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Subsurface drainage coefficient

Drainage coefficient is the volume of water per unit area to be removed in 24 h. The drainage coefficient is important in subsurface drainage design and a dependable drainage coefficient is difficult to obtain. It depends on land use, rainfall, runoff, infiltration and evapotranspiration. Incorporation of all these factors in a single physical parameter is difficult to measure effectively.

In arid regions, drainage coefficients are calculated on the basis of irrigation management and leaching requirements for salinity control. Since excess salts in the root zone are critical to the reclamation process than improvement in the aeration. In humid areas, the drainage coefficient is taken to be the depth of rainfall removed in 24 h. Drainage coefficients can be calculated from a soil water balance method. Selection and use of suitable drainage coefficients have always been problems in the design of subsurface drainage system. Use of a low value will reduce the effectiveness of the drainage system whereas use of a high value will raise its cost.

Drainage coefficient is the most important parameter that decides the lateral drain spacing, size of the laterals and collectors and capacity of the pump to dispose off the drainage effluent. The drain spacing is less in cases where drainage coefficient is more as compared to a case where drainage coefficient is less. As such, the cost of the system depends largely upon this parameter. Therefore, the need to select an appropriate value for this parameter has always been emphasized. The drainage coefficients for some of the sites in India have been observed to be in the rane of l-5mm. In the case of subsurface drainage design based on nonsteady state conditions, the drainage criterion is based on time to lower the water table from a predecided original to the final level. Usually the original water table is considered at the soil surface while the final level is taken as 30cm below the soil surface. As per the recommendation the capacity of the drainage system should be sufficient to lower the water table by 30 cm in 2 days time. In general, design rates area likely to be in the following DC ranges given in Table 6.

Table 6: Range of drainage coefficient suitable for various conditions.

DC, mm/d

Suitable conditions

<1.5

Soils low infiltration rate

1.5-3.0

Most soils, with higher rate for more permeable soils and where cropping intensity is high

3-4.5

Extreme conditions of climate, crop and salinity managements, an under poor irrigation practices.

>4.5

Special conditions, e.g. rice irrigation on light textured soils

Various drainage system layout. The objectives of layout include removing water from an isolated area, improving drainage in an entire field, intercepting a hillside seep, and so on

Figure 10 : Various drainage system layout. The objectives of layout include removing water from an isolated area, improving drainage in an entire field, intercepting a hillside seep, and so on.

 
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