Although Persian wheels and norias are mechanically quite efficient, the main source of
loss from these types of device is that some water is spilled from the buckets and also
there is a certain amount of friction drag caused when the buckets scoop up water,
which again reduces efficiency. Also, the Persian wheel is obliged to lift the water at
least 1 m (or more) higher than necessary before discharging it into a trough, which can
significantly increase the pumping head, particularly in the case of low lifts. The
traditional wooden Persian wheels also inevitably need to be quite large in diameter to
accommodate a large enough collection trough to catch most of the water spilling from
the pots; this in turn requires a large well diameter which increases the cost.