# Hooghoudt's equation

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Hooghoudt's equation is a drainage equation describing the relation between depth and spacing of parallel subsurface drains, depth of the watertable, depth and hydraulic conductivity of the soils. It is used in drainage design.
It is written as Q=(8*Kb*d(Di-Dd)(Dd-Dw)+4*Ka(Dd-Dw)^2)/L^2, where:

• Q = steady state drainage discharge rate
• Ka = hydraulic conductivity of the soil above the drain level
• Kb = hydraulic conductivity of the soil below the drain level
• Di = depth of the impermeable layer below drain level
• Dd = depth of the drains
• Dw = steady state depth of the watertable midway between the drains
• L = spacing between the drains
• d = equivalent depth

## References

Wikipedia (https://en.wikipedia.org/wiki/Drainage_equation)