Technical Abstract: Hortonian overland flow is well defined and predictable at the point scale, but is typically modeled at a much larger scale when deterministically predicting runoff for water use or pollution control purposes. The physical characteristics which control overland flow are spatially variable, however, and large-scale model parameters often do not represent this variability. In this research, measurements and observations made in a semi-arid environment were used characterize the overland flow process and form the basis of a distributed hydrodynamic model. The developed model was then used to explore issues of scale and spatial variability related to infiltration and microtopography. The model compared excellently to several numerical test cases.