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.

Rain clouds are born in the ocean, not over the land.

Rain in California originates in the Pacific Ocean. Which is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the shops, but that's just peanuts to space the ocean.

Irrigation channels are tiny compared to the ocean. Those channels barely affect the local microclimate near the the channel; they don't significantly impacting soil moisture more than a few steps the the source and certainly not via evaporation.

The water loss to evaporation is an economic problem. California loses about 65MM gallons of irrigation water a year to evaporation. As a result, they need to build extra dams (or desalination plants) to produce an extra 63MM gallons of fresh water, just to compensate for water that never reaches the destination.

They could build big pipes to prevent losses, but those are expensive too.

Canals and reservoirs are artificial; their use increases water loss in dry places like California over what it would be naturally. As others have noted, reducing evaporation from them would not significantly affect precipitation elsewhere, but it it would make a big difference for the local fresh water supply.

Water loss from a canal is negligible compared to the amount of water in the air.

What's interesting is that the way we build canals and other similar structures that allow natural runoff to pool on their uphill side instead of continuing downhill has led to large vegetation growth:

https://m.youtube.com/watch?v=jf8usAesJvo

The water percolating and increase in transpiration from the plants in these areas has way more effect on the local climate than the evaporation directly from the canal ever would.

[removed]

[removed]

[removed]

It might have a small effect on the local humidity but in general reservoirs and canals are not big enough to have any measurable effect on the climate, even locally. So it might have some very small effect by increasing dryness in plants or soil on the shore but the amount of evaporation is so small it wasn’t really adding to rain in any meaningful way.

Even the Great Lakes, large as they are only have effects in small, downwind regions, and the weather returns to what is typical for surrounding areas before you get far. And they are hundreds of times larger than even large reservoirs. Only the oceans are big enough to affect weather globally.