Editing Water cycle (section)

=== Residence times ===
{| class="wikitable" style="margin-left:1em; text-align:center" align="right"
|+ Average reservoir residence times<ref name="PHYS">{{Cite book |website=PhysicalGeography.net |chapter-url=http://www.physicalgeography.net/fundamentals/8b.html |chapter=Chapter 8: Introduction to the Hydrosphere |url-status=live |archive-url=https://web.archive.org/web/20160126072955/http://www.physicalgeography.net/fundamentals/8b.html |archive-date=2016-01-26 |access-date=2006-10-24|title=8(b) the Hydrologic Cycle }}</ref>
! style="text-align:left" | Reservoir || Average residence time
|-
| style="text-align:left" | Antarctica
| 20,000 years
|-
| style="text-align:left" | Oceans
| 3,200 years
|-
| style="text-align:left" | Glaciers
| 20 to 100 years
|-
| style="text-align:left" | Seasonal snow cover
| 2 to 6 months
|-
| style="text-align:left" | Soil moisture
| 1 to 2 months
|-
| style="text-align:left" | Groundwater: shallow
| 100 to 200 years
|-
| style="text-align:left" | Groundwater: deep
| 10,000 years
|-
| style="text-align:left" | Lakes (see [[lake retention time]])
| 50 to 100 years
|-
| style="text-align:left" | Rivers
| 2 to 6 months
|-
| style="text-align:left" | Atmosphere
| 9 days
|}

The ''[[Residence time (fluid dynamics)|residence time]]'' of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (''see table''). It is a measure of the average age of the water in that reservoir.

Groundwater can spend over 10,000 years beneath Earth's surface before leaving.<ref>{{Cite journal |last1=Maxwell |first1=Reed M |last2=Condon |first2=Laura E |last3=Kollet |first3=Stefan J |last4=Maher |first4=Kate |last5=Haggerty |first5=Roy |last6=Forrester |first6=Mary Michael |date=2016-01-28 |title=The imprint of climate and geology on the residence times of groundwater |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GL066916 |journal=Geophysical Research Letters |language=en |volume=43 |issue=2 |pages=701–708 |doi=10.1002/2015GL066916 |bibcode=2016GeoRL..43..701M |issn=0094-8276}}</ref> Particularly old groundwater is called [[fossil water]]. Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge. After evaporating, the residence time in the atmosphere is about 9 days before condensing and falling to the Earth as precipitation.

The major ice sheets – [[Antarctica]] and [[Greenland]] – store ice for very long periods. Ice from Antarctica has been reliably dated to 800,000 years before present, though the average residence time is shorter.<ref>{{cite journal |last1=Jouzel |first1=J. |last2=Masson-Delmotte |first2=V. |last3=Cattani |first3=O. |last4=Dreyfus |first4=G. |last5=Falourd |first5=S. |last6=Hoffmann |first6=G. |last7=Minster |first7=B. |last8=Nouet |first8=J. |last9=Barnola |first9=J. M. |last10=Chappellaz |first10=J. |last11=Fischer |first11=H. |last12=Gallet |first12=J. C. |last13=Johnsen |first13=S. |last14=Leuenberger |first14=M. |last15=Loulergue |first15=L. |last16=Luethi |first16=D. |last17=Oerter |first17=H. |last18=Parrenin |first18=F. |last19=Raisbeck |first19=G. |last20=Raynaud |first20=D. |last21=Schilt |first21=A. |last22=Schwander |first22=J. |last23=Selmo |first23=E. |last24=Souchez |first24=R. |last25=Spahni |first25=R. |last26=Stauffer |first26=B. |last27=Steffensen |first27=J. P. |last28=Stenni |first28=B. |last29=Stocker |first29=T. F. |last30=Tison |first30=J. L. |last31=Werner |first31=M. |last32=Wolff |first32=E. W. |title=Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years |journal=Science |date=10 August 2007 |volume=317 |issue=5839 |pages=793–796 |doi=10.1126/science.1141038 |pmid=17615306 |bibcode=2007Sci...317..793J |s2cid=30125808 |url=http://epic.awi.de/16356/1/Fis2007b.pdf }}</ref>

In hydrology, residence times can be estimated in two ways.<ref>{{Cite web |title=Hydrological Cycle |url=https://www.ldeo.columbia.edu/~martins/climate_water/lectures/hcycle.htm#:~:text=residence%20time:%20Tr%20=%20V/,the%20inflow%20or%20outflow%20rate. |access-date=2025-05-01 |website=www.ldeo.columbia.edu}}</ref><ref>{{Cite web |title=Copernicus Meetings - 404 |url=https://meetings.copernicus.org/404.html |access-date=2025-05-01 |website=meetings.copernicus.org}}</ref> The more common method relies on the principle of [[conservation of mass]] ([[water balance]]) and assumes the amount of water in a given reservoir is roughly constant. With this method, residence times are estimated by dividing the volume of the reservoir by the rate by which water either enters or exits the reservoir. Conceptually, this is equivalent to timing how long it would take the reservoir to become filled from empty if no water were to leave (or how long it would take the reservoir to empty from full if no water were to enter).

An alternative method to estimate residence times, which is gaining in popularity for dating groundwater, is the use of [[Isotope|isotopic]] techniques. This is done in the subfield of [[isotope hydrology]].