Annual Drainage Discharge

Drained-field area, A_d
(ac)

Drain spacing of 4-inch lateral, S
(ft)

Drain depth from soil surface to drain bottom, D_d
(ft)

Distance from soil surface to restrictive layer, D
(ft)

This value can be determined from Drain Spacing Tool. Click here.

Effective radius, R_ef
(in)

Average effective radius for 4-row perforated drains is 0.24 in.
Average effective radius for 8-row perforated drains is 0.71 in.
Average effective radius for 4-row perforated sand-slot drains (narrow-slot or knife-cut) is 0.61 in.
Average effective radius for 8-row perforated sand-slot drains is 0.59 in.
Average effective radius for regular-perforated drains wrapped with knitted sock envelope is 2.13 in.

Equivalent saturated hydraulic conductivity of the soil, K_e

This value can be determined from the Drain Spacing Tool. Click here.

Long-term average annual precipitation, P
(in)

This value can be determined from Drain Spacing Tool. Click here.