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Threats to Water Availability in Canada

10. Climate Variability and Change - Groundwater Resources

Alfonso Rivera,1 Diana M. Allen2 and Harm Maathuis3

1 Natural Resources Canada, Geological Survey of Canada, Quebec, QC
2 Simon Fraser University, Department of Earth Sciences, Burnaby, BC
3 Saskatchewan Research Council, Saskatoon, SK

 


Current Status

It has long been known that natural climate variability and climate change both affect water levels in aquifers. One can predict that as an important part of the hydrologic cycle, groundwater resources will be affected by climate change in relation to the nature of recharge, the kinds of interactions between the groundwater and surface water systems, and changes in water use (e.g., irrigation). We expect that changes in temperature and precipitation will alter recharge to groundwater aquifers, causing shifts in water table levels in unconfined aquifers as a first response (Changnon et al., 1988; Zektser and Loaiciga, 1993). Decreases in groundwater recharge will not only affect water supply, but may also lead to reduced water quality. There may also be detrimental environmental effects on fisheries and other wildlife as a result of changes to the baseflow dynamics in streams (e.g., Gleick, 1986). Other potential impacts include altering the equilibrium in coastal aquifers (e.g., Custodio, 1987; Lambrakis, 1997; Vengosh and Rosenthal, 1994), and reducing the volume of water stored in aquifers with associated potential for increased land subsidence (e.g., California, Mexico City).

From a regional or national perspective, our understanding of climate variability and change impacts on groundwater resources--related to availability, vulnerability and sustainability of freshwater--remains limited. Two important factors serve to complicate and limit our understanding and ability to measure these potential impacts directly.

Timing of Recharge: Climate variability is defined as the natural, often cyclic, and high frequency variation in climate. In contrast, climate change may be either natural or human-induced, and displays longer-term trends. While surface waters typically see rapid response to climate variability, the response of groundwater systems is often difficult to detect because the magnitude of the response is lower and delayed. Longer-term variations in climate are often well preserved in aquifers (e.g., Pleistocene climate impacts). Thus, the magnitude and timing of the impact of climate variability and change on aquifers, as reflected in water levels, are difficult to recognize and quantify. This is because of the difference in time frame that exists between climate variations and the aquifer's response to them.

Aquifer Character: Different types of aquifers respond differently to surface stresses. Shallow aquifers consisting of weathered or fractured bedrock or unconsolidated sediments are more responsive to stresses imposed at the ground surface compared to deeper aquifers. These tend to be more isolated from surface conditions by overlying aquitards (e.g., van der Kamp and Maathuis, 1991a). Similarly, shallow aquifers are affected by local climate changes, whereas water levels in deeper aquifers are affected by regional changes. Therefore, climate variability, being of relatively short term compared to climate change, will have greater impact on these shallow aquifer systems. In contrast, deep aquifers have an increased capacity to buffer the effects of climate variability, and are therefore able to preserve the longer-term trends associated with climate change. It is important to note, however, that deep aquifers can be vulnerable to climate variability. As shallow groundwater resources become limited or contaminated, deeper groundwater resources are often exploited (e.g., Texas).

Artesian Well: Groundwater is a precious resource and might be affected by land use and climate change.

Trends

Groundwater Level Data

Groundwater level data provide a direct means of measuring the impacts of both natural and anthropogenic changes to groundwater resources. The stresses caused by these changes affect recharge to, storage in, and discharge from aquifers (e.g., Taylor and Alley, 2001; Gilliland, 1967), and generally alter or disrupt the overall water balance. Most hydrogeologic assessments, which are conducted for the purpose of aquifer characterization, modelling, and yield assessment, include some analysis and interpretation of well hydrograph data.

Despite the fact that some groundwater level networks have been in operation for decades, there are only a few publications providing an interpretation of hydrographs. Gabert (1986) and Maathuis and van der Kamp (1986) provided a qualitative assessment of hydrographs for Alberta and Saskatchewan, respectively. In addition, van der Kamp and Maathuis (1991a) and van der Kamp and Schmidt (1997) showed that hydrographs for deep semi-confined aquifers can be used for assessment of soil moisture conditions at a regional scale. To date, little has been done in Canada to relate hydrographs to climatic variables (e.g., Rutulis, 1989; Chen and Grasby, 2001; Rivard et al., 2003), nor have hydrograph data been used systematically to specifically address the question of the impact of climate variability on aquifers and groundwater resources.

Potential Impacts

Some of the most important potential impacts to groundwater are described further below.

Recharge: Spatial and temporal changes in temperature and precipitation may act to modify the surface hydraulic boundary conditions of, and ultimately cause a shift in the water balance for, an aquifer. For example, variations in the amount of precipitation, the timing of precipitation events, and the form of precipitation are all key factors in determining the amount and timing of recharge to aquifers. Water levels in an aquifer are often observed to respond consistently to precipitation, although the nature of the response can be complex and depends on time of year and prior conditions, etc. These data may be used in calibrating numerical models because they provide a temporal record of the aquifer response to recharge. In most instances, the water level response to precipitation is positive, slightly delayed in the aquifer, attenuated with depth, and is more pronounced in unconfined than in semi-confined aquifers. However, recent studies have shown that increased annual precipitation does not necessarily correspond to an increase in recharge as would be anticipated (e.g., Rivard et al., 2003; Nastev et al., 2002).

The occurrence of droughts or heavy precipitation can also be expected to impact water levels in aquifers. Droughts result in declining water levels not only because of reduction in rainfall, but also due to increased evaporation and a reduction in infiltration that may accompany the development of dry topsoils. Extreme precipitation events (e.g., heavy rainfall and storms) may lead to less recharge to groundwater because much of the precipitation is lost as runoff. However, in Manitoba, infrequent heavy rainfall events, in the order of a 1-in-10 or 1-in-50 year storm, can "top up" aquifers that have suffered from years of decline (Betcher, personal communication). So, despite the fact that over the course of a year cumulative precipitation may be greater for an area, the total amount of recharge to the aquifer may be less.

Variations in temperature and precipitation, along with other factors, such as wind speed, vegetation, etc., will affect evapotranspiration. Determining evapotranspiration is difficult even when site-specific data are available. Predicting changes to rates and magnitudes of evapotranspiration that might accompany climate change, and thus impact groundwater resources in an aquifer will be particularly challenging. To date, there are few studies that address this issue.

Groundwater-surface water interactions: In many regions of Canada, groundwater interacts strongly with surface water. Thus, the interactions between surface and ground water are important mechanisms to consider since they play a vital role in supporting ecosystems. The intimate relationships between ground and surface water imply that these resources must be treated as an integrated resource rather than as separate ones.

Interactions between surface and ground water include, but are not limited to:

  • wetlands, which are supported and interact strongly with groundwater in some areas
  • streamflow, which is sustained by groundwater when contributions from direct precipitation are lacking (baseflow) (Arnell, 1996)
  • influent rivers, which contribute recharge to aquifers
  • springs, which are groundwater discharge features, and
  • coastal waters, which receive discharging fresh groundwater to support delicate ecosystems.

Therefore, climate variability as it affects any one of these environments will not only impact groundwater resources, but impacts to aquifers will affect each of these as well. For example, shifts in the nature of the stream discharge curves have the potential to influence significantly water levels in aquifers. Under various climate change scenarios for British Columbia, higher peak discharge and an earlier onset and more prolonged baseflow period are predicted for many rivers (Leith and Whitfield, 1998). For those aquifers that are in good hydraulic connection to rivers, rising water tables might accompany spring flood events and longer periods of declining water tables will occur through the late summer and early fall. Reductions to glacier runoff over the next several decades will also see a major impact on many river and aquifer systems as these rely on glacier melt during the late summer months to sustain baseflow and water levels in aquifers (Brugman et al., 1997).

Flow Regimes

Climate variability and change may be important considerations for overall changes to the flow regime. Coastal aquifers are used here as an example, but other complex aquifer systems (that might have fluids of different densities or levels of contamination, or natural quality variations) may be similarly affected. Coastal aquifers are sensitive to changes in water budget due to the interaction between fresh and salt water in the subsurface along the coast. When recharge is lowered, the position of the freshwater-saltwater interface will move inland at a rate that is proportional to the reduction in recharge, and water quality can be compromised to the extent that freshwater availability is limited. Similarly, changes in sea level that might accompany climate change affect the position of this interface. A complicating factor is land-use development, which has increased in most coastal regions of Canada (e.g., Nova Scotia, Prince Edward Island, lower mainland of British Columbia). Upconing or encroachment of the freshwater-saltwater interface due to increased pumping of groundwater is a potential threat.

Groundwater Storage

As the various inputs to (recharge) and outputs from (discharge) aquifers are affected, so too will be the overall storage of groundwater in an aquifer. Over the long term, and in the absence of any major changes to annual budgets that might, for example, be caused by groundwater pumping, the water budget for an aquifer is generally in dynamic equilibrium. This means that cyclic climate variability does occur and does impact water levels, but over the long term, the system is in dynamic equilibrium. Short-term (or moderate-term) variability is reflected in the hydrographs as cyclic variation. Long-period trends in hydrographs that are superimposed over the high-frequency variations reflect changes in groundwater storage that might be the result of groundwater overexploitation (excessive pumping) or climate change. When groundwater is removed from storage, water levels in the aquifer drop, and when water is added to storage, the water levels rise. In aquifers that contain layers or lenses of geologic materials that are easily compacted (e.g., clay), reductions in storage may potentially increase the effective stress on geologic units leading to compaction and land subsidence.

Increased Demand

Increased irrigation is perhaps the most obvious reason why there may be an increased demand for groundwater as an alternative or supplemental supply of water. From a global perspective, climate change impacts to other nations may limit their ability to grow food. An increase in Canadian food export will result in greater consumption of freshwater in Canada, some of which will most certainly be groundwater.

Changes in the demand for groundwater are also likely to occur as development increases and as land use changes or intensifies. While these effects will largely be driven by population increase, climate variability and change may play some role. For example, if shallow aquifers experience significant impacts, it may be necessary for some regions to seek additional groundwater resources from deeper aquifers. Therefore, while the overall demand may be lowered for some aquifers, it may increase for others.

Potential lower recharge to aquifers may prove to be limiting factors to water availability in many regions. This may occur particularly when evapotranspiration losses might be greater under climate change conditions, and when water use might rise to accommodate growing populations, increased use for agriculture and irrigation, etc.

Impact Studies

In Canada, most research on the potential impacts of climate change to the hydrologic cycle has been directed at forecasting the potential impacts to surface water, specifically the links between glacier runoff and river discharge (e.g., Whitfield and Taylor, 1998; Leith and Whitfield, 1998). Relatively little research has been undertaken to determine the sensitivity of aquifers to changes in the key climate change variables, precipitation and temperature. Internationally, only a few studies have been reported in the literature on the impacts of climate change (based on predictive scenarios) to groundwater resources (e.g., Vaccaro 1992; Rosenberg et al., 1999; McLaren and Sudicky, 1993). Aquifer recharge and groundwater levels interact and depend on climate and groundwater use; each aquifer has different properties and requires detailed characterization and eventually quantification (e.g., numerical modelling) of these processes and linking the recharge model to climate model predictions (York et al., 2002).

Three Groundwater Studies in Canada that Consider Climate Variability and Change

The Great Lakes Basin

The impacts of climate change on groundwater conditions within the Great Lakes basin have been estimated using the Grand River watershed and western southern Ontario as prototypes. Methods of analysis have varied from regional-scale groundwater modelling to the analysis of stream flow to determine groundwater levels and discharge (or base flow) as a function of historic and future climatic conditions.

The Government of Canada’s Climate Change Action Fund supported the studies that were conducted across western southern Ontario. They were conducted by the National Water Research Institute and Meteorological Service of Canada in collaboration with the Geological Survey of Canada and Ontario ministries of the Environment and Natural Resources (e.g., Piggott et al., 2001). Streamflow data for a network of 174 watersheds were separated into surface runoff and groundwater discharge components and analyzed in a time series context. The results of this analysis indicate that as much as 75 percent of melting snow and rain is available for runoff and groundwater recharge during the winter, while as little as 10 percent is available during the summer. The portion of this excess precipitation that recharges groundwater is a function of physiography and varies from 10 to 80 percent when calculated by watershed. The rate at which the recharge subsequently discharges to form base flow, and therefore the persistence of this flow is also a function of physiography.

These results were used in combination with two climate change scenarios to determine the potential impacts on groundwater conditions. Averaged over the watersheds, the two scenarios resulted in a 19% increase and a 3% decrease, respectively, in total annual base flow. The two scenarios also resulted in a significant, and more consistent, impact on the annual distribution of base flow. The study predicted increased flows during the winter due to a reduction in snow accumulation and decreased flows during the spring and early summer due to the corresponding decrease in snow melting. These results clearly indicate a potential for significant impacts on groundwater conditions and therefore water supplies, instream conditions, and aquatic habitat. These impacts are due to only the interaction of climate and groundwater; impacts due to changing water use are also probable.

Carbonate Aquifer in Winnipeg

Research on the regional carbonate aquifer in Winnipeg showed that freshwater hydraulic heads, measured southwest of Winnipeg, help keep a saline groundwater on the west side of the Red River from migrating eastward. It also showed that the saline/freshwater boundary is strongly controlled by river systems (Charron, 1965; Grasby and Betcher, 2002). Charron (1965) showed that the boundary was west of the Red River and south of the Assiniboine River in the early 1900s. He also suggested that heavy pumping in the freshwater zone in the early 1900s caused saline water to move across the rivers into freshwater zones in the Winnipeg area. Eastward movement of the boundary was also observed in response to dewatering during construction of the Winnipeg floodway (Render, 1970). Decreases in pumping in the last 30 years have resulted in the boundary moving back to its previous position. Chen and Grasby (2001) analyzed water-well levels in Winnipeg, built an analytical model, and performed simulations to evaluate the effects that long-term climate changes would have on water levels in the freshwater bearing portions of the aquifer.

An empirical relationship between key climate variables was used to relate precipitation and temperature with groundwater recharge, and thus, hydraulic head in the aquifer, based on historic monitoring well network data (Chen et al., 2002). The empirical model could predict heads based on precipitation and temperature. Using future climate change scenarios, projected temperature and precipitation trends were used to calculate hydraulic heads at various points in the aquifer for the year 2030.

Under some climate scenarios the freshwater zone showed net change in hydraulic head from the year 2000. Predicted drops in head near the Red River, southeast of Winnipeg, suggest that the saline/freshwater boundary could potentially move eastward in response to climate change, causing salinization of water wells.

Grand Forks Aquifer, British Columbia

Allen et al. (In press) attempted an integrated climate change sensitivity analysis for the Grand Forks aquifer in south-central British Columbia. They considered both climate variables and changes to surface hydrology. Projected ranges of precipitation and temperature, based on Global Climate Model results for the South British Columbia Mountains Climate Region, were used to estimate minimum and maximum values for recharge. Recharge values were subsequently input into a numerical hydrogeologic model of the aquifer. The sensitivity of water levels in the aquifer to changes in river stage was also investigated. Because the Grand Forks aquifer exhibits strong interaction between ground and surface waters of the Kettle and Granby rivers, variations in river stage water levels were shown to be much more sensitive to river stage variation than to recharge variation. Ongoing work is addressing the temporal/seasonal behaviour of the system in an effort to examine changes in storage that may prove important for assessing demand issues in the dry summer months.

Knowledge and Program Needs

With the purpose of increasing our knowledge on groundwater availability as a first step to assessing impacts from climate changes, several key areas require additional understanding and study in Canada.

Resource Inventory

Even without climate change, increased demand for water can be expected because of population growth, ongoing industrialization and agricultural demands. In addition, there is an increased need for protection of both surface and ground water resources by means of establishing land-use guidelines. Consequently, there will be an increasing need for aquifer resource inventories and aquifer characterizations, particularly in populated areas.

Knowledge Gaps

  • The impacts of climate variability and change will vary across Canada, not only due to differences in climate from region to region, but also due to the nature of the groundwater system being affected. Regional case studies involving detailed characterization of aquifers are required to gain a better understanding of the potential impacts on groundwater resources.
  • The impacts of climate variability and change on groundwater recharge are not well understood and are a major deficiency in current groundwater models.
  • The dynamics of the interaction between shallow aquifers and surface water are poorly understood and not well studied in Canada.

To address these knowledge gaps, historic and future climatic and hydrologic data will be of critical importance for describing changes to the overall water balance and flow regime within an aquifer system, and managing the resource into the future.

Monitoring

Groundwater level measurements from observation wells are the principal source of information on the effects of hydrologic stresses on groundwater systems. These data in combination with precipitation records, streamflow and withdrawal data are essential for monitoring the effectiveness of groundwater management and protection schemes. Similar to streamflow and climatic data, groundwater level data become progressively more valuable with increased record length and continuity. Groundwater level data from observation well networks are available for all provinces (Maathuis, In progress). However, in contrast to streamflow and climatic data, which may have records up to 100 years in length, groundwater level records are typically less than 25 years in length and seldom longer than 40 years. Furthermore, relatively few wells are strategically situated near climate and/or streamflow stations making analysis and comparison difficult. Also, in the past decade networks have suffered from budget cuts, resulting in a reduction in the number of groundwater level observation wells and interruption in the continuity of records.

Data on groundwater withdrawal are similarly critical in assessments of the behaviour of water levels in aquifers. Without withdrawal data, it is impossible to separate the impact of pumping from that caused by climatic variability and change. While in many parts of Canada groundwater withdrawal licences are required for non-domestic groundwater use, reliable withdrawal data are often absent.

The available data that could be used to support any evidence of impacts of climate variability and change on groundwater resources are insufficient and of very short duration. Therefore, the collection of the following long-term data is critical.

Water level data: A Canada-wide network of observation wells for long-term groundwater level monitoring should be established. The network should include wells completed in both stressed and natural environments. It also should be tied into the climate and streamflow networks.

Groundwater withdrawal data: Information about groundwater withdrawals (pumping) is critical to the proper interpretation of water-level data and a basic input parameter into groundwater models.

Modelling

Well-calibrated groundwater models could be used to simulate and anticipate the possible impacts of climate change on the sustainability of groundwater resources. Models should be built to simulate and predict:

  • groundwater changes due to human actions (pumping),
  • interactions with surface water bodies (rivers, streams, lakes and wetlands),
  • climate variability (hydrological cycle scale), and
  • climate change (long-term scale).

In addition to the above, models could be excellent tools for water management, when used for assessing the natural sustainable yield of aquifers and their vulnerability to contamination.

Institutional

The following institutional considerations are also recommended:

  • encouraging watershed approaches to water management and protection
  • increasing cooperation between federal and provincial agencies regarding implementation and operation of monitoring networks
  • fostering linkages between water scientists and water managers
  • promoting integrated water resource management
  • promoting a network of compatible (i.e., standardized) groundwater databases, and
  • promoting a groundwater resources inventory.

References

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Betcher, R.N. Personal communication.

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