Hydraulic Properties of Aquifers

 Several methods can be used to estimate the hydraulic properties of aquifers. These can be in the laboratory and the field. The laboratory methods are not as expensive as field methods but the information obtained will not be a true representation of the formation.  The physical properties of geologic materials control the storativity and the ability of fluids to move through them. The hydraulic properties control the storage and transmission properties of aquifers.

  • Porosity

Porosity determines the volume of water that can be taken into or released out from storage by an aquifer. It is a function of the size, shape, and arrangement of the particles. Porosity can be defined as the percentage of rock that is void of material. The larger the pore spaces/the greater the number of pores the higher the porosity and the higher the water holding capacity of the aquifer.

Porosity= Volume of void space/Total volume

Factors affecting the magnitude of porosity

  • Size, shape, and geometric arrangement in the packing of individual grains
  • Degree of sorting
  • Degree of cementation and compaction
  • Degree of fracturing
  • Solution and precipitation of salt

Types of Porosity

  • Primary porosity

This refers to the openings that occurred when the geologic material was formed.

  • Secondary porosity

Results from such processes as fracturing and dissolution. 

Not all the pore spaces in the geologic material are available for fluid flow. Effective porosity refers to the porosity available for fluid flow. It is abbreviated as ne. 

  • Specific yield and Specific retention 

Of the total porosity of geologic materials, there is a portion that will drain freely by gravity and an amount retained in the geologic materials. The volume of water that will drain by gravity per unit drop in the water table per unit volume of the aquifer is referred to as the specific yield (Sy). The amount of water that can be available for supply and consumption is represented by specific yield, whereas specific retention represents the water that remains clung to the surface of the solids in the pore spaces. Specific retention can be represented by the ratio between the volume of water retained and the total volume.

Specific retention and specific yield can be related using the following expression;

n=SR+Sy

  • Storativity/Storage Coefficient 

The amount of water an aquifer can release or take into storage per unit surface area when the head is lowered a unit distance is known as the storage coefficient.

It is usually determined by pumping tests. In a pumping test, the storativity represents the storativity from the saturated thickness contributing to the wellbore. Storativity of most unconsolidated (unconfined) and many loosely consolidated aquifers can be expressed as:

S=SY x SSb

SY is the Specific yield 

SS is Specific Storage.

The storativity of a confined aquifer is given by:

S=SSb

Ss is specific storage and b is aquifer thickness.

The volume of water that a unit of aquifer releases from storage under a decline in the head is known as specific storage. It is related to the compressibilities of water and the aquifer.

Where ρ is the mass density of water, g is gravitational acceleration, α is aquifer compressibility, n is total, and β is the compressibility of water.

  • Permeability

It measures the capacity of the medium to transmit a fluid. It is independent of fluid properties.

Where K is the hydraulic conductivity


µ is the dynamic viscosity

ρ is the fluid density

 g is the acceleration due to gravity.  

Factors affecting the magnitude of the permeability

  • Shape and size of sand grains
  • Lamination
  • Cementation
  • Hydraulic conductivity

It is also known as the coefficient of permeability and it indicates the ability of an aquifer material to conduct water through it under hydraulic gradients. It is a combined effect of the porosity of the material and the fluid flowing through it. 

  • Transmissivity

This is a measure of the amount of water that can be transmitted horizontally through a unit with the fully saturated thickness of an aquifer under a hydraulic gradient equal to 1. It is equal to the product of the hydraulic conductivity and the saturated thickness of the aquifer. 

T=Kb    [For confined aquifers] 

T=Kh    [For unconfined aquifers]

  • Specific capacity  

The specific capacity (Q/s) of a well is defined as the discharge rate (Q) per unit drawdown (s). It is calculated by dividing the yield of a well by the drawdown for a particular discharge and is a function of time and discharge.

  • Compressibility

Aquifers are elastic and the storativity of an elastic aquifer is the product of specific storage and thickness of the aquifer. Water that is released from elastic aquifers results from compression of the aquifer and expansion of water. Get more information from the link below

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