Agricultural Drainage and Salinity

In a homogeneous soil where drain rests directly on the impermeable barrier ($d = 0$), Hooghoudt's equation simplifies to $S^2 = \frac{4 K m^2}{q}$. For $K = 1.0\text{ m/day}$, $m = 1.0\text{ m}$, and $q = 0.004\text{ m/day}$, $S^2 = \frac{4 \times 1.0 \times 1.0^2}{0.004} = \frac{4}{0.004} = 1000$. Drain spacing $S = \sqrt{1000}$ in meters is (rounded off to one decimal place):