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Recharge or deep drainage is a hydrologic process where water moves downward from surface water to groundwater. This process usually occurs in the vadose zone below plant roots, and is often expressed as a flux to the water table surface.

This process is important for sustainable groundwater management, since the volume-rate abstracted from an aquifer should be less than or equal to the volume-rate that is recharged.

A recharge rate is difficult to quantify, since other related processes, such as evaporation, transpiration (or evapotranspiration) and infiltration processes must first be measured or estimated. Also, if there is a reliable direct or indirect estimation of recharge, evapotranspiration can be calculated.

Recharge can help move excess salts that accumulate in the root zone to deeper soil layers, or into the ground water system. This is evident through the salinity of the upper soil layers, where at least 25,000 km² of Australia and 5% of current cultivated land has elevated salinities. Another environmental issue is the disposal of waste through the water flux such as dairy farms, industrial, and urban runoff.

Physical methods use the principles of 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.

Chemical methods utilize the presence of water-soluble substances, such as isotopic tracer, moving through the soil, as deep drainage occurs.

Recharge can be estimated using numerical methods, using such codes as HELP, UNSAT-H, SHAW, and MIKE SHE. The 1D-program HYDRUS1D is available online. These codes generally use climate and soil data to arrive at a recharge estimate, and use Richards equation in some form to model groundwater flow in the vadose zone.

• Allison, G.B.; Gee, G.W.; Tyler, S.W. (1994). "Vadose-zone techniques for estimating groundwater recharge in arid and semiarid regions". Soil Science Society of America Journal 58: 6-14. OSTI:7113326. 
• Bond, W.J. (1998). Soil Physical Methods for Estimating Recharge. Melbourne: CSIRO Publishing. 

• Drainage
• Infiltration (hydrology)
• Salinity control by subsurface drainage
• Watertable control

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