Study of Water Table
Drainage investigation: It consists of getting necessary information regarding sources of water logging and ground water characteristics, extent and severity of water logging to decide the proposed line of caution and economical feasibility of soil. It includes topographical and ground water survey.
Contour map of the field:
Observation well: It gives depth of water table below the ground surface in water bearing strata. Water is at atmosphere pressure. An uncased an auger hole can be used for observation wells. However in sandy soils perforated casing may be provided to prevent collapsing of side wall.
Generally 2.5 cm diameter pipe with 3 mm diameter perforations are sufficient for the observation well however the perforation of stream depends upon particle size distribution of the surrounding formation.
Piezometer: When the soil strata, ground water character can be studied by installed piezometer. It indicates hydrostatic pressure of ground water at lower end of the part. The water enters through the opens bottom & there is no leakage through the sides.
Depending upon precisions required such observation well/piezometer are installed in grid of 30 to 300 m no of such piezometers can be installed at grid point, installed at different depth called as battery (piezometer battery.) The distance between individual piezometer should be minimum 60cm.
Observation of Data:
Record elevation of ground level at the piezometer / obs. well station with reference to the permanent bench mark on the farm.
Height of the pipe pre the level is to be noted (for observation wells piezometer)
Electrical depth gauge or tapes with chalked ends are used for measuring the depth of water table.
Observation are to be recorded periodically e.g. daily weekly seasonal etc. to study the water fluctuation etc.
e.g. R.L = 98.2m
Piezometer / obs. well is fixed at this height of pipe = 0.5m
Total height from ground = 98.2 to 0.5 = 98.7 m
If the height is measured by tape & if it obtain as 2m
Then the elevation of water table is 98.7-2 = 96.7m
Like this calculate for all points.
Water table contours:
In which direction water is flowing under the ground is known by water table contours. There are the lines of equal water table elevation above the datum. They are plotted similar to the ground surface contours on the base of map of the field it gives:
1) Visible (visual) information slope of water table
2) Information for analysis & solution of drainage problem.
Isobaths: Isobaths are the lines of equal depth from the ground level or they are the lines of equal depth to the ground water table lines.
These lines are plotted on map just like the controls & it helps to decide the surface drainage
1) Method of surface drainage
2) Method of sub surface drainage
3) Problems of conductivity drainage design in practical.
The process of removal of excess water from surface of field is termed as surface drainage. Generally the flat lands with low depressions & low infiltration rate requires surface drainage to remove the excess rainfall/excess irrigation water. Drainage can be achieved.
Land Smoothening for Surface Drainage:
Land smoothening also known as land grading is to produce plane land surface with uniform grade or slope. The finished surface is smooth & free from all minor depressions to prevent impounding of water & facilitate easy disposal of excess water along the slope within non-erosive velocity. The land smoothening is carried out two methods
1) Rough Grading-Bulldozers or Scrapers
2) Smoothening or Finishing-Float, Levelers
Drainage for pounded areas:
Low level sots accumulates run off water from adjoining areas can be removed out of the field by construction of fitted drainage. These drains are shallow with side field. Slop of 8:1 or more to facilities the crossing ditches by farm implements. This method is called random ditch system.
When the field operation (tillage) are performed parallel to ditch. The side slope of 4:1 for ditch can be preferred.
Drainage of flat lands (slope is less than 1.5%):
Bedding: In this method of surface drainage excess drains laterally from the crown strip of the land into the dead furrow & finally into the outlet. Area between two adjacent dead furrow is called bed.
The bed should be laid out dead furrow running in direction of greatest slope. Bed with is depends upon drainage characters of soil
i) 7-11-for very slow intend drained soil.
ii) 13-15-for slow internal drained soil.
Depth bed = 15-45cm.