What is residence time and what are the average residence time for some of the major water reservoirs on earth?

The average residence time varies greatly depending on the reservoir; some are very long, like Lake Tahoe in Nevada with a residence time of 650 years, or Lake Eucumbene in Australia with a residence time of 1953 years; others are very short, like Lake Arapuni in New Zealand with a residence time of just one week, or Switzerland's Lake Cristallina with a residence time of 2-4 weeks. I won't cover all of the major reservoirs and lakes, but I will highlight some of the most extreme ones.

Residence time refers to the average amount of time that a substance, such as water, spends within a particular reservoir or system before being exchanged or removed. In the context of water, it represents the average time that water molecules remain in a specific reservoir, such as oceans, rivers, lakes, or groundwater.

Here are the average residence times for some major water reservoirs on Earth:

1. Oceans: Approximately 3,000 to 3,200 years
2. Atmosphere: Approximately 9 days
3. Rivers: Typically ranges from days to weeks, depending on the size and flow rate of the river
4. Lakes: Varies widely depending on the size, depth, and circulation patterns of the lake, but can range from decades to centuries
5. Groundwater: Can range from days to thousands of years, depending on factors such as permeability of the aquifer and depth of the water table.