Agricultural Drainage

Parallel agricultural tile drains are laid at a depth of $1.8\text{ m}$ below the ground surface. The impermeable barrier is situated at a depth of $6.8\text{ m}$ below ground. The maximum permissible water table height midway between drains is $0.8\text{ m}$ below ground (hydraulic head midway $h = 1.8 - 0.8 = 1.0\text{ m}$). The soil hydraulic conductivity is $K = 1.5\text{ m/day}$, steady drainage flux $q = 0.005\text{ m/day}$, and equivalent aquifer depth $d = 3.0\text{ m}$. Using Hooghoudt's equation ($S^2 = \frac{8 K d h + 4 K h^2}{q}$), the required drain spacing $S$ in metres is ________ (round off to 1 decimal place).