Two design issues relating to an artificial catchment rainfall-runoff collecting system (ACRCS) are dealt with in this study; the first is the rainfall-runoff threshold value of an artificial catchment and the second is the definition and determination of the demand-design reliability. The rainfall-runoff threshold value of an artificial catchment, which is the critical element in determining the system efficiency, can be reduced to between 4 and 6 mm when surfaces are treated with chemical sealants. The current design threshold for south-western Australia is 10 mm. The influence of the time interval on the reliability was reinforced and the results demonstrate that monthly intervals are the most likely to provide appropriate design criteria to satisfy a targeted water demand.