Use of Downscaled GCM Data for Modeling Groundwater Recharge, Basin Runoff, and Groundwater Levels

A methodology is presented for generating spatially-distributed and temporally- varying recharge zonation and applied to a surficial valley aquifer situated in a mountainous, semi-arid area of southern British Columbia, Canada. Climate change scenarios from the Canadian Global Coupled Model 1 (CGCM1) model runs are downscaled to local conditions using Statistical DownScaling Model (SDSM), the change factors extracted and applied in the LARS-WG stochastic weather generator, and then input to the one-dimensional HELP hydrologic model, which is used to model groundwater recharge. Using a GIS in combination with HELP, recharge is modeled spatially, accounting for soil distribution, vadose zone depth and hydraulic conductivity, extent of impermeable areas, surficial geology, and vadose zone thickness. A three- dimensional transient groundwater flow model, implemented in MODFLOW and calibrated to historical groundwater levels, is then used to simulate four climate scenarios in 1-year runs (1961-1999 present, 2010-2039, 2040-2069, 2070-2099) and to compare groundwater levels to present. CGCM1 downscaling was also used to predict basin- scale runoff for a 26 km long river that meanders through the unconfined valley aquifer and exerts strong control on the groundwater levels in the aquifer. Stage- discharge relations for river segments were calculated using USGS BRANCH and calibrated to observed historical data. River schedules of high temporal resolution (1 to 5 days) were imported into the transient groundwater flow model for each climate scenario. Head differences were computed at each time step for historical and future, mapped in GIS and linked to the MODFLOW model. By the 2050's 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 to 25% increase in autumn, and a reduction in recharge in winter. Future climate predictions 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. Consequently, 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.