Fluxes (Inflows and Outflows) in Groundwater Systems
Fluxes (inflows and outflows) in Groundwater Systems: In order to define the water balance or water budget of an aquifer system, the individual processes that bring water into or out of the system must be quantified (Figure 37 on the next page).
Common inflows of water to a groundwater system include:
- Infiltration through the vadose zone that is not intercepted by evaporation, transpiration, or bound in the unsaturated zone, and thus becomes recharge. Infiltration may be distributed over large areas, or may be focused beneath surface water bodies or at geological features (e.g., sinkholes). Recharge may occur naturally or can be induced or enhanced by excavation and removal of low-permeability soils, and the construction of recharge pits, typically lined or filled with permeable sands (Figure 38 on the next page).
- Injection at wells, either for disposal of treated wastewater or as part of managed aquifer storage and recovery (ASR) programs. The latter is growing in popularity as one way to “bank” excess water in times of surplus, for example, wet seasons or wet years, and then tap the stored water when needed. Although energy-intensive because it requires pumping, ASR is not affected by evaporative losses whereas reservoirs are.
- Groundwater flow from areas outside of the region of interest – areas that are either up-gradient or above or below (i.e. flow across a confining layer).
Outflows from groundwater systems typically include:
- Evaporation or transpiration; this typically occurs in areas where the water table is shallow. Although direct evaporation of water from the water table is possible (in detail, this would occur by evaporation from the capillary fringe, and subsequent “wicking” of water upward from the water table), the upward flux (loss) of water from unconfined aquifers to the atmosphere is dominated by a family of plants known as phreatophytes, characterized by deep roots that extend to and below the water table.
- Water withdrawal by pumping from wells. As discussed in the previous section, pumping at wells induces radial flow toward the well. As the cone of depression grows, the well accesses water over a larger region of the aquifer. In some cases, as the cone of depression grows it may intercept water that would otherwise exit the aquifer via natural seeps or springs (e.g., Figure 37 on the next page), thus “redirecting” a flux that would have been an outflow somewhere else.
- Natural groundwater flow or discharge at springs or seeps, or to surface water bodies.