Groundwater is water stored beneath the Earth’s surface in saturated zones of soil and rock, occupying the void spaces in geologic strata to form an aquifer. As the largest reservoir of fresh, unfrozen water on the planet, groundwater is a key component of the global water cycle. Because it moves slowly through rock and sediment, groundwater movement is a long-term process. While surface water cycles quickly, groundwater can reside underground for periods ranging from days to millennia. The stored water must eventually return to the surface or atmosphere to complete its circulation through several distinct pathways.
Natural Flow into Surface Water Bodies
Groundwater naturally returns to the surface through discharge, flowing into lower-elevation surface water bodies. This constant flow from the saturated zone serves as the primary mechanism that sustains rivers and streams, particularly during dry periods. This sustained contribution is known as baseflow, which ensures that many surface waterways do not dry up when there is no recent rainfall or snowmelt.
The flow of water is dictated by the hydraulic head, which is the water pressure and elevation difference between the aquifer and the surface water body. Groundwater moves along flow paths from areas of high hydraulic head (recharge areas) to areas of low hydraulic head (discharge areas). The water table, the upper boundary of the saturated zone, typically mimics the overlying land topography, being higher under hills and lower near valleys and streams.
When the water table intersects the land surface, the groundwater is forced out, forming a natural discharge point. This discharge often happens most intensely near the shoreline of a river or lake, where the water table meets the surface. Springs are concentrated examples of this process, occurring where geologic formations allow water to exit the aquifer in a visible flow. The volume of water moving into the stream can vary along the length of the channel depending on the surrounding hydrogeology.
Atmospheric Return through Evapotranspiration
Groundwater can rejoin the water cycle by returning directly to the atmosphere as vapor through evapotranspiration. This pathway is relevant for water stored in shallow aquifers where the water table is close to the surface. Evapotranspiration is a collective term for two distinct physical and biological processes.
Evaporation
The physical process is evaporation, where liquid water converts directly into water vapor from a moist surface. This occurs in areas like wetlands where the water table is near the land surface, allowing water to wick up through the soil’s capillary fringe. Solar energy provides the heat needed for this phase change, moving water from the soil surface into the air.
Transpiration
The biological process is transpiration, which involves plants acting as natural pumps. Plants absorb liquid water through their root systems, transport it up, and then release it as water vapor through tiny pores in their leaves called stomata. Deep-rooted vegetation, such as riparian plants, can draw water directly from the saturated zone. This biological uptake is a significant mechanism for returning subsurface water to the atmosphere, where it eventually condenses to form clouds and precipitation.
Human Extraction and Reintegration
A major modern pathway for groundwater to rejoin the cycle is through human intervention, primarily via extraction using wells. Pumping water from aquifers for agricultural, municipal, or industrial use rapidly brings the subsurface water to the surface. This anthropogenic process bypasses the typically slow natural flow and significantly alters the timing and location of discharge.
Once extracted, the water follows several paths to reintegrate with the cycle. A substantial portion of the pumped water is used for irrigation in agriculture, where most of the water evaporates directly or transpires through crops, quickly returning to the atmosphere as vapor. This localized increase in evapotranspiration can significantly alter regional water balances.
Water used for municipal and domestic purposes also finds its way back, often after treatment. After use, the water is collected, treated at wastewater facilities, and then typically discharged into surface water bodies like rivers or lakes. This discharge directly augments surface water flow, allowing the water to continue in the cycle through natural processes.