Recharge is the addition of surface water to an aquifer. Natural recharge occurs when naturally occurring surface water infiltrates through the unsaturated zone (vadose zone) and is stored in an aquifer as groundwater. Artificial recharge is an engineered system (either natural or human designed system) designed to store surface water in an aquifer. Artificial recharge occurs in two ways: surface infiltration and direct injection.

Surface infiltration is the process by which surface water is diverted to basins, furrows, ditches, dry streambeds, or other retention structure and allowed to infiltrate through the unsaturated zone where it is added to an aquifer. The process is similar to natural recharge. Surface infiltration is the most common and economical method of artificial recharge.

Direct injection recharge is a process by which surface water is diverted into a well penetrating an aquifer, and the surface water is forced into the aquifer and stored. A hybrid method of artificial recharge is called "vadose zone wells". This process occurs when surface water is diverted into a well that penetrates the unsaturated zone above the aquifer. The surface water infiltrates through a portion of the unsaturated zone and is stored in the aquifer. Direct injection and vadose-wells are used in areas that lack sufficient land area for surface infiltration or where near-surface unsaturated materials are not conducive to infiltration. Direct injection and vadose-wells are significantly more costly than surface infiltration methods.

Artificial recharge is a water management tool commonly used to:

• Store excess surface water for future uses,
• Replenish groundwater supplies,
• Prevent or mitigate saltwater intrusion,
• improve water quality by natural filtration in the vadose zone,
• Prevent land subsidence.

Artificial recharge has been in use for many centuries for water management. Some of the earliest examples of recharge projects are structures constructed in wadis in historic Persia (Iran, Iraq, Syria, and Jordan) dated to at least 330 AD. These structures slowed surface water flowing in the wadis to increase infiltration and recharge aquifers supplying qanats (irrigation systems). Large-scale artificial recharge has been used in southern California to store excess surface water and replenish groundwater supplies since the early 1960's. In addition, artificial recharge has been used in northern California since the 1930's to mitigate salt water intrusion. Large-scale artificial recharge projects have been used in Arizona since the early 1990's to replenish groundwater and store excess surface water for future use.