What is Groundwater Recharge?

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Groundwater recharge schematic

Groundwater is water that is found underground in the cracks and spaces in soil, sand and rock. Groundwater is stored in--and moves slowly through--layers of soil, sand and rocks called aquifers. Aquifers typically consist of gravel, sand, sandstone, or fractured rock, like limestone. These materials are permeable because they have large connected spaces that allow water to flow through. Groundwater is potable water which is stored underground. Around 20% of the world's freshwater is groundwater. Ground water recharge is a vital part of the “hydrologic cycle”. The hydrologic cycle is a constant movement of water above, on, and below the earth's surface. It is a cycle that replenishes ground water supplies. It is not easy to estimate groundwater recharge rates because it is tough to track the amount of water which returns to subsurface water supplies, even though several different techniques can be used to arrive at estimates. It is important to understand how much water is entering a supply of groundwater, as this influences how much water can safely be taken from groundwater supplies for human use.

 

What are the Estimation methods?

Rates of groundwater recharge are difficult to quantify. Some of the methods are:

  • Physical: Physical methods use the principles of soil physics to estimate recharge. The direct physical methods are those that attempt to actually measure the volume of water passing below the root zone. Indirect physical methods rely on the measurement or estimation of soil physical parameters, which along with soil physical principles; can be used to estimate the potential or actual recharge. After months without rain the level of the rivers under humid climate is low and represents solely drained groundwater. Thus the recharge can be calculated from this base flow if the catchment area is known.
  • Chemical: Chemical methods utilize the presence of relatively inert water-soluble substances, such as chloride moving through the soil, as deep drainage occurs.
  • Numerical models: Recharge can be estimated using numerical methods, using such codes as HELP (Hydrologic Evaluation of Landfill Performance), SHAW, the Shaw Group, WEAP (Water Evaluation And Planning system). These codes generally use climate and soil data to arrive at a recharge estimate.

 

What is the importance of groundwater recharge?

Groundwater recharge is water that has soaked into (infiltrated) the ground, and moved through pores and fractures in soil and rock to the water table. The water table is the depth at which soil and rocks are fully saturated with water. Recharge maintains the supply of fresh water that flows through the groundwater system to wells, streams, springs, and wetlands, which support the plants and animals that are part of the surrounding ecosystem.

 

Why is Groundwater recharge important?

In some places (where there are no nearby rivers or large lakes), almost all water-supply needs are met by groundwater, and recharge is critical to maintaining the abundance and quality of groundwater. Groundwater contributes to wells as well as flow to streams, springs, and wetlands year-round, sustaining them during droughts and dry summer months.

 

How are the design objectives of groundwater recharge structures?

In locations where the withdrawal of water is more than the rate of recharge, an imbalance in the groundwater reserves is created. Recharging of aquifers is carried out with the following objectives:

  • To maintain natural groundwater as an economic resource.
  • To conserve excess surface water underground.
  • To combat progressive reduction of groundwater levels.
  • To battle unfavorable salt balance and saline water interruption.
  • Design of an aquifer recharge system: In order to achieve the objectives, it is crucial to plan out an artificial recharge scheme in a scientific manner. Thus it is very important that a proper scientific examination is carried out for selection of site for artificial recharge of groundwater.

 

What will a proper groundwater recharge system design consist of?

  • Selection of site: Recharge structures should be planned out after conducting proper hydro-geological investigations. Based on the analysis of this data (already existing or those collected during investigation) it should be possible to establish the maximum rate of recharge that could be achieved at that particular site. This will depend on the quality of soil, presence of underground aquifers, etc.,
  • Source of water used for recharge: Fundamentally, the potential of rainwater harvesting and the quantity and quality of water available for recharging, have to be assessed.
  • Rainwater harvesting: Building structure that can capture rainfall and hold it in place long enough for it to seep through to the underground water table

 

Can humans facilitate Groundwater recharge?

Yes, humans can also facilitate groundwater recharge. Public works agencies can reintroduce water to the ground with procedures such as specialized reservoirs to restore groundwater to previous levels or to keep groundwater levels stable. This method is used in regions where groundwater supplies are heavily utilized and authorities are worried about a dropping water table which makes the wells go dry, salt build-up in the soil, or about water scarcity. The earth also makes one of the best available places to store water, so groundwater recharge is utilized as a storage technique.

 

How to generate Groundwater map?

The computer model of groundwater recharge map calculates deep infiltration based on land cover, the water holding capacity of the soil, and the daily precipitation and temperature. The final result is an estimate of groundwater recharge in inches per year. 

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