Rainwater reuse for irrigation and flush discharges

In case a given project is predicted to feed both the irrigation system and the flush discharges with harvested rainwater, which criteria shall be used to "divide" the rainwater contribution for each system? There is any problem if the project team decide to adopt a "seasonal mode of operation" for this system, giving priority to the irrigation system, for example between April and September and to the flush discharges between October and March? Do you have any experience on documenting such an strategie?