As a residential water consumer on US city water, I am used to thinking about the marginal environmental cost (particularly energy cost) of my additional water use, based on considerations like allocating the energy cost of pumping and treatment proportionally to the users throughout the system.
If instead I wanted to think about the long-run cost/savings from increasing/decreasing my water use, what would I want to change in my thought process? These things probably are not going to be quantifiable, but I just want to "learn how to think," as they say.
Here are three hypothetical examples to motivate the distinction:
If my city was currently close to full utilization of our water treatment capacity, but if my city was also projected to quintuple in size by 2050: Then for the "long-run" evaluation I would want to account for the cost of new plants.
If I live in an area with abundant water, but a water-intensive process is discovered for creating zero-carbon sustainable biofuels. Then it might be socially desirable for some of my area's water to be used for the biofuels, and thus the "long-run" cost of my water use to society/ecosystem now needs to include the foregone biofuel production.
If my city was currently close to full utilization of our water treatment capacity, but that includes some cooling use by a nearby power plant. Maybe the plant has an alternative zero-water-use cooling method available, but hasn't yet adopted it because of fixed costs. Then if my (and neighbors') increased water use pushed that power plant to adopt the new cooling method, then the cost of my additional water use might be close to zero or even negative (maybe). On the other hand, maybe the plant has another another water source that it could switch to, but the hydrology dictated that the plant would end up using more energy or water with the new water source (I don't know why, maybe the new water source is warmer or needs to be filtered more or something). Then the cost of my water use would be higher would be higher.