Groundwater Drawdown Estimator
Estimates the steady-state drawdown of the water table at a chosen distance from a pumping well in a confined aquifer using the Thiem equation. Used by hydrogeologists and geotechnical engineers for first-pass well-yield, dewatering and cone-of-depression checks.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the pumping rate Q (m³/day) and the aquifer transmissivity T (m²/day).
- Enter the radius of influence R (m) — the distance at which drawdown effectively returns to zero.
- Enter the distance r (m) from the well where you want the drawdown; r must be less than R.
How it works
The Thiem equation gives the steady-state drawdown for radial flow to a fully penetrating well in a confined aquifer: s = Q / (2π·T) · ln(R / r). Drawdown is largest at the well and falls logarithmically to zero at the radius of influence R.
Transmissivity T = hydraulic conductivity × aquifer thickness controls how readily the aquifer transmits water — high T spreads the cone of depression wide and shallow, low T makes it deep and narrow. The solution assumes steady state, a homogeneous confined aquifer of constant thickness and horizontal radial flow.
Worked example
Drawdown 5 m from a well. With Q = 1000 m³/day, T = 500 m²/day, R = 300 m and r = 5 m: ln(R/r) = ln(60) = 4.094, and s = 1000 / (2π × 500) × 4.094 = 1.30 m of drawdown 5 m from the well.
Common mistakes
- Using this confined-aquifer form for an unconfined (water-table) aquifer — that needs the Dupuit head-squared form (h1² − h2²).
- Applying it before steady state is reached; early-time drawdown needs the transient Theis well function.
- Confusing transmissivity T (m²/day) with hydraulic conductivity K (m/day); T = K × aquifer thickness.
Frequently asked questions
What is the Thiem equation?
s = Q/(2π·T)·ln(R/r) gives the steady-state drawdown at distance r from a fully penetrating well pumping at rate Q in a confined aquifer of transmissivity T, with R the radius of influence.
Confined or unconfined aquifer?
This tool is for a CONFINED aquifer. An unconfined aquifer, where the saturated thickness itself drops as you pump, uses the Dupuit–Thiem form written in head-squared terms.
What is the radius of influence R?
R is the distance at which drawdown becomes negligible (the edge of the cone of depression). It is often estimated empirically (e.g. Sichardt) and depends on aquifer properties and pumping duration.
Why must r be less than R?
The equation gives zero drawdown at r = R and is only defined for r inside the cone of depression, so the tool requires r < R.
Is this the same as specific capacity?
No. Specific capacity is well yield divided by drawdown at the well (Q/s). This tool estimates the drawdown itself at any radius from the well.
Related tools
- Pore Pressure Calculator
- Piezometric Level Calculator
- Seepage Flow Calculator
- Hydraulic Gradient Calculator
- CPT Friction Ratio Soil Classification Tool
- Compression Index Calculator
Explore more in Geology, Geotechnical & Ground Engineering.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.