Estimation Of Irrigation Efficiency
Water use efficiency – The water utilized by crop is evaluated in terms of Water Use Efficiency. This water use efficiency can be classified into:
• Crop Water Use Efficiency
• Field Water Use Efficiency
• Physiological Water Use Efficiency, and
• Irrigation project efficiency
(i) Crop water use efficiency – It is the ratio of Crop yield (Y) to the amount of water used by the crop for evapotranspiration (ET).(Irrigation Efficiency)
CWUE =Y /ET and expressed as kg/mm/ha
(ii) Field water use efficiency (FWUE) – It is the ratio of crop yield (Y) to the total amount of water used in the filed (WR)
FWUE = Y/WR and expressed as kg/mm/ha
(iii) Physiological water use efficiency (PWUE) – The physiological WUE is calculated in terms of the amount of CO2 fixed per unit of water transpired
PWUE = Rate of photosynthesis/Rate of transpiration
(iv) Irrigation efficiencies of irrigation projects – Many irrigation projects throughout the World operate with 25-40 per cent overall efficiency.(Irrigation Efficiency) Thus perhaps one third of the water released at the Project headwork is actually beneficially used for evapotranspiration by crops. In many areas increased irrigation efficiency would result in increased irrigation average and production as well as decreased problems with salinity and drainage. The decrease in efficiency can be attributed to losses occurring at various stages. Some of the reasons are:
• Inadequate design of the project.
• Inadequate design of the Farm Irrigation System.
• Lack of maintenance.
• Inadequate management of the system.
Typical Irrigation System consists of
• head works
• main canals
• field channels
A. Water application efficiency (Ea)
The purpose of irrigation is to replenish the available moisture in the root zone depleted by evapotranspiration. Crop water requirement is defined by Doorenbos and Pruitt (1977) as “The depth of water needed to meet the water loss through evapotranspiration of a disease free crop, growing in large fields under non-restricting soil conditions including soil water and fertility and achieving full production potential under the given growing environment”. The application of the least amount of water required to bring the root zone moisture content up to field capacity is considered as efficient irrigation. (Irrigation Efficiency)If on the other hand, the amount of water applied grossly exceeds that actually needed for replenishment; the irrigator application efficiency is very low. To illustrate, consider a field, which needs 9 cm depth of water to bring the root zone to field capacity at the time of irrigation. To replace this amount it is necessary to deliver a total or gross depth of 12 cm of water to the field. Then the efficiency of application will be
9/12 × 100 = 75%
Application efficiency=Ea =Water required to bring soil to FC level/Water received at field inlet × 100
Primary factors for low application efficiency are:
• Improper Irrigation system design, construction.
• Poor maintenance of system.
• Inadequate farmers knowledge on crop water requirement.
Field application efficiency varies with type of soil and method of irrigation. Some observed efficiencies are given below:(Irrigation Efficiency)
Light soil- 55%
Medium soil- 70%
Heavy soil- 60%
Graded border irrigation- 53%
Basin irrigation- 58%
Furrow irrigation- 57%
Sprinkler irrigation- 67%
Drip irrigation- 80%
For rice cultivation, the efficiency is- 32%
B. Conveyance efficiency (Ec)
E=Water made directly available to the crop/Water released at head works × 100
Primary factors affecting conveyance losses are management aspects which cause fluctuations in the supply as well as physical factors such as seepage losses through canal banks and canal outlets. Some of the observed conveyance efficiencies are:
Continuous supply with no substantial change in flow – 90%
Rotational supply with no substantial change in rotation – 80%areas of 70–300 ha
Rotational Supply in projects more than 7,000 ha and less than 10,000 ha without effective management and communication – 65%–70% network(Irrigation Efficiency)
C. Project efficiency (Ep)
Ep=Water received at inlet to a block of fields/ Water released at project head works
The overall project efficiency represents the efficiency of the entire operation between diversion of source of flow and the crop zone. Water delivery system improvements and farm irrigation improvements would significantly improve the ability of the farmer to apply more uniform and efficient irrigation. The Project Efficiency can be increased through
• Lining of canals in areas of high seepage losses, proper alignment and sectioning of field canals.
• Maintenance of canals and drains, including an emphasis on farm drains.
• Inducing scarcity of water by limiting available water per unit area.
• Ensuring reliability of supply system down to farmer’s level. Farmers need to know when they can count on water and how much water much they can count on.
• Design farm systems so that release flow can be handled efficiently by the farmer.
• Install proper structures at outlets to maintain the flow constant.(Irrigation Efficiency)
• Encourage efficient design and construction of such system as level basins, contour borders and contour furrows and general land levelling and shaping.
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