Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from

surface water Surface water is water located on top of land forming terrestrial (inland) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean. The vast majority of surface water is produced by prec ...

to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in the vadose zone below plant roots and is often expressed as a

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

to the water table surface. Groundwater recharge also encompasses water moving away from the water table farther into the saturated zone. Recharge occurs both naturally (through the water cycle) and through anthropogenic processes (i.e., "artificial groundwater recharge"), where rainwater and or

reclaimed water Water reclamation (also called wastewater reuse, water reuse or water recycling) is the process of converting municipal wastewater Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produce ...

is routed to the subsurface.

Processes

Water is recharged naturally by rain and 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 Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidding, on-site processing, and loading of trees or logs onto trucks or skeleton cars. Logging is the beginning of a supply chain ...

. These activities can result in loss of topsoil resulting in reduced water infiltration, enhanced

surface runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...

and reduction in recharge. Use of groundwater, especially for irrigation, may also lower the water tables. Groundwater recharge is an important process for

sustainable Specific definitions of sustainability are difficult to agree on and have varied in the literature and over time. The concept of sustainability can be used to guide decisions at the global, national, and individual levels (e.g. sustainable livin ...

groundwater management, since the volume-rate

abstracted Abstraction in its main sense is a conceptual process wherein general rules and concepts are derived from the usage and classification of specific examples, literal ("real" or "concrete") signifiers, first principles, or other methods. "An abstr ...

from an 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 saturation into groundwater reducing water

runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock market ...

. Flooding temporarily increases river bed permeability by moving clay soils downstream, and this increases aquifer recharge. Artificial groundwater recharge is becoming increasingly important in India, where over-pumping of groundwater by farmers has led to underground resources becoming depleted. In 2007, on the recommendations of the

International Water Management Institute The International Water Management Institute (IWMI) is a non-profit international water management research organisation under the CGIAR with its headquarters in Colombo, Sri Lanka, and offices across Africa and Asia. Research at the Institute foc ...

, the Indian government allocated to fund dug-well 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.

Wetlands

Wetlands A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...

help maintain the level of the water table and exert control on the hydraulic head. This provides force for groundwater recharge and discharge to other waters as well. The extent of groundwater recharge by a wetland is dependent upon soil, vegetation, site, perimeter to volume ratio, and water table gradient. Groundwater recharge occurs through mineral soils found primarily around the edges of wetlands. 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.(Weller 1981) Groundwater recharge is typical in small wetlands such as prairie potholes, which can contribute significantly to recharge of regional groundwater resources. Researchers have discovered groundwater recharge of up to 20% of wetland volume per season.

Depression-focused recharge

If water falls uniformly over a field such that field capacity of the soil is not exceeded, then negligible water percolates to 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 pressure

Depression focused groundwater recharge can be very important in

arid region A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ar ...

s. 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

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

geological formations because water can eventually dissolve tunnels all the way to aquifers, or otherwise disconnected streams. This extreme form of preferential flow, accelerates the transport of contaminants and the erosion of such tunnels. In this way depressions intended to trap

water—before it flows to vulnerable water resources—can connect underground over time.

Cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...

of surfaces above into the tunnels, results in

pothole A pothole is a depression in a road surface, usually asphalt pavement, where traffic has removed broken pieces of the pavement. It is usually the result of water in the underlying soil structure and traffic passing over the affected area. Water ...

s or caves. Deeper ponding exerts 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 water table below and into the groundwater supply. Thus the quality of water collecting in infiltration basins is of special concern.

Pollution

Pollution in stormwater run-off collects in retention basins. Concentrating degradable contaminants can accelerate biodegradation. However, where and when water tables are high this affects appropriate design of

detention pond A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of rivers, streams, lakes or bays to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of tim ...

s, retention ponds and rain gardens.

Estimation methods

Rates of groundwater recharge are difficult to quantify since other related processes, such as

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

, transpiration (or evapotranspiration) and

infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...

processes must first be measured or estimated to determine the balance.

Physical

Physical methods use the principles of

soil physics Soil physics is the study of soil's physical properties and processes. It is applied to management and prediction under natural and managed ecosystems. Soil physics deals with the dynamics of physical soil components and their phases as solid ...

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

inert Inert may refer to: * Chemically inert, not chemically reactive ** Inert gas ** Noble gas, historically called inert gas * Inert knowledge, information which one can express but not use * Inert waste, waste which is neither chemically nor biol ...

water-soluble substances, such as an isotopic tracer or chloride, moving through the soil, as deep drainage occurs.

Numerical models

Recharge can be estimated using

numerical methods Numerical analysis is the study of algorithms that use numerical approximation (as opposed to symbolic manipulations) for the problems of mathematical analysis (as distinguished from discrete mathematics). It is the study of numerical methods th ...

, using such codes as

Hydrologic Evaluation of Landfill Performance The Hydrologic Evaluation of Landfill Performance (HELP) model is a quasi-two-dimensional hydrologic numerical model for conducting water balance analysis of landfills, cover systems, and other solid waste containment facilities; it was developed f ...

, UNSAT-H,

SHAW Shaw may refer to: Places Australia *Shaw, Queensland Canada *Shaw Street, a street in Toronto England *Shaw, Berkshire, a village *Shaw, Greater Manchester, a location in the parish of Shaw and Crompton *Shaw, Swindon, a List of United Kingdom ...

, WEAP, and

MIKE SHE MIKE SHE is an integrated hydrological modelling system for building and Computer simulation, simulating surface water flow and groundwater flow. MIKE SHE can simulate the entire land phase of the hydrologic cycle and allows components to be used ...

. The 1D-program HYDRUS1D is available online. The codes generally use climate and soil data to arrive at a recharge estimate and use the Richards equation in some form to model groundwater flow in the vadose zone.

Factors affecting groundwater recharge

Climate change

Climate change will impact on the availability of groundwater recharge in drainage basins. Groundwater recharge rates are different for moist, medium, and arid climates. Climate models project a series of various rainfall patterns. It is predicted that groundwater recharge rates will have the smallest impact on a climate of equal humidity and dryness. Research predicts the insignificant impact of groundwater recharge rates on a medium climate due to predictions of decreased basin size and rainfall. Precipitation trends are predicted to relay minimal change quantitatively in the near future, while groundwater recharge rates are subject to increase as a consequence of global warming. This phenomenon is explained through the physical attributes of vegetation. With increasing temperature as a result of global warming, leaf area index (LAI) decreases. This leads to higher rates of infiltration into the soil and less interception within the tree itself. A direct result of increasing infiltration into the soil is elevated rates of groundwater recharge. Therefore, with increasing temperatures and insignificant changes of precipitation patterns, groundwater recharge rates are subject to increase. Different mechanisms of groundwater recharge have different sensitivities in response to climate change. Increasing global temperatures generate more arid climates in some regions, and this can lead to excessive pumping of the water table. When rates of pumping are greater than the rate of groundwater recharge, there is an enhanced risk of overdrafting and hence lowering of the water table. This means deeper drilling would be required to access the groundwater.

Urbanization

Further implications of groundwater recharge are a consequence of urbanization. Research shows that the recharge rate can be up to ten times higher 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 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

flash flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing o ...

ing. The ecosystem will have to adjust to the elevated groundwater surplus due to groundwater recharge rates. Additionally, road networks are less permeable compared to soil, resulting in higher amounts of surface runoff. Therefore, urbanization increases the rate of groundwater recharge and reduces infiltration, resulting in flash floods as the local ecosystem accommodates changes to the surrounding environment.

Adverse factors

* Drainage * Impervious surfaces *

Soil compaction In geotechnical engineering, soil compaction is the process in which stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water (or other ...

* Groundwater pollution

References

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