Scale issues in connectivity
Scale issues in time and space are a significant issue in the assessment and management of stream-aquifer connectivity. In a spatial context there are three main scales relating to connectivity (Table 1):
- catchment-scale, where the stream is placed in context with the overall hydrogeological setting of the catchment;
- feature-scale, at the level of individual surface water features (such as a lake or a stream reach); and
- site-scale, where site-specific studies provide insights into processes particularly at the stream-aquifer interface.
There is a need to identify the overall hydrogeological setting for a surface water feature such as a stream. This is because hydrogeology can change across the catchment and there can be many groundwater flow systems developed which have the potential to interact with the stream, either directly or indirectly.
A groundwater flow system is defined by a recharge zone and a discharge zone and is separated from other groundwater flow systems by groundwater divides. In recharge zones there is a component of groundwater flow that is downward. In discharge zones, the vertical groundwater flow direction is upward.
Groundwater flow systems can operate at different scales and can overlay each other (Figure 1). In local flow systems, groundwater flow paths are relatively short (say <5km), where discharge is typically in the lowland adjacent to the more elevated recharge zone. Deeper regional flow systems have much longer flow paths where the recharge and discharge zones can be separated by tens (or hundreds) of kilometres. In addition to geology, topography plays a significant role in the scale of groundwater flow systems. Local flow systems dominate in areas of pronounced topographic relief, whilst regional flow systems develop in flat-lying landscapes. As local flow systems are the shallowest and the most dynamic, they tend to have the greatest interaction with surface water features. However, in the more subdued lowland parts of catchments, discharge from intermediate to regional-scale flow systems can be significant.
Figure 1: Groundwater flow systems of varying scale - local, intermediate and regional (Toth, 1963)
Also connectivity needs to be viewed at the catchment level as management targets as well as monitoring and reporting operate at this scale. Water management units (both surface water and groundwater) tend to be at this scale, so that connectivity properties need to be aggregated to this level to be incorporated into water management plans. Similarly, water quality targets (such as end-of-valley salinity targets) also operate in the catchment context.
| Scale | Typical Units | Relevance |
|---|---|---|
| Catchment-scale Regional | >100 km2 | Hydrogeological Setting Water Management Areas Catchment management targets Catchment monitoring and reporting |
| Feature-scale Intermediate | 1-100km | Water management decisions Environmental Planning |
| Site-Scale Local | <100 m | Process studies Ecosystem dependencies Water quality protection |
Connectivity also needs to be assessed at the feature level, in terms of individual surface water features such as lakes or stream reaches. This is because stream-aquifer connectivity can change significantly from reach to reach. As an example, Figure 2 shows the downstream changes in seepage in a typical Murray-Darling Basin catchment. Also, operational decisions are typically made at the reach-scale. For example, licensing and allocation of groundwater entitlements can be based on the likely impacts on the nearest stream reach. Mitigation of salt loads from groundwater discharge can involve mapping management zones on a reach-by-reach basis. Management plans can be developed for individual surface water features such as wetlands or lakes, particularly for the protection of important ecosystems.
Hydrogeologists tend to focus at the feature scale, but there is a need to rescale to a finer resolution (Woessner, 2000). Stream-aquifer processes are commonly much more complex than anticipated and this complexity can have important consequences. Investigations at the site-scale have historically been undertaken to resolve hydraulic, biological and chemical processes that relate to water quality issues (such as protection from contamination) or aquatic ecology. This involves direct investigation of the hyporheic zone, which is the portion of the saturated zone underlying and beside a surface water feature where mixing of surface water and groundwater occurs (after Woessner, 2000).
Changes in pressure gradients (and thereby seepage flux) can occur at fine scales along the stream bed. Such changes in gradients can occur near areas of positive relief on the stream bed such as point bars, boulders or ripples. In pool and riffle sequences, localised down-welling can occur upstream with up-welling at the riffle base (Figure 3a). Also, water flow in the hyporheic zone can be controlled by the geometry and position of the stream channel within the floodplain sequence and the groundwater flow regime (Woessner, 2000). Figure 3b shows shallow subsurface flow directions established in meandering streams.
This shallow mixing of groundwater and surface water with complex near-surface flow directions can make the investigation of seepage flux problematic. A particular issue is differentiating between this mixing and the actual groundwater flux in a gaining stream.
Figure 2: Schematic catchment section showing differences in connectivity for different river reaches, Murray-Darling Basin (Braaten and Gates, 2002)
Figure 3: Surface water-groundwater interaction in the hyporheic zone associated with (a) a pool and riffle sequence and (b) stream meanders (Winter et al, 1998)
Like the spatial context, variations in time scales relating to connectivity need to be recognised. Surface water groundwater interactions can be viewed over short, medium and/or long-term time frames (Table 2).
A long-term perspective needs to be taken when evaluating stream-aquifer connectivity. This is because groundwater input to streams can be derived from intermediate to regional-scale flow systems (Figure 1) where the transit time between recharge and discharge can span decades and longer. The response to changes in catchment management (such as land use change or groundwater development) can also take a long time to manifest as changes to groundwater discharge and stream flow. This is the case if such changes have occurred within the catchment but some distance from the stream. As groundwater-surface water interactions are sensitive to changes in the catchment water balance, long-term trends in climate (such as decreased rainfall reliability or increased evaporation) need to be accounted for.
| Scale | Typical Units | Relevance |
|---|---|---|
| Long-term | decades-centuries | Climate variation Land use change Groundwater extraction |
| Medium-term | seasons-years | Water management cycle Allocation and planning Water quality protection |
| Short-term | days-months | Episodic events Evapotranspiration Tidal effects Ecosystem dependencies |
Processes that effect stream-aquifer connectivity over the medium-term are important as these timeframes coincide with water management and operations. The evaluation of seasonal or annual fluctuations in groundwater inputs to streams is critical in water allocation, environmental water provisions and water quality management (such as salinity mitigation).
In certain settings, the understanding of short-term (eg daily) fluctuations in seepage flux is critical. In the arid zone, flooding of ephemeral streams can be an important recharge mechanism for the underlying aquifer that requires understanding of irregular and short-lived processes. Understanding tidal effects is important in assessing groundwater-surface water interactions in coastal areas. Evapotranspiration, particularly by near-stream vegetation that can access shallow groundwater, can cause daily variations in watertable depth expressed as fluctuations in seepage flux. An understanding of these processes is important when evaluating groundwater dependent ecosystems.
References
Braaten R, Gates G, 2002. Groundwater-surface water interaction in inland New South Wales: a scoping study. Water Science and Technology. 48(7): 215-224
Toth J, 1963. A theoretical analysis of groundwater flow in small drainage basins. Journal of Geophysical Research 68, 4785-4812.
Winter TC, Harvey JW, Franke OL, Alley WM, 1998. Ground water and surface water - a single resource', U.S. Geological Survey Circular 1139, Denver, Colorado. http://pubs.usgs.gov/circ/circ1139/
Woessner WW, 2000. Stream and fluvial plain ground water interactions: Rescaling hydrogeologic thought. Ground Water 38(3):423-429.