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Appendix 1: Relationship between Total Allocation and Ecological Flow Requirements in Rivers

Ideally, the effects of water abstraction or any other flow manipulation on the natural flow regime would be known in order to determine specific requirements of the ecological flow regime. For example, there might be a requirement for flushing flows below impoundments; or where the level of allocation results in extended periods of low flow, the minimum flow might be set higher or flow-sharing options considered. Unfortunately, ecological flow requirements must often be determined without knowing potential out-of-stream uses.

The most common situation is probably where a relatively small amount of water is taken from a river, usually for irrigation and sometimes for town supply. This might amount to 10–20% of summer low flows, but not be a significant proportion of high flows and not a sufficiently large total allocation to reduce the stream flow to a minimum for extended periods of time. Summer is the critical period for this type of abstraction because this is when maximum abstraction usually occurs and when low flows and high water temperatures may limit biological communities.

As the amount of abstracted water increases, so does the potential to reduce river flows to a minimum for extended periods. This is not necessarily deleterious as is often assumed. If instream conditions at the minimum flow are adequate (ie, provide optimal habitat quality or habitat levels that occur with annual natural low flows), then biota should not be detrimentally affected, provided the frequency of higher flushing and channel maintenance flows remains unchanged. However, if instream conditions at minimum flow provide less than optimal habitat quality (eg, average habitat suitability index), an increase in the duration of low flows increases the risk of detrimental effects. Few studies have examined the effects of extended duration of low flow. Jowett et al (2005) showed that in the Waipara River, where habitat is limited at low flow, the detrimental effect on fish numbers increased with the magnitude and duration of low flow. When summer flows were less than the mean annual low flow for about 30% of the time, there was a substantial decline in abundance of three of the four common native fish species in the river. When summer flows were less than mean annual low flow (MALF) for about 10% of the time, there was little change in native fish abundance. The effect was more severe on fast-water species (torrentfish and bluegill bullies) than species that prefer lower-velocity water (upland bullies and Canterbury galaxias).

If instream conditions at low flows are less than optimal, then increasing the duration of low flows through increasing total allocation increases the risk of detrimental effects. Obviously a reduction in the length of time that habitat is sub-optimal will reduce the detrimental effects. This can be done by increasing the minimum flow requirement (Figure A1.1) or by increasing the frequency of higher flows by a flow-sharing arrangement, whereby the amount of water available for abstraction at any particular time is some proportion of the natural flow less the minimum flow requirement. Either method reduces the total volume of water available for abstraction and the reliability of supply.

Figure A1.1: Conceptual relationship between minimum flow requirement and total allocation

Text description of figure

The graph shows a fast rising line where the minimum flow requirement reaches an asymptote which representes the optimal flow for instream objectives.

In practice, regional councils have imposed limits on the total allocation that do not significantly extend the duration of low flows and guarantee a certain reliability of supply to consent holders. This raises the question of what level of abstraction will significantly alter the duration of low flows, and in what type of river will the duration of low flow have a significant detrimental effect on biota? A river risks deleterious effect from flow reduction if habitat quality at natural low flows is less than optimal. Instream habitat analyses show clearly that small streams have less than optimal habitat quality for salmonids and many native fish species, but that flows in larger rivers can be reduced to create optimal habitat. In addition, a ‘small’ stream for salmonids is larger than a ‘small’ stream for native fish, so allocation levels, as well as flow requirements, will depend on the species present.

Allocation limits

Allocation limits have been used to manage water resources and a common approach is to limit total allocation to a proportion of a flow statistic such as the mean annual low flow. If the total allocation is low, the degree of hydrological alteration and thus ecological effect will be small. For example, if the total allocation is less than 10% of the MALF, abstractors will have high reliability of supply and there may be no need for any restriction such as a minimum flow requirement. This is because 10% of MALF is barely measurable with good flow measuring techniques and is therefore unlikely to have any biologically detectable effect.

The Motueka Conservation Order, 2004, limits abstraction to 12% of the instantaneous flow and presumably assumes that this will have negligible ecological effects. A 12% flow difference is just detectable by available flow measuring methods. This method of allocation guarantees that there is some water available for abstraction, even at lowest flows.

Another method of defining allocation limits is based on defining the acceptable level of risk to the environment and reliability of supply to the resource user. This is based on frequency and duration analyses of the hydrological record. For example, if the target reliability of supply is 95% and the frequency of the minimum flow is 1%, then the amount of water available for allocation is the difference between the flow that is exceeded for 94% of the time and the minimum flow. When the total allocation is being fully used, the frequency of occurrence of the minimum flow increases to the frequency of occurrence of the minimum flow plus allocation (ie, 6% of the time in the example). In terms of days, the frequency of the minimum flow increases from about four days per year to 22 days per year.

The effect of allocation extending the duration of low flows should be considered in terms of biological significance. Are there likely to be significant biological effects with the change in duration? In considering this, the quality of the habitat at low flow should be taken into consideration as described in the preceding section. If habitat at low flow is sub-optimal and limiting biota, extending the duration of low flows is likely to increase the detrimental effect on biota. Increasing the minimum flow can mitigate the detrimental effect; this calculation is described by Jowett and Hayes (2004). If habitat at the minimum flow is optimal or higher (Figure A1.1), then the biological effects of abstraction are likely to be minor or even beneficial; and there is no need to limit allocation from an ecological perspective until the volumes abstracted affect the magnitude and duration of minor freshes.

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