Flux or Water Flux-Flux or water flux is typically expressed as volume per area per unit of time. Flux is used to express the rate at which water permeates a reverse osmosis membrane. Typical units of measurement are gallons per square foot per day (i.e. GFD or GSFD) or litres per square meter per hour (l/m2/hr). The flux of a RO membrane is directly proportional to temperature and pressure.