Green infrastructure and techniques such as rainwater harvesting have been proposed as a means to reduce stormwater discharge in developed areas prone to floods. We examined the effects of rainwater harvesting on discharge cumulatively through the Perdido River drainage network in the US state of Florida, an area prone to routine rainfall-driven nuisance flooding. We considered scenarios where rainwater is stored in parcels with structures that use septic tanks (where tanks are retired and used as cisterns, volume approximately 5.7 cubic meters); and where a similar volume of water is stored at all developed parcels. To evaluate flow reduction through the drainage network, we modeled effects relative to a flow event with a 1.5-year recurrence interval using a spatial GIS-based cumulative-effects model. Our model predicted that retired septic tanks would reduce discharge by more than 10 percent in only a few areas in the study region, almost exclusively in headwater regions and where density of houses using septic tanks is high. Analysis of all developed parcels storing rainwater indicated that discharge in several areas would be reduced by more than 20 percent. Results indicate a spatially variable potential for rainwater harvesting to reduce routine storm discharge. Spatially continuous hydrologic tools such as the one we use here may be especially useful for managers seeking to prioritize limited resources at locations for maximum benefit.