Passive-wick water fluxmeters: theory and practice

Improvements in vadose-zone water-flux measurements are needed for a variety of reasons, including better water-use management for agriculture, for turf-grass (e.g., golf course) operations, and for monitoring the ground disposal of wastes from mining and other industries. For such purposes, we have developed and tested passive-wick water fluxmeters under a wide range of conditions, from non- vegetated desert settings in the USA to irrigated tea plantations in Sri Lanka and rain-fed squash plantations in the South Pacific. In desert settings, the drainage was found to depend upon the precipitation distribution, the surface soil and the type and amount of vegetation. In Washington State, USA, bare sands and gravels drained up to 60% of the annual precipitation while fine soils did not drain. In wetter environments, drainage was found to be closely linked to the rate and duration of precipitation events. Design calculations with a 2-D model show how divergence can be minimized for a wide range of soil conditions under expected transient fluxes. Model results show that for sands, the operational range of the water fluxmeter is from a few mm/yr to well above 10,000 mm/yr, for both steady state and transient conditions, while for silts and clays, the range is more limited and best operates in the range above a few hundred mm/yr. Passive-wick water fluxmeters provide a reliable, robust, and relatively inexpensive method to assess the quantity and quality of drainage waters over a wide range of conditions.