Autonomous shallow flow runoff detector for hillslope hydrological studies

Abstract This manuscript describes the development, calibration and validation of an autonomous overland flow detector (OFD) for the measurement and recording of shallow runoff depth at varying recording intervals based on measurement of runoff water electrical conductivity (EC). Using electronic components easily available and of moderate cost, approximately 367€ per unit, the proposed device can operate for a broad range of surface overland flow with EC from 0 to 435 µS cm−1 with a regular charge using a small built-in cell panel. It was originally intended for use in experiments related to runoff connectivity and efficiency of vegetated buffer strips at hillslope scale. Results show how, once calibrated for the expected range of runoff EC, the proposed device can provide reliable and replicable measurements among different units with an average root mean square error of 3 mm. When deployed in the validation test under rainfall simulation with water of high EC (435 µS cm−1) it allowed the determination of the evolution of the change in runoff starting time and the duration of the hydrograph after the end of rainfall in individual points as a function of slope length. In this test the differences in depth among OFD was not significant, since being lower than 4 mm overlaps with the average measurement error among units. This result highlights the need for careful interpretation of the measured depth in relation to the equipment measurement error in spatial variability of shallow flow. Further improvements will concentrate on remote communication for downloading data via a cell phone network, and additional ruggedness and autonomy for long-term deployment in the field under very cloudy conditions.