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24 UJ required to fill the water sample. Timing of each of the runs -...l ru started when water introduction was complete and the water surface CO was in contact with the rotating disc. EC was measured by withdrawing a water sample from the lowest port (just above the bed) at specified time intervals, Samples were taken at 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 105 and 120 minutes and their temperature was also recorded, They were returned to the flow cylinder by pouring over the disc as in the beginning of the run. The volume required for a measurement was approximately 0.5 liters or about 13% of the total water in the cylinder. Each sample took between thirty and forty-five seconds from the time the sample was removed (and the flow stopped) and returned. Losses during the sampling were generally insignificant, At the completion of the run, the soil surface was changed by as much as 20 mm from the original level, This was mostly the result of water intro- duct ion and returning of samples. A separate run (W-4b) with only one sample taken at completion (120 minutes) was done to check this effect. Repeatability was also checked by doing three runs at the same speed (W-3a, b, c). Between each EC sample, the motor speed was measured so that a consistent value could be maintained, The motor was connected to a variable-speed controller for this purpose. 2.l.lc Velocity Measurement Average velocity for the depth of flow was measured by injecting dye through a port and timing its movement, The window in the side of the flow cylinder was adjacent to the upstream side of the injec- tion point, The dye was viewed just as it completed one revolution.