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1 Introduction

1.1 Purpose and scope

This report provides scientific guidelines for the selection of methods to determine ecological flows and water levels for rivers, lakes, wetlands, and groundwater resources.

The process of establishing environmental flows and water levels is nested within wider environmental flow decisions. Environmental flows and water levels describe the water that remains in waterbodies to provide for ecological, tangata whenua, cultural, recreational, landscape and other values.

An environmental flow includes an ‘ecological flow’. An ecological flow or water level is defined as:

the flows and water levels required to provide for the ecological integrity of the vegetation and fauna present within waterbodies and their margin.

Therefore the ecological function of a waterbody must always be provided for when setting environmental flow management objectives; although other critical values may need to be taken into account in order to meet community expectations.

Environmental flow and water level decisions are made within the framework of the Resource Management Act (1991), national and regional policy statements, and the objectives and policies of the relevant regional plan.

In the setting of environmental flows there are two distinct elements:

  • a robust scientific methodology for assessing the ‘needs of freshwater ecosystems’ over a range of flow and seasonal conditions

  • a clear approach to taking into account the ecosystem values alongside other natural and development values of Māori and the wider community.

This report deals with the first of these elements and concentrates on ecological values. It does not take into consideration economic, social or cultural values, nor does it discuss RMA ‘process’ issues, which are beyond its scope.

1.2 The context around use of tables within this report

As described above, the process of establishing ecological flows is nested within wider environmental flow decisions. Ecological assessment should include the steps shown blue in Figure 1.1 below, but these are nested within other components that are more related to process and qualitative decisions.

Figure 1.1: The process of assessing ecological flows within wider environmental flow decision-making

See figure at its full size (including text description).

The purpose of this report is to provide a framework for robust and scientific technical assessment methods. These should be applied to the assessment of hydrological requirements for freshwater biota in rivers, lakes and wetlands, and other important values in groundwater. The technical assessments assist the process of determining ecological flow and water level requirements.

1.3 Framework of the approach

Traditionally, ecological flow methods have been used in rivers to define a minimum flow, below which there should be no human influence on river flow. However, taking into account the needs of all freshwater systems, the current trend is away from methods that set one minimum flow towards more holistic methods that consider the hydrological regime and aspects that, with some degree of hydrological variability, are needed to maintain the system morphology and ecologically-based values. Long-term solutions to flow and level management need to take a holistic view, taking into consideration geology, fluvial morphology, sediment transport, riparian conditions, biological community, habitat and interactions, connections between rivers, groundwater and wetlands, and water quality, both in a temporal and spatial sense. Similar holistic considerations apply to the social, economic and cultural aspects of environmental flows.

Technical methods need to be cost-effective and take a risk-based approach, with simple methods where the risk or environmental consequences of not achieving goals is low and more complex methods where aquatic values are high or the hydrological regime is highly modified.

When setting ecological flows, we need to acknowledge that the amount of hydrological variation required to maintain a healthy aquatic ecosystem is poorly understood. The complexity and unknown natural variation of many aquatic systems need to be acknowledged, and our relative lack of understanding of how different regimes will affect them. We need a cautious approach to setting flows that builds in buffers for risk and unknown outcomes. It is also important that no analytical method (eg, a model) should become a substitute for common sense, critical thinking about stream ecology, or careful evaluation of the consequences of flow modification.

An important component of ecological flows is that they quantify the amount the water available for allocation and also address requirements for both high and low flows/water levels throughout the year. The amount of water that can be allocated, and the manner in which it is used or regulated, determines the degree of hydrological alteration (the degree to which the natural hydrological regime could be potentially modified): this in turn will determine the technical methods to be used. For example, simple methods can be used to assess ecological flow and water level requirements where there are small changes to the natural hydrological regime. However, for substantial allocations or situations involving major flow/level regulation (eg, from impoundments), more complex and holistic methods are necessary. Similarly, the methods used will depend on the biota present or potentially present, so that there is no one method that fits all situations.

These draft guidelines:

  1. provide a base methodology or process for establishing the relevant factors relating to a ecological value (for example, native fish)
  2. set an appropriate technical method or methods for describing how the value changes with changing hydrological patterns
  3. identifies methods and tools that are appropriate given the value and the level of allocation of a resource.

In summary, it aims to get a robust process for selecting methods that can be applied to assessing ecological flows and water levels.

The process involves:

  1. assessing aquatic values and their relative significance
  2. determining the degree of hydrological alteration that could be expected from water allocation
  3. choosing an appropriate method for the assessment of ecological flow and water level requirements.

In the following sections, we describe this process applied to rivers, lakes and wetlands, and groundwaters, respectively. These descriptions provide the background and justification for our recommendations on the approach to selecting technical methods for the assessment ecological flows and water levels in freshwater systems under varying circumstances. The Executive Summary gives a brief overview of our recommendations.

1.4 Connected systems

The document is structured into sections concerning with rivers, lakes/wetlands and groundwaters, each providing separate recommendations. However, it is well recognised that these systems are often inter-connected. For example, a groundwater ecological flow or water level may well be set to maintain flows in an adjacent river or in a spring. Similarly, flows in rivers can be set to ensure adequate ecological flows or water levels in wetlands along their margins or on their floodplains. For connected systems, it is recommended that the resource-specific tables are used in combination with each other; the most sensitive or significant value will drive then the selection of methods for all resources.

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