The discipline of hydroecology integrates biological and hydrological dynamics for the purpose of understanding and projecting watershed processes. Fire is a disturbance that both shapes and responds to ecosystem dynamics and impacts the provisioning of ecosystem services, yet few hydroecological models integrate dynamically with fire. This chapter presents the preliminary development and uncertainty assessment of a stochastic model of fire spread, with falsifiable components that make it possible to characterize uncertainty in model structure and parameter estimates. It compares simulated fire to the spatial pattern of observed spread for a wildfire in Washington state (USA) and discovers some issues in parameter identifiability. Sources of uncertainty in ecosystem models fall into three broad categories: data uncertainty, model-structure uncertainty, and natural variability including stochastic events. The regional hydroecologic simulation system (RHESSys) is a GIS-based hydroecological model of water-carbon dynamics including water, carbon, and nutrient cycling.