Studies of water velocity in the capillary fringe: the point velocity probe

The point velocity probe (PVP) is a device that can measure groundwater velocity at the centimeter scale, and unlike devices that measure velocity within well screens, the PVP operates while in direct contact with the porous medium. Because of this feature, it was postulated that the PVP could be effective in measuring velocity within the capillary fringe. This hypothesis was tested using a laboratory flow-through cell filled with a medium-fine sand from Canadian Forces Base Borden. The cell was constructed to simulate conditions such that the PVP was positioned from 2.5 cm below the water table to 79 cm above the water table. As the water table was lowered, the PVP gave highly consistent values of velocity over the range equivalent to 2.5 cm below the water table to 44 cm above the water table, the approximate extent of the capillary fringe. The average measured velocity was 11.3 cm/d +/- 11.6%, somewhat higher than that calculated based on the measured discharge through the cell (7.5 cm/d +/- 5.5%). With a further decline in the water table there was a progressive decrease in the measured velocity values, consistent with the declining hydraulic conductivity as the sand material drained. Readings could not be made beyond about 57 cm, where the water content was approximately 75% of saturation. These experiments showed that the PVP is capable of measuring groundwater velocity within the saturated zone above the water table and possibly into the unsaturated zone. Currently, this is the only instrument available with this capability.