Why More Canal Water Is Making the Fields White: Diagnosing a Sodic Patch
A block in the Ganga–Yamuna doab has been canal-irrigated since 1972. Sugarcane covers 62% of the net sown area and paddy 21%, both grown by flood irrigation. In 1975 the pre-monsoon water table stood 14 m below ground level; by 2023 it had risen to 2.1 m over 40% of the block, and 1,240 ha now carry white salt encrustation in the rabi season, with paddy yields falling from 5.2 to 3.4 t/ha and cane from 70 to 48 t/ha. Soil tests in the affected patch show pH 8.9, electrical conductivity of the saturation extract 4.6 dS/m, sodium adsorption ratio 18, and free calcium carbonate (kankar) nodules at 60–90 cm. The block is a level interfluve with no natural surface outlet; the main canal runs on an embankment 2–3 m above the fields and feeds 14 distributaries. Of 96 tube wells, only 22 are used, because canal water is free. Farmers want to apply more canal water to 'wash the salt away'.
1. Diagnose the condition: is this a saline soil, a sodic (alkali) soil, or simply waterlogging? Use the test values.
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Separate the three symptoms. Waterlogging is the physical cause (water table at 2.1 m, no surface outlet); salinity and sodicity are the chemical results, and they need different remedies.
Read the numbers. pH 8.9 is above 8.5; sodium adsorption ratio 18 is well above the threshold of about 13; free calcium carbonate is present. A high SAR with a high pH means sodium has displaced calcium and magnesium on the clay complex, so the clay is dispersed, the soil swells and seals, and permeability falls. That is a sodic (alkali) soil. In a purely saline soil, pH stays below about 8.5, SAR is low, the crust is white and fluffy, and the soil remains well structured.
Explain the mechanism. Canal on an embankment → seepage into the block; free water and two water-hungry crops → over-irrigation; no outlet → water table rises; capillary rise from 2.1 m brings sodium salts to the surface; evaporation leaves the white crust; the kankar layer at 60–90 cm acts as a partly impermeable pan that traps water above it.
Conclusion: irrigation-induced secondary sodicity with waterlogging, aggravated by a calcareous subsoil - not a naturally saline soil.
2. Explain why adding more canal water cannot 'wash the salt away', and set out a reclamation package in the correct order.
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Why more water fails. Leaching needs two conditions: a place for the salt to go (drainage) and a source of calcium to replace sodium. Adding water without an outlet raises the water table further, increases capillary rise, and delivers more sodium; the dispersed surface clay then seals and infiltration drops, so the water evaporates rather than leaching. Water alone converts a sodic patch into a bigger sodic patch.
Order of operations.
- Stop the input: line and repair the canal, reduce the running time of the distributaries, and prescribe irrigation doses instead of flooding.
- Lower the water table: use the 96 existing tube wells for conjunctive use, pumping to a drawdown of 2–3 m, and lay subsurface field drains or open drains leading to a natural drain.
- Restore calcium: apply gypsum or phosphogypsum according to the gypsum requirement (SAR 18, pH 8.9 means a substantial dose), incorporated into the root zone.
- Leach: only after drainage and gypsum, leach with good-quality water, in the pre-monsoon.
- Change the cropping: dhaincha as green manure (adds organic matter and calcium), barley, mustard or sugarbeet on the worst patches; cut paddy, which is the heaviest water user and worsens waterlogging.
Test of the answer: each step is placed where it is because the next step fails without it.
3. Canal water is free, so farmers use it heavily. Show how this economic signal interacts with the physical process, and propose a policy that goes beyond banning canal irrigation.
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The link. A zero marginal price for canal water pushes demand above the crop's actual requirement. The surplus percolates, recharge exceeds natural discharge, the water table rises, capillary rise increases, and salt accumulates. Free water thus does not merely waste a resource; it drives the chemical process that is destroying the yields. Because canal water is also needed for aquifer recharge and for diluting salts in good years, a simple ban is the wrong instrument.
Policy package.
- Price canal water volumetrically above a basic entitlement, with warabandi (rotational) supply so that a farmer gets a reliable share rather than a free flood.
- Meter tube-well electricity so that pumping for drainage/conjunctive use is not free while canal water is; equalise the signals.
- Subsidise gypsum and field drains, since the benefit of reclamation is long-term and the cost is immediate for a smallholder.
- Line the canal and check seepage from the embanked section - this is the cheapest single intervention.
- Pay for a shift away from paddy on the waterlogged patches, through procurement and extension.
Judgement: the block needs less water applied and more water removed. Pricing plus drainage achieves both; a ban achieves only a conflict with farmers.