Abstract. There is a growing recognition among water resources managers that sustainable watershed management needs to not only account for the diverse ways humans benefit from the environment, but also incorporate the impact of human actions on the natural system. Coupled natural-human system modeling through explicit modeling of both natural and human behavior can help reveal the reciprocal interactions and coevolution of the natural and human systems. This study develops a spatially scalable, generalized modeling framework consisting of a process-based distributed hydrologic model (SWAT) and a decentralized water systems model (ABM) to simulate the impacts of water resources management decisions that affect the food-water-energy-environment (FWEE) nexus at a watershed scale. Agents within a river basin are geographically delineated based on both political and watershed boundaries and represent key stakeholders of ecosystem services. Agents decide about the priority across three primary water uses: food production, hydropower generation and ecosystem health within their geographical domains. Agents interact with the environment (streamflow) through the SWAT model and interact with other agents through a parameter representing willingness to cooperate. The innovative two-way coupling between the water systems model and SWAT enables this framework to fully explore the feedback of human decisions on the environmental dynamics and vice versa. This generalized ABM framework is tested in two key transboundary river basins, the Mekong River Basin in Southeast Asia and the Niger River Basin in West Africa, where water uses for ecosystem health compete with growing human demands on food and energy resources. We present modeling results for crop production, energy generation and violation of eco-hydrological indicators at both the agent and basin-wide levels to shed light on holistic FWEE management policies in these two basins.

Review of ABM applications Table S1: Selected relevant existing studies of ABM applications in surface water management and ecological modeling Topics Literature Agent types Agents' decisions Agents' goal Hierarchical agents Direct agent interaction Link with other models Watershed management-Surface water Barreteau et al. (2004) farmers farm activities simulate cultivating activities Yes Yes No Berger et al. (2007) water users bid on land/water resources max incomes No Yes Yeshydrological model Brady et al. (2012) farmers farm activities max incomes No Yes No Farolfi et al. (2010) farms/villages water demand simulate water demand No No No Giacomoni et al. (2013) consumers/policy makers water use/water restrict simulate water demand Yes Yes Yes-SWAT Giuliani and Castelletti (2013) reservoirs/delta reservoir operation max HP/delta preservation No Yes No Giuliani et al. (2015) farm/city water use max profit No No No