Fig. 3. Interrelationships between two social-ecological systems of reference with two different sizes, Z1 and Z2, and their relation to their total system growth limits. In the World Cellular Model computer interface that was applied to the Ebro River basin in Spain, each subsystem is related to an agent with a differentiated capacity to use water resources, process information, generate rules of behavior, and create social-ecological change in a co-evolutionary fashion (Tàbara 2003, Tàbara et al. 2006). S = structure and ruling institutions, E = energy and resources, I = information and knowledge, and C = social-environmental change.