Factors affecting sensitivity of Wetting Front Detectors

Wetting Front Detectors (WFD) are simple instruments that provide a yes/no response when a front of certain strength passes a set depth. This paper describes two different designs of wetting front detector and determines their sensitivity under experimental conditions. The FullStop WFD comprises a specially shaped funnel that distorts the downward flow of water through the soil, producing saturation at its base. The sensitivity was determined by applying water via intermittent misting at rates of 0.1 to 0.8 mm/h. For the soil type under study, the minimum flux at which water was collected by the WFD was 0.2 mm/h. The experiment was repeated for drip irrigation, and the FullStop WFD collected water at application rates less than 0.05 L/h, corresponding to a soil suction of about 2 kPa. Whereas these application rates are lower than what can physically be applied in the field, wetting fronts do weaken with depth and time after the irrigation is turned off. Fronts can then move at rates below 0.2 mm/h or suctions drier than 2 kPa, which could go undetected. A second WFD design, termed a LongStop, was also tested to determine if it would respond to lower flux rates. When fluxes are low, convergence by the funnel is less important than the need to counteract capillary emptying by the surrounding soil. A pipe-like design is therefore more appropriate than a funnel. A LongStop having a diameter of 50 mm and length of 600 mm was able to detect wetting fronts up to 6 kPa suction, and the volume of water in the LongStop was well correlated to the suction in the soil around the opening.