In places where there is no hydrometric data (watershed with dam or reservoir), the value '-9999' was assigned to that polygon.
Data are contiguous, topologically correct polygons as built and enforced using ArcGIS software.
No effort was made to ensure continuity of features with existing data in adjacent study areas.
The study area was delineated based watershed boundaries defined by Agriculture and Agri-Food Canada (2012) but constrained to the following geological boundaries; the western edge of the Paskapoo Formation (Fm), the northern edge of the Wapiti Fm, the eastern edge of the Horseshoe Canyon Fm and the southern edge of the Scollard Fm.
The AAFC (2012) incremental drainage areas were the basic unit for the aquifer yield assessment. Zones were created within the study area based on incremental drainage areas that had corresponding hydrometric gauging station data (HYDAT database, ECCC 2016). When a station did not have flow data, incremental drainage areas were merged together at the sub-sub-basin level or sub-basin level depending on available data.
Zone delineation:
The Agriculture and Agri-Foods Canada (AAFC, 2012) incremental drainage areas are the basic unit for the groundwater yield assessment. Zones were created within the study area based on incremental drainage areas that had corresponding hydrometric gauging station data (HYDAT database, ECCC 2016). When a station did not have flow data, incremental drainage areas were merged together at the sub-sub-basin level or sub-basin level depending on available data.
Recharge:
Baseflow at each hydrometric gauging station is determined using recorded streamflow rates from the HYDAT database. The HYDAT database was brought into an internal statistical spreadsheet tool that aggregates daily data into weekly and monthly data to supplement record gaps. Monthly 5th percentiles were calculated from average monthly flows for each year on record, this value is expressed as a percent exceedance and is referred to as the Q95. The lowest monthly Q95 streamflow value was chosen to be the representative of baseflow for each hydrometric gauging station within the study area.
The Q95 determined for each hydrometric gauging station is not representative of baseflow generated solely within that zone; rather, it is an accumulation of baseflow up until that point and may include many zones depending on how many tributary zones are located upstream. Therefore Q95 value(s) from hydrometric gauging station(s) immediately upstream of each zone were subtracted from the Q95 of that zone. For zones derived from incremental drainage areas that are partly outside of the study area, the resulting value was scaled based on the percentage of the incremental drainage area within the study area border in order to represent baseflow for that particular zone.
Discharge:
Assuming the baseflow determined for each zone is representative of current natural discharge, 50% of natural discharge is permitted for pumping under the maximum sustained yield class and 10% of natural discharge is permitted for pumping under the permissive sustained yield. These values can be modified to be site specific, depending on how conservative an approach is necessary.
Volume:
Aquifer yield was determined considering the topmost unit of bedrock and overlying sediment. Topmost bedrock unit was determined using bedrock geology Map 600 (Prior et al., 2013). Bedrock thickness data were obtained using a 3D geological model from Lyster and Andriashek in addition to internal data sources at the AGS. The 3D model extent did not cover the whole study area, therefore in areas where the model cover was absent, the thickness of the topmost bedrock unit was assumed based on the average bedrock thickness along the boundary of the 3D geological model for each zone. Thickness of the overlying sediment was obtained from MacCormack et al. (2015).
To calculate the amount of water stored in each zone, the topmost bedrock unit(s) volumes and overlying sediment volume were multiplied by an effective porosity value. Aquifers, aquitards, and overlying sediment were assigned porosity values of 0.3, 0.1 and 0.2, respectively.
Compilation into yield matrix:
The three parameters (recharge, discharge, and volume) were plugged into the groundwater-yield continuum equations outlined by Pierce et al. (2013) in order to populate the five classes (permissive sustained yield, maximum sustained yield, sustained yield, permissive mining yield and maximum mining yield) of the yield matrix.