The manner in which water is applied to the land is commonly referred to as method of irrigation. These methods are adopted to apply irrigation water to the crop depending on the landscape, amount of water and equipment available, the crop and method of cultivation of crop. The main aim of these methods is to store water in the effective root zone uniformly and in maximum quantity ensuring minimum water loss and to get optimum yield.
Various methods of irrigation are:
In the surface system, water flows by gravity either through furrows basins or borders. In this type, water loss by conveyance and deep percolation is heavy and the efficiency of irrigation is only 40 to 50% at field level. Efficiency can be improved by lining the canal and by proper leveling of the field.
The method is suitable for specific types of soils. Evaporation and other losses are reduced considerably since water is applied below the surface through porous pipes.
Water is applied in the form of rain. Water is conveyed through pipes and sprayed though sprinklers. This method is most suited for steep sloppy and sandy soils.
Drip irrigation is adopted in water scarce areas for conserving water. In this method, water is applied in drops around the root zone through a pipeline with appropriate drippers.
The common methods of irrigation are indicated schematically as follows:
Whatever be the method of irrigation, it is necessary to design the system for the most efficient use of water by the crop.
Surface method of irrigation
Surface irrigation (gravity irrigation) is the most ancient method of irrigation and this method still holds good for more then 95 per cent of the irrigated area in the world. It can be defined as the process of introducing a stream of water at the head of a field and allowing gravity and hydrostatic pressure to spread the flow over the surface throughout the field. To move forward, the flowing water must have a downward slope in the direction of flow. This is, generally, provided by running water over a sloping land surface, but in the irrigation of level land, the water must build its own slope (from deepest at entry to near zero depth at advancing front). The soil surface thus serves the dual role of water conveyance and distribution. As it conveys the water, it controls the spreading pattern and hence the opportunity time for water to infiltrate. Spatial and temporal variability in infiltrability of the soil translates into non uniformity of water distribution into the root zone.
Field area nearest to the water inlet receives the greatest opportunity time and hence the greatest depth of infiltration, whereas the down field farthest from inlet receives least. This non—uniformity is most pronounced in coarse (sandy) soils, in which the infiltration rate is so high that much of the water entering the field infiltrates near the inlet and relatively little water remain for farther reaches of the field. On the other hand, fine textured soil (clayey) exhibit low infiltrability leading to significant flow of applied water to the lower sections of field, while higher section (near the inlet) remains insufficiently watered. The distribution of water is obviously affected by the slope and length of run. Major components of a typical surface irrigation system and the idealized distribution of water in an almost level border are illustrated in (Fig. 5.2).
Water application efficiency is usually higher on fine textured (clayey) soils than on coarse textured (sandy) soils because of their lower infiltration rates and more water retention per unit depth within the root zone. However, clayey soils are more prone to excessive wetness, compaction and impeded aeration. Land leveling and smoothing are essential operations...