Drainage rate (mm/day), initial 55 mm

Rice tolerates standing water only within limits of depth and duration, so the drainage rate is chosen such that the field water level during and after the design storm stays within these limits. The field is a bunded rice field in the plains (Terai).

Given data and assumptions

• Initial depth $h_0$ = 55 mm; design 3-day rainfall $R$ = 400 mm, falling uniformly, so the rain rate $r = R/3$ = 133.3 mm/day
• Maximum depth 400 mm may persist for 1 day; depth above 250 mm may persist for 2 days
• ET and deep percolation neglected (safe side); drainage $D$ (mm/day) starts with the rain and is constant

Water level history

During the storm the level rises at $(r-D)$ per day, and after it the level falls at $D$ per day:

$$\begin{aligned} h_{peak} &= h_0 + R - 3D \\ t_1 &= \frac{h_e-h_0}{r-D} \quad (\text{time to reach } h_e) \\ t_2 &= \frac{h_{peak}-h_e}{D} \quad (\text{time to fall back to } h_e) \\ T_e &= (3-t_1)+t_2 \le 2 \end{aligned}$$

The duration condition above 250 mm governs. Trial values:

Solving $T_e$ = 2 days gives $D$ = 47.9 mm/day. Then:

• $h_{peak}$ = 55 + 400 − 3 × 47.9 = 311 mm, which is below 400 mm, so the 400 mm level is never reached and the 1-day limit is satisfied
• $t_1$ = 2.28 days, $t_2$ = 1.28 days, $T_e$ = (3 − 2.28) + 1.28 = 2.00 days

Conversion: 1 mm/day = 10 m³/ha/day = 0.1157 l/s/ha, so

$$D = 47.9\times\frac{10}{86.4} = 5.54\ \text{l/s/ha}$$

Answer: drainage rate ≈ 47.9 mm/day (≈ 5.5 l/s/ha); adopt 48 mm/day.