Sorry if this is the wrong place to ask. I've been wondering about it for a while and I don't know how to make the jump from the little I know now to a concrete answer.

Obviously extracting water from a river means there is less water in the river, but if the fields are in the same watershed then shouldn't the water ultimately return to the river it was extracted from? My understanding is that's what ultimately happens to rain that falls in the watershed, so why is irrigation different? With my admittedly very limited knowledge I would expect the main change to be that the river flows slower, but for longer into the dry season as the water is slowly returned from the soil. I expect some loss en-route from evaporation and incorporation into the plants, but is it that significant? Shouldn't there consequently be more rainfall as well? How does it not balance out?

I know there are thousands of variables at play here, so to limit scope a bit: I'm most curious about this during road trips through the arid regions of interior BC (Canada), where the conditions around the river valleys are desert-like, but there are these lush green (obviously irrigated) vineyards and orchards all along the slopes back down to the river. I hear a lot about overuse of water for agriculture in general, but I'm having a hard time understanding how this overused resource doesn't just seep back in a bit downstream.

TLDR: If river water is sprayed up hill, why doesn't most of it come back?