Enter the thickness, hydraulic conductivity, and the recharge into the calculator (Recharge method tab) to determine the radius of influence. You can also use the Dewatering (Sichardt) and Pumping test (Cooper-Jacob) tabs for common pumping-related estimates.
Radius of Influence Formula
The following simplified formula is used in the Recharge method tab to estimate a radius of influence from aquifer thickness, hydraulic conductivity, and a uniform recharge rate. (For pumping-related radius estimates, use the Sichardt or Cooper-Jacob tabs.)
RoI = b*SQRT(K/(2*N))
- Where RoI is the radius of influence (length)
- b is the thickness (m)
- K is the hydraulic conductivity (e.g., m/s)
- N is the recharge rate (same length/time units as K, e.g., m/s or m/day)
To calculate the radius of influence, multiply the thickness by the square root of the hydraulic conductivity divided by two times the recharge rate. Make sure K and N use compatible time units (for example, both in per-second units or both in per-day units).
What is radius of influence?
Definition:
A radius of influence is an approximate distance from a pumping well or recharge source beyond which the change in groundwater level (drawdown or mounding) is considered negligible or below a chosen detection/impact threshold.
How to calculate radius of influence?
Example Problem:
The following example outlines how to calculate a radius of influence.
First, determine the thickness. For this example, the thickness is 100m.
Next, determine the hydraulic conductivity. In this problem, the hydraulic conductivity is .000005 (m/s)
Next, determine the recharge rate. In this case, this is measured to be .00000002378 m/s.
Finally, calculate the radius of influence using the formula above;
RoI = b*SQRT(K/(2*N))
RoI = 100*SQRT(.000005/(2*.00000002378))
RoI ≈ 1025.20 m
FAQ
What factors can affect the hydraulic conductivity in a radius of influence calculation?
Hydraulic conductivity can be influenced by several factors including the porosity and permeability of the soil or rock, the presence of fractures or voids, the saturation level of the material, and the temperature and viscosity of the fluid. Changes in any of these factors can lead to variations in hydraulic conductivity, thereby affecting the calculation of the radius of influence.
How does the thickness of the aquifer layer impact the radius of influence?
The thickness of the aquifer layer directly impacts the radius of influence as it is a multiplier in the formula. A thicker aquifer can potentially extend the radius of influence since it allows for a greater volume of water to be stored and transmitted, assuming other factors such as hydraulic conductivity and recharge rate remain constant.
Can the radius of influence change over time?
Yes, the radius of influence can change over time due to variations in recharge rates, changes in hydraulic conductivity due to sediment deposition or erosion, alterations in groundwater extraction rates, or shifts in the hydrological characteristics of the area. Monitoring and recalculating the radius of influence periodically can help in understanding and managing the effects of these changes.
