Direct measurements of water velocity also have been used to
measure induced circulation. In an aerated pond, Boyd and Watten
(1989) introduced 60, l-liter plastic bottles filled with water so that they
floated with only their caps above the surface. Drawings were made of
the paths followed by the bottles. This technique is applicable only to the
measurement of surface water currents. Petrille (1984) attached drogues
to floats in an attempt to measure underwater currents, but the lines
supporting the drogues often became tangled with each other. Moore
(1992) attempted to use a pigmy current meter to show water velocities
at different locations in a 0.4-ha, aerated pond, but water velocities in
most parts of the pond were below the limit of detection by the current
meter used. Current meters were only useful for measuring exit velocity
of water from aerators.