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Microtopography, consisting of small-scale excursions in the elevation of the land surface on millimeter to centimeter scales, is ubiquitous on hillslopes, but its effects are rarely incorporated into hydrological analyses of rainfall-runoff partitioning. To progress toward a hydrological theory that accounts for microtopography, two research questions are considered: (1) Does microtopography change the partitioning of rainfall into runoff and infiltration compared to a background case that lacks these small-scale excursions? and (2) how do soil, mean slope, storm properties, and microtopographic geometric attributes influence this partitioning? To address these questions, a simplified one-dimensional hillslope with uniform sinusoidal microtopography is considered, and several rainfall-runoff scenarios are examined with a numerical model. The results indicate that for a range of realistic conditions, microtopography increases the proportion of rainfall infiltrating by 20-200% relative to an equivalent "background state" in which microtopography is absent. Additional theoretical development addressing issues of connectivity and improved representations of flow hydraulics over microtopographic surfaces are needed to refine these estimates and extend them to less idealized conditions. If confirmed, the results suggest that microtopography may have a significant impact on streamflow generation, plant water availability and the co-evolution of geomorphic, hydrological and ecological systems, with important implications for land management, especially in arid ecosystems.