The reduction and depletion of stream flow and lake levels due to groundwater pumping is an underappreciated impact of groundwater pumping with often devastating effects on ecosystems. California’s new groundwater law (the 2014 Sustainable Groundwater Management Act, SGMA) is one of few state and international water laws that explicitly attempt to protect “interconnected surface waters”. We have been developing novel integrated hydrologic modeling approaches to provide decision-support to local, regional, and state regulatory agencies. Developing these decision-support tools has been a two-step process: development of a trusted baseline model capable of reproducing and explaining the observed hydrologic history; and development of future model scenarios to inform decision-making. This provides the basis for the design of appropriate and effective monitoring networks, for prioritizing potential future projects and management actions, and for setting regulatory limits on groundwater-surface water interaction. The modeling development is part of a transdisciplinary collaboratory with a wide diversity of outreach and extension efforts with the affected community’s various actors. Through their interactions with these models, they participate in the design of the model. Community-engagement shapes planning and management design decisions and must be considered a critical element of model building. Clear, open, transparent, consistent, and educational communication with strong integrity is critically important to this process, between scientists and community/actors and between opposed factions of actors. A trusted hydrological model can disassemble some barriers to consensus building. But value-decisions remain as relevant to management design as scientifically based information. Our efforts in this arena are documented here, including publications, reports, research, and available software.