This study presents the development and evaluation of a low-cost sensor-based system to optimize the management of surface irrigation at the field level. During a surface irrigation event, water flows according to the slope of the field and it is difficult and time-consuming to predict the optimal time when inflow should be stopped. In such systems, measurement tools are uncommon and those existing are far too complex and expensive to be used as decision support tools on small farms. This article presents the development of an Open Source system based on low-cost technologies that allows: (1) detection of water at the sensor location in the field; and (2) sending an alert by phone to the user. The metrological characteristics of the system and its suitability were tested in real conditions during one irrigation season of hay fields in the Mediterranean region.