  There has been little exploration of the explicit simulation of the set of
options of actors in agent-based models and its evolution over time. This study
proposes to use affordances as intermediate entities between agents'
environment and agent actions. We illustrated the approach on a typical
gravity-fed network in the South-East of France to explore how the abandonment
of traditional sharing of water changes the irrigators' options to irrigate. We
simulated a typical dry year irrigation season under two institutional
arrangements (i.e. traditional coordination through daily slots and its
abandonment). Simulation results are consistent with field surveys, and reveal
an increase in the number of internal conflicts among irrigators as the
counterpart of the abandonment of traditional sharing of water. They also
highlight the consequences of the heterogeneity of the irrigators' interests
within the collective institution. The sensitivity analysis of the model
allowed identification of optimal modalities of coordination, and a potential
compromise between past and current institutional arrangements. The key
benefits of using affordances in ABM lie in the study of their population
dynamics for characterizing the interaction situations between actors and their
environment and for better understanding the model dynamics.
