The rate of take refers to the speed at which water is withdrawn from a source, such as groundwater (e.g., from a bore) or surface water (e.g., from a stream). This rate is typically measured in litres per second (L/s).
The volume of water refers to the total amount extracted from the source, measured in cubic metres (m³) or litres.
When setting resource consent conditions for water abstraction, authorities usually specify:
- The maximum rate of take (L/s)
- The maximum volume (m³) that can be taken over specific periods
To calculate the total volume of water taken within a given timeframe based on the rate of take, you can follow this simple formula:
- Multiply the rate of take (L/s) by 3.6 to get the volume in cubic metres per hour (m³/hr).
- Multiply this result by the number of hours you are abstracting water.
Example Calculation:
If the rate of take is 5 L/s and water is abstracted for 3 hours:
5 L/s × 3.6 = 18 m³/hr
18 m³/hr × 3 hours = 54 m³
To convert cubic metres to litres, multiply by 1,000, to give you (54,000 litres).
The formula can also be reversed to determine how many hours of abstraction are required to reach a specific volume.
For example, if your consent allows you to take 55,980 cubic metres per year at a maximum rate of 5 L/s, the total hours needed can be calculated as:
55,980 ÷ (5 L/s × 3.6) = 3,110 hours
Sue Clark
Environmental Consultant