Te Horo Stream: allocating water in a dry February
Te Horo Stream is a spring-fed stream that runs through 14 dairy farms, a town of 1,100 people, and a lagoon that is a breeding site for long-finned eels. In a dry February the flow at the council's gauge falls to 320 litres per second. The regional council's minimum ecological flow is 250 L/s. Town supply takes 45 L/s. Nine farms hold irrigation consents totalling 180 L/s, but use is unmetered and estimated at 140 L/s. Iwi hold customary values for the lagoon and report that eel numbers have fallen sharply. The council must set a summer allocation. Options: (A) cut all irrigation by 30% whenever flow drops below 350 L/s; (B) install meters, charge per cubic metre and cap use at current estimated levels; (C) build an off-stream storage dam to capture winter flows for summer (cost $4.8 million, three years to consent and build).
1. Using the figures, show why the current allocation cannot be sustained at 320 L/s.
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Step 1: work out the water actually available for abstraction.
Flow (320 L/s) - minimum ecological flow (250 L/s) = 70 L/s that can be taken without dropping below the environmental minimum.
Step 2: subtract the town.
70 - 45 = 25 L/s left for irrigation.
Step 3: compare with demand.
Consented demand is 180 L/s, roughly 7 times the 25 L/s available. Even the estimated actual use of 140 L/s is about 5.6 times what the stream can give.
Step 4: interpret. The consents were granted on the assumption of higher flows, so the stream is over-allocated: the paper entitlements add up to more water than exists in a dry year. The 250 L/s minimum is also a floor, not a target; it is the point below which habitat and the lagoon start to degrade. If all farmers irrigated at the consented rate, the flow would fall to 320 - 45 - 180 = 95 L/s, well below the ecological minimum.
That is why a cut is unavoidable: the only question is how to share the 25 L/s and how to protect the eel habitat at the same time.
2. Recommend an option, or a combination, and justify it using environmental, economic and social considerations.
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Environmental: the eel lagoon and the 250 L/s floor are non-negotiable; any option must keep flow at or above that level in a dry February. Option A does this by triggering cuts on flow; Option B does not guarantee a flow outcome, only a use cap; Option C would help in three years but does nothing this summer.
Economic: a 30% cut (Option A) hits all nine farms regardless of efficiency, so the farmer who already uses less is penalised and there is no incentive to improve. Metering (Option B) is cheap, gives the council real data, and charging per cubic metre rewards efficiency. Option C is $4.8 million and three years away, and only works if winter flows are genuinely surplus.
Social: town supply must be protected, and Iwi values need to be recognised in the allocation, not treated as an afterthought.
Recommendation: combine B and A. Meter immediately, cap total irrigation at the 25 L/s available, and set an efficiency-based cut triggered at 350 L/s. Use the data to redesign consents. Start consenting the storage dam (C) as a long-term fix only if winter flow data shows a genuine surplus. Punishing all farms equally is simpler but wastes water and goodwill; metering creates the information needed to allocate fairly.
3. Explain how unmetered water use makes the council's decision harder.
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Unmetered use turns a measurable problem into an estimate, and that has three effects.
- The size of the cut is uncertain. The council is told use is 'about 140 L/s', but the range might be 100-180 L/s. A 30% cut based on 140 could leave the stream above the minimum, or could still breach it. You cannot calculate the shortfall reliably.
- Fairness is impossible to demonstrate. Without meters, a farmer who has already reduced use cannot prove it and will still be cut, while a high user is protected by the average. That invites non-compliance just when the council needs cooperation.
- No feedback or learning. Meters show which farms, soils and irrigation methods use least water per hectare, which is the information needed to redesign consents. Without it the council will be making the same decision with the same guesses next summer.
There is also a monitoring gap at the environmental end: if the flow gauge is accurate but abstraction is unknown, a drop in flow cannot be attributed to irrigation or to drought. The council may act on the wrong cause.