In Hooghoudt's drainage spacing equation $S^2 = \frac{8 K_2 d h + 4 K_1 h^2}{q}$, the parameter $d$ ('equivalent depth') is introduced primarily to account for:
Options:
(A) Radial flow resistance and convergence of flow lines entering the drain tubes
(B) Unsaturated capillary fringe above the phreatic surface
(C) Chemical clogging of drain filter envelopes
(D) Evaporation losses from the ground surface
Answer Key:
A
Step-by-Step Solution & Explanation
Hooghoudt assumes horizontal flow. However, near drain tubes flow lines curve and converge radially. To correct for this radial flow resistance without complex conformal mapping, Hooghoudt replaced actual aquifer thickness $D$ below the drains with an effective smaller 'equivalent depth' $d$.