Waterlogging and Salinization in an Irrigated District
An irrigation district in the lower reaches of a semi-arid river basin has expanded from 12,000 to 38,000 hectares over 25 years. Winter wheat and cotton are grown, and irrigation applies about 1,400 mm per year against an estimated evapotranspiration demand of 1,050 mm. The district sits on a thick clay layer and has no artificial drainage; the water table has risen from 18 m below the surface to 1.8 m. Farmers report white crusts on the surface, and yields on the oldest fields have fallen by 40% while waterlogging appears in patches after every irrigation. Upstream cities have begun to claim more of the river flow, and the district's allocation is likely to be cut by 20%. The district board is considering three options: (a) lining the main canals, (b) installing tile drains and leaching the fields, (c) switching to a more salt-tolerant crop without changing the water regime.
1. Explain the process that produced the white surface crust and the 40% yield decline. Use the water-table, evapotranspiration, and salt-transport evidence.
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Irrigation applies about 1,400 mm a year against an evapotranspiration demand of about 1,050 mm, so roughly 350 mm of excess water enters the soil each year. The clay layer restricts downward percolation, so the excess raises the water table; it has risen from 18 m to 1.8 m below the surface. At 1.8 m, capillary rise can move soil water to the surface, where evaporation discharges it. Evaporation leaves dissolved salts behind, so salts accumulate in the root zone and form the white crust. The salts come from the irrigation water and from soil and parent material. The yield decline follows from osmotic stress, toxic ion effects, and waterlogging: in the patches, saturated soil lacks oxygen, roots cannot respire, and after each irrigation the water table rises further. The 40% loss on the oldest fields is consistent with cumulative salt loading over decades.
2. Rank the three options-canal lining, tile drains plus leaching, and a salt-tolerant crop without changing the water regime. Which addresses the root cause?
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Rank tile drains plus leaching first, canal lining second as a support, and the salt-tolerant crop last as a temporary measure. Tile drains plus leaching addresses the root cause: the water table is too high and salts are accumulating in the root zone. Drains lower the water table, and a controlled leaching fraction flushes salts downward through the profile. This requires an outlet and safe disposal of saline drainage water, and it requires water, which is scarce if the allocation is cut by 20%. Canal lining reduces conveyance losses and may slow recharge, but it does not remove the excess water already applied to fields or the salt already in the root zone; it can help at the margin. A salt-tolerant crop without a change in the water regime only postpones the problem, because the water table keeps rising and salinity keeps increasing. The sustainable package is drainage plus leaching, plus reduced irrigated area or more efficient application to fit the lower allocation.
3. How does the likely 20% cut in the district's water allocation change the analysis?
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A 20% cut reduces the water available for leaching, which is the very operation that makes tile drainage work. The district cannot continue applying 1,400 mm; it must reduce net application toward the 1,050 mm evapotranspiration demand plus a modest leaching fraction. That likely means retiring or fallowing the most salt-affected and waterlogged fields, switching to less water-intensive crops, and improving on-farm efficiency so that the water saved is used for leaching rather than lost. Canal lining becomes more attractive because it stretches the reduced supply, but the district must account for the return flows that lining removes from downstream users; at basin scale, saved water is not always new water. The cut also weakens the salt-tolerant crop option: it may survive, but without leaching the salt store continues to grow. The board should plan for a smaller, drained, and carefully leached irrigated area rather than trying to maintain 38,000 hectares.