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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

4.5.5 Time series analysis

Framework for use (Table 4.2)

a. Description

This method includes statistical analyses of: groundwater levels, groundwater system inputs, groundwater system outputs and statistical analysis of groundwater quality over time. The method assists with: maintenance of groundwater flow, maintenance of groundwater outflows that sustain surface water, maintenance of groundwater ecology (flora and fauna), controlling saltwater intrusion, maintenance of groundwater quality.

An advantage of the method is that it is relatively easy to apply and scenarios of environmental variability and groundwater use can be tested. Disadvantages include:

• dependence on observed data – poor quality results can come from poor data

• the duration of groundwater level data may be short and not sufficient for time series analysis

• groundwater level measurements may be too infrequent for time series analysis

• observation wells be in a poor location for identifying the drivers of groundwater level variation

• groundwater system inputs and outputs may be poorly known and adequate data on groundwater system inputs and outputs may not be available.

The disadvantages of the method for assessing groundwater quality data include groundwater quality data that are not collected with standard sampling techniques and poor laboratory analysis as indicated by ion balances.

Time series analysis contributes to the applications through identifying the effects of groundwater inputs on groundwater level, or groundwater discharge. For example simple correlation of base flow discharge in streams with rainfall recharge may be useful to assessment of ecological flows/water levels where rainfall recharge is declining in the long term.

b. Methodology

Time series analysis aims to relate responses of groundwater systems to stressors on groundwater systems. Groundwater system responses include: groundwater level (ie, hydraulic head), groundwater discharge (eg, base flow stream discharge) and groundwater quality. Groundwater system stressors include groundwater recharge from rainfall, or from rivers, and groundwater use.

Many approaches are available, including:

• simple correlation

• Fourier time-series analysis (White 1994)

• Principal Component Analysis (Cameron and White 2004)

• river recharge and rainfall recharge separation (White and Brown 1995)

• neural networks (White et al 2003)

• the ‘eigenvalue’ model (Bidwell and Morgan 2001).

Typically, the analysis process follows:

• potential groundwater system inputs (eg, recharge from rainfall and from rivers) or outputs (eg, groundwater discharge to rivers and lakes) are identified in the conceptual model. The characteristics of inputs and outputs are identified, or estimated, over time. For example rainfall recharge is a seasonal input to groundwater on the east coast of New Zealand because rainfall recharge during summer seasons is typically lower than rainfall recharge during winter seasons (White et al 2003)

• groundwater levels, and groundwater quality are plotted and commonly interpolated into a constant time base for ease of analysis – the time base chosen is relevant to identification of groundwater level response over short, seasonal, medium and long-term time scales

• groundwater level variations on short, seasonal, medium and long-term time scales are identified. The causes of these level variations are then identified. For example a groundwater level variation over a week, or less, duration may be due to pumping groundwater from a neighbouring well

• relationships between groundwater system inputs (eg, magnitude and period) and groundwater level response (eg, magnitude, period and time lags of response) are expressed as equations

• a relation between groundwater level response and a groundwater system input may then be explored.

Statistical analysis of groundwater quality over time is used to assess groundwater quality changes (eg, in response to land use change). Typically the changes of relevant individual ionic species are assessed (eg, nitrate-nitrogen for land use and chloride for salt water intrusion) in time-series plots; Piper diagrams (Rosen 2001) may be used to assess changes of suites of ions over time.

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 time series analysis 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.

4.5.6 Analytical models

Framework for use (Table 4.2)

a. Description

This method typically uses spreadsheet-based models that use groundwater flow and groundwater transport equations. The method assists with:

• maintaining groundwater flow and maintaining groundwater outflows that sustain surface water

• maintenance of groundwater ecology (flora and fauna)

• controlling land subsidence and aquifer consolidation

• controlling saltwater intrusion, and maintaining groundwater quality.

Advantages of this method include: ease of use, moderate skill level, moderate data requirements. Disadvantages of the method include: a common requirement for simple assumptions; cumulative effects of groundwater use may be ignored.

Analytical models assist the applications by assessing:

• groundwater pumping to ensure that ecological groundwater flows or levels are maintained, eg, to prevent saltwater intrusion

• groundwater pumping to ensure that ecological surface water flows are maintained;

• groundwater pumping and aquifer consolidation

• land use so that groundwater quality and surface water quality are maintained where surface water is linked to groundwater.

b. Methodology

Analytical models are solutions to the equations governing groundwater flow and solute transport in groundwater. These solutions are equations that are often implemented in spreadsheets.

Groundwater flow is assessed by analytical models that solve for groundwater flow, solute transport and boundary conditions. Boundary conditions represent the type of problem, including:

• pumping tests where groundwater flow to a well is assessed (Kruseman and de Ridder 1991) to estimate properties of the formation relating to water flow in porous materials and properties of the well

• groundwater–surface water interaction where surface water flow may be reduced when groundwater is pumped (Hunt 1999; Jenkins 1977; Pulido-Velazquez et al 2005)

• solute transport to estimate chemical dilution in groundwater (Environment Canterbury 2007)

• ground consolidation associated with groundwater pumping at the local scale or regional scale.

Analytical models use parameters related to groundwater flow, solute transport and boundary conditions, including:

• aquifer type (unconfined or confined)

• well diameter

• pumping rate

• aquifer porosity

• aquifer hydraulic conductivity

• aquifer thickness

• aquifer storability

• distance between surface water and pumping well

• dispersion coefficients and land use coefficients.

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 analytical 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.

4.5.7 Numerical quantity models – steady state

Framework for use (Table 4.2)

a. Description

This method represents aquifers and groundwater flow with a computer model based on groundwater flow equations and boundary conditions where model properties are constant in time. The method is relevant to:

• maintaining outflows that sustain surface water

• maintenance of groundwater ecology (flora and fauna)

• controlling saltwater intrusion.

Numerical models of groundwater quality are built on estimates of groundwater flows and boundary conditions provided by numerical models of groundwater quantity.

Advantages of the method include:

• numerical models allow a complex representation of the real world and allow an assessment of many types of groundwater issues

• numerical models allow representation of groundwater flow in multi-aquifer groundwater basins.

Disadvantages of the method include:

• numerical models can be time-consuming to develop and they can be complex

• numerical models are intensive users of data yet data may be of poor, or unknown, quality

• model properties are commonly assumed because the steady-state data are often collected at a scale that is coarser than the model grid

• data collected at the local scale (eg, results from pump tests) may not be representative of model properties at the regional scale

• model boundary conditions may be poorly defined

• estimates of groundwater system behaviour, in a model application, may be poor even where model calculations agree well with steady-state data.

b. Methodology

Numerical models are solutions to the equations governing groundwater flow (Anderson and Woessner 1992) that aim to represent the variability of natural systems in two dimensions or three dimensions. Steady state numerical models aim to represent groundwater flow in average conditions, eg, average annual conditions of groundwater recharge and groundwater discharge. Numerical models are often developed with a graphical user interface and often applied in spreadsheets.

The method of numerical modelling may follow Anderson and Woessner (1992) to include:

1. development of a conceptual model of the system
2. development of model datasets representing components of groundwater hydrology including:

• geology

• aquifer type (eg, unconfined or confined) and aquifer properties

• steady-state boundary conditions such as groundwater recharge (eg, from rainfall, irrigation and rivers)

• steady-state boundary conditions such as groundwater discharge (eg, to springs, streams, lakes and sea) and groundwater use

3. calibration of the model that aims to provide a good representation of steady-state data
4. calibration sensitivity analysis that aims to assess uncertainty in the calibrated model
5. model application.

Steady-state numerical models may contribute to the applications, at local and regional scales, by assessing the:

• effects of pumping on groundwater discharge and flow in spring-fed streams so that ecological flow in spring-fed streams remains at acceptable levels

• assessment of cumulative pumping on groundwater levels and groundwater flows so that ecological flows and water levels within groundwater are maintained

• effects of groundwater pumping on the potential for salt water intrusion so that groundwater levels are maintained above sea level.

c. Decision pathway to setting ecological flows and water levels

1. Apply the numerical quantity models – steady-state method.
2. Complete an uncertainty analysis that assesses the effect of variability in model properties and model stressors (recharge and groundwater pumpage) on model estimates of groundwater level, groundwater discharge and groundwater flow.
3. 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 uncertainty in model calculations

• consider groundwater allocation options by location

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

• include limits on groundwater allocation.

4.5.8 Numerical quantity models – transient

Framework for use (Table 4.2)

a. Description

This method represents aquifers and groundwater flow with a computer model based on groundwater flow equations and boundary conditions where model properties are variable in time. Numerical models of groundwater quality are built on estimates of groundwater flows and boundary conditions provided by numerical models of groundwater quantity.

Advantages of the method include:

• complex representations of real world are allowed

• many types of groundwater issues may be assessed

• transient numerical models allow representation of groundwater flow in multi-aquifer groundwater basins

• consideration of groundwater storage effects such as the time-dependent response of groundwater discharge to surface water such as may be observed in short and long term depletion of baseflow.

Disadvantages of the method include:

• models are time-consuming to develop and are commonly complex

• models are intensive users of data yet data may be of poor, or unknown, quality

• model properties are commonly assumed because the model data are often collected at a scale that is coarser than the model grid

• data collected at the local scale (eg, results from pump tests) may not be representative of model properties at the regional scale

• transient data is often smoothed or interpolated from observations

• model boundary conditions may be poorly defined

• estimates of groundwater system behaviour, in a model application, maybe poor even where model calculations agree well with transient data.

The method is relevant to:

• maintaining outflows that sustain surface water

• maintenance of groundwater ecology (flora and fauna)

• controlling saltwater intrusion.

b. Methodology

Numerical models are solutions to the equations governing groundwater flow (Anderson and Woessner 1992) that aim to represent the variability of natural systems in two dimensions or three dimensions over time. Transient numerical models aim to represent groundwater flow in typical conditions, eg, time-variable groundwater recharge and groundwater discharge. Numerical models are often developed with a graphical user interface.

The method of numerical modelling may follow Anderson and Woessner (1992) to include:

1. development of a conceptual model of the system
2. development of model datasets representing components of groundwater hydrology including:

• geology

• aquifer type (eg, unconfined or confined) and aquifer properties

• transient boundary conditions such as groundwater recharge (eg, from rainfall, irrigation and rivers)

• transient boundary conditions such as groundwater discharge (eg, to springs, streams, lakes and sea) and groundwater use

3. calibration of the model that aims to provide a good representation of transient data
4. calibration sensitivity analysis that aims to assess uncertainty in the calibrated model
5. model application.

Transient numerical models may contribute to the applications, at local and regional scales, by assessing the:

• effects of pumping on groundwater discharge and flow in spring-fed streams over time so that ecological time-varying flow in spring-fed streams remains at acceptable levels

• assessment of cumulative pumping on groundwater levels and groundwater flows over time so that time-varying ecological flows and levels within groundwater are maintained

• effects of groundwater pumping on the potential for salt water intrusion over time so that groundwater levels are maintained above sea level.

Numerical models of groundwater quality are built on estimates of groundwater flows and boundary conditions provided by numerical models of groundwater quantity.

c. Decision pathway to setting ecological flows and water levels

1. Apply the numerical quantity models – steady-state method.
2. Apply the numerical quantity models – transient method.
3. Complete an uncertainty analysis that assesses the effect of variability in model properties and model stressors (recharge and groundwater pumpage) on model estimates of groundwater level, groundwater discharge and groundwater flow.
4. 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 uncertainty in model calculations

• 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.

4.5.9 Numerical quality models – transport

Framework for use (Table 4.2)

a. Description

Numerical quality models, more commonly known as ‘contaminant transport models’ are used to solve the partial differential advection dispersion equations for an entire flow field of interest. Algebraic equations for each model sub-area (or cell) are solved numerically through an iterative process for various transport options such as advection, dispersion, and chemical reactions including bacterial decay. This is also true for density-dependent models as commonly applied to assessing coastal saltwater intrusion. Such numerical quality models are usually coupled to flow models.

This method may model groundwater quality, groundwater temperature or groundwater age, based on groundwater transport equations and boundary conditions. Applications of the method include:

• controlling saltwater intrusion

• maintaining groundwater quality and

• maintaining surface water quality.

Advantages of transient numerical transport models include, that they allow:

• a complex representation of the real world and allow an assessment of many types of groundwater issues

• representation of multi-species transport in multi-aquifer groundwater basins and may also account for density-dependent groundwater flow as occurs with saline intrusion.

Disadvantages of the method include:

• these models are time-consuming to develop and they can be complex

• they are intensive users of data yet data may be of poor, or unknown quality

• obtaining a good calibration for the model is often difficult

• model properties are commonly assumed because the model data are often collected at a scale that is coarser than the model grid

• data collected at the local scale (eg, dispersion properties) may not be representative of model properties at the regional scale

• model transient data is often smoothed or interpolated from observations

• model boundary conditions may be poorly defined.

The advantage of using numerical contaminant transport models over their analytical counterparts is that they allow a greater variability of physical flow and transport parameters (which dominate transport movement) to be represented. It should be noted that specific ‘site’ concentrations might not be accurately predicted by these models due to errors in measurement and spatial variability in transport parameters. However, geostatistical tools may be used to interpret model results and reduce predictive uncertainty.

Examples of such applications of transport models are:

• management and remediation of a contaminated site where an existing or pre-existing contaminant source is identified and sufficient hydro-geological and geochemical information exists to predict local and/or regional impact of the specific contaminant plume. This type of application may require a management limit on source flow, concentration and/or groundwater level

• predicting the coastal saline interface for an unconfined or confined aquifer to determine minimum pumping levels or ecological levels in aquifers near the coast.

The transport modelling approach may be applied as a comprehensive assessment to appreciating groundwater responses of water quality to recharge or discharge, or introduced contaminant levels. This approach may allow an appropriate ecological level of groundwater flow. Land use effects on groundwater quality (Bidwell 2005), and on the quality of groundwater discharge to surface waters (White and Daughney 2002), may be assessed with spreadsheet models based on groundwater flow models and nutrient transport in groundwater.

b. Methodology

The process of numerical transport modelling may follow Anderson and Woessner (1992) to include:

1. development of a conceptual model of the system
2. development of model datasets representing components of groundwater transport and quality including (but not limited to):

• all components of physical flow for steady state or transient numerical; quantity models as specified above

• aquifer porosity distribution

• aquifer dispersion properties

• source recharge and discharge concentrations

• chemical reactions including microbial decay within the aquifer.

3. calibration of the model that aims to provide a good representation of transient monitoring data
4. calibration sensitivity analysis that aims to assess uncertainty in the calibrated model
5. model application
6. making decisions on ecological groundwater flow rates 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 uncertainty in model calculations

• consider groundwater allocation options by location and in time

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

• maintain, or improve, groundwater quality

• include limits on groundwater allocation.

c. Decision pathway to setting ecological flows and water levels

1. Apply the numerical quantity models – steady-state method if the transport model is steady state.
2. Apply the numerical quantity models – transient method if the transport model is transient.
3. Complete an uncertainty analysis that assesses the effect of variability in model properties and model stressors (recharge and groundwater pumpage) on model estimates of groundwater level, groundwater discharge and groundwater flow.
4. Make decisions on ecological groundwater flow rate and groundwater level 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)

• maintain groundwater discharge across the coastal boundary to prevent salt water intrusion

• consider uncertainty in model calculations

• consider groundwater allocation options by location, if the transport model is steady state, and in both location and time if the transport model is transient

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

• include limits on groundwater allocation.

4.5.10 Consolidation models

Framework for use (Table 4.2)

a. Description

This method aims to estimate settlement of ground materials caused by groundwater depressurisation. The method is applicable to controlling land subsidence and controlling aquifer consolidation.

An advantage of the method is that it allows quantification of consolidation. Disadvantages of the method include: much input data is required by sophisticated models; sophisticated models require experienced modellers; field measurements may not represent the variability of ground over short distances.

Applications of consolidation models include assessment of settlement risk to services and structures from groundwater level variations over time within the parent aquifer. Management of groundwater levels may be required where consolidation poses sufficient risk to infrastructure or to aquifer. This application may require a management limit on groundwater level, and groundwater use, to protect aquifer integrity and maintain groundwater levels in an aquifer.

The consolidation modelling approach could be applied as part of a comprehensive assessment of responses to groundwater level variation over time.

b. Methodology

Consolidation models generally utilise output from analytical or numerical flow models to estimate groundwater level variability over time. Consolidation within overlying materials, or within an aquifer, is assessed from groundwater level and from the properties of materials within a target area.

Assessment of consolidation is mainly in the form of:

• broad assessments – initial risk analysis from the comparison of groundwater level variation in relation to local unconsolidated geology

• engineering calculation – conventional consolidation calculations (Bowles 1996) and to determine site-specific or regional problems. Output in the form of consolidation settlement at a specific site or geo-statistical contoured distribution of consolidation settlement over a specified area

• application of 2D and 3D approaches with the use of finite element or finite difference iterative numerical modelling techniques. This type of modelling environment is generally limited to structural design assessments.

Broad assessments may be included in conceptual modelling whereby a first estimate of risk is made. Engineering calculation is defined as an analytical consolidation model. 2D and 3D approaches using finite element or finite difference numerical modelling techniques are likely to require conceptual and analytical approaches as a prerequisite.

c. Decision pathway to setting ecological flows and water levels

1. Development of a conceptual model of the system.
2. Development of model datasets representing components of groundwater hydrology and sediment properties including:

• geology

• sediment compaction factors (Lambe & Whitman 1969)

• aquifer type (eg, unconfined or confined) and aquifer properties

• boundary conditions such as groundwater recharge (eg, from rainfall, irrigation and rivers)

• boundary conditions such as groundwater discharge (eg, to springs, streams, lakes and sea) and groundwater pumpage.

3. Calibration of the model that aims to provide a good representation of observed data.
4. Calibration sensitivity analysis that aims to assess uncertainty in the calibrated model.
5. Model application.
6. Make decisions on ecological groundwater flow rate and groundwater level that:

• are suitably conservative

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

• 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)

• maintain groundwater discharge across the coastal boundary to prevent salt water intrusion

• constrain sediment compaction so that the risk of infrastructural damage is within acceptable limits

• include limits on groundwater allocation.

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