Long‐Term Soil‐Water Tension Measurements in Semiarid Environments: A Method for Automated Tensiometer Refilling

Abstract

Core Ideas We provide details of an automated tensiometer refilling system. The systems monitors soil water tension in a rugged, steep, and arid post‐fire debris flow source area. Vadose zone processes are looked at through remote data acquisition and SCADA applications. Tensiometer‐equipped data acquisition systems measure and record positive and negative soil‐water pressures. These data contribute to studies in hillslope hydrology, including analyses of rainfall runoff, near‐surface hydrologic response, and slope stability. However, the unique ability of a tensiometer to rapidly and accurately measure pre‐ and post‐saturation subsurface pressures requires maintenance techniques that have precluded their application to unattended sensor networks in semiarid regions. Under suction, the de‐aired water in the tensiometer is drawn from a porous cup. Under positive pressure, dissolved gases from pore water infiltrates the cup. Over time, both contribute to unreliable readings and/or poor signal response through cavitation. To address this problem, we used commercially available equipment to develop a simple system of solenoid valves and a water reservoir that enable automated in situ tensiometer refilling. We tested the system at two post‐wildfire hydrologic monitoring sites in the Angeles National Forest, southern California. We present example results from 3 mo of monitoring and show how the tensiometers can be refilled by a remote trigger. By remotely refilling the tensiometer, we were able to continuously monitor quasi‐saturated soil pore‐water pressures without making repeated and costly maintenance visits.