system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a country downstream from a dam suffer a hit to its water supply? Doesn't the net water flow stay the same?

Take Egypt and Ehtiopia. Wouldn't the water volume coming out of the renaissance dam be the same as the water volume going in and the net flow to Egypt remain constant?
gpt
One big factor is evaporation.  A reservoir generally creates a greater water temperature in the upper layer and a larger surface area, which increases evaporation and reduces downstream water availability.  Also infiltration can be a problem, with water becoming groundwater due to the fact that it's sitting in the reservoir for so long.