Canal discharge for dry and wet crops (30000/40000 ha)

Formula: $Q = \dfrac{A\,d}{T\times 86400}$ with $A$ in m$^2$, $d$ in m, $T$ in days.

Dry-season crops: $A = 30000$ ha $= 3\times10^8$ m$^2$, $T = 6\times7 = 42$ days, $d = 0.148$ m

$$Q_{dry} = \frac{3\times10^8\times 0.148}{42\times 86400} = 12.235\ \text{m}^3/\text{s}$$

Wet-season crops: $A = 40000$ ha $= 4\times10^8$ m$^2$, $T = 4\times7 = 28$ days, $d = 0.115$ m

$$Q_{wet} = \frac{4\times10^8\times 0.115}{28\times 86400} = 19.015\ \text{m}^3/\text{s}$$

The seasons are different, so the canal is designed for the larger of the two.

Answer: canal discharge = 19.01 cumec (wet season governs; dry season needs 12.24 cumec).