The mariotte reservoir supplies water at a constant hydraulic pressure by self-regulation of its internal gas pressure. Automated outflow measurements from mariotte reservoirs are generally difficult because of the reservoir's self-regulation mechanism. This paper describes an automated flow meter specifically designed for use with mariotte reservoirs. The flow meter monitors changes in the mariotte reservoir's gas pressure during outflow to determine changes in the reservoir's water level. The flow measurement is performed by attaching a pressure transducer to the top of a mariotte reservoir and monitoring gas pressure changes during outflow with a programmable data logger. Using a simple linear relation between reservoir gas pressure and water-level changes with time, the data logger converts the transducer signal into outflow-flux values. To demonstrate the usefulness of the new technique, two constant-head experiments are described that have vastly different flux ranges and time durations. The first experiment was a 1-h infiltration run in which infiltration rates dropped from 0.6 to 0.2 cm/min. The second experiment was a 3-week evaporation experiment in which the evaporation rate ranged from 1.0 to 3.0 cm/d. Results indicate that the automated flow measurement technique performed well when compared to a manual sight-tube technique for flux measurements; the difference between the two methods was never more than 9% for the infiltration experiment and 5% for the evaporation experiment. The advantages of the new technique over previously available automated flow measurement techniques include: (i) the ability to rapidly record a large range of fluxes without restricting outflow, and (ii) the ability to accurately average the pulsing flow, which commonly occurs during outflow from the mariotte reservoir.