any time. You can buy indoor outdoor retractable pool enclosures from various web sources.
It should be noted that if you have a ground pool above then you have two options. You can leave the pool after the cold outside and put the cover on it, or you can lower it depending on the type of pool.
If it is one that doesn't have anything holding it (so the sides are inflatable), then you cannot record it for winter. Each ground pool must have a pool cover for winter. In-ground pools tend to be bigger than the land, and you have to take care of care for years. It's much easier to put the cover on them to forget it for winter.
Using a pool cover can make you not have to clean the bottom of the pool before using it again. This is especially true if you store water in the pool during the winter months, which can be very dirty especially if found.