Irrigation Engineering

Definition of Irrigation

Irrigation may be defined as the science of artificial application of water to the land, in accordance with the „crop requirements‟ throughout the „crop period‟ for full-fledged nourishment of the crops.

Following are the factors which govern the importance of irrigation
• Insufficient rainfall
• Uneven distribution of rainfall
• Improvement of perennial crops
• Development of agriculture in desert area

Advantages of irrigation
• Increase in food production
• Optimum benefits
• Elimination of mixed cropping
• Improvement of cash crops
• Source of revenue
• General prosperity
• Generation of hydroelectric power
• Domestic water supply
• Facilities of communications
• Inland navigation
• Afforestation

Disadvantages of irrigation
• Rising of water table: water-logging
• Problem of water pollution (nitrates seepage into GW)
• Formation of marshy land
• Dampness in weather
• Loss of valuable lands

Types of Irrigation
 Surface Irrigation
 Sub-surface irrigation

Surface irrigation
In the surface methods of irrigation, water is applied directly to the soil surface from a channel located at the upper reach of the field. Water may be distributed to the crops in border strips, check basins or furrows. Two general requirements of prime importance to obtain high efficiency in surface methods of irrigation are: distribution systems to provide adequate control of water to the fields and proper land preparation to permit uniform distribution of water over the field. They are also designed to minimize labor and capital requirements. Effective management practices are dependent on the type of irrigation, and the climate and topography of the region.
System of Irrigation

Subsurface irrigation
In sub-irrigation, water is applied below the ground surface by maintaining an artificial water table at some depth, depending upon the soil texture and the depth of the plant roots. Water reaches the plant roots through capillarity action. Water may be introduced through open ditches or underground through pipelines such as drains or mole drains. The depths of open ditches or trenches vary from 30 cm to 100 cm and they are spaced about 15 m to 30 m apart. The water application system consists of field supply channels, ditches or trenches suitably spaced to cover the field adequately and drainage ditches for the disposal of excess water.

Methods of Irrigation
• Free Flooding
• Border Flooding
• Check Flooding
• Basin Flooding
• Furrow irrigation method
• Sprinkler irrigation method
• Drip irrigation method

Free Flooding or Ordinary Flooding
o Ditches are excavated in the field
o Water is applied from field ditches without any levee to guide its flow.
o Movement of water is not restricted, it is sometimes called “wild flooding”
o It is suitable for close growing crops, pastures etc.
o It is practiced large where irrigation water is abundant and inexpensive.
o It involves low initial cost of land preparation, extra labour cost in the application of water.
o Application of efficiency is low.
o This method may be used on rolling land (topography irregular) where borders, checks, basins and furrows are not feasible.

Border Flooding
o The farm is divided into a number of strips (width 10 ~ 20 m and length 100 ~ 400 m) separated by low levees or borders.
o Water is turned from the supply ditch into these strips along which a flow slowly toward the lower end, wetting the soil as it advances. When the advancing water front reaches the lower end, the stream turned off.
o The surface is essentially level between levees and lengthwise slope is somewhat according to natural slope of the land (0.2 ~ 0.4%)
o It is suitable to soils having moderately low to moderately high infiltration rates and to all closely growing crops.
o Uniform distribution and high water application efficiencies are possible.
o Large streams can be used efficiently.
o It involves high initial cost.
o Ridges between borders should be sufficiently high
o The land should be perpendicular to the flow to prevent water from concentrating on either side of the border

Determination of required time to irrigate in border flooding
A relationship between the discharge through the supply ditch (Q), the average depth of water flowing over the strip (y), the rate of infiltration of the soil (f), the area of the land irrigated (A), and the approximate time required to cover the given area with water (t), is given by the equation:

Considering small area, dA of the border strip of area (A)
Depth of water, y over this area (A)
Assume that in time dt, water advances over this area (dA).
Now, the volume of water that flows to cover this area = y.dA ------------------------ (1)
During the same time dt
The volume of water that percolates into the soil over the area (A) = f.A.dt --------------- (2)
The total quantity of water supplied to the strip during time (dt) = Q.dt ------------------- (3)
From equation (1), (2) & (3)
Q .dt = y. dA + f. A. dt

For getting time required to irrigate the whole land, we have to integrate the above equation and
considering y, f, and Q as constants

After integrating the above equation, we get

Further, considering the maximum value of

Problem:

About Admin MC3

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