Connectivity Processes

Connectivity categories

There are many different approaches to categorising the connectivity between surface water features and their associated groundwater systems. This is reflected in a wide range of terms currently being used to describe connectivity. In general, surface water-groundwater interactions can be classified on the basis of the four key aspects of contiguity, seepage direction, conductance and impact. Figure 3 outlines this classification in the case of streams, but this can equally be applied to other surface water features.

Contiguity describes whether or not the groundwater system is in direct hydraulic contact with the surface water feature. A stream is contiguous with the groundwater system if the water-saturated zone is continuous between the two. This is consistent with the general use of the term which describes when two entities are in contact and share a common boundary, in this context a hydrological one. Figure 1 shows a situation where there is a direct hydraulic connection between the stream and unconfined aquifer via the shallow watertable. This situation is commonly called connected in many classifications (eg Winter et al, 1998). In contrast, a stream is perched if an unsaturated zone separates it from the underlying groundwater system, as indicated by the example of Figure 2. This situation is commonly termed disconnected which is misleading as it suggests that there is no possibility of actual movement of water between stream and aquifer. In Figure 2, this is not the case as downward infiltration from the stream through the unsaturated zone has formed an underlying watertable mound. It can be difficult to explain the concept that a disconnected stream can still lose water to the aquifer.

Figure 1: A contiguous losing stream (graphic from Winter et al, 1998)

Figure 2: A perched losing stream (graphic from Winter et al, 1998)

Whether a connected water resource is contiguous or not is significant (Table 1). Pumping groundwater from a contiguous stream-aquifer system will impact on the local stream flow hydrology, reducing water availability for surface water users and riverine ecosystems. Pumping of shallow groundwater near a perched stream generally does not affect stream flow, although unsaturated zone storage of water may play a role.

Figure 3: Categorisation of stream-aquifer connectivity

Table 1: Impacts of stream-aquifer connectivity (modified from REM, 2002)

Contiguity	Seepage Direction	Synonyms	Potential impact of groundwater on surface water	Potential impact of surface water on groundwater
Contiguous	Gaining	Effluent Upwelling Groundwater-fed Aquifer discharge	High	Low
Contiguous	Losing	Influent Down-welling Stream-fed Aquifer recharge	Medium	High
Contiguous	Underflow		Low	Medium
Perched	Losing		No Impact	Medium
Contiguous	Fluctuating	Variable Gaining/Losing Seasonal	Medium	Medium
Contiguous	Throughflow	Flowthrough	Medium	Medium

Connected water resources can also be classified in terms of the direction of seepage. However, this is not straight-forward as seepage direction can be variable in both space and time due to changes in the relativity of the stream stage to the shallow watertable. Gaining streams which receive inflows of groundwater and losing streams which leak water to the underlying aquifer are two ends of the spectrum. In many studies, gaining streams are also called effluent and losing streams are influent. Situations intermediate to these end-members are common and include fluctuating seepage streams where the seepage direction changes through time. These are also commonly called variably gaining/losing as they either receive from, or lose to, the groundwater system, depending on the time of year. Seasonal effects can alter the type of connection between groundwater and surface water systems, with streams gaining in summer months and losing in winter in some regions and vice-versa in other regions. Streams are termed variable seepage, if the direction of seepage flux varies, so that one part of the reach loses water and another part gains groundwater. These are called throughflow in the situation where groundwater flow is perpendicular to the stream and groundwater enters the stream on one side but stream leakage occurs on the other. Figure 4 shows this setting for a throughflow lake. Underflow streams are a special situation where groundwater flows parallel to, but does not actually discharge into, the stream. As we are dealing with a spectrum, rather than distinct independent categories, it is suggested that a nominal threshold of 70% is used (eg. stream reaches are termed gaining if they receive groundwater for more than 70% of the time for more than 70% of their length).

Figure 4: A contiguous throughflow lake which both receives groundwater and loses water to the aquifer (graphic from Winter et al, 1998)

Contiguous streams can span the whole spectrum of seepage direction from gaining to losing depending on the relative elevation of the surrounding watertable. Perched streams tend to be losing, as by definition the watertable is deeper than the stream stage. However, it is possible for a perched stream to be gaining over the short-term, due to the release of perched bank storage immediately following a high-stage event. This can occur for ephemeral streams where underlying permeability barriers (such as clay horizons) encourage shallow laterally outward subsurface flow during floods which become return flows during the flood recession. Figure 5 shows this bank storage mechanism for the contiguous case.

Figure 5: A contiguous fluctuating stream, with stream gaining during low-stage period but losing during high-stage period (graphic after Winter et al, 1998)

Classification can also be applied on the basis of conductance, which is the ability of the geological material to transmit water. This includes the aquifer itself and any intervening geological features, such as stream bed profiles which may impede or encourage interaction. Hence, a systems approach is taken where classification is at the scale of the overall groundwater flow system, accounting for the entire groundwater flow path between aquifer and stream. Simple categories of high, medium and low-conductance can be used to describe the extent of seepage flux (Figure 3). Table 2 provides some guidance for categorisation. Highly conductive streams are associated with highly permeable lithologies (such as gravels) that allow large seepage fluxes to occur and are reflected in rapid hydrological responses. In contrast, weakly conductive streams have very low seepage flux due to the presence of extensive low-permeability material (eg clay). In some studies, this end-member is termed an insulated stream where the reach neither contributes water to, nor receives water from, the saturated zone, due to an intervening impermeable horizon. The seepage flux thresholds defined in terms of m³/d/km in Table 2 are those suggested in a study of stream-aquifer connectivity in the Murray-Darling Basin (SKM, 2003). This requires extrapolating flux rates over the stream reach using known stream dimensions of width and length. When describing seepage flux for other surface water features (such as lakes and wetlands) the units m³/d/km2 would be more appropriate. Also, the potential for flux is assessed, so semi-arid streams can still be deemed to be highly conductive although flux may not exceed 1000 m³/d/km. This recognises that the existing seepage flux relates to water availability. In this context, the ratio of seepage to the total flow regime is a better criterion.

Table 2: Typical features of various conductance categories for stream-aquifer systems

Features	High Conductance	Moderate Conductance	Low Conductance
Typical lithologies	Gravels, coarse sands, karst	Fine sands, silts, fractured rock, basalt	Clay, Shale, Fresh Unfractured Rock
Typical Hydraulic conductivities (k)	>10 m/d	10-0.01 m/d	<0.01 m/d
Typical seepage flux	>1000m3/d/km	10-1000 m3/day/km	<10m3/d/km
Ratio of seepage to total flow	>0.5	0.1-0.5	<0.1
Typical Near-Stream Response Times	Days-Months	Years	Decades+

Lastly, connectivity can also be classified in terms of the potential impact on the combined water resource and its use and management. In this respect, a relative classification (high, medium, low-impact) is the simplest approach to describe the significance of stream-aquifer connectivity for water resource management. When assigning an impact category the following aspects need to be considered:

1. Both the water quantity and water quality issues of the catchment need to be included in the analysis. Accounting for groundwater discharge to streams is important in the water allocation process, to avoid the double accounting of the water resource. Equally, groundwater discharge can also have an impact on stream water quality, in terms of ingress of salt, acid, nutrients or contaminants or changes to the in-stream temperature regime. In this regard, the connectivity for a stream reach may be deemed to be high impact although having a relatively low seepage flux, because of the input of very poor quality groundwater.
2. Connectivity can have implications for both the surface water resource and the groundwater resource. A stream can be losing water to the underlying aquifer which may be viewed as a minor transmission loss, but such seepage can be providing a valuable mechanism for groundwater recharge.
3. The likely impacts on all users of the resource need to be assessed. This includes environmental needs (the ecosystem dependencies of both surface water and groundwater systems) as well as future consumptive users.
4. A long-term (>50-year) view of the likely impacts should be taken as well as accounting for the effects within a seasonal or annual timeframe. Depending on the catchment setting, the management impacts associated with changes to connectivity may be significant but only manifest themselves after several decades.
5. The significance of connectivity needs to be assessed in terms of the overall objectives of the catchment plan, particularly in terms of management targets. The thresholds demarcating the different impact categories are highly debatable. Nominally, connectivity can be viewed as low-impact if the predicted effect on a long-term (>50-years) target is <1%. Likewise, a high-impact connectivity has a highly significant impact on whether a long-term catchment management target can be met, say >10%. This prioritisation approach (albeit with different thresholds) has been proposed in a national framework for managing the impacts of groundwater and surface water interaction (SKM, 2006).

The overall stream-aquifer connectivity can be described on the basis of conductance and impact. Surface water and groundwater systems are highly connected if attributes of high conductance and/or high impact are evident.

A highly connectedsystem is indicated by: short-term near-stream response times, with the response in one system (eg stream flow) due to change in the other (eg watertable decline) occurring over a timeframe of days to months; or seepage flux having a significant (>10%) impact on catchment management targets, particularly over the medium-term (eg 1-5 years).

This definition is similar to the approach used by New South Wales for developing operating rules for highly connected systems, which uses the definition:
"Highly connected river-aquifer systems are ones where groundwater extraction results in equivalent streamflow depletion within a relatively short time frame of days to weeks".

Groundwater-surface water interactions need to be assessed and managed for highly connected water resources to protect dependent economic, social, and environmental values. Examples of highly connected water resources include:

1. unregulated systems where groundwater seepage defines the low-flow stream conditions;
2. shallow alluvial systems where groundwater recharge is reliant on leakage from the overlying stream;
3. where seepage of saline groundwater contributes significantly to in-stream salinity; and
4. lakes (or wetlands) where the input of nutrient-rich groundwater can initiate algal blooms.

Surface water and groundwater systems are weakly connected if:

1. potential movement of water between these systems is limited and the near-stream response times are slow (e.g. decades); or
2. seepage flux has no significant impact (say <1%) on water quantity and/or quality over the long-term (>50 year) timeframe.

Examples of weakly connected water resources include where:

1. stream and aquifer are separated by impermeable layers (such as clay) that impede the movement of water; and
2. the hydraulic gradient between stream and aquifer is low.

Weakly connected water resources still need to be assessed and monitored in terms of changes in hydraulic conditions. For example, watertable mounding due to land use change (such as irrigation development) can alter hydraulic gradients and induce groundwater discharge to the stream. Also weakly connected water resources can be used to advantage. For example, the aquifer component could be used in water banking without significant losses from storage due to leakage to surface water features.

The significance of connectivity is coached in terms of two key questions:

1. Does the seepage flux between stream and aquifer have a role (either positive or negative) in meeting existing management targets that relate to water quality or quantity? This relates how existing flux rates contribute to management goals. A 10% threshold is suggested in terms of defining significance. For example, if there is a 10% variance when seepage flux is included in assessment and when it is not, then the system is deemed to be highly connected.
2. Will changes in seepage flux due to predicted changes in the surface water or groundwater regime have a significant impact on water quality and quantity targets over a long-term (>50 year) timeframe? This looks at the potential status for flux and focuses on the effect of future hydrological stresses such as increased water extraction, irrigation accessions, land use change or climate change.

Stream-aquifer connectivity is controlled by the relativity between stream stage and the shallow watertable. Both of these can change due to catchment activities, so that post-development connectivity may be quite different to that operating under undeveloped conditions. For example, stream regulation can artificially maintain high stream stages and induce stream losses to the underlying aquifer. Excessive groundwater extraction can deepen the watertable so that a previous contiguous stream becomes perched, due to the development of an underlying unsaturated zone. Hence, the categorisation of stream connectivity should be placed in context in terms of the extent of catchment modifications. This involves the use of descriptors such as:

1. whether the stream is regulated or unregulated involving the artificial manipulation of stream flow using infrastructure such as dams or weirs;
2. water development terms based on the percentage ratio of water extraction to defined sustainable yields such as weakly developed (<30%), moderately developed (30-70%), highly developed (70-100%) or overdeveloped (>100%). These are the thresholds used in the National Land and Water Resources Audit (NLWRA, 2001), refer Table 3; and
3. whether the stream (and near-stream environment) is modified or unmodified in terms of works (such as engineering, drainage or mining) which has altered stream morphology.

Table 3: Development categories for water resource management areas (NLWRA, 2001)

Category	Description	Extraction/allocation	Development Status1	Examples
1	No to low levels of resource use, direct management interventions and information requirement is low.	<30%	Low development	Victoria River (Northern Territory, SWMA) Burnie (Tasmania, GMU)
2	Moderately developed, management and resource information requirement is moderate.	30 - 70%	Moderate development	Mary River (South Australia, SWMA) Ti Tree (Northern Territory, GMU)
3	Close to, or at, their extraction limit and require a high level of management inputs. Resource information and monitoring is vital for these systems. Development depends on putting in place appropriate water markets to move water to higher value use and to provide surplus for development or the environment through efficiency gains.	70 - 100%	Highly developed	Pioneer (Queensland, SWMA) Woongarra (Queensland, GMU)
4	Over-committed in water allocation and/or use-insufficient provision has been made for environmental and non-consumptive uses, management intervention and information requirements are substantial.	>100%	Overdeveloped	Wimmera-Avon Rivers (Victoria, SWMA) Neuarpur GSPA (Victoria, GMU)
1 Development status is water use as a percentage of sustainable flow regime (surface water) and sustainable yield (groundwater).

References

NLWRA 2001, Australian Water Resources Assessment 2000, Surface water and groundwater - availability and quality, National Land and Water Resources Audit, Canberra. http://audit.ea.gov.au/anra/water/docs/national/Water_Contents.html

REM 2002. Watermark: Sustainable groundwater use within irrigated regions. Project 2: Conjunctive resource management, milestone 2 final report. Prepared for the Murray Darling Basin Commission, Australia.

SKM 2003. Projections of groundwater extraction rates and implications for future demand and competition for surface water. Murray Darling Basin Commission Publication 04/03. Sinclair Knight Merz.

SKM, 2006. Towards a national framework for managing the impacts of groundwater and surface water interaction in Australia. Natural Heritage Trust, Sinclair Knight Mertz. http://www.nht.gov.au/ncc/ground-surface-water.html

Winter, TC, Judson, WH, Franke, OL, Alley WM. 1998. Groundwater and surface water a single resource. Circular 1139, U.S. Geological Survey, Denver. http://pubs.usgs.gov/circ/circ1139/