Fig. 1. Water systems are a distinct class of coupled systems because they present a finite set of recurrent management challenges: disturbances of too much, too little, or wrong-quality water. These challenges can manifest alone or in combination at different temporal and spatial scales and have differential impacts within and across social groups. Thus, water systems provide us with a long historical record of institutional innovation in response to management challenges at increasingly large scales of social organization, from small farming settlements to nation states.