2.3 Application of water to fields - Conventional Method

Every irrigation method has advantages and disadvantages. Method should be selected keeping in view local conditions. Following points must be considered while selecting irrigation method.

· Method should be such that uniform water distribution for (light irrigation) small water depth as 6 cm should be possible.

· It should afford heavy uniform application of 15 to 20 cm water depth.

· It should not allow large concentrated water flows for reduction of conveyance losses and labor cost.

· It should facilitate mechanized farming.

Conventional methods of irrigation

Uncontrolled or wild flooding- In this method water is spread or flooded on a rather smooth flat land without much control or prior preparation. It is wasteful use of water and is practiced largely where irrigation water is abundant and inexpensive. This wild flooding is most suitable for close growing crops, pastures etc. This method is employed for irregular topography where other methods are not feasible.

Controlled flooding

Free flooding or ordinary flooding

Also known as irrigation by plots. In this method ditches are excavated in the field along the contour or up & down the slope. The field is divided into number of small sizes plots which are practically level. Water is admitted to these plots at higher ends and supply is cut off as soon as the lower part of the plot has received sufficient depth of water. Water is allowed to flow in these ditches across the field. No controlling to flowing water is made by using any means.

Contour Laterals

This method is applicable for steeper terrain. The field is cut by relatively dense network of small contour laterals, the spacing of which depends upon the prominent grade of the field between two adjacent ditches or laterals, the uniformity of slope and the soil type.

Border flooding

All land is divided into number of strips separated by low levees called borders. Each strip is 10 to 20 m wide and 100 to 400 m in length. Ridges between borders should be sufficiently high to prevent overtopping during irrigation.

Land is leveled perpendicular to the flow. Water advances confined and guided by two borders in a thin sheet towards the lower end of strip. The water flows slowly towards the lower end and infiltrates into soil as it advances. Slope is 0.5 to 1.5 %. When this water reaches lower end of strip, supply water is turned off. This method is especially suited for forage crops. With highly mechanized farming large area can be irrigated within short time by border strip method.

Check flooding

Check flooding is similar to ordinary flooding except that water is controlled by surrounding the check area with low and flat levees. Levees are constructed along contours which are connected with cross levees. Levees are generally 2 to 3m wide at the base and not more than 25 to 30 cm high so that it does not obstruct operation of farm machinery. The confined plot area varies from 0.2 to 0.8 hect generally rectangular with some initial slope. The check is filled with water at fairly high rate and allowed to stand until the water infiltrates.

Basin flooding

This method is special form of check flooding adapted for orchard trees. One or more trees are generally places in the basin and the surface is flooded as in check method by ditch water. Water is supplied to these basins through supply ditch. In some cases number of basins may be interconnected. Portable pipes or large hoses may also be used in place of ditches.

Zigzag flooding method

In this method water is made to take circulous route before reaching dead end of each plot. The whole area is divided into a number of square or rectangular plots: each plot is then subdivided with the help of low bunds or levees. This method is suitable for relatively level plots. This method is highly unsuitable for mechanized farming.

Furrow irrigation method

This method is very much used for row crops like maize, jowar, sugarcane, cotton, tobacco, groundnut, potatoes etc. In all other methods discussed above entire land surface is wetted but in this method one half to one fifth of the surface is wetted.. It results in less pudding soil & permits cultivation sooner after irrigation.

Furrows are narrow field ditches excavated between rows of plants & carry irrigation water through them. Furrows vary from 8 to 30 cm deep and 400 m long. Furrow spacing for corn, potatoes, sugarcane etc is determined by the proper spacing of plant rows. Furrows are made before, at the time of planting or after plants are grown. Often young plants are irrigated by small furrows until a good root system is developed. Thereafter furrow is made larger.

Advantages of furrow Irrigation

• Water contacts only ¹/₅ to ¹_/2 of the land surface, reducing pudding and crusting of soil.
• Evaporation losses are also reduced.
• Earlier cultivation is possible in heavy soil.
• Especially suitable for those crops which gets affected by over irrigation.
• Labor requirement is much reduced.
• No wastage of land in field ditches.

Corrugation Method

Corrugations are channels of smaller cross sections and run down the predominant slope. Corrugations are small furrows and are used for grain and other forage crops such as alfalfa. Corrugations are advantageous when the available irrigation streams are small and also for land of uneven topography.