Question Statement:
Referring to the subsurface tile drainage cross-section shown in the accompanying diagram, parallel pipe drains are installed at spacing $S$ above an impermeable layer. The hydraulic head midway between the drains is $m$ and the steady drainage recharge rate is $q$. Which of the following statements regarding Hooghoudt's equation $S^2 = \frac{8 K_2 d m + 4 K_1 m^2}{q}$ are TRUE?
Options:
- (A) $K_1$ is the hydraulic conductivity of the soil layer above the drain level
- (B) $K_2$ is the hydraulic conductivity of the soil layer below the drain level
- (C) The equivalent depth $d$ is smaller than the actual barrier depth $D$ to compensate for radial flow convergence near the drains
- (D) When the water table is below the drain level ($m < 0$), subsurface drainage discharge continues to increase