Land having water table is high
Water logging lands. (When the water stands on the land surface for long period e.g. 2-4 hours for vegetable)
Excessive moisture content above the field capacity.
Humid regions where rainfall is less than evaporation.
Humid regions having high rainfall continuous or intermittent.
Lands with fine textured soils.
Drainage Properties of Soil:
The artificial drainage is required to be provided for two reasons
Lowering of high table.
Removal of excess accumulated water
Soil parameters plays imp. Role in deciding the extent & type of drainage system required. This includes following:
Permeable soils do not require artificial drainage unless the water table is high slow permeable soil often requires drainage specially when rainfall is high & field is leveled.
Texture: fine texture soil requires artificial drainage where as coarse textured soils may not require the artificial drainage.
Structure: platy structure soils poor drainage characteristics whereas blocky & granular soil structure exhibits good drainage property.
Soils having low infiltration rate & soils horizontal having less permeability require the drainage facility.
Types of drainage:
1) Surface drainage
2) Sub surface / internal drainage
The direction of ploughing should be paralleled to dead furrow, whereas tillage operation likes sowing, planting, perpendicular to dead furrow.
The collected drains collect the water from dead furrows to carry them out from the field. The spacing of collected drains is generally 90m for flat land to 300m for sloppy lands.
Parallel field ditch system:
Ditch is speed further apart and has greater capacity than the dead furrows. The ditches are not at equidistant and such system is adapted to flat. Poorly drained soil with numerous shallow depressions
Generally, ‘V’ shaped trapezoidal or parabolic drains are constructed having minimum depth 22.5 cm & cross at area 0.5 m2. The spacing is around 360m when water is moving towards both the sides in the drains.
Surface drainage: flat sloppy
Surface drainage: 1) flat flow 2) interminable strata with pervious soil
Subsurface drainage: 1) tile drains 2) open ditch
Subsurface drainage will essentially required when the land is flat or when surface drainage is not possible also if is pervious, underlined by impervious strata, subsurface-drainage is required.
Deep trenches or tile drainage are the two essential means for subsurface drainage. Many times under field condition combination of surface & subsurface drainage may be required.
Types of subsurface drainage:
Random subsurface drainage:
This method issued to drain the scattered wet spots in the field. The lines (drained files) are laid some what randomly to drain these depressions generally the main line follows largest natural depression of the field and sub main & lateral connects the scattered spot with the main.
For drainage areas, individual low depressions, spots can be drained using herringbone or grid iron system.
Herring bone system: it consists of parallel laterals that enter the main from either side at angle. The main line or sub main lies in the narrow depression, particularly suitable where laterals are long & required area to be thoroughly drained.
Laterals enters the main only from one direction hence the cost of this system is comparatively less. This system is used on flat land/regularly shaped field on uniform soil.
Placing the main on each side of depression serves a dual purpose intercept the seepage & provide outlet for the laterals.
Interceptor: Deep drenches are tiles, are used to intercept seepage water from the hillside. The interceptor should be laid along bottom of permeable layer.
Factors affecting flow into tile drains:
1) Hydraulic conductivity of soil horizons
2) Depth of drain below the ground surface
3) Spacing of the drain
4) Diameter of the drain
5) Joint spacing between tile drains (generally3 mm)
6) Depth of impervious layer below the ground surface.
Hooghouts for spacing of subsurface drain:
4 kh (2d + H)
S2 = ———————-
k = hydraulic conductivity
d = depth of impervious layer below the drain
V = rate of replenishment of water by irrigation or rainfall in cm/sec
H = maximum water table from base of drain as shown in fig
S = spacing between drain
Benefit of drainage:
Provides better environmental for plant growth
Depth of plant rooting zone increased hence have larger rooting system
Improves the soil structure & infiltration rate of soil
Reduces soil erosion
Provides opt. tillage condition even in rainy season
Crop damage harvest can be reduced by removal of water from the wet lands
Makes the soil well created, maintains the soil temp, which enhances microbial activities.
Promotes leaching of undersible salts beyond the root zone of the crop.
Provides leaching climate & contributes for general prosperity of the region.