Groundwater recharge
This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface.
Diffuse recharge occurs when precipitation infiltrates through the soil to the water table, and is by definition distributed over large areas.
Recharge can help move excess salts that accumulate in the root zone to deeper soil layers, or into the groundwater system.
The extent of groundwater recharge by a wetland is dependent upon soil, vegetation, site, perimeter to volume ratio, and water table gradient.
In 2007, on the recommendations of the International Water Management Institute, the Indian government allocated ₹1,800 crore (equivalent to ₹54 billion or US$630 million in 2023) 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.
This is of great concern in regions with karst geological formations because water can eventually dissolve tunnels all the way to aquifers, or otherwise disconnected streams.
There are no widely applicable method available that can directly and accurately quantify the volume of rainwater that reaches the water table.
The direct physical methods are those that attempt to actually measure the volume of water passing below the root zone.
After months without rain the level of the rivers under humid climate is low and represents solely drained groundwater.
Chemical methods use the presence of relatively inert water-soluble substances, such as an isotopic tracer[14][15][16] or chloride,[17] moving through the soil, as deep drainage occurs.
Recharge can be estimated using numerical methods, using such codes as Hydrologic Evaluation of Landfill Performance, UNSAT-H, SHAW (short form of Simultaneous Heat and Water Transfer model), WEAP, and MIKE SHE.
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.
The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased evapotranspiration.
Research shows that the recharge rate can be up to ten times higher[22] 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.
Therefore, urbanization increases the rate of groundwater recharge and reduces infiltration,[23] resulting in flash floods as the local ecosystem accommodates changes to the surrounding environment.
