1.1 Surface Irrigation
1.2 Sprinkler Irrigation
1.3 Drip Irrigation
1.4 Functioning of Irrigation Systems
An adequate water supply is important for plant growth. When rainfall is not sufficient, the plants must receive additional water from irrigation. Various methods can be used to supply irrigation water to the plants. Each method has its advantages and disadvantages. These should be taken into account when choosing the method which is best suited to the local circumstances.
A simple irrigation method is to bring water from the source of supply, e.g. a well, to each plant with a bucket or a watering can (see Figure 1).
Figure 1 Watering plants with a can
This can be a very time-consuming method and involves very heavy work. However, it can be used successfully to irrigate very small plots of land, such as vegetable gardens, that are close to the water source.
More sophisticated methods of water application are used when larger areas require irrigation. There are three commonly used methods: surface irrigation, sprinkler irrigation and drip irrigation.
Surface irrigation is the application of water by gravity flow to the surface of the field. Either the entire field is flooded (basin irrigation) or the water is fed into small channels (furrows) or strips of land (borders).
Basins are flat areas of land, surrounded by low bunds (see section 2.2.2). The bunds prevent the water from flowing to the adjacent fields. Basin irrigation is commonly used for rice grown on flat lands or in terraces on hillsides (see Figure 2a). Trees can also be grown in basins, where one tree is usually located in the middle of a small basin (Figure 2b). In general, the basin method is suitable for crops that are unaffected by standing in water for long periods (e.g. 12-24 hours).
Figure 2 Basin irrigation on the hillside
Figure 2 Basin irrigation for trees
Furrows are small channels, which carry water down the land slope between the crop rows. Water infiltrates into the soil as it moves along the slope. The crop is usually grown on the ridges between the furrows (see Figure 3). This method is suitable for all row crops and for crops that cannot stand in water for long periods (e.g. 12-24 hours).
Figure 3 Furrow irrigation, using siphons
Irrigation water flows from the field channel into the furrows by opening up the bank of the channel, or by means of siphons or spiles (see Annex 1).
Borders are long, sloping strips of land separated by bunds. They are sometimes called border strips.
Irrigation water can be fed to the border in several ways: opening up the channel bank, using small outlets or gates or by means of siphons or spiles. A sheet of water flows down the slope of the border, guided by the bunds on either side. (see Figure 4).
Figure 4 Border irrigation, using siphons
Sprinkler irrigation is similar to natural rainfall. Water is pumped through a pipe system and then sprayed onto the crops through rotating sprinkler heads.
Figure 5 Sprinkler irrigation
With drip irrigation, water is conveyed under pressure through a pipe system to the fields, where it drips slowly onto the soil through emitters or drippers which are located close to the plants. Only the immediate root zone of each plant is wetted. Therefore this can be a very efficient method of irrigation (Figure 6). Drip irrigation is sometimes called trickle irrigation.
Figure 6 Drip irrigation (A)
Figure 6 Drip irrigation (B)
Whatever irrigation method is being chosen, its purpose is always to attain a better crop and a higher yield. Therefore proper design, construction and irrigation practice are of utmost importance. Maintenance, the after-care of the system to keep it functioning as well as possible, is often neglected. This always results in a lower irrigation efficiency (see also Volume 4), and thus less benefit from the irrigation system.
It is recommended to give canals, structures and methods a regular check-up and to repair damage immediately. Maintenance of canals and structures is dealt with in the Volumes concerning these subjects; maintenance of surface methods is discussed in the appropriate sections (2.5, 3.6, 4.4). Maintenance of sprinkler and drip systems is usually described in handbooks supplied by the manufacturers of the systems.