In this article we will discuss about:- 1. Benefits of Subsurface Drainage 2. Subsurface Drainage Methods 3. Installations 4. Groundwater Studies 5. Subsurface Drainage of Paddy Fields.
Benefits of Subsurface Drainage:
Provision of subsurface drainage in agricultural lands results in the following benefits:
1. Aeration of rootzone for maximum development of the plant roots.
2. Opportunity for desirable soil micro-organisms to develop through aeration and higher soil temperatures.
3. Availability of the soil for early cultivation and thus increased crop growth period.
4. Improvement of soil moisture conditions for operation of farm machinery.
5. Removal of undesirable salts from the rootzone.
6. Greater storage of rainwater in the rootzone because of a low initial watertable before the rains.
In agricultural lands with high watertable problems, if subsurface drainage is not provided, crop growth is adversely affected because of negative aspects of the above listed benefits.
Subsurface Drainage Methods:
In subsurface drainage water moves under the influence of gravity to suitable outlets.
This is accomplished using one of the following:
(1) Tile drains including perforated pipes,
(2) Mole drains,
(3) Drainage wells,
(4) Deep open drains, and
(5) Combination of tile and open drains.
Tile drains are a subsurface drainage method and consists of short length pipes (30 cm to 90 cm) installed at a particular depth from the land surface. The pipes are made of concrete or burnt clay. After digging the trench to the desired depth the pipes are held end to end without any jointing.
They are covered with an envelope material in certain cases and the soil is backfilled. Water enters the tile drains through the opening available between the pipes. A network of tile lines laid with a grade will remove the subsurface water.
Perforated pipes are like tile drains except that they are continuous and water enters the pipes through openings provided on the pipe. PVC pipes are most commonly used for the purpose. These are also laid below the soil surface by digging a trench to the required depth and backfilling the soil after the pipes are laid.
Mole drains are cylindrical channels formed at a desired depth below the soil surface. There is no lining material and the inherent stability of the soil at the depth gives stability to the mole drains. Water enters throughout the mole drains and is guided to the outlet.
Installation for Subsurface Drainage:
For the design and installation of subsurface drainage system the following information will be needed:
1. Topographic map of the area.
2. Data on soil salinity and alkalinity, drainable porosity etc.
3. Position and fluctuations of watertable levels relative to the ground surface and artesian pressures.
4. Groundwater quality.
5. Logs of soil and subsoil materials.
6. Hydraulic conductivity measurements.
7. Crops proposed to be grown and their drainage requirements.
8. Irrigation practices and requirements.
A topographic map of the area is needed both for planning the surface and subsurface drainage systems. The topographic map gives the details of land slope, possible outlets, existing drainage pattern, undulating land areas etc., and serves as the base map for preparing the watertable contour maps. Information on soil salinity and alkalinity is needed if surface drainage systems are to be planned along with reclamation of such soils.
Groundwater Studies for Subsurface Drainage Systems:
In planning subsurface drainage systems information about the groundwater depths, fluctuations, and quality are needed. The groundwater conditions are observed by installing piezometers and observation wells. The piezometers or observation wells are small diameter pipes (2.5 to 5 cm) installed vertically into the ground. In the piezometer there is no leakage around the pipe and all entrance of water into the pipe is through the open bottom.
The piezometer indicates the hydrostatic pressure of groundwater at the specific point in the soil where the lower end of the tube is located. In case of observation wells, water enters into the well through the entire section of the pipe located below the watertable. For installation of the observation wells, holes are drilled on the pipe and a coir rope or any filter material is wound around it to prevent sand particles entering into the well.
The piezometers or observation wells are installed in position in the same manner as tubewells are drilled but on a small scale. Depending upon the depth to which the piezometers or observation wells are to be located, the installation could be done either by driving in or using a high pressure water jet or by the percussion method.
The observation of watertable levels in piezometers located at different depths indicates the existence of hydraulic pressure causing either upward or downward movement of water.
The observation wells are useful for determining the depth of watertable from ground level. In areas where subsurface drainage systems are planned, observation wells could be located in a grid pattern, the grid size depending on the size of the area and the accuracy of the information needed. From the data obtained from observation wells, watertable contour maps and watertable isobath maps can be plotted.
Watertable contour maps show the configuration of the watertable surface in the same way as the contour map of an area show the configuration of the land surface. The watertable contours are plotted in the same way as surface contour lines arc plotted.
The watertable contour maps are useful in determining the direction of groundwater flaw, the location and extent of the high watertable areas and the configuration of the watertable surface within and adjacent to, any wet area.
From the observation well data, one can prepare a map what is known as a water table isobath maps. Isobath lines are lines of equal depth to water table. They are prepared in the same manner as water table contour maps. Isobath maps indicate at a glance the areas affected by high water table problems.
Prepared at regular intervals, those maps indicate the seasonal variation of affected areas. In areas provided with subsurface drainage, the isobath maps indicate the effectiveness of the drainage system in different parts of the area.
The depth of water in the observation wells can be measured using different methods:
(A) In case of shallow water levels, the depth could be measured by lowering a tape or rod coated with chalk and noting the point upto which it has been wetted.
(B) In case of deeper water levels, a small hollow brass cylinder is attached to, the end of the tape. As the tape is lowered in the observation well, mild jerks are given to, the tape. When the brass cylinder hits the water level a characteristic ping noise is heard and at this point the depth is noted. Same practice is needed for measuring the depth by this method.
(C) Portable battery operated instruments are available to measure the watertable depths quickly and accurately. These instruments consist of a metallic cylinder with two insulated terminals, attached to, the end of the tape.
The terminals are connected to a circuit which includes a storage battery and an ammeter. As soon as the two terminals in the cylinder touch the water level, the circuit is complete and an indication is given on the ammeter. At this point the depth is noted.
(D) Continuous record of the water levels can also be maintained using a water level recorder. For installing these recorders, larger diameter wells are needed for the float mechanism.
Subsurface Drainage of Paddy Fields:
Subsurface drainage systems in paddy fields have been found to be successful in Japan.
The features of the subsurface drainage system as adopted in Japan are as follows:
1. Design of Drain Discharge:
The design discharge depends upon the allowable time period of removal of surface water from the fields. It is assumed that land grading can achieve precision up to 5 cm and as such, a maximum of 5 cm of water has to be removed from the fields.
With 2 to 5 cm of water and 1 to 2 days of allowable discharge period the design discharge for subsurface drainage becomes 10 to 50 mm/day. Total volume of discharge is calculated by multiplying the design discharge by drainage area.
2. System Layout:
A subsurface drainage system generally consists of lateral drains, collecting drains, relief wells, outlets and ancillary structures. Typical layout of the system is shown in Figure 18.24. Lateral drains are generally of PVC, but at some locations bamboos and wood are used.
3. Spacing, Depth, Grade and Size:
The spacing of the laterals is generally about 10 m in paddy, fields with heavy clay soils. Depths are in the range of 0.8 to 1 m. Grades adopted range from 1/100 to 1/600 in order to keep sufficient velocity in the pipes. Diameter of the pipes, range from 50 mm to 100 mm.
4. Supplementary Drains:
When the average hydraulic conductivity of the soil is very low (less than 1 x 10–5 cm s–1) normal subsurface drainage cannot be very effective. In suet, situations, supplementary drains at relatively shallow depth and close drain spacing are necessary to provide adequate drainage.