Recharging Groundwater

Groundwater recharge is one of the most important functions of wetlands

Water that infiltrates and recharges groundwater contributes to the local and regional groundwater flow net, thus contributing to higher base flows and improved distribution of seasonal flows. Recharge is important for replenishing aquifers used for water supply.

Source: A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Prairie Potholes (HGM), Army Corps of Engineers

Characteristics and processes that influence the groundwater recharge

The attributes of wetlands that allow them to recharge groundwater are not completely understood. Many studies indicate that wetlands, especially in humid climates, are principally discharge areas. The complexities of groundwater interactions with depressional wetlands make it difficult to model groundwater functions. The recharge/discharge function of pothole wetlands has been shown to change seasonally, annually, cyclically through drought and pluvial cycles, and some wetlands have been shown to function as both hydrologic sources and sinks simultaneously.

The ability of any portion of the earth’s surface to be a groundwater recharge area can be simplified to two components: hydraulic head and hydraulic conductivity. Hydraulic head is provided by two characteristics— the elevation of the wetland relative to the groundwater surface (elevation head) and the mass and pressure of water (pressure head). In the depression focused recharge that occurs in prairie potholes, the pressure head is provided by the ability of the basin to collect and pond water (both within and above the wetland “boundary”) and the elevation head generally depends on the basin’s position in the groundwater flow path. The overall hydraulic conductivity depends on soil infiltration and hydraulic conductivity and by the underlying geologic materials.

Source: A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Prairie Potholes (HGM), Army Corps of Engineers

Subsurface Drainage

Local groundwater is directly influenced by the presence of nearby subsurface drainage (e.g., ditches, tile drains, etc.), which, in turn, influences surface water and, therefore, the amount of seasonal water that the depression can capture and hold.

Finally, the elevation of the surface outlet directly affects the height of the water level and, therefore, the ability of the depression to provide the pressure head necessary for depression focused recharge. If a wetland that recharges groundwater is drained, the recharge function of the wetland will no longer exist.

Source: A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Prairie Potholes (HGM), Army Corps of Engineers

Wetland Size and Surrounding Soils

The size or area of the wetland and the soil texture in the surrounding upland are two factors controlling the wetland’s water budget.

A large wetland with a proportionately small watershed may indicate subsidization of its water budget by groundwater discharge. The probability of groundwater discharge occurring may thus increase as the wetland/watershed ratio increases. The wetland size also controls the amount of recharge potential.

The more fine-grained the soil texture in the surrounding uplands, the more water will flow to the wetland via overland flow and less likely water is to flow to the wetland via groundwater discharge. Sandy and loamy upland soils allow more infiltration of precipitation than clayey soils. The infiltrated water will percolate downward vertically and/or flow laterally becoming groundwater discharge where wetlands intersect the water table.

Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms. The four hydrologic soil groups are as follows:

SOIL GROUP and INFILTRATION RATE TABLE

Source: Minnesota Routine Assessment Method (MnRAM), Version 3.4, Minnesota Board of Water and Soil Resources

Hydroperiod

Permanently flooded, semi-permanently flooded, and saturated water regimes, especially in regions having high evaporation rates, often indicate groundwater discharge to a wetland. Exceptions are saturated wetlands on flats and/or bogs that are precipitation-driven systems. Wetlands that are seasonally- or temporarily-flooded are more likely to recharge groundwater.

Source: Minnesota Routine Assessment Method (MnRAM), Version 3.4, Minnesota Board of Water and Soil Resources

Inlet/Outlet for Groundwater

A wetland with a permanent stream inlet but no permanent outlet is more likely to recharge groundwater than one with an outlet. Several factors support this ranking. First, a higher hydraulic gradient will likely be present in an area with no outlet, especially if an inlet is present. Second, the longer water is retained in an area, the greater the opportunity for it to percolate through the substrate. Third, wetlands without outlets generally experience more water-level fluctuations, resulting in inundation of unsaturated soils. Finally, lack of an outlet suggests that water is being lost either through recharge or evapotranspiration, especially if an inlet is present. A wetland with a permanent outlet and no inlet is more likely to discharge groundwater than one with other combinations of inlets and outlets. Continuous discharge of water (i.e. permanent outlet) without surface water feeding the wetland through an inlet suggests an internal source of groundwater (e.g., springs or seeps). Flow-through wetlands would likely be considered discharge wetlands.

Source: Minnesota Routine Assessment Method (MnRAM), Version 3.4, Minnesota Board of Water and Soil Resources

Topographic Relief

This refers to landscape-level topography at a large, subwatershed scale. Groundwater discharge is more likely to occur in areas where the topographic relief is characterized by a sharp downslope toward the wetland (i.e. wetland is located at the toe of a slope). Groundwater recharge is more likely in wetlands where the topographic relief is characterized by a sharp downslope away from most of the wetland. The slope of the water table with respect to the wetland influences the hydraulic gradient for groundwater movement. The water table usually slopes roughly parallel to the land surface topography. Thus, when local topography slopes sharply toward the wetland, the result is typically a hydraulic gradient favorable for groundwater discharge.

Source: Minnesota Routine Assessment Method (MnRAM), Version 3.4, Minnesota Board of Water and Soil Resources