Scibek1 D M Allen1 P H Whitfield2

1 Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada

V5A 1S6 (e-mail: [email protected])

2Meteorological Service of Canada, Environment Canada, Pacific and Yukon Division, Vancouver, British Columbia, Canada V6C 3S5

Abstract: A three-dimensional transient groundwater flow model, implemented in MODFLOW, is used to quantify the impacts of climate change on groundwater in an unconfined aquifer with demonstrated strong connection to surface water (Kettle and Granby Rivers). The Grand Forks aquifer is located in a semi-arid region of south-central British Columbia, Canada. Distributed recharge is modelled using HELP, driven by the LARS-WG stochastic weather generator, and stage-discharge curves for rivers are modelled using BRANCH and calibrated to historical data. For recharge modelling, three year-long climate scenarios were run, each representing one typical year in the present, and future (2020s and 2050s), by perturbing the historical weather according to the downscaled CGCM1 global climate model results. By the 2050s the largest increase in recharge relative to present occurs in late spring, by a factor of three or more, a 50% increase in summer months in most areas of the aquifer, a 10-25% increase in autumn, and a reduction in recharge in winter. CGCM1 downscaling was also used to predict basin-scale runoff for the Kettle River. Future climate scenarios suggest a shift in the hydrograph peak to an earlier date, although the peak flow remains the same, and baseflow level is lower and of longer duration. Groundwater levels near the river floodplain are predicted to be higher earlier in the year due to an earlier onset of peak flow, but considerably lower during the summer months. Away from rivers, groundwater levels increase slightly due to the predicted increase in recharge.

Water resources are central to any study on climate change. In areas that rely heavily on groundwater, for example for agricultural, domestic or industrial use, it is important that the potential impacts of climate change be assessed so that adaptation measures can be taken, if needed. One concern of water managers and government officials is the potential decrease of groundwater supplies under future climate change. Another is the potential impact to streams that are fed by groundwater at periods of low flow.

Most research to date has been directed at forecasting the potential impacts to surface water hydrology (e.g. Whitfield & Taylor 1998), while only large, regional and coarse-resolution models have been undertaken to determine the sensitivity of groundwater systems to changes in critical input parameters, such as precipitation and runoff (York et al. 2002; Yusoff et al. 2002). There are a few exceptions of very small aquifers and detailed investigations of potential impacts of climate change (scenarios) on unconfined aquifer water levels (e.g. Malcolm & Soulsby 2000). Thus, there is still a need for high-resolution, local-scale, realistic models, linked to most up-to-date predictions, which can be useful to groundwater managers.

It is expected that changes in temperature and precipitation will alter groundwater recharge to aquifers, causing shifts in water table levels in unconfined aquifers as a first response to climate trends (Changnon et al. 1988; Zektser & Loaiciga 1993). Traditionally, aquifer recharge has been difficult to estimate for large areas; however a variety of methods have been used (Simmers 1998), from statistical empirical models linking precipitation trends to aquifer recharge and groundwater levels (Chen et al. 2002), to spatially distributed recharge applied to three-dimensional groundwater flow models (Jyrkama et al. 2002). For the purposes of climate change impacts modelling, the recharge rates must be as accurate as possible to accurately represent the small shift from present to future climatic conditions.

Also of interest in climate change impacts studies are coupled hydrologic systems, where changes in surface flow regime and changes in recharge to groundwater interact to affect ground-water and surface water levels. Groundwater may contribute to baseflow in streams; therefore, a change in the groundwater regime could have detrimental environmental effects on fisheries and other wildlife due to the altered baseflow dynamics (Bredehoeft et al. 1982; Gleick 1986).

Each aquifer has different properties and requires detailed characterization and, eventually, quantification (e.g. numerical modelling) of these processes, as well as linking of the recharge model to climate model predictions (York et al. 2002). In practice, any aquifer that has an existing and verified conceptual model, together with a calibrated numerical model, can be assessed for climate change impacts through simulations. The accuracy of predictions depends largely on scale of project and availability of hydrogeologic and climatic datasets.

This paper summarizes the methodology and describes the results of a climate change impacts study of an unconfined aquifer that is strongly influenced by surface water. We present a case study of a small regional unconfined aquifer (34 km2), contained within the mountainous valley of the Kettle River near the City of Grand Forks in south-central British Columbia (BC), Canada (Fig. 1). The aquifer consists of glaciofluvial sediments overlying glaciolacustrine sediments, which partially infill steep and variable bedrock topography. The climate is semi-arid and most precipitation occurs in summer months during convective activity. Groundwater is used extensively for irrigation and domestic use (Wei et al. 1994).

The study was motivated by the Canadian government's initiative to assess impacts of climate change and develop adaptive strategies for climate change under the auspices of Natural Resources Canada's Climate Change Action Fund. The goal of the study was to permit a more comprehensive evaluation of water budgets, incorporate seasonal

Fig. 1. Shaded relief map of the Grand Forks valley in British Columbia (BC), Canada. Inset maps show the location of the study area within BC and within Canada.

changes in demand for groundwater, and provide a better understanding of the direct impact of climate change on alluvial aquifers. This study extends the methodology used by Allen et al. (2004a), through the use of spatial analysis tools in a GIS environment and the development of a transient groundwater flow model.

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