View source for Earth:Groundwater recharge

{{short description|Groundwater that recharges an aquifer}}
[[Image:Surface water cycle.svg|thumb|Water balance]]
'''Groundwater recharge''' or '''deep drainage''' or '''deep percolation''' is a [[Earth:Hydrology|hydrologic]] process, where [[Chemistry:Water|water]] moves downward from [[Earth:Surface water|surface water]] to [[Earth:Groundwater|groundwater]]. Recharge is the primary method through which water enters an [[Earth:Aquifer|aquifer]]. This process usually occurs in the [[Earth:Vadose zone|vadose zone]] below plant [[Biology:Root|root]]s and is often expressed as a [[Physics:Flux|flux]] to the [[Earth:Water table|water table]] surface. Groundwater recharge also encompasses water moving away from the water table farther into the saturated zone.<ref>{{cite book |last1=Freeze |first1=R.A. |last2=Cherry |first2=J.A. |date=1979 |title=Groundwater |publisher=Prentice-Hall |isbn=978-0-13-365312-0 |oclc=643719314 |url=https://archive.org/details/groundwater00free}} Accessed from: http://hydrogeologistswithoutborders.org/wordpress/1979-english/ {{Webarchive|url=https://web.archive.org/web/20200406061947/http://hydrogeologistswithoutborders.org/wordpress/1979-english/ |date=2020-04-06 }}</ref> Recharge occurs both naturally (through the [[Physics:Water cycle|water cycle]]) and through anthropogenic processes (i.e., "artificial groundwater recharge"), where rainwater and or [[Earth:Reclaimed water|reclaimed water]] is routed to the subsurface.

The most common methods to estimate recharge rates are: chloride mass balance (CMB); soil physics methods; environmental and isotopic tracers; groundwater-level fluctuation methods; water balance (WB) methods (including groundwater models (GMs)); and the estimation of baseflow (BF) to rivers.<ref name=":1">{{Cite journal |last1=MacDonald |first1=Alan M |last2=Lark |first2=R Murray |last3=Taylor |first3=Richard G |last4=Abiye |first4=Tamiru |last5=Fallas |first5=Helen C |last6=Favreau |first6=Guillaume |last7=Goni |first7=Ibrahim B |last8=Kebede |first8=Seifu |last9=Scanlon |first9=Bridget |last10=Sorensen |first10=James P R |last11=Tijani |first11=Moshood |last12=Upton |first12=Kirsty A |last13=West |first13=Charles |date=2021-03-01 |title=Mapping groundwater recharge in Africa from ground observations and implications for water security |journal=Environmental Research Letters |volume=16 |issue=3 |pages=034012 |doi=10.1088/1748-9326/abd661 |bibcode=2021ERL....16c4012M |s2cid=233941479 |issn=1748-9326|doi-access=free }}[[File:CC-BY icon.svg|50px]] Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License]</ref>

==Processes==

=== Diffused or focused mechanisms ===
Groundwater recharge can occur through diffuse or focused mechanisms. Diffuse recharge occurs when precipitation infiltrates through the soil to the water table, and is by definition distributed over large areas. Focused recharge occurs where water leaks from surface water sources (rivers, lakes, wadis, wetlands) or land surface depressions, and generally becomes more dominant with aridity.<ref name=":1" />

=== Natural recharge ===
[[File:Groundwater flow.svg|thumb|Natural processes of groundwater recharge. Adjustments affecting the water table will drastically enhance or diminish the quality of groundwater recharge in a specific region.]]
Water is recharged naturally by rain and [[Physics:Snow|snow]] melt and to a smaller extent by surface water (rivers and lakes). Recharge  may be impeded somewhat by human activities including paving, development, or logging. These activities can result in loss of [[Earth:Topsoil|topsoil]] resulting in reduced water infiltration, enhanced [[Earth:Surface runoff|surface runoff]] and reduction in recharge. Use of groundwater, especially for [[Earth:Irrigation|irrigation]], may also lower the water tables. Groundwater recharge is an important process for sustainable groundwater management, since the volume-rate abstracted from an [[Earth:Aquifer|aquifer]] in the long term should be less than or equal to the volume-rate that is recharged.

Recharge can help move excess salts that accumulate in the root zone to deeper soil layers, or into the groundwater system. Tree roots increase water [[Chemistry:Soil#Soil moisture|saturation]] into [[Earth:Groundwater|groundwater]] reducing water [[Earth:Surface runoff|runoff]].<ref name="trees water infiltration">{{cite web | url=https://www.agronomy.org/news-media/releases/2008/1117/221/ | title=Urban Trees Enhance Water Infiltration | publisher=The American Society of Agronomy | work=Fisher, Madeline | date=November 17, 2008 | access-date=October 31, 2012 | url-status=dead | archive-url=https://web.archive.org/web/20130602034025/https://www.agronomy.org/news-media/releases/2008/1117/221/ | archive-date=June 2, 2013 }}</ref> Flooding temporarily increases [[Earth:Stream bed|river bed]] [[Earth:Permeability (earth sciences)|permeability]] by moving clay soils downstream, and this increases aquifer recharge.<ref name="flood">{{cite web | url=http://www.science.unsw.edu.au/news/major-floods-recharge-aquifers/ | title=Major floods recharge aquifers | publisher=University of New South Wales Science | date=January 24, 2011 | access-date=October 31, 2012}}</ref>

==== Wetlands ====
Wetlands help maintain the level of the water table and exert control on the hydraulic head.<ref>O'Brien 1988</ref><ref>{{cite journal |first=T.C. |last=Winter |title=A conceptual framework for assessing cumulative impacts on the hydrology of nontidal wetlands |journal=Environmental Management |volume=12 |issue= 5|pages=605–620 |date=1988 |doi=10.1007/BF01867539 |bibcode=1988EnMan..12..605W |s2cid=102489854 |url=https://lab.jonesctr.org/wp-content/uploads/2021/06/4_impacts_on_hydrology.pdf}}</ref> This provides force for groundwater recharge and discharge to other waters as well. The extent of groundwater recharge by a wetland is dependent upon [[Earth:Soil type|soil]], [[Biology:Vegetation|vegetation]], site, perimeter to volume ratio, and water table gradient.<ref>{{cite book |last1=Carter |first1=V. |last2=Novitzki |first2=R.P. |chapter=Some Comments on the Relation between Ground Water and Wetlands |chapter-url= |title=The Ecology and Management of Wetlands |publisher=Springer |volume=1 |date=1988 |isbn=978-1-4684-8378-9 |pages=68–86 |doi=10.1007/978-1-4684-8378-9_7}}</ref><ref name=Weller81>{{cite book |first=M.W. |last=Weller |title=Freshwater Marshes: Ecology and Wildlife Management |publisher=University of Minnesota Press |date=1994 |orig-date=1981 |edition=3rd |isbn=978-0-8166-8574-5 |oclc=476093538 |url={{GBurl|Uf1VTku_ID8C|pg=PR9}}}}<!-- Guess at ref, please confirm --></ref> Groundwater recharge occurs through [[Earth:Soil type|mineral soils]] found primarily around the edges of wetlands.<ref>{{cite journal |last1=Verry |first1=E.S. |last2=Timmons |first2=D.R. |title=Waterborne nutrient flow through an upland‐peatland watershed in Minnesota |journal=Ecology |volume=63 |issue=5 |pages=1456–67 |date=1982 |doi=10.2307/1938872 |jstor=1938872 |bibcode=1982Ecol...63.1456V |url=https://www.nrs.fs.usda.gov/pubs/jrnl/1982/nc_1982_verry_001.pdf}}</ref> The soil under most wetlands is relatively impermeable. A high perimeter to volume ratio, such as in small wetlands, means that the surface area through which water can infiltrate into the groundwater is high.<ref name=Weller81/> Groundwater recharge is typical in small wetlands such as prairie potholes, which can contribute significantly to recharge of regional groundwater resources.<ref name=Weller81/> Researchers have discovered groundwater recharge of up to 20% of wetland volume per season.<ref name=Weller81 />

=== Artificial groundwater recharge ===
Managed aquifer recharge (MAR) strategies to augment freshwater availability include streambed channel modification, [[Engineering:Bank filtration|bank filtration]], water spreading and recharge wells.<ref name="WWDR2022">United Nations (2022) [https://unesdoc.unesco.org/ark:/48223/pf0000380721 The United Nations World Water Development Report 2022: Groundwater: Making the invisible visible]. UNESCO, Paris [[File:CC-BY_icon.svg|50x50px]] Text was copied from this source, which is available under a Creative Commons Attribution 3.0 International License</ref>{{rp|110}}

Artificial groundwater recharge is becoming increasingly important in India, where [[Earth:Overdrafting|over-pumping]] of groundwater by farmers has led to underground resources becoming depleted. In 2007, on the recommendations of the [[Organization:International Water Management Institute|International Water Management Institute]], the Indian government allocated {{INRConvert|1800|c|year=2007}} to fund dug-well [[Earth:Aquifer storage and recovery|recharge projects]] (a dug-well is a wide, shallow well, often lined with concrete) in 100 districts within seven states where water stored in hard-rock aquifers had been over-exploited. Another environmental issue is the disposal of waste through the water flux such as dairy farms, industrial, and urban runoff.

Pollution in stormwater [[Earth:Surface runoff|run-off]] collects in [[Earth:Retention basin|retention basin]]s. Concentrating degradable contaminants can accelerate biodegradation. However, where and when water tables are high this affects appropriate design of detention ponds, retention ponds and [[Engineering:Rain garden|rain garden]]s.

===Depression-focused recharge===
If water falls uniformly over a field such that [[Earth:Field capacity|field capacity]] of the soil is not exceeded, then negligible water percolates to [[Earth:Groundwater|groundwater]]. If instead water puddles in low-lying areas, the same water volume concentrated over a smaller area may exceed field capacity resulting in water that percolates down to recharge groundwater. The larger the relative contributing runoff area is, the more focused infiltration is. The recurring process of water that falls relatively uniformly over an area, flowing to groundwater selectively under surface depressions is ''depression focused recharge''. Water tables rise under such depressions.

Depression focused groundwater recharge can be very important in arid regions. More rain events are capable of contributing to groundwater supply.

Depression focused groundwater recharge also profoundly effects contaminant transport into groundwater. This is of great concern in regions with [[Earth:Karst|karst]] geological formations because water can eventually dissolve tunnels all the way to [[Earth:Aquifer|aquifer]]s, or otherwise disconnected streams. This extreme form of preferential flow, accelerates the transport of contaminants and the [[Earth:Erosion|erosion]] of such [[Engineering:Tunnel|tunnel]]s. In this way depressions intended to trap [[Earth:Surface runoff|runoff]] water—before it flows to vulnerable water resources—can connect underground over time. [[Physics:Cavitation|Cavitation]] of surfaces above into the tunnels, results in potholes or caves.

Deeper ponding exerts [[Physics:Pressure|pressure]] that forces water into the ground faster. Faster flow dislodges contaminants otherwise adsorbed on soil and carries them along. This can carry pollution directly to the raised [[Earth:Water table|water table]] below and into the [[Earth:Groundwater|groundwater]] supply. Thus the quality of water collecting in [[Earth:Infiltration basin|infiltration basin]]s is of special concern.

==Estimation methods==

Rates of groundwater recharge are difficult to quantify.<ref>{{Cite journal|last1=Reilly|first1=Thomas E.|last2=LaBaugh|first2=James W.|last3=Healy|first3=Richard W.|last4=Alley|first4=William M.|date=2002-06-14|title=Flow and Storage in Groundwater Systems|journal=Science |volume=296|issue=5575|pages=1985–90|doi=10.1126/science.1067123 |pmid=12065826|bibcode=2002Sci...296.1985A|s2cid=39943677}}</ref><ref name=":1" /> This is because other related processes, such as [[Physics:Evaporation|evaporation]], [[Earth:Transpiration|transpiration]] (or [[Earth:Evapotranspiration|evapotranspiration]]) and [[Earth:Infiltration (hydrology)|infiltration]] processes must first be measured or estimated to determine the balance. There are no widely applicable method available that can directly and accurately quantify the volume of rainwater that reaches the water table.<ref name=":1" />

The most common methods to estimate recharge rates are: chloride mass balance (CMB); soil physics methods; environmental and isotopic tracers; groundwater-level fluctuation methods; water balance (WB) methods (including groundwater models (GMs)); and the estimation of baseflow (BF) to rivers.<ref name=":1" /> 

Regional, continental and global estimates of recharge commonly derive from global hydrological models.<ref name=":1" />

===Physical===

Physical methods use the principles of [[Earth:Soil physics|soil physics]] to estimate recharge. The ''direct'' physical methods are those that attempt to actually measure the volume of water passing below the root zone. ''Indirect'' physical methods rely on the measurement or estimation of soil physical parameters, which along with soil physical principles, can be used to estimate the potential or actual recharge. After months without rain the level of the rivers under humid climate is low and represents solely drained groundwater. Thus, the recharge can be calculated from this base flow if the catchment area is already known.

===Chemical===

Chemical methods use the presence of relatively [[Chemistry:Chemically inert|inert]] water-soluble substances, such as an isotopic tracer<ref>{{Cite journal |last=Gat |first=J. R. |title=Oxygen and Hydrogen Isotopes in the Hydrologic Cycle |date=May 1996 |url=https://www.annualreviews.org/doi/10.1146/annurev.earth.24.1.225 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=24 |issue=1 |pages=225–262 |doi=10.1146/annurev.earth.24.1.225 |bibcode=1996AREPS..24..225G |issn=0084-6597}}</ref><ref>{{Cite journal |last=Jasechko |first=Scott |date=September 2019 |title=Global Isotope Hydrogeology―Review |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2018RG000627 |journal=Reviews of Geophysics |language=en |volume=57 |issue=3 |pages=835–965 |doi=10.1029/2018RG000627 |bibcode=2019RvGeo..57..835J |s2cid=155563380 |issn=8755-1209}}</ref><ref>{{Cite journal |last1=Stahl |first1=Mason O. |last2=Gehring |first2=Jaclyn |last3=Jameel |first3=Yusuf |date=2020-07-30 |title=Isotopic variation in groundwater across the conterminous United States – Insight into hydrologic processes |url=https://onlinelibrary.wiley.com/doi/10.1002/hyp.13832 |journal=Hydrological Processes |language=en |volume=34 |issue=16 |pages=3506–3523 |doi=10.1002/hyp.13832 |bibcode=2020HyPr...34.3506S |s2cid=219743798 |issn=0885-6087}}</ref> or [[Chemistry:Chloride|chloride]],<ref>{{cite journal |last=Allison |first=G.B. |author2=Hughes, M.W. |year=1978 |title=The use of environmental chloride and tritium to estimate total recharge to an unconfined aquifer |journal=Australian Journal of Soil Research |volume=16 |pages=181–195 |doi=10.1071/SR9780181 |issue=2}}</ref> moving through the soil, as deep drainage occurs.

===Numerical models===

Recharge can be estimated using [[Numerical analysis|numerical methods]], using such [[Source code|codes]] as Hydrologic Evaluation of Landfill Performance, UNSAT-H, SHAW (short form of Simultaneous Heat and Water Transfer model), WEAP, and [[MIKE SHE]]. The 1D-program [[Software:Hydrus|HYDRUS1D]] is available online. The codes generally use [[Earth:Climate|climate]] and [[Chemistry:Soil|soil]] data to arrive at a recharge estimate and use the [[Richards equation]] in some form to model groundwater flow in the [[Earth:Vadose zone|vadose zone]].

== Factors affecting groundwater recharge ==

=== Climate change ===

{{excerpt|Groundwater#Climate change|paragraphs=1-4|file=no}}

=== Urbanization ===
Further implications of groundwater recharge are a consequence of [[Earth:Urbanization|urbanization]]. Research shows that the recharge rate can be up to ten times higher<ref name=":3">{{Cite web |url=https://water.usgs.gov/edu/gwdepletion.html |title=Groundwater depletion |date=2016-12-09 |website=USGS Water Science School |publisher=United States Geological Survey}}</ref> in urban areas compared to rural regions'''.''' This is explained through the vast water supply and sewage networks supported in urban regions in which rural areas are not likely to obtain. Recharge in rural areas is heavily supported by precipitation<ref name=":3" /> and this is opposite for urban areas. Road networks and infrastructure within cities prevents surface water from percolating into the soil, resulting in most surface runoff entering storm drains for local water supply. As urban development continues to spread across various regions, rates of groundwater recharge will increase relative to the existing rates of the previous rural region. A consequence of sudden influxes in groundwater recharge includes [[Earth:Flash flood|flash flood]]ing.<ref name=":4">{{Cite web|url=https://pubs.usgs.gov/fs/fs07603/|title=Effects of Urban Development on Floods|website=pubs.usgs.gov|access-date=2019-03-22}}</ref> The ecosystem will have to adjust to the elevated groundwater surplus due to groundwater recharge rates. Additionally, road networks are less [[Earth:Permeability (earth sciences)|permeable]] compared to soil, resulting in higher amounts of surface runoff. Therefore, urbanization increases the rate of groundwater recharge and reduces infiltration,<ref name=":4" /> resulting in flash floods as the local ecosystem accommodates changes to the surrounding environment.

==Adverse factors==
* Drainage
* [[Earth:Impervious surface|Impervious surface]]s
* [[Engineering:Soil compaction|Soil compaction]]
* [[Earth:Groundwater pollution|Groundwater pollution]]

==See also==
* [[Earth:Aquifer storage and recovery|Aquifer storage and recovery]]
* [[Earth:Contour trenching|Contour trenching]]
* Depression focused recharge
* [[Earth:Dry well|Dry well]]
* [[Earth:Groundwater model|Groundwater model]]
* [[Earth:Groundwater remediation|Groundwater remediation]]
* [[Earth:Hydrology (agriculture)|Hydrology (agriculture)]]
* [[Earth:Infiltration (hydrology)|Infiltration (hydrology)]]
* [[Earth:International trade and water|International trade and water]]
* [[Earth:Peak water|Peak water]]
* [[Engineering:Rainwater harvesting|Rainwater harvesting]]
* [[Earth:Soil salinity control|Soil salinity control]] by subsurface drainage
* [[Earth:Subsurface dyke|Subsurface dyke]]
* [[Earth:Watertable control|Watertable control]]

==References==
{{Reflist}}

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{{Agricultural water management}}
{{Natural resources}}
{{Wastewater}}

[[Category:Hydraulic engineering]]
[[Category:Hydrology]]
[[Category:Soil physics]]
[[Category:Water and the environment]]
[[Category:Water]]

{{Sourceattribution|Groundwater recharge}}