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If you have any questions on the information presented, or require additional report data or attachments, please contact the Report Contact
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Preliminary monthly groundwater budgets were prepared for eleven mapped aquifers in the South Cowichan region of Vancouver Island.
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Author: Harris, M. and Steven Usher
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Date Published: Oct 2017
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Report ID: 52917
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Audience: Government and Public
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This report describes the development of a monthly groundwater budget model for eleven aquifer areas in the South Cowichan region of Vancouver Island. Specifically water budgets were developed for five overburden aquifer areas (aquifer numbers 0197, 0199, 0201, 0205, and 0206); and six bedrock aquifer areas (aquifer numbers 0198, 0200, 0202, 0203, 0204, and 0207).
The water budgets are based on a conceptual hydrogeologic model developed for each aquifer area based on understanding of the geologic setting to identify the groundwater pathways into and out of each aquifer.
Aquifer recharge was calculated for average annual climate conditions established from 30-years of climate records (1977-2006). In addition, hot/dry and wet/cold periods were represented with the most extreme three year period for each. Recharge of the water table comes from the surplus portion of precipitation (surplus =precipitation - evaporation) that does not run off (recharge = surplus - runoff). The estimated average annual surplus is 761 mm for the 30-year average condition, 512 mm for a representative hot/dry year (1989), and 1263 mm for a representative wet/cold year. All three conditions were used to determine the water budget for each aquifer.
Groundwater recharge is highly seasonal, with the bulk of the infiltration of surplus water occurring between November and February. The deficit season in the summer is about six months in a normal year and eight months in a dry year. Low groundwater level conditions are accentuated when there is reduced precipitation in the preceding wet season.
Climate change predictions indicate temperatures will climb into the future, increasing the length of the summer season, and resulting in significantly more precipitation in the winter months. Recharge during the winter will potentially increase. Conversely, the summer dry periods are expected to grow in length. Water demand in the hotter summers is also expected to increase, further deepening the contrast between seasons. Water storage will become increasingly important.
An Excel spreadsheet has been developed to summarize the water budget calculations for each aquifer study area. This spreadsheet allows interaction between the eleven aquifers and surrounding area. It is equipped with a dashboard feature that allows one to see the monthly water budget both graphically and in a tabulated fashion.
The water budget for each aquifer area has been assessed for water quantity stress levels by examining the ratio of existing consumptive use to available groundwater (R=Use/Availability). Groundwater use was conservatively assumed to be 100% consumptive. A water quantity stress level was considered to be low if R was less than 10%, moderate when R is between 10 and 25%, and a significant when R is greater than 25%. Aquifers identified with significant stress levels were overburden aquifers 0197 and 0206, and aquifers 0198, 0204 and 0205 exhibit a moderate stress level rating. In general, the inland areas receive more recharge than the coastal areas and also have less consumptive use, so are less stressed.
The study revealed significant data gaps necessitating use of assumptions that increase uncertainty in the estimates of groundwater discharge and availability. The hydraulic conductivity values are the single most sensitive factor and need to be assessed in more detail. Groundwater use is reasonably estimated, but should be measured to establish actual water use, and estimates of consumptive use should be refined. The groundwater to surface water interaction is poorly understood, and a program of baseflow measurements should be undertaken. The geologic model developed herein is based on water well records and a more detailed geologic model should be developed for each aquifer area. The report outlines recommendations for further work to better inform the water budgets and to support groundwater allocation decisions.
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Report Type
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Water Information - Groundwater |
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