This can be determined by a percolation test or by the grain size and type of soil.
For a percolation test (may depend on jurisdiction), it is necessary to excavate a minimum of 3 holes having a dimension of approximately 6" dia x 12-18" deep. The bottom of the excavation is filled (approx 1/2" fine granular mat'l to provide a base. The hole is filled with water to a minimum depth of 12" and an initial time is taken. The level of the water is measured over a period of time and water is added until steady state conditions are reached. The time is taken (minutes) and the water level is measured in centimetres. The time divided by the change in water level is called the percolation time. This is determined for 3 holes in the area of the distribution bed and the maximum value determined. This value is used to size the distribution bed. There are other issues if the soil is too free draining, etc., but the above is a bit of a guide.
Similarly, the grain size and type of soil can be used, although my personal preference is a percolation test.
Dik
