firstbest instruments may involve entry costs or cause frictions that make them much less affordable than more ad hoc contractual solutions taking into account preexisting regulatory frameworks that address the core constraints of water quantity and quality the marginal benefit of action may not cover its significant marginal cost in this setting ad hoc smallscale contractual arrangements which have the advantage of addressing a particular city s specific constraints and involve a limited number of actors may have much more appeal than welldefined but complex or challenging institutional responses