At the EEA, we discussed a good measure of water quality:
"What about nitrate levels in the water? We can measure those."
"Sure, but nitrates don't matter if they are carried into an area where abundant water dilutes it. We care about eutrophication that's caused by too many nutrients in the water feeding algae that take all the dissolved oxygen and kill aquatic life."
In other words, we care about impacts, not inputs (or outcomes, not outputs).
Why is this important? Because a goal of reducing nitrates may lead to the construction of water treatment plants that remove harmless nitrates or inaction on the other contributors to eutrophication.
Bottom Line: It's important to get incentives right, because what gets measured gets managed.