Modeling the Water–Food Composite Index in a Brazilian Peri-Urban Watershed: An Agent-Based Simulation Under Climate Change Scenarios

The accelerating expansion of Brazilian urban areas intensifies competition for water, energy, and land in peri-urban zones where smallholder agriculture, residential development, and distributed solar generation coexist. Brazil’s resource governance remains institutionally fragmented with no formal nexus-level coordination. The water–energy–food nexus is gaining interest but there are few agent-based models regarding the water–food implications and they are practically absent for Latin American peri-urban contexts. This study develops an ABM of WF interactions in the Alto Tietê basin, integrating open data from ANA, ANEEL, MapBiomas, and IPCC AR6 climate scenarios (SSP2-4.5 and SSP5-8.5). After implementing photovoltaic pumping, heterogeneous farmers become rural solar prosumers within municipality-level land-use, water-allocation, and climate settings. We contrast four governance regimes from a fragmented rule—emergency restrictions triggered once stress exceeds a threshold—to coordinated rules in which allocation responds proportionally to stress and solar eligibility is conditioned on stress. Technical efficiency is treated as a separate factor applied identically across the regimes, so they differ only in the allocation rule, not in the efficiency they are granted. The model reproduces the broad land-cover trajectory across 1985–2024, but underrepresents late period urbanization (simulated 21.8% vs. observed 28.6% urban area in 2024, 6.8-point residual). Three interacting dynamics emerge from the model: (i) a solar-pumping rebound; (ii) a land-use displacement cascade; and (iii) a governance-conditional trade-off. Under climate stress the regimes diverge at the pillar level, not in the aggregate: fragmented governance preserves the water pillar by activating restrictions that reduce delivered irrigation and depress the food pillar, while integrated governance redistributes it, holding the food pillar at the expense of the water pillar. Because the sign of the composite contrast depends on the pillar weighting, the substantive result is this pillar-level trade-off rather than any single weighted index; integrated governance also reduces between-replicate variance several-fold, acting as a source of predictability under climate stress. A Morris screening indicates that the composite is driven by demand-side efficiency and its reabsorption when the efficiency parameters are allowed to vary, rather than by the coordination parameters.