Framework

Comparison of Assessment Methods

Table 1 presents a summary that compares different methods of assessing connectivity. These tools are described in the context of:

spatial scale, classified in terms of local (ie at a point or site), intermediate (at the scale of a feature such as a stream reach) and regional (at the catchment scale);
temporal scale, classified in terms of short-term (over the timeframe of days to months such as tidal, evapotranspiration or discrete episodic processes), medium-term (at the seasonal to yearly scale) and long-term (exceeding the decadal timeframe such as influences of climate change);
cost, associated with collection, analysis and interpretation of data;
ease of use, focusing on the accessibility of technology and the extent of prior expertise required;
advantages, the inherent benefits of applying the methodology;
limitations, the potential constraints and limiting assumptions; and
application, outlining the extent that the method has been used in Australia.

Table 1: Summary of tools to assess stream-aquifer connectivity
Method	Spatial Scale	Temporal Scale	Cost	Ease of use	Advantages	Limitations	Application
Desktop Tools
Hydrographic Analysis Processing of time-series stream flow monitoring to define baseflow (groundwater discharge) component	Intermediate to Regional. Hydrograph represents water balance for subcatchment above gauge	Medium to Long-term. Depends on length of monitoring record	Low.	High. Many analysis techniques and software tools available. Stream flow data routinely collected	Uses existing flow monitoring data. Can be undertaken as a desktop study prior to detailed field investigations. Provides information of seepage changes through time	Applicable to gaining stream conditions only. Assumption that baseflow is groundwater discharge may not be valid. Baseflow effected by water use and management activities (eg regulation). Does not provide spatial distribution of groundwater input along stream	Commonly applied method for unregulated Australian catchments
Hydrogeological Mapping Mapping of groundwater systems including flowpaths, groundwater quality, aquifer structure and properties and geomorphology	Intermediate to Regional. Typical mapping scales of 1:100,000 to 1:250,000	Short to Medium- term. Usually 'average' conditions at time of mapping. Some parameters such as aquifer transmissivity or structural contours are time-insensitive	Medium to High. Depends on data availability. Expensive if drilling required to supplement existing data	Low to Medium. Knowledge of hydrogeological principles required	Provides conceptual understanding of groundwater systems around stream and hydrogeological controls on connectivity	Compiling and interpreting hydrogeological data can be time consuming and complex. Limited borehole data can lead to misinterpretation	Groundwater flow system, surface geological and hydrogeological mapping available at a coarse scale for many groundwater management areas across Australia
Modelling Simulate water flow regime around stream using mathematical equations	Intermediate to Regional. Typical models are 2D profiles or 3D grids	Medium to Long-term. Used to predict future events	Low to High. Depends on data availability and model complexity	Low to Medium. Requires good conceptual understanding of hydrological processes and modelling expertise	Useful predictive tool for management and policy. Helps define information gaps. Transient 3-D models can estimate changes in seepage through time and space	Oversimplified models may not be adequately robust. Over-complex models can be data hungry, costly and time-consuming	Commonly, surface water models for a catchment are developed in isolation to groundwater models
Field Tools
Field Indicators Visual indications of seepage such as water clarity, springs, aquatic plant species, chemical precipitates etc	Local. Site specific observation of seepage indicators	Short-term. Current at time of observation	Low.	Medium to High. Easily incorporated into field work. Depends on familiarity with indicators	Can identify seepage hotspots quickly. Return visits can provide information on seasonal changes in seepage flux. Field indicators can form basis for mapping (eg airphoto interpretation)	Limited in quantifying seepage flux. Effectiveness varies with observer's knowledge of field indicators (eg plant or aquatic biota)	Used in specific settings such as acid groundwater (eg iron precipitates, lilies) and karstic streams (eg travertine deposits). Assessment of groundwater-dependent ecosystems not routine
Artificial Tracers Monitoring movement of introduced tracers such as fluorescent dye to track water flow	Local to Intermediate.	Short to Medium term. Typical tracer studies over days to weeks	Medium. Need to establish monitoring network	Medium. Conceptually simple but needs expertise in field measurement and data interpretation	Can provide direct evidence of water movement between stream and aquifer. Aquifer parameters and fluid transport properties can be quantified	Tracer studies require careful planning including meeting environmental regulatory controls. Processes such as degradation, precipitation or sorption can affect tracer performance	Not routinely applied in connectivity studies in Australia. Overseas focus on karstic aquifers or investigations of contaminated sites
Geophysics and Remote Sensing Use of geophysics (eg resistivity, EM, radiometrics) or remote sensing (eg Landsat) to map landscape features that indicate or control connectivity	Local to Regional. Range from site specific (eg downhole surveys) to intermediate (eg run-of-river EC imaging), to catchment scale (eg satellite imagery).	Short-term. Measures conditions at the time of survey. Multiple surveys can provide trends through time.	Medium. Per hectare cost depends on technology and platform (eg ground, airborne)	Low. Needs technical expertise in field equipment operation and data interpretation	Allows rapid, non-invasive mapping of landscape parameters with good spatial resolution. Some techniques provide information at depth	Requires specific equipment, technical expertise and logistical support. Can require complex data processing and calibration with other datasets. Ground surveys can encounter obstacles such as rough terrain, vegetation cover etc	Opportunities exist to use geophysical data collected for other purposes eg. mineral exploration. Satellite imagery commercially available, some free in public domain
Hydrochemistry and Environmental Tracers Use of chemical constituents of water (such as major ions, stable isotopes, radon) to track water flow	Local to Regional. Depends on scope of water sampling survey	Short to Medium-term. Defines chemistry at time of sampling. Time-series monitoring (eg EC, pH) possible	Medium to High. Can be expensive due to sampling logistics and cost of analyses	Low. Requires expertise in appropriate sampling and data interpretation	Useful in quantifying seepage flux and defining key hydrological processes (such as groundwater recharge and discharge)	Can have long lead times between sample collection and final analytical results	Commonly used in Australia to identify hydrogeological processes including groundwater seepage to streams
Hydrometrics Measurement of hydraulic gradient between aquifer and stream and the hydraulic conductivity of intervening aquifer material. Based on Darcy's Law	Local to Regional. Can range from in-stream studies, to borehole transects to regional flow net analysis	Short to Medium-term. Possible to compare hydrographs of stream and groundwater levels	Low to Medium. Can use existing data but costly if drilling of bores is required	Medium to High. Comparison of groundwater and stream levels simple. Estimation of hydraulic conductivity more difficult	Comparison of stream and groundwater levels a simple guide to seepage direction. Installation of minipiezometers in stream bed allows direct local measurement of potential seepage direction	Relies on reasonable estimate of hydraulic conductivity to quantify seepage flux. Assumption of simple groundwater flow conditions may not be valid. Point measurement. Need to correct for density effects	Comparison of stream levels with nearby groundwater levels commonly used to define direction of potential seepage
Seepage Measurement Direct measurement of water flow between stream and aquifer using seepage meters	Local. Point measurement of seepage. Many measurements required to map spatial variations	Short-term. Meters typically installed over days/weeks. Measures aggregate seepage over time of operation	Low to Medium. Can be time consuming if measuring at multiple sites	Low. Simple concept with meters easy to use and no prior technical knowledge required	Direct measurement of seepage flux. Meters are simple and inexpensive to construct and provide a semi-quantitative measurement	Potentially significant measurement errors due to meter design and operation. Unsuitable for high stream flow, gravel and heavy clay sediment beds	Main application to date in Australia has been investigating leakage from irrigation channels or studying aquatic ecosystems
Temperature Monitoring Monitor variations in stream and sediment temperatures to trace seepage	Local. Multiple measurements required to map spatial variability in seepage	Short-Medium term. Temperature can be included in time-series monitoring	Low. Temperature loggers are cheap and widely available	Medium to High. Temperature simple to measure. Heat transfer modelling to quantify seepage more difficult	Temperature loggers are simple, robust and cheap. Heat transfer models that can compliment flow models to quantify seepage are available	Only measures at a point. Interpretation of monitoring requires confirmation using other assessment methods	Not specifically applied to study stream-aquifer connectivity in Australia to date. Opportunities to incorporate real-time temperature monitoring into existing hydrographic network
Water Budgets Quantification of stream reach water balance to define seepage component	Intermediate to Regional. Does not provide spatial variability of seepage along reach being investigated	Short to Medium Term. Possible to use time-series monitoring of stream flow at multiple stations	Low to Medium. Can be expensive if data collection required for estimating water balance components	Medium to High. Conceptually simple using existing monitoring data. Water balance components such as extraction or diversions can be difficult to quantify	Simple water balances estimated rapidly using existing stream flow monitoring. Provides estimate of aggregate seepage along reach	Measurement errors in stream flow data can be significant, hence more suited to long reaches. Can be misleading if water balance component (eg extraction) is not adequately accounted for	Routinely applied, particularly for regulated rivers or irrigation channels