Interpreting a Water Budget in a Semi-Arid Catchment
A 1,200 km² catchment receives 480 mm of rainfall annually. Evapotranspiration accounts for 78% of precipitation; 12% runs off; the remaining 10% infiltrates to groundwater. A new 90-hectare irrigation scheme proposes to extract 18,000 ML per year from the catchment's river. Current river discharge at the catchment outlet averages 52,000 ML per year. Downstream, 14,000 people and a wetland of national significance depend on environmental flows of at least 30,000 ML per year. The scheme's proponents argue the catchment 'wastes' water to the sea.
1. Calculate the annual runoff volume in ML and assess whether the proposed extraction is sustainable. Show your working.
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Step 1 - Convert catchment area to a volume of rainfall.
- 1,200 km² = 1,200 × 1,000,000 m² = 1.2 × 10⁹ m².
- Rainfall depth = 480 mm = 0.48 m.
- Annual rainfall volume = 1.2 × 10⁹ × 0.48 = 5.76 × 10⁸ m³.
- Convert to ML (1 ML = 1,000 m³): 5.76 × 10⁸ ÷ 1,000 = 576,000 ML.
Step 2 - Calculate runoff.
- Runoff = 12% of 576,000 ML = 69,120 ML/yr.
Step 3 - Compare with observed discharge.
- Observed discharge is 52,000 ML, which is less than the calculated 69,120 ML. The difference (about 17,120 ML) reflects transmission losses, storage in soils and wetlands, and the fact that the 12% figure is an average across wet and dry years. This gap is a caution: in dry years runoff will be well below the average.
Step 4 - Assess sustainability.
- Required environmental flow = 30,000 ML.
- Available for extraction = 52,000 − 30,000 = 22,000 ML in an average year.
- Proposed extraction = 18,000 ML, which is 82% of the available margin.
Verdict: marginally sustainable in an average year but highly risky. In a dry year, discharge could fall to, say, 35,000 ML, leaving only 5,000 ML of extractable water - the scheme would take 3.6 times the safe yield and breach environmental flows. Sustainability therefore depends on a cap tied to actual annual flows, not a fixed 18,000 ML entitlement. A strong answer shows the arithmetic, notes the average-versus-dry-year distinction, and does not simply compare 18,000 with 52,000.
2. Critique the proponents' claim that the catchment 'wastes' water to the sea, using the concept of environmental flows.
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The claim treats water reaching the sea as lost, but this reflects a supply-side, storage-oriented view of water that ignores the ecological and social functions of environmental flows.
1. Environmental flows are a legitimate use, not a loss. The 30,000 ML required downstream maintains the nationally significant wetland, sustains fish and bird breeding, flushes salt and nutrients, and supports the estuary. Reducing flows below this threshold causes channel contraction, acidification of soils, algal blooms and loss of biodiversity - costs that are difficult and expensive to reverse.
2. Downstream users are real users. 14,000 people depend on the river. Upstream extraction reduces their reliability, effectively transferring water from downstream communities to the irrigation scheme. This is an equity issue, not an efficiency gain.
3. The 'wasted' volume is smaller than claimed. Of 576,000 ML of rainfall, 78% is lost to evapotranspiration before it ever reaches a stream, and only 12% runs off. The discharge of 52,000 ML is not surplus; it is the residual after the catchment has already used most of its water. Calling it waste misreads the water budget.
4. Variability matters. In dry years the same discharge supports the same wetland with less water. A fixed 18,000 ML entitlement would take a larger share of a smaller flow, breaching environmental thresholds precisely when the ecosystem is most stressed.
Conclusion: the claim is not supported. A defensible scheme would set extraction as a share of actual annual flow above the environmental-flow threshold, with a cap and a review trigger, rather than asserting that water reaching the sea is wasted.