PNNL

Abstract

Unsaturated water flux densities are needed to quantify water and contaminant transfer within the vadose zone. However, there are few reliable and accurate methods available for direct measurement. A water flux meter was designed, constructed and tested to directly measure drainage fluxes in field soils. The flux meter consists of collector, designed to minimize divergence, which concentrates flow into a narrow sensing region filled with a fiberglass wick of known hydraulic properties. The wick applies suction, equal in magnitude to its length, to the collector to passively drain the meter. The meter can be easily installed below the root zone in augured borehole. Water flux through the sensing region can be determined from the time-of-flight measurements or with a self-calibrating tipping bucket. In this study, sensitivity of the tipping bucket was 0.167 mm per tip. Laboratory measurements confirmed predictions of a 2-D numerical model, which showed for optimal performance, precise control of the ratio of the collector height (H) to its width (W) and collector surface area to that of sensor region is required. In sandy soils, a H/W ratio of 1.5 to 2.0 reduced divergence to less than 10% of the applied flux for all fluxes above 10 mm yr-1. Because greater divergence can expected in fine-textured soils, larger H/W ratios will be required to achieve a similar levels of performance. The water flux meter is simple in concept, inexpensive, easy to install, and provides continuous and reliable monitoring of unsaturated water fluxes ranging from less than 10 mm yr-1 to more than 1000 mm yr-1.