Executive Summary:A drainage meter, for determining the vertical downward flux of soil water past a given depth has been developed, through a series of prototypes.This report provides the information necessary to understand the principles and operation of the current version of the drainage meter, as well as how to install and use it.The drainage meter is based on two tube tensiometers to determine the vertical hydraulic gradient.Combined with information about the hydraulic conductivity at the prevailing soil water content (or suction), the instantaneous drainage flux can be calculated.The drainage meter design allows for the in situ measurement of hydraulic conductivity, but a satisfactory method for doing this has yet to be perfected.The device is packaged in a single unit 90 mm in diameter and 1350 mm long.Two versions of the drainage meter have been produced to make them suitable for different measurement and logging systems; one has analog output and the other has the industry standard SDI-12 protocol.The drainage meter is installed by drilling a 92 mm diameter hole to a depth 1350 mm below the depth at which the measurement of drainage flux is required.After inserting the drainage meter, the upper end of each tube tensiometers is covered with firmly packed diatomaceous earth that forms the sensing tip of the tensiometer and makes good contact with the surrounding undisturbed soil.The sensing tips of the two tube tensiometers, one 200 mm above the other, are hydrologically isolated with a layer of bentonite.Coupled with an appropriate data logger, the whole data acquisition, download, calculation, graphing and website updating process has been automated.No manual intervention is required for graphs with updated information about the drainage flux to appear on a website at any desired interval, usually daily.
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