I think you are underestimating several very large quantities. Surface area of the earth covered in water, volume of the atmosphere, the amount of water in the atmosphere at any given time, and the sheer amount of energy striking the surface of the Earth every second. These are big, huge, human-brain-defying concepts and our minds can only reify numbers on paper to a certain degree.

The Earth is, generally speaking, BIG. Precipitation may feel like a purely local phenomenon, because we generally perceive it ourselves in a very limited area around us, but I think you’re underestimating the interconnectedness of the water cycle and globe-spanning air/ocean currents.

The oceans are likewise big, like break-your-brain-massive. Forget volume for a moment; the surface area alone is a mind-boggling 70% of the globe. The surfaces of Earth’s oceans are our main source of evaporation for the water cycle, and preventing your local canal from evaporating is not going to make a dent. By comparison, the surface of the ocean is also our main source of oxygen, but you don’t seem too worried about that.

We have plenty of water. The H2O molecule is not in short supply. The problem with local water sources is never the amount of water on the planet, it is having usable/drinkable freshwater nearby. Preventing your reservoir from evaporating quite as fast is not going to mean the local farmers won’t get rain.

TLDR: the reason storms roll in is because the water in those clouds is from somewhere else.