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Irrigation, Salinization, and the Cost of Growing Food

You’ll be able to

Four irrigation methods, ranked

The exam asks you to compare, so hold the ranking with reasons. Flood (furrow) irrigation floods the field or the furrows: cheapest, least equipment, and the least efficient — much water evaporates or percolates below the root zone. Spray or center-pivot irrigation sprays from above: more efficient than flooding, but substantial evaporative loss, especially in heat and wind. Drip irrigation delivers water directly to the root zone through perforated tubing: by far the most efficient, with the least evaporation and the lowest salinization risk, and the most expensive to install. Flood irrigation from a furrow and drip differ by a factor of roughly two in the share of water that actually reaches a plant. The trade-off to state is always cost against efficiency, and the reason drip is not universal is capital, not ignorance.

Salinization and waterlogging

Both are consequences of irrigating without adequate drainage, and they are different processes. Salinization: irrigation water carries dissolved salts; when the water evaporates the salts remain and accumulate in the topsoil, eventually reaching concentrations that draw water out of roots by osmosis and kill the crop. It is worst in arid regions with high evaporation and with flood irrigation, and it has ended agriculture on land that had been farmed for millennia. Waterlogging: over-irrigation raises the water table until it saturates the root zone, excluding oxygen and drowning roots. The remedies differ — salinization is addressed by drainage and by flushing with excess water, waterlogging by drainage alone and by irrigating less — so a question naming crop death plus white crusting on the surface is salinization, while one naming saturated ground is waterlogging.

The Green Revolution, both halves

From the 1940s, high-yielding dwarf varieties of wheat and rice, combined with synthetic fertilizer, pesticides, irrigation and mechanization, raised yields enough to avert the mass famines widely predicted for South Asia; India became a grain exporter. That is a genuine achievement and should be stated as one. The costs belong in the same answer: dependence on purchased inputs favored farmers with capital and drove smallholders into debt; groundwater depletion followed intensive irrigation, notably in the Ogallala aquifer and the Indian Punjab; fertilizer runoff caused eutrophication and dead zones; pesticides damaged health and non-target species and drove resistance; and monoculture narrowed the genetic base, which is the vulnerability the biodiversity unit warns about. The best answers treat this as a trade-off with a real numerator, not as a failure.

Irrigation efficiency ranking
drip > spray/center-pivot > flood/furrow cost runs in the opposite order
Efficiency and salinization risk move together: the method that loses the least water also leaves the fewest salts.
Worked example

A farm in an arid region uses flood irrigation. Crops are failing and a white crust has appeared on the soil surface. Diagnose the problem and propose two responses.

  1. 1.Identify the symptom: a white surface crust in an arid irrigated field is accumulated salt.
  2. 2.Diagnose the mechanism: irrigation water carries dissolved salts, high evaporation removes the water and leaves the salts behind, and flood irrigation applies large volumes that maximize the effect.
  3. 3.Explain the crop damage: high soil salinity draws water out of root cells by osmosis, so plants cannot take up water even when soil is wet.
  4. 4.Response one: switch to drip irrigation, which delivers less water directly to roots and greatly reduces evaporative concentration of salts.
  5. 5.Response two: install drainage and flush the soil with excess water to carry accumulated salts below the root zone — noting that this requires a place for the drainage to go.
Answer: The problem is soil salinization from flood irrigation in a high-evaporation environment. Two responses: convert to drip irrigation, which cuts evaporative loss and therefore salt accumulation; and install subsurface drainage so the soil can be flushed, moving salts below the root zone rather than concentrating them at the surface.
Watch out

Do not treat the Green Revolution as simply good or simply bad. It averted famines that were confidently predicted and it produced groundwater depletion, eutrophication and debt. An answer that states only one half is incomplete in whichever direction it leans.

Checkpoint

Salinization of irrigated soil occurs because

Checkpoint

Compared with flood irrigation, drip irrigation primarily reduces water loss by

Answer the 2 checkpoints as you read.

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