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Draft Guidelines for the Selection of Methods to Determine Ecological Flows and Water Levels

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

Executive Summary – Recommendations

Introduction

The Ministry for the Environment (MfE) is assessing the need for a National Environmental Standard (NES) on methods for establishing ecological flows and water levels for rivers, lakes, wetlands, and groundwater resources. As a part of this process, MfE sought scientific guidelines for selecting appropriate methods for determining ecological flows and water levels. Beca Infrastructure Ltd (Beca) was commissioned to coordinate the ‘capture’ of this advice from some of New Zealand’s top experts on the science of assessing the ecological requirements for ecological flows and water levels. This executive summary documents which approach the expert group recommends to be taken in selecting an appropriate method. The full report provides the underlying logic behind the recommendations.

It should be noted that this report relates only to method selection for establishing ecological flow requirements. Ecological flows are defined here as “the flows and water levels required in a waterbody to provide for the ecological integrity of the flora and fauna present within waterbodies and their margins”. This report offers no guidance on the process of how to set environmental flows (defined as “the flows and water levels required in a waterbody to provide for a given set of values which are established through a regional plan or other statutory process”) or the management implications of environmental flow decisions.

Methodology

Beca facilitated a two-day workshop in Christchurch on 19–20 December 2006. The workshop participants:

1. listed the ecological management objectives/values relating to the ecological flow/level of the river, lake, wetland or groundwater resource being considered, together with factors that might affect the ability to achieve that objective
2. listed the technical methods applicable to the setting of ecological flows and water levels for the type of water body under consideration and debated the pros and cons of each method
3. developed a matrix of methods applicable depending on the significance of the values perceived for the water resource under consideration, and the degree of hydrological alteration being considered for that water resource.

Subsequent to the workshop, lead writers – for each of: rivers, lakes and wetlands, and groundwaters – drafted documents intended to support the recommendations. Each of these documents was reviewed by three members of the workshop team as well as by the Department of Conservation (in the case of rivers and lakes) before being consolidated by Beca.

Recommendations: rivers

It is proposed that the approach to selecting technical methods to determine the ecosystem flow requirements of rivers be based initially on the risk of deleterious effects on instream habitat according to the species present and natural mean stream flow (Table 1). The risk of abstraction decreasing available habitat depends on stream size and the species present in the stream, with higher risks of deleterious effects in small streams than in larger streams and rivers.

Table 1: 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).

• The extent to which abstraction affects the duration of low flows is a useful measure of the degree of hydrological alteration. A high degree of hydrological alteration is assumed to occur when abstraction increases the duration of low-flow conditions to 30 days or more, with moderate and low levels of hydrological alteration corresponding to increases of about 20 days and 10 days, respectively.

The degree of hydrological alteration for a river can be determined, first by determining the risk based on mean flow and species present (Table 1), then using Table 2 to determine how the total abstraction (in terms of mean annual low flow, MALF) affects the degree of hydrological alteration for the stream and its risk category and its baseflow characteristics. In Table 2, 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. Further details are given in the supporting document.

Table 2: Relationship between degree of hydrological alteration and total abstraction expressed as % of mean annual low flow for various risk classifications (Table 1) based on stream size and species composition

View a relationship between degree of hydrological alteration and total abstraction expressed as % of mean annual low flow for various risk classifications (Table 1) based on stream size and species composition (large table).

• Once the degree of hydrological alteration is determined, Table 3 lists the technical methods that should be used to assess ecological flow requirements. One or more of the methods listed within each cell of Table 3 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 are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. Specific recommendations of the use of each of the methods are given in the supporting document.

Hydrological alteration of rivers involves an examination of a number of hydrological statistics, including flow variability of the system, which affects the quality of instream habitat, and the connectivity of rivers with riparian wetlands, springs and groundwater. Potential critical factors include magnitude and duration of low flows or levels, timing, frequency and magnitude of floods and the 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.

Table 3: Methods used in the assessment of ecological flow requirements for degrees of hydrological alteration and significance of instream values

Recommendations: lakes and wetlands

a. Lakes

The distribution and occurrence of healthy lake littoral habitats and communities vary with lake size, depth and water clarity. The risk of changing lake levels decreasing available habitat or adversely affecting communities depends on the lake bed profile (bathymetry), substrate type, water clarity, wave action as well as size and depth. The risks of deleterious effects are greater in shallower systems than in deep water bodies. Within a lake level range, impacts arise from changing seasonality in levels and the proportion of time spent at different levels (level duration).

• It is proposed that for lakes, the risks for a potential change to lake level may be defined as follows:
  • Low. Less than 0.5 m change to median lake level in lakes greater than 10 m depth, and less than 10% change in annual lake level fluctuation in lakes greater than 10 m depth; and less than 10% change in median lake level and annual lake level fluctuation in lakes less than 10 m depth; and, patterns of lake level seasonality (relative summer vs winter levels) remain unchanged from the natural state.
  • Medium. Between 0.5 and 1.5 m change to median lake level and less than 20% change in annual lake level fluctuation in lakes greater than 10 m depth; and between 10 and 20% change in median lake level and annual lake level fluctuation in lakes less than 10 m depth; and, patterns of lake level seasonality (relative summer vs winter levels) show a reverse from the natural state.
  • High. Greater than 1.5 m change to median lake level, and greater than 20% change in annual lake level fluctuation in lakes greater than 10 m depth, and more than 20% change in median lake level and annual lake level fluctuation in lakes less than 10 m depth; and, patterns of lake level seasonality (relative summer vs winter levels) show a reverse from the natural state.
• The risks for a potential change to lake level must also be defined in relation to seasonal and inter-annual level variability as determined by the methods shown in Table 4 below and documented in full in the main report.
• Once the risk of potential change to lake level has been established (degree of hydrological alteration) the technical methods that should be used to assess level requirements should be selected from Table 4. One or more of the methods listed within each cell of Table 4 should be used to assess ecological flow and level requirements for the given combination of degrees of hydrological alteration and significance of instream values. In situations with high lake values, two or more methods from each cell should be used, because the risks to ecology of making an incorrect ecological flow decision are greater. The methods within each cell are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. Specific recommendations of the use of each of the methods are given in the supporting document.
• The proposed categorisation of risks associated with potential changes in lake levels are based on the professional judgement/experience of lake experts within this team. We recommend that work be commissioned to provide scientific justification for this categorisation and provide an equivalent of MALF (and other flow statistics) based on level duration curves. Profiles of level duration demonstrate graphically and quantitatively the lake level regime, however there is currently no easy way to use these in a general rule-based format as they are calculated from absolute altitude. It will be possible to convert these to a relative level based on variance from a mean (or median) lake level. In this way curves between lakes could be compared and a general set of rules on level duration derived.

Table 4: Methods used in the assessment of ecological flow and water level requirements for degrees of hydrological alteration and significance of lake values

b. Wetlands

The distribution and occurrence of healthy wetlands varies with size and depth and connectivity to other hydrological systems. The risk of changing wetland levels decreasing available habitat or adversely affecting communities depends on the depth and the bathymetry and the dominant species present. Wetlands are generally shallow with wide littoral ephemeral areas that are dependent on a number of different flow-dependent variables. Therefore risks to wetlands are perhaps greatest compared with any other freshwater ecosystem. The risks of deleterious effects are greater in shallower than in deepwater wetlands, and wetlands without permanent connections to freshwater sources. The effect of changing inflows and/or outflows and therefore changing levels depends not only on the magnitude of change but also the timing, periodicity (hydroperiod) and duration of the levels.

• It is proposed that for wetlands the potential risk of ecological change associated with changes in levels may be defined as follows:
  • Low. Less than 0.2 m change in median water level; and, patterns of water level seasonality (summer vs. winter levels) remain unchanged from the natural state (summer relative to winter).
  • Medium. Greater than 0.2 m and less than 0.3 m change to median water level; and, patterns of water level seasonality show a reverse from the natural state (summer relative to winter).
  • High. Greater than 0.3 m change to median water level; and, patterns of water level seasonality show a reverse from the natural state (summer relative to winter).
• The risks for a potential change to wetland level must also be defined in relation to seasonal and inter-annual variability in hydroperiod as determined by the methods shown in Table 5 below and documented in full in the main report.
• Once the risk of potential change to wetland level has been established (degree of hydrological alteration) the technical methods that should be used to assess level requirements should be selected from Table 5. One or more of the methods listed within each cell of Table 5 should be used to assess ecological flow and level requirements for the given combination of degrees of hydrological alteration and significance of wetland values. In situations with high wetland value, two or more methods from each cell should be used, because the risks to ecology of making an incorrect ecological flow decision are greater. The methods within each cell are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. Specific recommendations of the use of each of the methods are given in the supporting document.

Table 5: Methods used in the assessment of ecological flow and water level requirements for degrees of hydrological alteration and significance of wetland values

Recommendations: groundwater

Typically, knowledge of groundwater systems is less certain than knowledge of surface waters. Therefore, the approach for groundwater differs slightly from the approach for rivers, lakes and wetlands. A ‘cumulative approach’ to groundwater methods application is used in response to uncertainty and the unknowns associated with groundwater systems. A ‘cumulative approach’ to methods application follows the typical groundwater investigation process whereby simple models are used to build more complex models.

• It is proposed that for groundwaters the potential risk for changes in levels may be defined as follows:
  • Low: Less than 10% of average annual recharge
  • Medium: 11% to 25% of average annual recharge
  • High: Greater than 26% of average annual recharge.
• Once the risk of potential change to groundwater levels has been established (degree of hydrological alteration) the technical methods that should be used to assess level requirements should be selected from Table 6. One or more of the methods listed within each cell of Table 6 should be used to assess ecological flow requirements for the given combination of degrees of hydrological alteration and significance of the resource values. The methods within each cell are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. Specific recommendations of the use of each of the methods are given in Chapter 4.
• Potential changes to flow regimes relate to the percentage allocation of aquifer recharge. It is acknowledged that these allocation thresholds from low to high may vary depending on the nature of the groundwater system. However the recharge percentages as presented, provide a conservative approach to groundwater allocation in most circumstances. ‘Significance of values’ should be used as the main criterion for determining methods most suitable for water level requirements when the relationship between groundwater allocation and the potential change to the flow regime is uncertain (eg, in deep confined aquifer systems where recharge and discharge are not well defined.

Table 6: Methods used in the assessment of water level requirements for degrees of hydrological alteration and significance of groundwater values

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