Rainwater harvesting (RWH) is a decentralized practice that provides both water supply and runoff reduction benefits that are often difficult to assess. To assist in this evaluation, a model was developed that simulates a single RWH system in Richmond, Virginia, using storage volume, roof area, irrigated area, an indoor nonpotable demand, and a storage dewatering goal as independent design variables. Water supply and runoff capture reliability are assessed for a wide variety of cases. Tradeoff curves were developed to evaluate the design variable substitution when reliability was held constant. A reliability function was fit to the simulation results, and a solution method was developed to solve for an unknown variable as a function of the others. This method evaluates different design cases that provide the same water supply and/or runoff reliability, demonstrating that the design variables can be substituted for each other, using care to restrict substitutions between functional inputs or (separately) functional outputs. This method can provide guidance for designers in selecting equivalent RWH systems and regulators in assessing runoff reduction goals. Results indicate that the dewatering goal enhances runoff capture reliability but reduces water supply reliability moderately. Increases in storage volume increased both water supply and runoff capture reliability. Irrigated area has a much larger, negative effect on water supply reliability, and roof area has a similar negative effect on runoff capture reliability. As irrigated area increases for the same population, runoff capture reliability increases but eventually remains constant, reflecting the dominance of indoor demand, which in turn reflects the simulation's assumption of seasonal irrigation. Applications indicate that land uses that provide larger demands, such as offices, commercial sites, and high-density residential sites, may be better suited than lower-density residential lots where RWH is more commonly employed.