The ability to manage an irrigation system is contingent on an accurate estimate of the percentage of water pumped that becomes available for crop use. No irrigation system delivers water at 100 per cent efficiency. Water may be lost through delivery systems or pipelines and some water may remain in the soil, but not be used by the crop. To know how much water to pump, these losses must be totalled and added to the amount of water needed by the crop.
Water application efficiency accounts for how uniformly the water is applied and can be used for other assessments. If a farmer is trying to decide whether switching to a new sprinkler package would be economical, the change in water application efficiency could be a major factor. If water becomes limited, changing to a system with a higher water application efficiency will provide more useable water to the crop and reduce pumping costs.
Benefits and barriers of using irrigation efficiency for designing a project
Irrigation efficiency improves agricultural production greatly. It reduces the amount of water extracted for irrigation purposes. It also reduces the costs related to extraction and transport for irrigation. Water availability is also increased for other uses, especially during dry periods. Change in irrigation practices may change the soil-water balance and reduce groundwater recharge. Agricultural water costs are heavily subsidised in many countries, providing little incentive to change from conventional (high water consuming) farming methods.
Irrigation efficiencies
Conveyance Efficiency (Ec) is the ratio between the water received at inlet to a block of fields and that released at the project head and Field Canal/Pipeline efficiency (Eb) is the ratio between the water received at the field inlet and that received at the inlet of the block of fields.
Due to evaporation and seepage loss, conveyance efficiency and field canal efficiency for a canal system is observed as 90 per cent and 75 per cent respectively. But as the piped networks are closed system, this is nearly of 95 per cent and always on a higher side than that of a canal system.
The ratio between the water directly available to the crop and that received at the field inlet is known as field application efficiency (Ea) and project efficiency (ep = Ea. Eb. Ec) is the ratio between water made directly available to the crop and that released at the head works.
An example of irrigation efficiency
In Table 1, the quantity of water discharged in different locations of an irrigation system has been calculated considering a discharge of 100 units of water in an irrigation field. The resulting quantity of water available at the root zone of the crop is shown in Table 1.
Calculation of various efficiencies based on the water discharge available in different locations in an irrigation system is shown in Table 2.
Conclusion
From the above table it is clearly understandable that there is always an increase in the overall efficiency (%) from a canal-based system to a piped-based system. For a drip-based irrigation system, it is 24.1 per cent, for a sprinkler-based irrigation system, it is 20.1 per cent, and for a surface-based irrigation system, it is 16.1 per cent. So, it can be concluded that the increase in project efficiency of a piped irrigation network is about 20 per cent as compared to a canal distribution network.
Sabarna Roy, Business Development, Applications Technology, Engineering and Strategy Department, Senior Vice-President; and Rajat Chowdhury, Business Development, Applications Technology, Engineering and Strategy Department, Assistant Manager, Electrosteel Group, Kolkata 700017