Design discharge for surface drainage, bunded field

In bunded rice fields the design discharge of the surface drain is fixed from the drainage rate that satisfies the tolerance of the crop to depth and duration of flooding.

1. Design rainfall: find the maximum 3-day rainfall $R$ for the chosen return period (5–10 years), using the frequency analysis or the mean annual value × growth factor.
2. Initial depth: take the standing water $h_0$ in the bunded field (40–50 mm).
3. Crop tolerance: fix the maximum depth $h_{max}$ and its permissible duration, and the depth $h_e$ and the duration $T_e$ for which it may persist (for example 300–400 mm for 1 day, 200–250 mm for 2–3 days).
4. Water-level history: assuming the rain falls uniformly in 3 days, ET and percolation neglected, and drainage $D$ constant: $$h_{peak}=h_0+R-3D, \qquad T_e=\left(3-\frac{h_e-h_0}{R/3-D}\right)+\frac{h_{peak}-h_e}{D}$$ Find $D$ (mm/day) by trial so that the limits are just satisfied; convert: 1 mm/day = 0.1157 l/s/ha.
5. Design discharge: $$Q = D\,A$$ with $D$ in l/s/ha and $A$ in ha (gives l/s). For large areas, a reduction factor for the travel time is sometimes applied.