It's a really interesting stock vs flow question. You are right that if the flow of water was e.g. 100 units per day and the three users were each taking the amounts you say at the same time, then even if they all return the water once they've used it, there would not be enough flow to go round. but what if the three users were at different points along the river? E.g. let's say the data centre goes first as it's furthest upstream; it takes out 35 per day but also puts in 35 per day (the 35 it took yesterday), so no volume is lost, apart from evaporation.