Commemorating the 50th anniversary of the Freeze and Harlan (1969) Blueprint for a physically-based, digitally-simulated hydrologic response model

The year 2019 marks the 50th anniversary of a pioneering publication in hydrology. Allan Freeze and Richard Harlan published their Blueprint for a physically-based, digitally-simulated hydrologic response model (Freeze and Harlan, 1969) in this journal. Their vision was for a futuristic model that would integrate key processes and compartments in the hydrologic cycle: precipitation, evapotranspiration, overland runoff, infiltration and groundwater exchange (into and out of) surface water bodies, such as rivers and lakes. Today, the original Blueprint is a reality. Our paper commemorates the 50 year anniversary of the original Blueprint paper. Through personal communications with Allan Freeze, we document the history and genesis of this paper for the first time. We reflect on the uptake of the Blueprint into modern hydrology, the development of numerical models that enabled this, and the range of challenges being tackled by these models. Finally, we consider challenges and opportunities for the future of this area of modelling and hydrologic science.