Source
https://www.orc.govt.nz/media/15680/memo-default-minimum-flow-and-allocation-limits-for-otago.pdf — original source (opens in a new tab; the file is not redistributed)
Cawthron / ORC — default minimum flow and allocation limits for Otago (Hayes 2021)
The regulatory frame behind the model's environmental-flow rule (RT_329, supporting)
Cawthron Institute advice memo (John Hayes, 17 Sept 2021) to Otago Regional Council — a paired Cawthron/NIWA advice letter on default minimum-flow and primary-allocation limits for the Regional Plan: Water. 19 pp. Technical advice feeding a regional plan, not a regulation itself. Filed alongside OT_099 (the observed drought data).
Summary
Cawthron’s advice to ORC on default minimum-flow and allocation limits — the limits that apply where a regional plan has set none — expressed as percentages of naturalised 7-day MALF. It reviews the 2008 proposed National Environmental Standard for Flows and Water Levels (NES), the Beca (2008) method-selection guidance, and Richter et al.’s (2012) international presumptive flow standard, then proposes defaults for Otago.
For Neobiome it does three jobs. (1) It confirms the 90%-of-MALF minimum flow for small streams (mean flow ≤5 m³/s — i.e. essentially every stream the D26 hydro gate admits) from a regulator-facing primary, where the wiki previously rested on a single MBIE report. (2) It shows the engine’s 10%-of-MALF usable flow is conservative against the 20–30% allocation rates actually contemplated in practice. (3) It supplies the high-baseflow vs rain-fed vocabulary that has elsewhere been mistakenly attributed to HBRC.
Key claims
- claim: "2008 proposed NES interim defaults, as percentages of naturalised 7-day MALF (verbatim): 'The 2008 proposed NES recommended a default minimum flow of 90% of MALF, and a total allocation rate of 30% of MALF, for rivers with a mean flow of less than 5 m3/s. For rivers with a mean flow greater than 5 m3/s, it recommended a minimum flow limit of 80% of MALF and an allocation rate of 50% of MALF.'"
source_location: "Section '2008 proposed NES', p.3"
- claim: "Cawthron's PROPOSED default limits for Otago (Table 1), expressed as % of naturalised 7-day MALF: surface water body with mean flow <= 5 m3/s — minimum flow 90%, allocation rate 20%. Mean flow > 5 m3/s — minimum flow 80%, allocation rate 30%. (The allocation rates are TIGHTER than the 2008 NES's 30%/50%.)"
source_location: "Table 1, p.8"
- claim: "Beca (2008) high-alteration threshold (verbatim): 'Abstraction of more than 40% of naturalised 7-d MALF, or any flow alteration using impoundments, would be considered a high degree of hydrological alteration, irrespective of region or source of flow.' A total allocation of 20-30% of MALF 'could be considered a high degree of hydrological alteration in rivers and streams with mean flow less than 5 m3/s, depending on the instream values and baseflow characteristics.'"
source_location: "Section '2008 proposed NES', p.3-4"
- claim: "High vs low baseflow definition (verbatim, Appendix 2 footnote): 'A high baseflow river is one where the low flows are relatively high compared to the mean flow, such as in rivers with frequent freshes, rivers with their sources in hilly or mountainous areas or rivers fed from lakes, or springs. A low baseflow river is one where the low flows are very much lower than the mean flow, such as occurs in rain-fed rivers in areas that are not subject to orographic rainfall. Many of the rivers under abstraction pressure in the dry country of Otago will be of this character.'"
source_location: "Appendix 2, Table 2 footnote 1, p.17"
- claim: "Ecological risk rises as streams get smaller (verbatim): 'Risk of deleterious effect is related to stream size; the smaller the mean flow, the greater the risk presented by the same flow alteration when defined as a percentage of MALF.'"
source_location: "Appendix-2 framework discussion, p.5"
- claim: "Richter et al. (2012) presumptive flow standard: daily flow alterations of no more than 10% give a high level of ecological protection; 11-20% a moderate level; 'Alterations greater than 20% will likely result in moderate to major changes in natural structure and ecosystem functions.' Unlike a minimum-flow method, it allows some take at all flows and has no flow below which take ceases."
source_location: "Section 'Richter et al.'s (2012) presumptive flow standard', p.5"
- claim: "NIWA regional hydrology model (Singh et al. 2021, for ORC): provides naturalised hydrological statistics and daily flow time-series for ALL Otago streams of Strahler order 3 or higher — including 1-in-5-year low flow, 7-day mean annual low flow, mean flow, median flow, proportion of flow in February (driest month), and FRE3. Fitted from 59 of 250 gauging sites meeting record-completeness and minimal-abstraction criteria; the memo cautions those 59 'did not represent an unbiased sample of river locations'."
source_location: "Appendix 1, 'Regional hydrology model', p.10"Neobiome Intelligence relevance
Corroborates the D26 environmental-flow cap from a second, regulator-facing direction. micro_hydro previously cited the 90%-of-MALF residual rule only to OT_075 (an MBIE opportunities report); this memo confirms it in the words of the technical adviser to a regional council, and pins the <5 m³/s band it applies to — which covers essentially every community-scale stream the D26 gate admits.
It also shows the engine’s usable-flow rule is cautious, not aggressive:
| Rule | Fraction of MALF a scheme may use |
|---|---|
| 2008 NES allocation rate (mean <5 m³/s) | 30% |
| Cawthron’s proposed Otago allocation rate (≤5 m³/s) | 20% |
| Beca “high degree of hydrological alteration” threshold | >40% |
Engine usable_q = 0.10 × MALF (D26) | 10% |
The 90%-of-MALF minimum flow leaves exactly 10% headroom at MALF, so the engine’s 10% is the conservative reading of the binding constraint. No model change — corroboration.
It does NOT resolve RT_333 — and it slightly sharpens the question
The memo contains no run-of-river, non-consumptive, or hydro-generation language anywhere; it is entirely about consumptive water resource use (takes, allocation). It therefore neither exempts nor bans a micro-hydro diversion. But Beca’s threshold explicitly names “any flow alteration using impoundments” alongside abstraction — evidence that the framework regulates flow alteration, not only consumption. That makes a blanket “run-of-river is exempt from low-flow bans” assumption less safe, not more. RT_333 stays open.
Separately, the memo’s finding that ecological risk grows as streams get smaller (“the smaller the mean flow, the greater the risk presented by the same flow alteration”) is an independent reason for caution on exactly the small streams the model gates. Note this is a different mechanism from RT_334: RT_334 asks whether small streams fall further in drought (unevidenced); this says the same percentage abstraction does more ecological harm on a small stream (evidenced). Both point the same way — be conservative on small streams — but they are not the same claim and should not be conflated.
Research targets
Documents to retrieve
- RT_336 — NIWA regional hydrology model, Singh et al. (2021) for ORC. Naturalised 7dMALF, 1-in-5-year low flow, mean/median flow and February flow proportion for all Otago streams of Strahler order ≥3. This is a genuine small-stream low-flow dataset — the thing OT_099’s large-river analogue lacks (advances RT_334) — and a potential validation set for the GIS sampler’s per-site MALF. NIWA runs equivalent national models, so the method may generalise beyond Otago. ⚠ Sampling caveat the memo raises itself: the 59 gauged sites used to fit it “did not represent an unbiased sample of river locations”.
Research gaps
- None new. (RT_333 sharpened but unresolved; RT_334 unchanged.)
Notes
Authoritative Cawthron/ORC document (not AI-prepared) — data_quality: high. Retrieved 2026-07-14; figures read verbatim via pdftotext -layout.
Provenance correction carried from OT_099: the “rain-fed vs high-baseflow” framing originates here (Appendix 2 footnote), not in the HBRC report. HBRC attributes the Wairoa River’s high 2019-20 drought ratio (0.92) to that catchment escaping the regional rainfall deficit, not to baseflow character. The two should not be merged.
Connections
Links to
Technologies (1): Micro-hydro (run-of-river, community-scale)
Referenced by
Sources (4): CR_053 · OT_099 · OT_101 · OT_160
EDT domains (1): D03: Water, Waste & Circular Systems
Technologies (1): Micro-hydro (run-of-river, community-scale)