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Acknowledgements

Executive Summary – Recommendations

1 Introduction

2 Rivers

3 Lakes and Wetlands

4 Groundwaters

5. References of Main Text and Appendices

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

Appendix 2: Effects of Flow Regime on Stream Ecology

Appendix 3: Methods Used in Ecological Flow Assessments of Rivers

2 Rivers

2.1 Assessment of instream values and critical factors

An assessment of instream values is an important part of selecting an ecological flow regime for rivers. It establishes the biological communities, amenity and other values that could be affected and thus the methods of assessment to be used; it also establishes baseline data for the consideration of environmental effects. The identification of critical factors is also an important part of the process because instream values may be indirectly affected by flow-related factors that have a high flow requirement, such as provision of food and connectivity for some fish species.

Instream values may be grouped into:

• ecological or intrinsic values

• landscape, scenic and natural characteristics of the river

• amenity values – recreational angling and fishing

• amenity values – boating and other recreational activities undertaken in, on or near the river

• Māori values, including traditional fishing

• commercial value for fishing.

There are, of course, overlaps and linkages among these values and in this report we focus on ecological or biological values, particularly ‘flow-related values’ that change in a discernible way as flow changes. Flow requirements of amenity values – such as fishing, boating, and swimming – can be determined using some of the same methods presented for biological values (eg, such as instream habitat models). Factors like water quality, water temperature and the micro-distribution of turbulence and velocity also change with flow; yet these flow-related changes are often small and the biological effects are difficult to predict because of the large natural variation in these factors and the tolerances of aquatic organisms. Table 2.1 lists the ecological (instream) values relevant to rivers and streams and possible factors that might affect the values and are considered to be flow-related to some degree. Some of the possible factors are closely related to flow, some of the factors apply only in special circumstances (eg, seston supply is specific to lake outlets), and in some instances the relationships between flow and the factors have not been established (eg, effects of flow on pH): further research is required to determine the relationship between the biological community, factor and flow requirements.

Table 2.1: Examples of biological instream values and management objectives

Section 88 in the fourth schedule of the Resource Management Act (1991) states that an assessment of effects “shall be in such detail as corresponds with the scale and significance of the actual or potential effects ...”, and section 92(4) RMA allows a consent authority to require further information from a consent applicant if necessary to “... better understand the nature of the activity ..., the effect it will have on the environment, or the ways in which any adverse effects will be mitigated”. In particular, the relevant biological matters are: section 5(2)b “safeguarding the life-supporting capacity of air, water, soil and ecosystems”, section 6(c) “the protection of areas of significant indigenous vegetation and significant habitats of indigenous fauna”, section 7(d) “the intrinsic values of ecosystems”, and section 7(h) “the protection of the habitat of trout and salmon”.

The level of biological assessment for fish, benthic invertebrate, periphyton and macrophyte communities will depend on the degree of hydrological alteration. Where the degree of alteration is small, adequate data may already be available. Where the degree of alteration is high, detailed and quantitative biological information is required to assess biological significance and establish a baseline for evaluation of potential effects.

2.1.1 Significance of instream values

The significance or relative importance of the instream values that are associated with a river informs the level of protection that should be considered, and also the technical methods used to assess ecological flow requirements. As the relative importance of instream values increases, the consequences of not meeting the environmental goals also increase. Because of this risk, the most robust and biologically supportable technical methods should be used to assess ecological flow requirements in highly valued rivers.

The significance of instream values can be judged by:

• national or regional significance of biological assemblage (biodiversity)

• species abundance, rarity or scarcity

• popularity (eg, for trout angling or whitebaiting).

A rigorous and consistent process should be used for determining national or regional significance. For example, it is very easy to say that a set of factors makes a river ‘unique’ because all rivers are different in some way. Most rivers are considered ‘locally important’ because they provide a source of food, water, valued biological community, or recreational activity and it is necessary to compare rivers over a reasonably large area to determine their relative importance.

The determination of significance is assisted by national databases, such as the Department of Conversation’s electronic threatened species list (http://www.doc.govt.nz/templates/ MultiPageDocumentTOC.aspx?id=39578), the New Zealand Freshwater Fish Database, and national surveys such as trout abundance (Teirney and Jowett 1990), Fish & Game Council trout angling surveys (eg, Unwin and Image 2003), and national inventories of rivers (eg, Teirney et al 1982).

2.2 Determination of degree of hydrological alteration

2.2.1 Components of hydrological alteration and flow variability

The biologically important components of a hydrological regime are:

• magnitude and duration of minimum flow: for streams and rivers these set the limit to habitat quantity and can influence connectivity to other habitats such as wetlands

• magnitude, frequency, and duration of high flows: in streams/rivers it is the magnitude of high-flow events sufficient to cause substantial movement of fine particles (fine gravel or smaller) that is most relevant in this context. These occur moderately frequently and contribute to maintaining habitat ‘quality’ through flushing away accumulations of silt and periphyton from coarse sediments. The magnitude of such flow perturbations is usually about 3–6 times the median flow (or 3–6 times the low flow in a highly regulated river) (Biggs and Close 1989; Clausen and Biggs 1997). While they cause sand and fine gravel movement, they seldom move larger bed sediments such as cobbles where invertebrates and fish hide during such events. Some designed ‘flushing flow’ events below storage dams may need to be higher in magnitude for removal of some periphyton and silts deeper in the gravels of regulated rivers

• magnitude, frequency and duration of flood flow sufficient to cause substantial movement of the armour layer and erosion of banks in rivers: these flows cause large disturbances to the river and its floodplain and often wash most periphyton, macrophytes and invertebrates from rivers, together with a large proportion of young introduced fish species (McIntosh 2000). Most native fish species appear to have evolved to cope with these floods and may take temporary refuge in more sheltered bank areas. Studies of New Zealand rivers indicate that flows of more than about ten times the mean flow or 40% of the mean annual maximum flow begin to move a substantial portion of the river bed (Clausen and Plew 2004).

The above discussion is focussed on hydrological alteration from the viewpoint of how instream habitat is affected. Consideration should also be given to whether hydrological alteration of rivers will affect connectivity of rivers with riparian wetlands, springs and groundwater. Potential critical factors include timing, frequency and magnitude of inundation (as referenced to water level) of wetlands, surface–groundwater exchange, and maintenance of fish passage. This requires knowledge of the pattern and ecological significance of water level variation in wetland and groundwater systems (see chapters 3 and 4).

The biologically important parts of a hydrological flow regime show that variability above the minimum flow is usually required to maintain healthy ecosystems. Flow variability is determined from the overall pattern of low and high flows during the year. Figure 2.1 shows the relative magnitude of the different high flow events during 2004/05 in the Waiau River, in southern New Zealand. Low flows that set habitat quantity are in the range of 70–100 m³/s, whereas high flows that help maintain habitat quality are in the range of 300–600 m³/s (ie, 3–6 times the low flows). Flood flows that can alter channel morphology and transport bed sediments are greater than about 1,000 m³/s.

The biological effects of hydrological alteration will depend on source of flow, stream size, and biological community. The importance of low/minimum flows for the provision of adequate habitat quantity is fairly clear, as are the effects of large flood flows on channel structure and aquatic fauna. However, through extensive laboratory and field experimental studies we have now begun to understand the link between habitat quality and ecosystem productivity/health of New Zealand rivers, and how these are driven by the magnitude, frequency and duration of small floods/flushing flows.

Lake outlets and spring-fed streams have few floods and freshes, but their biological communities are usually not considered degraded, with many spring-fed streams and lake outlets containing very high densities of trout and benthic invertebrates. In this type of river, natural low flows are usually relatively high for the channel shape (ie, the channel is relatively full), habitat quality is high, and sediment transport is low.

However, the source of flow in many streams is rainfall and this generates flows that are more variable than those from lake or groundwater sources. Rain-fed streams transport more fine sediment than spring- or lake-fed streams, so floods and freshes are necessary to remove fine sediment that accumulates during steady flows. In rain-fed streams, habitat quality in streams with frequent high flows tends to be higher than in streams with infrequent floods. This is simply because the magnitude of the low flows depends on the recession rate and time between high flows – the stream seldom has enough time between events to reach low flows.

Although the biological communities in streams with frequent floods and freshes are often considered to be more desirable than those in streams with infrequent floods, the biota reflect the quality of the habitat at low flow rather than flow variation. For instance, some lake outlet rivers, which have low flow variability and good water quality, have abundant populations of macroinvertebrate species which provide good trout food, and also provide good habitat for trout at low flow (Jowett 1992; Harding 1994). In contrast to lake and spring-fed rivers, long flow recessions that occur in rain-fed rivers with infrequent floods can result in poor-quality habitat, with aquatic communities that are associated with low-velocity environments (Jowett and Duncan 1990). Thus, stream biota will benefit from flow variation only in cases where habitat quality at low flow is poor and any flow variation increases the amount and quality of habitat, provided that there is sufficient time for a biological response.

The amount and quality of habitat at low flow varies with stream size and the flow recession rate and time between high flows, as described above. Habitat quality in small streams is often relatively poor at low flows and any further reduction in flow results in deterioration in habitat quality and consequent biological response. In comparison, flow reductions in large rivers will not necessarily result in a decline in habitat quality. Whether a river is considered small or large in this context depends on the biological community, because habitat and consequently flow requirements depend on the biological community that is to be supported.

Although flow variability is often thought of as an essential element of the flow regime that should be maintained, there is still relatively little evidence that flow variability affects instream communities either directly on indirectly in New Zealand rivers. Valued biological communities can be maintained in rivers where the flow regime has been extensively modified, but the needs of the instream values have been specifically identified and targeted in the management regime (Jowett and Biggs 2006) by some of the methods discussed here.

2.2.2 Degree of hydrological alteration and flow variability

The degree of hydrological alteration will depend on the way in which water is managed within the catchment. Water use can be divided into three categories of increasing hydrological alteration.

a. Consumptive use or abstraction

Water is taken from the river and used for activities such as water supply and irrigation, often with seasonally varying demand. The biologically relevant component affected is the magnitude of low flows, with a minor effect on duration. For example, abstraction of up to 10% of the mean annual low flow (MALF) is barely measurable and therefore unlikely to result in significant biological effects in any stream. Abstraction of up to 20% of MALF is unlikely to result in significant biological effects in lake- or spring-fed streams or in streams with frequent floods and freshes, such as those draining mountainous regions exposed to the prevailing westerly winds. When total abstraction exceeds these limits, the magnitude and duration of low flow may have significant effects on biota. In that case, measures such as a higher minimum flow (Dewson et al 2007) or various flow-sharing regimes can be taken to avoid adverse biological effects.

b. Diversion or large scale abstraction

Water is diverted from the river on a relatively large scale and may be returned to the river downstream or discharged into another catchment. A diversion or abstraction is considered large-scale when it is able to divert more than 90% of the MALF out of a river. The biologically relevant components affected are the magnitude and duration of low flows. The frequency of flushing flows may also be affected if the capacity of the diversion is sufficiently large (eg, > 1.5 times the mean flow). With large-scale diversions or abstractions, the quality and amount of habitat at minimum flow will directly affect the biological communities because flows are at the minimum for substantial periods of time. Consequently, the minimum flow required to support these communities should be higher than the minimum flow that would be applied to situations with short-duration low flows (Dewson et al 2007).

c. Storage regulation

River flows are modified by storage with potential change to the seasonality of flows, minimum flows, and high flows. Storage regulation can be consumptive (water supply or irrigation) or non-consumptive (hydro-electricity). The potential degree of regulation will depend on the storage volume in the impoundment. Storage regulation can affect all biologically important components of the flow regime.

2.2.3 Determination of the degree of hydrological alteration

In order to develop a relationship between the potential abstraction (degree of hydrological alteration) and the effects on instream management objectives or values, one first needs to understand the risks involved. For management or preservation of fish communities, these risks are well understood and are related to stream size (decreasing available habitat) and the preferred flow requirements of the fish species present. For example, many small galaxids and bullies prefer low velocities and shallow water, juvenile salmonids tend to prefer moderate water velocities, and adult trout are commonly found in fast-flowing and deep water. Because water velocity and depth tend to increase with stream size, optimal stream size for various fish species can be broadly categorised (Table 2.2). The risks of deleterious effects on fish communities in small streams are higher than in larger streams and rivers. However, for other values such as invertebrates or river birds (Table 2.1), there is currently no strong quantitative relationship with flow. However, as explained in Section 2.1 (p.5) the critical factors associated with fish community values can also be applied generically, which enable us to assess flow-related risk for some of these other values, based on their known relationship with fish communities.

In addition, flow in-river may well be managed (and therefore flows set) to provide flows to connected systems such as riparian wetlands, springs and groundwater flows.

The risks to fish communities (and where they can be applied generically to other biotic values) are categorised in Table 2.2 in terms of mean flow. Although other measures of flow (eg, median and mean annual low flow) could be used, mean flow is the measure that can be estimated with the greatest accuracy.

The extent to which abstraction affects the duration of low flows is a useful measure of the degree of hydrological alteration. Increasing the duration of low flows increases the risk of detrimental effects, and if low flows persist for 30–50 days per year, there will be noticeable growth of algae and changes to invertebrate communities and potential effects on fish (eg, Suren and Jowett 2006; Jowett et al 2005).

A high degree of hydrological alteration is assumed to occur when abstraction increases the duration of low flow to about 30 days or more, with moderate and low levels of hydrological alteration corresponding to about 20 days and 10 days, respectively.

Table 2.2: Assessment of risk of deleterious effects on instream habitat according to fish species present and natural mean stream flow (and generic application to other values/management objectives)°

* Access to and from the sea is necessary.

+ Access to spawning and rearing areas is necessary.

° Actual degree of impact will depend on the degree of hydrological alteration whether or not the level of risk is high or low.

Note: The data in the column for ‘Salmonid spawning and rearing, torrentfish, bluegill bully’, may be generically applied to invertebrates and riverine bird feeding (eg, wading birds, blue duck, black fronted tern).

In areas with frequent rain such as those exposed to the south or to the prevailing westerlies, flows are relatively reliable; and the mean annual low flow (usually defined as the average of the annual seven-day minimum flows) is a relatively high proportion of the mean flow and occurs for less than 3% of the time (11 days a year). Low flows are also relatively high compared to the mean flow in rivers fed from lakes or springs or pumice catchments. Abstraction of water increases the percentage of time flows are low, with the number of days per year at low flow (below MALF) increasing by 2–3 days for each 10% of MALF abstracted. In these rivers with reliable flows, abstraction of 40% of MALF will extend the duration of low flows to about 30 days.

In drier regions, such as eastern rivers with their source of flow in the rain shadow of ranges, flows are less reliable. The MALF is less than 1/20th of the mean flow and occurs for 6–10% of the time (c. 30 days per year). In these rivers, abstraction increases the duration of low flows by about 5–8 days per year for every 10% of MALF and abstraction of 20% of the MALF will extend the duration of low flows to about 40 days.

Table 2.3 lists the degree of hydrological alteration that would be caused by abstracting various amounts (10–40%) of the MALF for rivers with high and low baseflows for categories of risk based on stream size and species present. In this table, a high baseflow river is one where the mean flow is less than 20 times the MALF, such as occurs 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 mean flow is more than 20 times the MALF. The degree of hydrological alteration for a river can be determined, first by determining the risk based on mean flow and fish species present (Table 2.2), then using Table 2.3 to determine how the total abstraction (in terms of MALF) affects the degree of hydrological alteration with the risk category and stream source of flow.

Table 2.3: Relationship between degree of hydrological alteration and total abstraction expressed as % of MALF for various risk classifications (Table 2.2) based on stream size and species composition

View relationship between degree of hydrological alteration and total abstraction expressed as % of MALF for various risk classifications (Table 2.2) based on stream size and species composition (large table)

Figure 2.2 shows the geographic distribution of rivers classified in the River Environment Classification (REC) as either high or low baseflow rivers based on the ratio of mean flow to mean annual low flow. Flow statistics in the REC are based on regional models and do not necessary account for spring sources; in some situations it may be necessary to assess baseflow status using local knowledge. Note that rivers with their sources in mountain areas can flow through areas containing rivers with low baseflows.

Once the degree of hydrological alteration is determined, appropriate technical methods can be selected to assess flow regime requirements (Section 2.3).

Abstraction of more than 40% of MALF, or any flow alteration using impoundments would be considered a high degree of hydrological alteration – irrespective of region or source of flow.

A discussion of the relationship between total allocation and the ecological flow requirements of rivers is given in Appendix 1.

2.3 Which method? Decision-making framework

The decision as to which method to apply for assessing the ecological flow requirements of a particular river depends firstly on the significance of the value to be managed and the critical factors affecting those values (Section 2.1). Gauging the significance of values requires stakeholder input (eg, iwi, Department of Conservation, Fish & Game Council, interested general public). Identification of the critical factors affecting those values may also require expert input (eg, to assess whether water temperature is affecting or likely to affect fish or invertebrates).The decision on method depends secondly on the potential for hydrological alteration, which can be determined using the procedure outlined in Section 2.2. This framework (Table 2.4) allows selection of a methodology appropriate to the significance of the instream values and the potential for hydrological alteration. One or more of the methods listed within each cell of Table 2.4 should be used to assess ecological flow requirements for the given combination of degrees of hydrological alteration and significance of instream values. In situations with high instream values, two or more methods from each cell should be used, because the risks to stream ecology of making an incorrect ecological flow decision are greater.

The methods within each cell in Table 2.4 are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. For example, if elevated water temperatures affecting fish passage was the main concern under a medium alteration/high values scenario, then there would be little sense in using a hydraulic habitat model and vice versa.

Table 2.4: Methods used in the assessment of ecological flow requirements for degrees of hydrological alteration and significance of instream values

The rationale behind this framework is that techniques that are simple to apply and require no or little additional data, are appropriate where both instream values and potential degree of hydrological alteration are low; but when either values or hydrological alteration increases, then more complex methods are justified. It can be seen from Table 2.4 that some methods listed are specific to establishing the ecological flows needed to manage critical factors (eg, temperature, seston flux, bank stability) whereas others are aimed more generally at maintaining aquatic habitat (eg, generalised habitat models, one-dimensional (1D) or two-dimensional (2D) hydraulic habitat models). A summary of the methods listed in Table 2.4 is given in Section 2.4, including the advantages and disadvantages of each method.

2.4 Summary of technical methods

2.4.1 International approaches

Organisations responsible for water management are becoming increasingly aware of their responsibilities for environmental protection, creating an increasing interest in methods of assessing flow requirements for different instream uses. In Europe, there are attempts to rehabilitate large rivers that have been controlled and channelised for centuries. In the United States, attempts are being made to rehabilitate the lower Mississippi River and remove dams elsewhere and in Australia, the extensive flow regulation of the Murray-Darling River system is being questioned. Although operating on a smaller scale, water managers in New Zealand are required to assess the impact of water use on the stream environment through regional plans; and whenever development of the water resource is proposed or when the consents for use for that resource are reviewed.

The approach to flow assessment currently favoured in Australia and South Africa is a ‘holistic’ approach that maintains a natural flow regime, low flows, seasonal variation, and flood frequency in order to protect aquatic fauna. A minimum-flow policy that restricts abstractions to the level of naturally occurring low flows and maintains major elements of the natural flow regime will maintain stream fauna, essentially in a natural state. This is a ‘safe’ environmental policy and one that will ensure the protection of aquatic resources in most situations. In this report, we suggest an approach that is cognisant of the holistic natural flow paradigm, while maintaining the biologically important elements of the flow regime.

Rivers will have different flow requirements depending upon the species that are supported by the river and their life cycle requirements. The challenge is to determine the aspects of the flow regime that are important for the various biota associated with their rivers, and to develop flow regimes that meet those needs. Experience in six New Zealand rivers has shown that flow regimes that are very different from the natural flow regime can sustain excellent fish and invertebrate populations and achieve instream management objectives (Jowett and Biggs 2006).

Annear et al (2002) discuss instream assessment tools used in the United States and Canada and describe and comment on 29 different methods relating to hydrology, biology, geomorphology, water quality and connectivity. In the United States, the most commonly used method of assessing flow requirements is the instream flow incremental methodology (IFIM). This method is considered the most defensible method available at present and is particularly useful in ‘trade-off’ situations (Table 2.5). Low flow is not necessarily the factor that limits aquatic populations. Many studies have attempted to link stream fauna and its abundance to flow magnitude and most have failed to show any relationship. However, intuitively there must be a point at which there is too little water in a stream for the continued survival of aquatic species. This minimum flow is difficult to determine, and at present, instream habitat methods are the most biologically defensible approach to their determination.

An alternative ‘standard setting’ approach has been outlined by Richter et al (2006) who use a range of variability approach (RVA) to derive a range of recommended flows for the low flows in each month, high flow pulses throughout the year, and floods with targeted inter-annual frequencies. The hydrological modelling behind RVA is a four-step process which characterises the streamflow record using 32 different hydrological parameters and the range in variation of these at plus or minus one standard deviation from the mean.

The latest iteration of the RVA method includes the addition of a number of new parameters, designed to deal with problems which had become apparent with the use of the method (these parameters were incorporated into the Indicators of Hydrological Alteration (IHA) software in 2005). These new parameters are grouped into five ‘environmental flow components’(EFCs): extreme low flows, low flows, highflow pulses, small floods and large floods. The RVA method has been used the United States mainly in regulated systems to maximise the benefit of high-flow pulse releases of water from dams at a targeted magnitude, frequency, timing, duration and rate-of-change (Mathews and Richter 2007). To date the method has not been used in New Zealand for setting ecological flows and levels and is therefore not included in our recommended methods (Table 2.4). However, as the discussion document on the proposed National Environmental Standard for Ecological Flows makes clear, this technical document can be updated to reflect any new methods when their usefulness has been demonstrated in New Zealand. Further research is required on the relationship of RVA parameters to the biology, water quality, and geomorphology of river systems. Also the utility of the RVA method for setting ecological flows in New Zealand, particularly relating to abstraction, needs to be demonstrated.

Until the research discussed above is carried out, we propose that ‘analysis of hydrological variation’ should be included in the schedule of methods for rivers with a high significance of instream values. While analysis of hydrological variation will not by itself allow the setting of ecological flows, it will act as a ‘flag’ to other methods to illustrate the extent of hydrological change, and how these hydrological parameters may be affected by the ecological flow decision. Analysis of hydrological variation can be carried out using the RVA software or any other standard hydrological software that calculates flow statistics. Similarly, simple flow duration curves can be used where the proposed degree of hydrological alteration is low. Both analysis of flow variability and flow duration curves are standard hydrological techniques and are ‘flags’ to the potential importance of flow variability rather than ecological flow setting methods in their own right; therefore they are not discussed further in the description of individual methods (Section 2.5).

2.4.2 Habitat-based methods in New Zealand – uses and criticisms

Minimum flow assessments based on hydraulic habitat have been used in New Zealand for 25 years. In that time there have been considerable improvements to the survey and analysis techniques, and to our knowledge of habitat preferences of New Zealand aquatic fauna and flora. The effectiveness of New Zealand flow assessments based on habitat methods has been examined and generally the response of aquatic communities has been consistent with habitat change predictions. Jowett and Biggs (2006) showed that an increase in minimum flow resulted in expected improvements for trout numbers in the Waiau River, but not in the Ohau River. Similarly, increases in minimum flow improved the benthic invertebrate communities in the Monowai and Moawhango rivers. They showed that a decrease in flow probably improved the trout fishery in the Tekapo River. Jowett et al (2005) showed that low flows in the Waipara River were particularly detrimental to high velocity native fish species in the Waipara River. Richardson and Jowett (2006) showed that fish community changes in the Onekaka River were in accordance with habitat changes, with a decrease in minimum flow decreasing the abundance of koaro, but not redfin bullies. These are the only known case studies of how flow regime change has affected biological communities in New Zealand rivers, other than instances where rivers or streams have been completely dewatered with obvious consequences. One objective of the National Institute of Water and Atmosphere (NIWA)’s current research programme is to monitor and report the effects of flow regime changes so that we can learn from these experiences.

Habitat methods are based on hydraulic models that predict how water depths and velocities change with discharge. The same hydraulics models can also be used to evaluate the effects of flow regime changes on many aspects of the riverine environment, including sediment entrainment (for flushing flow and channel maintenance flow requirements), fish passage, water quality, sediment or seston deposition, and fish bioenergetics.

Habitat methods, although often described as micro-habitat, are in fact evaluating meso-habitat. The variation of flow with habitat can be determined from relatively few cross-sections and the selection of an appropriate tool will depend on the type of river and extent to which data are extrapolated. Habitat analyses based on simple hydraulic geometry, one-dimensional (1D) or two-dimensional (2D) surveys will often produce useful and similar results. The survey techniques described here are capable of predicting depths and velocities to the scale of the survey, which is usually measurements spaced at 0.1–3 m. This is consistent with habitat suitability observations that usually describe meso-habitats – the characteristics of the area in which the organism lives – rather than the micro-hydraulics of its precise location. In assessing suitability for one target species, we are often assessing conditions for a number of species that live in that area. Riffle-dwelling fish and invertebrates are an example, where the habitat suitability curves describe riffle conditions, rather than micro-habitat of the location of an individual organism. The selection of a target species (fish or invertebrate) as an indicator of stream health is a concept that can be applied to flow assessment.

The derivation and use of habitat suitability models are the most important aspects of flow evaluation. The tasks of survey, calibration, habitat suitability and analysis, and finally the interpretation of results require a good knowledge of river mechanics, hydraulics, and ecology. Survey (habitat mapping) and hydraulic calibration used in river hydraulic habitat simulation (RHYHABSIM) are relatively robust; techniques such as water surface modelling and 2D modelling, are more complex.

Habitat suitability curves can be derived and used inappropriately. Although habitat suitability criteria are available for many New Zealand aquatic organisms, they can be improved by collecting more data and recalculated habitat suitability models. The question of hydraulic scaling, or transferability between rivers of markedly different size and gradient, for benthic invertebrate and rainbow trout habitat is a problem that has yet to be solved.

Although the functions of flow regime components (such as low flow, flow variability, flushing flows, and channel maintenance flows) are known, we do not know the degree to which the frequency and duration of these events affect biota; and we do not have any quantitative method of assigning acceptable frequencies and durations, other than mimicking nature. However, for periphyton and benthic invertebrates it is possible to provide rough guidance on an appropriate flushing flow frequency based on periphyton growth rates and reported invertebrate colonisation times.

Hudson et al (2003) were critical of the application of IFIM and physical habitat simulation/ river hydraulic habitat simulation (PHABSIM/RHYHABSIM) in resource management decisions in New Zealand. Their biggest concerns were the lack of knowledge around the habitat preferences of many of New Zealand’s freshwater biota. As stated earlier, without good knowledge of these requirements it is difficult to make predictions about how changes in flow will affect available habitat for those species. Hudson et al (2003) went so far as to say “... habitat suitability curves have been developed for a very limited range of conditions.” As noted earlier, habitat suitability curves are very important, and although we believe that the existing curves are based on hydraulic conditions commonly experienced in New Zealand rivers, we support additional collection of information. Native fish preference curves are being revised using a database of over 6,000 observations.

Hudson et al (2003) were also concerned that the effects of other habitat characteristics, (such as water temperature and water quality) on freshwater biota that will also be affected by flow regimes, were rarely considered. However, IFIM involves considering all aspects of the instream environment that change with flow and this is recommended in this report for situations where there is a high degree of alteration.

Finally, hydraulic-habitat modelling is a tool to assist the decision-making process. No flow will maintain maximum habitat for all aquatic organisms. The selection of an appropriate flow regime for a river requires clear goals and target management objectives, with levels of protection set according to the relative values of the in- and out-of-stream resources. The process of establishing target management objectives is not a wish-list: management objectives should be relevant, important, flow-dependent and hierarchical. Failure to establish clear management goals and to carry out wide consultation will lead to conflict.

2.4.3 Summary of technical methods – categories, situations of use, pros and cons

Annear et al (2002) discuss instream flow assessment tools that are used in the United States. They divide them into three categories: standard setting where the method defines a flow; incremental where the method shows how stream characteristics vary incrementally with flow; and monitoring or diagnostic methods that assess conditions over time and compare the two broad categories of flow assessment (Table 2.5). The monitoring and diagnostic methods include indices of biotic integrity and hydrological alteration, and are not discussed in this report.

Table 2.5: Relative attributes of the two broad categories of flow assessment

Source: Annear et al 2002.

Although the more complex incremental methods based on hydraulic models could be used in every situation, it would not be cost-effective where values are low, nor would it be necessary to evaluate effects for aspects of the natural flow regime that would not be changed with proposed water allocation. Thus, the methods we suggest for assessing ecological flow requirements depend on the degree of hydrological alteration and the value of the instream resource that is affected (Table 2.4).

Technical methods of flow assessment that are currently available and are used in New Zealand are either standard setting methods (historical flows or expert panel assessments) or incremental methods (hydraulic-habitat). Traditionally, and appropriately, standard setting methods have been used to assess ecological flow requirements in situations where the natural flow regime is not expected to change markedly (except at low flows) and instream values are low to moderate. Hydraulic-habitat methods have tended to be used where the flow regimes are expected to change significantly, such as below a large-scale diversion or impoundment, or where instream values are high. Situations where each method might be used (related to type of hydrological alteration) are given in Table 2.6, and the pros and cons of each method are summarised in Table 2.7. A description of each individual method is given in Section 2.5.

Table 2.6: Methods used in the assessment of ecological flow requirements and the types of hydrological alteration for which they could be used

View methods used in the assessment of ecological flow requirements and the types of hydrological alteration for which they could be used (large table).

Table 2.7: Pros and cons of flow assessment methods

View pros and cons of flow assessment method (large table).

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