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MATHEMATICS Parameters of a suburface drainage system: The Hooghoudt equation for groundwater flow to subsurface drains reads: Q = {8.Kb.d.(HE) + 4.Ka.(HE)^2} / S^2 where: Q = drain discharge (m/day), Kb = hydraulic conductivity below drain level (m/day), Ka = hydraulic conductivity above drain level (m/day), d = the equivalent depth of the impermeable layer below drain level (m), H = height of the water table above drain level midway between the drains (m), E = height of the water table above drain level near the drain (m), S = drain spacing (m). The equivalent depth d depends on the actual depth D, the drain radius R,and the drain spacing S. It can be calculated using the method described by W.H. van der Molen and J, Wesseling 1981. A solution in closed form and a series solution in Hooghoudt's drain spacing formula. In: Agricultural Water Management 52, pp 336  346. Rewriting the quation as: Q.S^2/(HE) = 8.Kb.d + 4.Ka.(HE) gives the opportinity to determine the values of 8.Kb.d and 4.Ka from a linear regression of Q.L^2/(HE) on HE, so that the values of Ka and Kd.d are found, while with the value of d, the Kb value can be determined, 
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