Salinity and the Irrigation Block
A 240-hectare irrigation district sits on a clay-loam floodplain. For 30 years growers applied about 9 ML/ha per year of irrigation water with an electrical conductivity (EC) of 0.4 dS/m. The water table, once 14 m deep, is now 1.8 m below the surface and rising about 0.3 m per year. Soil sampling shows a saline-sodic layer beginning at 1.2 m, with EC of the saturated extract (ECe) of 11 dS/m and an exchangeable sodium percentage (ESP) of 19 in the top 0.6 m. Winter rainfall averages 420 mm, summer evaporation exceeds rainfall by roughly 900 mm. Wheat yields have fallen 35% over eight years. A neighbouring block that was laser-levelled and switched to 6 ML/ha with subsurface drainage installed at 1.5 m depth now yields 20% above the district average. Growers want to know whether to drain, change crops, or reduce water application.
1. Using the water and salt balance, explain why the district is salinising despite relatively low-EC irrigation water, and calculate the approximate salt load added annually per hectare.
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The key is that salt enters with irrigation water and leaves almost only by drainage below the root zone. With EC = 0.4 dS/m, the salt concentration is roughly 0.4 × 640 ≈ 256 mg/L, i.e. about 0.256 kg/m³. At 9 ML/ha (9,000 m³/ha), the annual salt input is about 9,000 × 0.256 ≈ 2,300 kg/ha per year, or roughly 2.3 tonnes of salt per hectare annually. Over 30 years that is about 69 t/ha - enough to substantially raise ECe in a clay loam with restricted drainage.
Because the water table has risen to 1.8 m and is rising, capillary rise brings dissolved salts into the root zone, and summer evaporation (rainfall minus evaporation deficit ≈ 900 mm) concentrates them near the surface. The low irrigation EC does not protect the block: the problem is the accumulation, driven by insufficient leaching fraction and a shallow, rising water table. The saline-sodic layer (ECe 11, ESP 19) confirms accumulation and structural degradation. Without drainage to export salt, reducing irrigation alone would slow the rise but not remove the stored salt.
2. Evaluate the three options (subsurface drainage, changing to more salt-tolerant crops, reducing irrigation water) and recommend a strategy, justifying it with the evidence provided.
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Subsurface drainage directly addresses the cause: it lowers the water table, prevents capillary discharge into the root zone, and provides a pathway for salt export. The neighbouring block's evidence - drainage at 1.5 m plus reduced application, yielding 20% above average - supports this. However, drainage requires an outlet for saline effluent, or the problem is displaced downstream, and capital cost is high.
Changing crops (e.g., to barley or salt-tolerant pasture) manages the symptom. With ECe around 11 dS/m in the subsoil and ESP 19, only tolerant species will yield, and the sodic component still degrades structure, so gypsum may be needed to displace sodium. It buys time but salt continues to accumulate.
Reducing irrigation to 6 ML/ha without drainage would lower the water table more slowly and risk dryland salinity concentrating salts, because leaching would be minimal and the stored salt remains. It is only viable combined with drainage.
Recommendation: install subsurface drainage at about 1.5 m, apply gypsum to address sodicity, reduce applications toward 6 ML/ha with a calculated leaching fraction, and monitor ECe and water table depth. Pair with salt-tolerant rotations during recovery. This treats cause, not symptom, and is supported by the comparison block.