Irrigation efficiencies: conveyance, application, storage
Measuring irrigation efficiency: conveyance, application, storage and distribution, uniformity, water productivity and the basin view, with sums.
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Only part of the water released from a dam or pumped from a well ever reaches the roots of a crop. Some seeps out of canals, some evaporates, some runs off the end of the field, and some drains below the root zone. Irrigation efficiencies measure these losses at each step, so engineers and farmers can find where water is wasted and how to save it. They are also central to debates about whether "efficient" irrigation really saves water.
Efficiency definitions and calculations are a standard exam topic in irrigation engineering and agronomy.
Where water goes
The classic efficiencies
Water conveyance efficiency (Ec)
Losses are mainly seepage through unlined canals, evaporation, leaks and operational spills. Lining canals and using pipelines raise Ec.
Water application efficiency (Ea)
Losses are surface runoff from the field end and deep percolation below the root zone. Ea is typically about 40 to 70 per cent for surface methods, 70 to 85 per cent for sprinklers and 85 to 95 per cent for drip.
Water storage efficiency (Es)
It measures whether irrigation adequately refilled the root zone. Light irrigations can have high application efficiency but low storage efficiency, leaving the crop short of water.
Distribution uniformity and Christiansen's coefficient
Water should be spread evenly. Christiansen's coefficient of uniformity (1942), used especially for sprinklers:
where x are depths caught in cans spread across the field, x̄ is their mean and n the number of cans. Values above about 85 per cent are considered good for sprinklers. The distribution uniformity (DU, the average of the lowest quarter of catches divided by the overall average) is used for surface and drip systems.
Overall (project) efficiency
The product of the efficiencies along the chain: Ep = Ec × Ea (with field canal efficiency included where relevant). Large surface irrigation projects often have overall efficiencies of only 30 to 50 per cent.
Water productivity
Efficiency ratios say nothing about how much crop the water produces. Water productivity does:
- Crop water productivity = yield ÷ water consumed (evapotranspiration), in kg per m3.
- Irrigation water productivity = yield ÷ irrigation water applied.
Typical crop water productivity values range from about 0.6 to 1.7 kg of grain per m3 for wheat and rice and up to about 2.5 for maize, varying with climate, variety and management.
The basin view: classical versus effective efficiency
Water "lost" from one field is not always lost to the basin. Canal seepage and deep percolation often recharge groundwater or flow back to rivers, where others use them. Researchers such as David Seckler distinguished:
- Classical efficiency, which treats all non-crop water as lost;
- Effective or basin efficiency, which counts return flows that are reused, and treats as truly lost only water that evaporates unproductively or flows to saline sinks or the sea.
This explains a paradox: converting to drip can raise field efficiency while increasing total water consumption, if farmers use the "saved" water to irrigate more land or switch to thirstier crops, and if the former percolation was being pumped by others downstream. Real savings require reducing consumptive use, for example by cutting non-beneficial evaporation, and often caps on water allocations.
Calculating efficiencies
From the figure: 100 units diverted, 70 delivered, 45 stored.
- Ec = 70 ÷ 100 = 70 per cent.
- Ea = 45 ÷ 70 = 64 per cent.
- Overall = 0.70 × 0.64 = 45 per cent.
Storage efficiency. Before irrigation, the root zone needed 60 mm to return to field capacity, and 45 mm was stored: Es = 45 ÷ 60 = 75 per cent. The crop still has a deficit of 15 mm.
Uniformity. Six catch cans under a sprinkler collected 8, 10, 12, 10, 9 and 11 mm.
- Mean = 60 ÷ 6 = 10 mm.
- Deviations from the mean: 2, 0, 2, 0, 1, 1; sum = 6.
- CU = 100 × [1 − 6 ÷ (6 × 10)] = 100 × 0.90 = 90 per cent, a good uniformity.
Water productivity. A wheat crop yields 5,000 kg/ha using 450 mm of evapotranspiration (4,500 m3 per hectare): 5,000 ÷ 4,500 ≈ 1.1 kg per m3.
Improving efficiency
- Conveyance: line canals, use pipes, maintain structures, and improve operation and scheduling.
- Application: level land, adjust stream size and cut-off time, use furrow surge flow, convert to sprinkler or drip where suitable, and schedule irrigation by soil moisture or crop water requirement.
- Storage: apply enough water to refill the root zone without exceeding it.
- Uniformity: correct pressures, spacings and nozzle sizes, and replace worn nozzles.
Key points
- Ec = delivered ÷ diverted; Ea = stored in root zone ÷ delivered to field; Es = stored ÷ needed to fill root zone.
- Overall efficiency is the product of efficiencies along the chain.
- Christiansen's CU = 100 [1 − Σ|x − x̄| ÷ (n x̄)]; above about 85 per cent is good for sprinklers.
- Water productivity = yield ÷ water consumed (kg per m3).
- Field losses may be reused downstream; real savings need reductions in consumptive use.
What the exams ask
- School-leaving and entrance papers: meaning of irrigation efficiency, and ways to reduce water losses.
- Degree and research-fellowship entrances: definitions and calculations of conveyance, application, storage and distribution efficiencies, uniformity coefficients, and water productivity.
- Agricultural engineering and policy examinations: project efficiency, basin-scale water accounting, and the paradox of irrigation efficiency.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can application efficiency be 100 per cent?
In practice no. Some runoff, deep percolation or uneven distribution always occurs, and some leaching is needed to control salinity in dry climates.
Why is high application efficiency sometimes bad?
If water is applied too lightly, the root zone is not refilled, so the crop suffers even though little water is lost. Storage efficiency checks for this.
Does lining canals save water?
It reduces seepage, which saves water if the seepage would otherwise be lost to saline groundwater or unusable sinks. Where seepage recharges aquifers that farmers pump, the basin-wide saving is smaller.
Sources
- Michael, A. M. Irrigation: Theory and Practice, 2nd edition. Vikas Publishing, 2008.
- Christiansen, J. E. Irrigation by Sprinkling. University of California Agricultural Experiment Station Bulletin 670, 1942.
- Burt, C. M. et al. "Irrigation performance measures: efficiency and uniformity." Journal of Irrigation and Drainage Engineering 123, 423–442 (1997).
- Perry, C. "Efficient irrigation; inefficient communication; flawed recommendations." Irrigation and Drainage 56, 367–378 (2007).
- Grafton, R. Q. et al. "The paradox of irrigation efficiency." Science 361, 748–750 (2018).
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