The following points highlight the eight main methods of surface irrigation. The methods are: 1. Wild or Uncontrolled Flooding 2. Free Flooding 3. Contour Laterals 4. Border Strip Method 5. Check Flooding Method 6. Basin Flooding 7. Zig-Zag Method of Flooding 8. Furrowed Method.
Method # 1. Wild or Uncontrolled Flooding:
In this method water is applied to rather unprepared fields without exercising any control on the direction of spread of water. No levees are generally made to guide the spread of water. This method of irrigation is adopted in inundation irrigation wherein water from streams during floods is taken and allowed to spread on vast areas lying adjacent to the stream. This is a very wasteful method of irrigation and can be used only where water is available abundantly at nominal or not cost.
Method # 2. Free Flooding:
This method is a somewhat improved method than wild or uncontrolled flooding method. This method is mainly adopted where fields have not been properly prepared and field soil is so hard that it will allow very small amount of water to be absorbed by top soil before flooding water gets evaporated.
In this method, fields are divided into large plots and water is allowed to flood or spread on whole of the field so as to give time to the soil to absorb sufficient amount of water. The fields are provided with permanent supply water course. This method is very much prevalent in most parts of U.P. where soil is of very heavy type.
Analysis of Time to Cover a Strip Area by Irrigation Water:
Depending upon the rate of infiltration of water into the soil, the size of the outlet supplying water to the fields is decided. Total area in the command of an outlet also is a deciding factor in fixing the size of the outlet. When a large stream of water is applied to unit area of the soil of low infiltration rate, excessive run off occurs. Conversely when a small stream is applied to a unit area of soil of high infiltration rate, excessive depth of water lost by deep percolation.
The relation between size of water stream and time rate of water application over a given area of land can be established easily as follows:
Let A = Area of land covered at any time t in hectares
I = Rate of infiltration in m/hr
q = Quantity of water in ha-m/hr
t = Time in hours
y = Average depth of sheet of flowing water in metres
Total quantity of water flowing in small time interval dt is equal to the quantity of water infiltrated in this time dt over the area A plus the quantity of surface flow over the area dA.
Thus
qdt = yd A + IA dt
or (q – IA) dt = ydA
Equation (3) gives the maximum area that can be irrigated with a stream of discharge q.
Example:
0.04 cumecs of water is being discharged by an outlet at the head of a field of area 0.2 hectare. If the average depth of flow is expected to be 80 mm and average infiltration rate for the soil is 50 mm/hr determine the time required to cover this area.
Method # 3. Contour Laterals:
This method may be used for irrigating steeper terrains. In this method a number of small contour laterals (drains) are constructed. The spacing of these ditches depends upon the grade of field between adjacent ditches. Spacing is less for steep slope and more for flat terrains. This is also not a very good method of irrigation.
Method # 4. Border Strip Method:
This method is very much adopted in canal irrigation in Northern India. In this, the farm is divided into series of strips. The strips are 10 to 20 m wide and 100 m to 300 m long. In sandy soils, the size of the strip is usually 8 to 10 m wide and about 50 m long. In case of sandy soils, strips are made of shorter lengths as otherwise head reaches of strips absorb excessive water and thus affect duty of water.
The method has very scientifically laid system of water courses. The water from the water courses is led to the strips and water moves in each strip in form of a thin film, uniformly spread along the width. As soon as one strip gets filled, water flowing into it, is stopped.
The water is then opened to the adjoining strip. Size of the water course may vary from 0.015 to 0.3 cumec and slope of the strip in the direction of flow of water may be 0.5 to 2% depending upon the porosity of the soil. The whole of the form is divided into long strips with the help of low or small earthen levees.
Method # 5. Check Flooding Method:
This method consists of a comparatively large stream which discharges water into a well prepared and levelled plot. The plot is surrounded by checks or levees about 30 cm in height. The levees are broad based at the bottom so that water filled in the plot may not escape.
This method is found suitable for very permeable soils, as plot gets filled up very soon without allowing much water to percolate. It is equally suitable for heavy soils also. The water filled in the field can remain held up in the levees and allows heavy hard soil sufficient time to absorb sufficient water.
In the levelled farm, the plots or kiarsies are generally rectangular. But if the ground has substantial slope the checks or levees may follow the contours at vertical intervals of 5 to 12 cm. Contour levees may be connected with cross-levees at convenient places. In this method plots having areas varying from 0.2 to 0.8 hectares have been found to give best results.
Method # 6. Basin Flooding:
It is also a sort of deep check flooding method. It is mostly adopted for the irrigation of orchards. The basins are formed for each individual plant or for a group of plants. All these basins remain connected with each other. The water from supply ditch flows to the basins. Sometimes portable pipes or large hoses may be used in place of ditches.
Method # 7. Zig-Zag Method of Flooding:
In this method a large plots is subdivided into a number of small size plots and all the plots remain connected with one another.
Water is led into the first part which after filling takes a circuitous route and floods the second, third and so on, small plots. This method is not much practiced as number of levees hampers farming operations with modern farm machinery.
Method # 8. Furrowed Method:
In this method of irrigation the fields is divided into ridges and furrows. Duty of canal water is very much increased by this method as only part of the surface of the fields has to be flooded. Crops are sown on ridges and watering is done in furrows. This method of irrigation is adopted for certain crops which give better yield in this way.
All the crops which give their yield from under the soil are usually grown by this method. Length of furrow or ditch varies from about 3 m for gardens to as much as 500 m for field crops. The most common length of the ditch varies from 100 m to 200 m. The general slope of the furrows varies from 1 per cent to 3 per cent.
This method of irrigation is not suitable for very sandy soils, as in that case losses by percolation become excessive. Spacing of the adjacent furrows depends upon the type of soil and also the crop to be sown in it. Depth of furrows varies from 20 cm to 30 cm.
Furrows are called corrugations if they are of smaller cross-section and run with more longitudinal slope. Corrugations are used for grain and other forage crops. This method of irrigation is preferred when available irrigation streams are small and topography of land uneven.