Ayo, hope you are having a good day :))
Its kind of basic, we know that swimming pools are exposed to sunlight, and gets heat. Thus, the water will get evaporated with time. And same is the case with Sarah's swimming pool. Thus the answer becomes :
☆ Evaporation is the reason why Sarah's father has to add water to the swimming pool every month. The water in the pool gets evaporated due to the exposure to sunlight and hence, the amount of water gets decreased. Thus, to maintain the same Sarah's father has to do the same.