In this webcast, Diane Cooper, with the Southern Region Headquarters of NOAA’s National Weather Service, provides a basic scientific description of the physical processes, mathematical equations, and data issues with respect to distributed hydrologic models. Ms. Cooper first explains the background of hydrologic modeling and how that influences the current state-of-the-art for distributed hydrologic modeling. She then describes the physical process that distributed hydrologic models are attempting to capture and covers a few basic mathematical equations related to these models. She also identifies modeling challenges related to the complexity, calibration, and large data requirements, and gives an overview of the results to date of distributed hydrologic models used at the NWS. The target audience for this module is NWS forecasters who have little or no training in hydrology but can benefit from knowing how distributed hydrologic models work.