Design Considerations for Water Fluxmeters.

Two dimensional modeling was used to assess performance of passive-wick fluxmeters over a wide range of drainage rates and soil types. For these fluxmeters, soil-filled tubes are placed above the wick and used to control convergent and divergent flow. Length of wicks and control tubes can be optimized for known soil types and flow conditions. Low flux rates (1 to 10 mm/yr) can be measured accurately with standard passive-wick fluxmeters (60-cm wick, 60-cm control tube) only in sandy soils and gravels. However, for drainage rates between 100 and 10,000 mm/yr, standard fluxmeters will produce collection efficiencies of ~ 100% and should be adequate for all but very fine soils or in soils where there is a high degree of preferential flow. Where year-round data collection is desired, passive-wick fluxmeters are limited to sites which have water tables in excess of 1.5 m deep.  Instantaneous flux rates are readily captured with the present fluxmeter design in a robust manner, not obtained by other means.