One of the most important components in an assessment of groundwater supply or aquifer vulnerability is the groundwater recharge rate.
Yet it is the least understood and constrained component because it is so variable and recharge rates are difficult to directly measure.
The sustainability of groundwater supply is dependent on the recharge rate. The rate of recharge from incident precipitation or streams may affect the mass flux of contaminants to groundwater. Proper quantification of groundwater recharge and its uncertainty are essential for the successful management of water resources and for modeling groundwater flow and contaminant transport within the subsurface.
In a previous blog I discussed the benefits of integrated hydrologic modeling, which links surface water (watershed) modeling and groundwater modeling.
One of the benefits is better estimation of recharge to groundwater. Good recharge estimates are fundamental to groundwater modeling because the calibrated model hydraulic conductivity is typically very sensitive to recharge rates. If you are relying on a groundwater model to make informed decisions about a remediation technology, potential environmental consequences of mining or the effects of pumping groundwater on surface water and groundwater resources, then you want to be confident that a critical model input – recharge – is well constrained. A host of factors influence groundwater recharge, which varies significantly in space and time at all scales.
How Does Groundwater Recharge Occur?
Groundwater recharge is the primary method way which water enters the aquifer. Through natural and artificial means, water moves from surface water to groundwater.
Recharge is the rate of groundwater replenishment, which depends on but is not the same as the infiltration rate. The infiltration rate is a measure of the capacity of soil to absorb and transmit surface water under a given condition. Recharge is the net infiltration rate after the effects of other processes on the infiltrating water.
Gravity and Capillarity
Infiltration is caused by two forces: gravity and capillarity. The rate of infiltration is determined by soil characteristics, including ease of entry, storage capacity and the saturated transmission rate through the soil. The soil texture and structure, vegetation types and cover, water content of the soil, soil temperature and rainfall intensity all play a role in controlling infiltration rates.
For example, coarse-grained sandy soils have large pore spaces between each grain, which permit water to infiltrate quickly under the force of gravity. Finer-grained soils with smaller pores offer greater resistance to gravity, and fine-grained soils with very small pores pull water through the soil by suction (capillary forces) with the help of gravity. Under many conditions, capillary forces work against the force of gravity.
The process of infiltration can only continue while pore space is available at the ground surface for entry of additional water. The availability depends on the porosity of the soil and the rate that previously infiltrated water moves away from the ground surface through the soil. If the rate of arrival of water at the soil surface is greater than the infiltration capacity, the water is ponded or redirected as overland flow.
Infiltration capacity rapidly declines during the early part of a storm and approaches a constant value. In fine-grained soils, capillary forces diminish and permeability increases as water saturates the pores.
Factors Affecting Recharge Rates
Infiltrating water is designated potential recharge because it may return to the atmosphere by evapotranspiration; migrate as near-surface interflow and emerge as runoff; or remain suspended in the vadose zone. Infiltration and net recharge vary temporally and spatially by season, storm water intensity, stream stage, soil type, vegetation type and cover, elevation, slope, temperature, solar radiation and other factors, including the presence of buildings, paved surfaces and drainage culverts.
For example, spatial variations in soil types may be related to lithologic differences in the rocks intersecting the ground surface or variations in mineralization. Soil type not only directly affects infiltration, it influences the vegetation type, which affects the rate of evapotranspiration.
Slope & Direction
Vegetation is also a function of topographic slope and direction. The north facing slope in a semi-arid environment may be covered in conifer, while the south facing slope is dominated by desert scrubs. In mountainous areas, precipitation and vegetation vary dramatically with elevation to the extent that precipitation is greatest and vegetation is non-existent at the highest elevations.
Infiltration and recharge occur through diffuse and focused mechanisms. Diffuse recharge, also called direct recharge, occurs by the distributed infiltration of precipitation through the soil surface over large areas. Focused recharge occurs through the infiltration of water from surface water bodies such as streams, canals and lakes, as well as from runoff water collecting in small depressions or passing through low slope areas and ephemeral drainages.
Focused recharge varies spatially more than diffuse recharge. Generally, diffuse recharge dominates in humid settings, and focused recharge is more important in arid climates. Focused recharge is also the main mechanism in urban areas, where pavement and buildings cover a large percentage of the ground surface, occluding local infiltration and generating runoff that is directed to discharge basins and streams. Leaks in water supply lines and sewers are another form of focused recharge.
Cost of Inaccurate Models and Unreliable Assessments
Groundwater recharge is a complex function of many variables. Application of a spatially uniform recharge rate in a groundwater model, a common practice, can result in the development of inaccurate model properties and unreliable assessments of groundwater flow and transport, especially at sites of limited data on the hydraulic conductivity of the subsurface and/or extreme variability, as in fractured bedrock. Wrong decisions can ensue with costly ramifications, such as:
- Underestimating impacts of mining or industrial contaminants on surface water and groundwater resources.
- Overdraft by municipal, industrial or mining groundwater supply wells that affects other users and/or causes land subsidence with attendant structural damage.
- Erroneous conclusions about the effects of land development on aquifer recharge and contamination
The Importance of Watershed Modeling
Watershed models are increasingly utilized for improved estimates of recharge and uncertainty analysis. These models address the numerous variables in groundwater recharge and can be linked with groundwater models for integrated hydrologic modeling. The feedback provided by the groundwater model improves the watershed model, which in turn improves the groundwater model. Iteration between the watershed and groundwater models leads to optimization of surface water and groundwater simulations.
Ask your groundwater consultant about the sensitivity of your project or problem to the groundwater recharge rate. Ask about the known and potential unknown factors controlling recharge. A good groundwater modeling consultant will explain his method of estimating recharge and inform you of the consequences of error in the estimate. Consider watershed or integrated hydrologic modeling, if spatial and temporal variations in recharge at your site may be important.