Fresh and clean water is one of the scarcest and most vital resources to humankind. Agriculture is the largest global water user. Irrigation managers must orchestrate water use at the catchment scale, balancing the management of supply and demand and taking into account the benefits from water use, its distribution among water users and environmental concerns. We present results from the integrated modeling of irrigation water use at catchment scale. An extended hydrological runoff model WaSiM-ETH that depicts inefficient surface irrigation was integrated with and coupled to a parametric model for irrigation water distribution, which is linked to the bio-economic multi-agent model MP-MAS that represents farmers as water users. Models were calibrated empirically, first as standalone models and then with increasing complexity of interactions. The integration of process across such long chain of reasoning resulted in an improved system understanding along disciplinary boundaries. The case study presented is irrigation water use within the Chilean Region of Maule. We analyze how farmers whose endowment with formal water rights is insufficient may depend on spillover water, and specifically the distribution of benefits from improvements in canal conductive efficiency across the farming community.