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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

4 Groundwaters

The purpose of this section is to describe technical methods for assessing ecological flow and water level requirements for groundwater systems. Such requirements are set in the context of environmental, social, cultural and economic values of a water body (aquifer). Our approach concentrates on the aspects of groundwater systems related to ecological values (in the groundwater system and connected surface water systems) and physical properties of the aquifers such as structure and water quality. It does not include an assessment of wider economic and social factors such as reliability to water users.

For groundwaters, an ecological flow regime may include an allocation limit, water level or pressure limits, or other measures to ensure management objectives (such as adequate surface water flows or prevention of salt water intrusion) are met. Therefore, the ecological flow or water level regime includes allocation limits but often includes other measures as well.

The ecological flow regime in groundwater may vary in different circumstances. For example the ecological flow regime in groundwater may consist of one of a:

• simple groundwater allocation limit

• groundwater level limit with a groundwater allocation limit

• minimum flow restriction in a stream and a groundwater allocation limit.

The process of setting ecological flows and water levels in groundwater systems involves three steps:

1. assessment of resource values and their relative significance
2. assessment of the degree of hydrological alteration that could be expected from groundwater allocation
3. identification of an appropriate method, or methods, for the assessment of ecological flow/water level requirements.

The process of setting ecological flows and water levels in groundwater systems should consider uncertainty and the unknowns associated with groundwater systems.

Therefore the approach for groundwater aims at:

• a conservative approach to method identification

• applying, at least, the basic groundwater assessment method (conceptual model / simple water balance) for groundwater systems

• applying more complex methods for higher resource values and higher degrees of hydrological alteration

• adopting a ‘cumulative approach’ to methods application.

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.

4.1 Assessment of resource values and their relative significance

An assessment of resource values is an important part of selecting an ecological flow regime. It establishes the natural water body systems that could be affected and thus the methods of assessment to be used, as well as establishing baseline data for the consideration of environmental effects.

Values may be broadly grouped into:

• aquifer integrity including water use values

• aquifer outflow values

• ecological or water quality values.

There are ‘flow-related values’ that change in a discernible way as flow changes within aquifers from variations to aquifer recharge, groundwater abstraction, or modifications to aquifer outflows. Table 4.1 lists some groundwater values relevant to management of aquifer systems.

Table 4.1: Some groundwater values, or management objectives, for aquifer systems and factors to be considered in achieving the management objectives

It is often not possible to detect change in aquifer conditions as groundwater flows are reduced or the pattern of flows is changed. The inability to detect change arises from the high natural variability and the complexity of aquifer-surface water systems. It is only once springs stop flowing or wells dry up, that it becomes clear that values cannot be sustained. Management approaches need to reflect the associated uncertainty in aquifer response.

4.2 Determination of the degree of hydrological alteration

Natural groundwater flow is altered by groundwater use. Natural groundwater flow is altered at the local scale (eg, pumping of groundwater from a well inducing groundwater flow towards the well) and altered at the regional scale (eg, cumulative groundwater use reducing flows in spring-fed streams). The impacts on natural groundwater flows will depend on the amount of use, the location of use, the timing of use, and aquifer properties.

The setting of ecological flows and water levels controls the amount, the location, and the timing of groundwater use. Groundwater ecological flows or water levels are linked with surface water ecological flows where effects of groundwater use impacts on surface water.

The amount of groundwater allocated by resource consent is typically greater than groundwater use. For example the annualised groundwater allocation is approximately seven times greater than groundwater use in the area between the Ashley and Ashburton rivers in Canterbury (White et al 2003). This approach is based on the amount of water allocated, rather than the amount used. It is important that over time, allocation approaches change to better reflect actual use and seasonal volumes.

The degree of hydrological alteration of a groundwater system is related to the amount of groundwater allocated. Hydrological alteration of a groundwater system is related to groundwater allocation in three classes:

• low, where the allocation is a small proportion of recharge and therefore ecological effects of groundwater use are likely to be minor

• medium, where the allocation is a moderate proportion of recharge and therefore ecological effects of groundwater use are likely to be moderate

• high, where the allocation is a large proportion of recharge and therefore ecological effects of groundwater use may be significant.

These classes also relate to the security of supply for groundwater users and the conditions of groundwater allocation. For example groundwater users will have high security of supply where the hydrological alteration of a groundwater system is low and resource consents may include minor restrictions on groundwater use. However groundwater users may have less security of supply where the hydrological alteration of a groundwater system is high because the resource consent may include major restrictions on groundwater use. The conditions on groundwater consents are commonly linked to effects of groundwater use, including: local effects such as drawdown in a neighbouring well or induced flows from streams; and regional effects such as cumulative groundwater use reducing flows in spring-fed streams.

Hydrological alteration of a groundwater system is related to groundwater allocation in three classes by the portion of groundwater allocation to recharge from surface water sources (ie, with percentages rounded):

• low, where allocation is up to 10% of recharge from surface water sources

• medium, where allocation is from 11% to 25% of recharge from surface water sources

• high, where allocation is greater than or equal to 26% of recharge from surface water sources.

Existing groundwater allocation is typically assessed using regional council, or district council, resource consent databases. Surface recharge is typically assessed using methods outlined in Section 4.5. Estimates of median recharge (rather than mean) should be used if recharge estimates are available as time series because median estimates are more conservative than mean estimates.

These figures are based on experience rather than research and are conservative. They recognise some experiences of the effects of groundwater allocation in New Zealand. For example the approach would have the area between the Rakaia River and the Waimakariri River on the Canterbury Plains classed with a ‘high’ degree of hydrological alteration based on existing groundwater allocation. Allocation is approximately 119% of recharge from surface water sources because: estimated annualised allocation is around 43 m³/s (White et al 2003) and estimated groundwater recharge from surface water is around 36 m³/s (made up of around 24 m³/s rainfall recharge (White et al 2003), around 7 m³/s recharge from the Waimakariri River, and up to 5 m³/s recharge from the Rakaia River (Bowden 1983)). Groundwater levels in the area are commonly observed below their long-term average (eg, NIWA 2004) possibly because groundwater use is a significant portion of groundwater recharge.

4.3 Which method? Decision-making framework

4.3.1 Principles for selecting methods

Groundwater flow, or level, assessment tools are commonly used in the assessment of impacts of groundwater abstraction in New Zealand. These assessment methods are summarised in Section 4.4, and described in detail in Section 4.5.

Table 4.2 outlines the selection process of methods based on ‘resource values and their relative significance’ and ‘potential degree of hydrological alteration from groundwater allocation’. Resource values, and the degree of hydrological alteration from groundwater allocation, need to be carefully considered in the context of the management objectives outlined in Table 4.1 to evaluate ecological flows/water levels with appropriate methods.

Each method (Section 4.5) includes a ‘decision pathway to setting ecological flows’. Application of this pathway results in a cumulative application of methods as development pressure increases on a groundwater system. For example the pathway to applying the ‘historical levels’ method (Section 4.5.2) includes application of the ‘conceptual model/simple water balance’ method (Section 4.5.1).

The selection process aims to have ‘resource values and their relative significance’ as the main criteria for identifying methods most suitable for ecological flow requirements when the relationship between the potential change to the flow regime and groundwater allocation is uncertain (eg, in deep confined aquifer systems where groundwater recharge and groundwater discharge are not well defined).

Table 4.2: Methods used in the assessment of water level requirements for degrees of hydrological alteration and significance of groundwater values

The classification of ‘degree of hydrological alteration from groundwater allocation’ (Table 4.2) considers groundwater allocation and recharge. This classification aims to:

• provide a consistent approach to setting ecological flows and water levels

• provide an increasing knowledge base for decisions on ecological flows and water levels as development pressure increases.

Method selection should aim at a conservative approach after considering an analysis of uncertainty. For example methods will be in the ‘Medium’ category for ‘potential degree of hydrological alteration from groundwater allocation’ where groundwater allocation is 20 ± 5%) of groundwater recharge, but qualifies for a ‘High’ classification where groundwater allocation is 20 ± 8% of recharge. In other words, the degree of uncertainty associated with ‘allocation as a percentage of groundwater recharge’ needs to be taken into account when determining the appropriate ‘hydrological alteration’ category. Where doubt exists, users should defer to the highest appropriate category.

Method selection should also consider the surface water methods, where surface water is linked to groundwater.

4.3.2 Decision pathway to setting ecological flows and water levels

The overall process used in application of the approach to a given situation is as follows:

1. identify the groundwater system, boundaries and stresses
2. consider linkages between groundwater and surface water
3. identify the values or management objectives of the system (Table 4.1)
4. set management objectives and criteria significance, based on the values, ie, decide the significance of values and the potential change to the flow regime for groundwater, and for surface water if appropriate
5. identify critical factors in relation to groundwater system allocation and level, and for surface water if appropriate
6. select the potential methods from Table 4.2 based on ‘significance of groundwater resource values’ and ‘degree of hydrological alteration’. For example potential methods are conceptual model/simple water balance, historical levels and expert panel for the cell in Table 4.2 where the ‘significance of groundwater resource values’ is medium and ‘degree of hydrological alteration’ is low.
7. Select an appropriate method from the potential methods to set groundwater ecological flows and water levels in the aquifer system based on:

• the class of problem

• data availability.

  The class of problem may be either groundwater quantity or groundwater quality, or both. For example maintenance of base flow in spring-fed streams is a groundwater quantity problem so a numerical quantity model is a relevant method. Data availability is a key issue in method selection: a conceptual models/simple water balance can require little data, whereas a credible transient groundwater flow model has a large data requirement. Methods in each cell in Table 4.2 are listed in order of increasing data needs. A method should be chosen that is consistent with available data. For example the method ‘numerical quantity model – steady state’ is appropriate where:

• ‘significance of groundwater resource values’ is high and ‘degree of hydrological alteration’ is low

• the class of problem is groundwater quantity

• data is available to build a credible model.

8. Apply the method, considering critical factors and knowledge requirements, following the ‘detailed decision pathway’ of each method (Section 4.5), to:

• set a groundwater ecological flow/water level, and/or the groundwater level regime, to protect or manage the relevant objective(s)

• set a surface water ecological flow, if appropriate.

4.4 Summary of methods

Table 4.3 summarises the methods shown in the decision-making framework (Table 4.2) and their advantages and disadvantages.

Table 4.3: Summary of methods for groundwaters from the decision-making framework (Table 4.2), with advantages and disadvantages for use

4.5 Description of individual methods

Ten methods to assess groundwater ecological flows and water levels are described briefly, with advantages and disadvantages summarised. Each methodology is outlined and ‘decision pathway to setting ecological flows’ is provided. A recommendation on the circumstances in which the method should be applied is also made, cross-referencing to Table 4.2.

Decisions on ecological flows and water levels in groundwater systems require increasing scientific knowledge as the significance of resource values increases and as the user pressure on the groundwater system grows. Therefore, the ‘decision pathway’ commonly includes the application of more than one method so that scientific knowledge of a groundwater system builds in response to increasing knowledge demands.

4.5.1 Conceptual model/simple water balance

Framework for use (Table 4.2)

a. Description

A conceptual model, inclusive of a simple water balance, is a basic representation of the components of the aquifer system (Anderson and Woessner 1992). The model includes all readily available information, and idealised concepts, referenced from literature such as: system boundaries, the hydrogeology and physical nature of the aquifer, the components of groundwater recharge, components of groundwater flow and components of groundwater discharge.

This method is an initial approach to assessing ecological flows and water levels in groundwater and provides a basis for further assessment. The ‘decision pathway to setting ecological flows’ includes a conceptual model at an early stage of all methods because conceptual models are the first step in groundwater assessment. Advantages of conceptual models include simplicity of data needs and an ability to provide an overall hydrological framework around the setting of ecological flows. A disadvantage of this method is that information availability is poor for many New Zealand aquifer systems, resulting in uncertainty in application as well as in outcomes.

b. Methodology

The most basic information can provide an assessment of groundwater system behaviour and it is likely that a range of information may be available to provide for conceptual models and water balances. The physical parameters and boundaries of the aquifer system (grouped as a single hydrogeological unit or as separate aquifers for groundwater management) should be identified.

The conceptual model includes a characterisation of a groundwater system including the following components:

• geology (eg, type of aquifer, basement)

• aquifer type (eg, unconfined, confined)

• aquifer extents (lateral and vertical)

• likely recharge sources

• likely discharge locations (including streams, lakes, sea, and groundwater abstraction)

• groundwater level

• flow directions

• inter-aquifer groundwater transfer

• groundwater quality.

The simple water balance includes a characterisation of a groundwater system including the following components for estimating:

• likely recharge rates (eg, from rivers, rainfall and irrigation)

• likely flow rates with some simple approaches (eg, based on Darcy’s law)

• likely discharge rates (eg, to rivers, lakes, sea and from groundwater; abstraction) and using some simple approaches (eg, stream flow gaugings)

• groundwater volumes

• inter-aquifer groundwater transfer

• errors in rates of: groundwater recharge, groundwater discharge and groundwater flow.

It is important that the conceptual model and simple water balance are simplified, or approximated, in the correct context based on system complexity and scale. The water balance must provide for conservation of flow. Typically the water balance will represent average conditions of recharge, flow and discharge. However, changes in groundwater storage may also be identified in terms of time-variant water balances.

c. Decision pathway to setting ecological flows and water levels

1. Identify the boundaries of the groundwater system including top, bottom and lateral boundaries.
2. Apply the conceptual model/simple water balance method.
3. Identify sources of groundwater recharge and estimate rates of groundwater recharge.
4. Identify locations of groundwater discharge and rates of groundwater discharge.
5. Identify rates of groundwater flow.
6. Make decisions on ecological groundwater flows and levels that:

• are suitably conservative

• represent average conditions of recharge, discharge and flow in the aquifer

• may represent time-varying conditions of recharge, discharge and flow in the aquifer

• consider errors in rates of: groundwater recharge, groundwater discharge and groundwater flow

• maintain ecological flows in surface water potentially linked to groundwater (see rivers, lakes and wetlands)

• maintain inter-aquifer groundwater flows

• include limits on groundwater allocation.

4.5.2 Historical levels

Framework for use (Table 4.2)

a. Description

Historical groundwater levels and groundwater level variations are examined in this method. Groundwater level data may be grouped to form piezometric contour information to provide an understanding of spatial variability of groundwater level and an understanding of the variability of groundwater level with time. The data may be plotted in a time series to assess level trends over time.

The typical applications of assessing historical levels are to:

• maintain groundwater outflows that sustain surface water

• maintain groundwater ecology (flora and fauna)

• maintain groundwater quantity

• assistance with maintaining groundwater quality

• and to prevent saltwater intrusion via direct saline ingression or up-welling.

Critical factors to be assessed from this method include:

• groundwater levels

• groundwater level variation in time

• groundwater level variation within aquifers and aquifer systems.

The advantages of the method include simplicity of approach as levels are directly related to aquifer recharge and discharge. Disadvantages of the method include a commonly unknown quality of historical data: groundwater levels alone may not be sufficient to determine allocation for the aquifer system.

The method can contribute to:

• maintaining outflows that sustain surface water by ensuring that the local and regional groundwater level is at a sufficient height above the level of the receiving surface waters in connection with the aquifer for a sufficient (to be determined) period of time

• maintaining groundwater ecology (flora and fauna) by ensuring that the local and regional seasonal groundwater level variation remains within a suitable range and that long-term groundwater levels are within a suitable range to provide for ecological requirements

• preventing saltwater intrusion by ensuring for a suitable duration that the groundwater level is of sufficient height above levels in coastal surface waters

• maintaining groundwater quantity, and assist with maintaining groundwater quality, by ensuring that groundwater levels remain within a suitable range to ensure that ecological flows and water levels in aquifers are maintained; and that relatively poor-quality groundwater, or poor-quality surface water, is not drawn into an aquifer.

b. Methodology

• Collate historical groundwater level (ie, hydraulic head) data; assess the quality of this data and assess its data for analysis of groundwater levels and groundwater level variations on short-term, seasonal, medium-term and long-term time scales.

• Collate historical surface water level information and assess the suitability of this data for analysis of surface water levels and surface water level variations on short-term, seasonal, medium-term and long-term time scales.

• Assess the groundwater level and surface water level data together for suitability of identifying interaction between groundwater and surface water.

• Assess errors in groundwater level measurements.

• Assess errors in surface water level measurements.

• Establish time series plots of historical groundwater level measurements.

• Establish time series plots of historical surface water level measurements.

• Establish groundwater level maps including levels of relevant surface water features – identify groundwater flow directions and identify if possible the relationships between groundwater and surface water.

Ecological water levels in the aquifer system may be set by assessing groundwater level time series, or by visual inspection of a groundwater level map, to identify:

• short-term, or local, groundwater levels that relate to groundwater abstraction

• seasonal, or medium-term groundwater levels that relate to seasonal groundwater abstraction or seasonal groundwater recharge

• long-term, regional groundwater level trends that relate to long-term groundwater sustainability

• identification of surface waters that potentially receive groundwater or lose water to groundwater and that are possibly dependent on groundwater as a source of baseflow.

Confidence limits of groundwater level time assessments should be assessed by considering errors or gaps in groundwater level measurements.

c. Decision pathway to setting ecological flows and water levels

1. Identify the boundaries of the groundwater system including top, bottom and lateral boundaries.
2. Apply the conceptual model/simple water balance method.
3. Apply the historical levels method.
4. Identify sources of groundwater recharge and estimate rates of groundwater recharge.
5. Identify locations of groundwater discharge and rates of groundwater discharge.
6. Identify rates of groundwater flow from the simple water balance.
7. Make decisions on ecological groundwater flow rate and ecological groundwater level that:

• are suitably conservative

• represent average conditions of recharge, discharge and flow in the aquifer

• may represent time-varying conditions of recharge, discharge and flow in the aquifer

• consider errors in rates of: groundwater recharge, groundwater discharge and groundwater flow

• maintain relative levels of groundwater and surface water so that natural groundwater recharge, or natural groundwater discharge, continues

• maintain relative levels of groundwater in aquifers so that natural inter-aquifer groundwater transfers continue

• maintain ecological flows in surface water potentially linked to groundwater (see rivers, lakes and wetlands)

• include limits on groundwater allocation.

4.5.3 Expert panel

Framework for use (Table 4.2)

a. Description

An independently appointed panel of experts can provide advice on ecological flows and levels in groundwater and on ecological flows in surface waters supported by groundwater discharge.

This method has the advantages that it is quick, cheap, has credibility (dependent on experts), and can help overcome mistrust if well managed. The disadvantages of this method include that the expert panel can be used as a political tool and that implied consensus of the expert panel can lead to poor environmental outcomes.

Typical applications of expert panels are for:

• maintaining outflows that sustain surface water

• maintenance of groundwater ecology (flora and fauna)

• controlling land subsidence and aquifer consolidation

• controlling saltwater intrusion

• maintaining groundwater quality.

The critical factors that need to be considered are:

• aquifer head/variation

• gradient

• point and non-point sources

• flow

• land use.

b. Methodology

An expert panel may be called to assess a groundwater system with respect to ecological flows and levels in groundwater, and on ecological flows in surface waters supported by groundwater discharge. This type of assessment should include all the aspects, and include all existing knowledge, of the groundwater system.

Assessments of this nature are generally on an agreed approach by more than one expert appointed to the panel. The panel reviews all existing information including: conceptual model, water balance and detailed models of geology, recharge, flow and chemistry.

The panel aims to address ecological flows and levels in groundwater and in groundwater that supports surface water flows by assessment of:

• factors included in the conceptual model/simple water balance method

• factors included in the historical levels method

• any other relevant information considered important by the Expert Panel such as groundwater flow models and groundwater quality models.

An expert panel may be called at any stage of an assessment of ecological flows and water levels. The above approach is particularly suitable initially to provide sufficient review of available hydrogeological data for the setting of ecological flows, or levels, in groundwater; and in setting ecological flows where groundwater discharge supports surface water flow. A team approach to initial assessments commonly provides good results, particularly with initial assessments, because the experience of a range of groundwater experts and surface water experts can efficiently identify all relevant technical requirements. The expert panel should also assess gaps in knowledge and consider future research needs; in this way the knowledge base will improve over time to meet future requirements for improved knowledge of a groundwater system.

The expert panel provides a quick and cheap method that builds consensus with stakeholders. However, its effectiveness is limited by the credibility of the experts and poor outcomes can result from the need for consensus.

c. Decision pathway to setting ecological flows and water levels

1. Identify the boundaries of the groundwater system including top, bottom and lateral boundaries.
2. Apply the expert panel method.
3. Identify sources of groundwater recharge and estimate rates of groundwater recharge.
4. Identify locations of groundwater discharge and rates of groundwater discharge.
5. Identify rates of groundwater flow from the simple water balance.
6. Make decisions on ecological groundwater flows and levels that:

• are suitably conservative

• represent average conditions of recharge, discharge and flow in the aquifer

• may represent time-varying conditions of recharge, discharge and flow in the aquifer

• consider errors in rates of: groundwater recharge, groundwater discharge and groundwater flow

• maintain relative levels of groundwater and surface water so that natural groundwater recharge, or natural groundwater discharge, continue

• maintain relative levels of groundwater in aquifers so that natural inter-aquifer groundwater transfers continue

• maintain ecological flows in surface water potentially linked to groundwater (see rivers, lakes and wetlands)

• include limits on groundwater allocation.

4.5.4 Detailed water balance

Framework for use (Table 4.2)

Critical factors: aquifer head-water volumes.

a. Description

This method is used for quantifying groundwater flows, including inflows and outflows and has the advantage that it may be applied to all situations. Disadvantages of the method include that assumptions about groundwater flows are commonly required: some components of the balance such as subsurface recharge and subsurface discharge are generally unknown or very uncertain. A detailed water balance approach is more complex than a simple water balance approach, in that: specific recharge investigations and outflow measurements, including the metering of groundwater abstraction, may be made; and water balances over time are calculated. Thus, the detailed water balance is likely to include aquifer geometry, hydrogeologic parameters, boundaries and stresses.

Specific requirements for the water balance include:

• recharge sources and quantification of spatial and temporal water input

• aquifer discharge zones and the relationship to adjoining streams and rivers

• the size of the groundwater resource including volume of groundwater storage

• the effects of adjacent boundaries (possibly recharge and discharge boundaries)

• system stresses including groundwater abstraction over time.

The detailed water balance refers to the conceptual model and includes estimates of recharge from rivers (White et al 2001) rainfall, and irrigation (Thorpe 2001) and discharge to surface water (White et al 2001).

b. Methodology

Technical approaches to estimating groundwater recharge include (but are not limited to) the following:

• rainfall recharge to groundwater may be estimated by infiltration analyses, soil water balance models, catchment runoff analyses, environmental isotopes and chemical tracers

• recharge to groundwater from surface water (eg, river, lake, sea) may be estimated by stream flow gaugings, Darcy’s law or stream tube analysis supported with sufficient hydraulic data, environmental isotopes and chemical tracers

• recharge to groundwater from irrigation may be estimated by infiltration analysis, environmental isotopes and chemical tracers

• recharge to groundwater through the sub-surface may be estimated by Darcy’s law or flow net analysis with sufficient hydraulic data, environmental isotopes and chemical tracers.

Technical approaches to estimating groundwater flow and groundwater storage include (but are not limited to) the following:

• groundwater flow may be estimated by using Darcy’s law or flow net analysis coupled to estimates of hydraulic conductivity or transmissivity from aquifer pumping tests, environmental isotopes and chemical tracers

• groundwater storage volumes may be estimated by aquifer pumping tests or regional estimates based on recharge analysis or aquifer response to air pressure or sea level variation where sufficient groundwater level measurements are available.

Technical approaches to estimating groundwater discharge to surface waters include (but are not limited to) the following:

• groundwater discharge to rivers and streams water may be estimated by stream flow gaugings or using Darcy’s law or flow net analysis supported with sufficient hydraulic data

• groundwater discharge to lakes and wetlands may be estimated by using Darcy’s law or flow net analysis supported with sufficient hydraulic data.

Detailed information, such as stream gaugings, may indicate whether a stream is gaining water from, or losing water to, an aquifer. The physical parameters and boundaries of the aquifer system (grouped as a single hydrogeological unit, or individual aquifer, for groundwater management) should be specifically identified, including groundwater inflow and natural recharge from rainfall or stream flow, artificial recharge, vertical leakage and excess irrigation water. Groundwater losses should also be identified from the system including evapotranspiration, vertical leakage and abstraction, groundwater discharge and change in storage.

The water balance obtained must provide for conservation of flow/volume, be calibrated against known measurements and for modelling purposes, must have convergence. Changes in water storage may also be identified in terms of transient approaches to the conceptual model/water balance.

Application of the detailed water balance is at a regional scale and it includes:

• assessment of effects of cumulative groundwater abstraction on groundwater discharge and flow in spring-fed streams over a relatively coarse time scale so that ecological flows in spring-fed streams remain at acceptable levels

• assessment of cumulative groundwater use on groundwater levels and groundwater flows over time so that ecological flows and water levels within groundwater systems are maintained.

c. Decision pathway to setting ecological flows and water levels

1. Identify the boundaries of the groundwater system including top, bottom and lateral boundaries.
2. Apply the conceptual model/simple water balance method.
3. Apply the historical levels method.
4. Apply the detailed water balance method.
5. Identify sources of groundwater recharge and estimate rates of groundwater recharge.
6. Identify locations of groundwater discharge and rates of groundwater discharge.
7. Identify rates of groundwater flow from the simple water balance.
8. Make decisions on ecological groundwater flow rate and groundwater levels that:

• are suitably conservative

• consider errors in rates of: groundwater recharge, groundwater discharge and groundwater flow

• maintain relative levels of groundwater and surface water so that natural groundwater recharge, or natural groundwater discharge, continues

• maintain relative levels of groundwater in aquifers so that natural inter-aquifer groundwater transfers continue

• maintain ecological flows in surface water potentially linked to groundwater (see rivers, lakes and wetlands)

• consider groundwater allocation options by location and in time

• consider potential effects of groundwater allocation options on ecological flows and water levels

• include limits on groundwater allocation.

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