Development of a recording water flow meter using ultrasonic measurement of water levels in a slotted U-pipe

The accurate measurement and recording of water flow rates is necessary in the performance of field experiments in irrigation, overland flow or drainage studies. There is a lack of reliable, low-cost commercial flow meters that are able to operate under “non-pressure” conditions. This work describes a simply designed recording meter based on the ultrasonic measurement of water levels in a slotted U-pipe. A three-width slot design allows accurate flow measurements in the range 0–3 L s−1 and the ultrasonic probe allows for high resolution measurements. Six replications of the instrument have been performed and calibrated (water levels against water flows) in laboratory experiments. After that, they have been successfully tested under field conditions, monitoring water flows which were recorded by using a data logger in the drainage system of an experimental farm in southwest Spain during several irrigation episodes in summer 2005.