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Report: Comparison of Two Methods for Determining Long-Term Well Yield in British Columbia
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If you have any questions on the information presented, or require additional report data or attachments, please contact the
Ministry.
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Long-term water supply well capacity can be evaluated by both the 100-day and the modified Moell Q20 methods. The 100-day method is the safe available drawdown multiplied by the specific capacity of the well and assumes no recharge for 100 days. The Q20 method, projects drawdown to 20 years using a mathematical model, assumes continuous pumping for 20 years.
The choice of method should be dictated by the conceptual model. The 100-day capacity will always be greater than that of the Q20 method.
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Author: David van Everdingen
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Date Published: Jun 2024
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Report ID: 63044
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Audience: Government and Public
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Well performance and aquifer capabilities must be assessed to Provincial standards when providing supplementary information for groundwater licence applications in British Columbia. Two acceptable methods for evaluating long-term capacities for water supply wells are the 100-day and the modified Moell Q20 methods.
The 100-day method is the safe available drawdown multiplied by the specific capacity (SC) of the well. SC is the pumping rate divided by the projected drawdown at 100 days using the Cooper-Jacob straight line semi-log plot. This assumes Theis assumptions are valid. The 100-days of pumping represent a period where no recharge occurs (e.g., typically summer months in coastal regions and winter months in the interior). It was originally designed to be a graphical solution to calculating long-term well yield. Because it uses an extrapolation of the actual drawdown data from the pumping test, monitored in the pumping well, it implicitly accounts for well loss. This method was developed in British Columbia where precipitation is expected to occur after a temporary period when groundwater recharge is minimal.
The Q20 is used to calculate the sustainable yield of the well based on the projected drawdown after 20 years using a proven mathematical model (e.g., Theis but can also use any other model), assuming continuous pumping for the duration. The sustainable well yield (Q20) equation is essentially the same as the 100-day equation, except that SC equals test pumping rate divided by the projected drawdown after 20 years modified by the well loss during the first 100 minutes of pumping. This method was developed for aquifers in the prairie provinces where recharge is likely much lower.
The choice of method should be dictated by the conceptual model (i.e., aquifer characteristics) and both current and predicted future climatic conditions. If the methods are applied correctly, the 100-day estimated long-term well yield will always be greater than that of the Q20 method.
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Report Type
Subject
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Region - Province Wide |
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Water Information - Groundwater |
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Water Information - Water Supply |
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