Please
Measure vertical head in inches, feet, yards, meters. Head is the vertical
distance from top of water source to bottom of discharge.
Only the military would measure the head with a level head.

The flow rate will vary and must be controlled as with a water gate or valve.
Throw a floater into the stream. Measure the distance the floater moves in ten
seconds. Divide by ten to find feet per second or meters per second.
Measure the cross section area of the stream. Make sure to use common units of
measure - all in inches, feet, meters, mm, etc.

Multiply cross section area by water speed to find flow capacity.
10 feet per second x 1/2 ft x 1/2 ft (for a 6"x6" stream) results in 10/4 cu ft
per sec (2.5 cufps)water flow.

If you have a water gate, the flow will vary with the height of water head and
gate setting. Measure several settings to establish the setting that maintains
constant head.

Come back after a rain and measure the maximum flow.
Come back during a drought and measure the minimum flow.

Please forgive Scumbag.com clint meyers for bleeding the lilly for his way
wandering flock.