Open-surface drainage is defined as the diversion or orderly removal of drainage water by means of improved natural or constructed channels, supplemented when necessary by the shaping and grading of land surfaces of such channels.

Parts of surface drainage system:

The surface drainage system consists of three parts, (1) collection system, (2) conveyance or disposal system, and (3) outlet (Figure 2).

Figure 2: A typical drainage system.

Water to be drained from the individual field is collected through collection system (consists of field drains) arid moves through disposal system (consists of intermediate and main drains) to outlet.

Design criteria for field drainage

The very purpose of a good surface drainage system is to prevent the harmful effects of waterlogging on crops. Selection of an appropriate drainage coefficient is the key to design a successful surface drainage system. Drainage coefficient is defined as the amount of water that runs off from a given area and is to be removed in 24 hours. While designing surface drainage system, a low value of the drainage coefficient will lead to partial improvement in drainage though the cost of design may be relatively low, whereas a high value would increase the cost substantially without any additional gain in the removal of surface congestion. The waterlogging tolerance of a crop should be considered while estimating the drainage coefficient for a surface drainage project. Although there are several methods for estimation of drainage coefficients, a simple method involves following steps for its estimation:

1. Estimation of 24 hr rainfall depth that might occur with a probability level generally of 20% or a return period of 5 years should be considered for agricultural drainage

2. Evaluate the basic infiltration rate of the soil and determine the expected potential evapotranspiration.

3. Estimate the crop tolerance to surface congestion at sensitive growth stages in days. Multiply the infiltration rate and expected potential evapotranspiration with crop tolerance period.

4. Subtract the value calculated in step (3) from estimated probable rainfall value.

The resulting value when divided by crop tolerance period (days) will give the drainage coefficient in depth of water/day. These steps could be described mathematically in the form of an equation as:

Q= [R-n(E + I)}/n ...(1)

Here, q is the drainage rate in mm/day, R is the rainfall in mm, E and I are potential evapotranspiration and infiltration rate in mm/day and n is number of days. When n is greater than one, it may be useful to increase the duration of rainfall in step 1 also from 1-day maximum to n- day maximum. The drainage coefficient thus calculated will be closer to the actual field values. For rice crop, the drainage coefficient would be less as at the end of drainage period some depth of water is allowed to stand in the fields. This depth is subtracted before dividing by number of days, n.