During the timing phase of the percolation test, how much of a drop in water level is measured to calculate the rate?

Answer

One inch

After pre-wetting, the hole is refilled, and the measurement involves timing how long it takes for the water level to drop by exactly one inch.

During the timing phase of the percolation test, how much of a drop in water level is measured to calculate the rate?
ratewatersoildrainagepermeability