Written by Eric Azulay — SSP Engineer (contaminated land), Managing Director of SARL G.M.E.P · Published on July 3, 2026
Principle of groundwater drawdown
Drawdown refers to the lowering of the piezometric level of a groundwater body caused by pumping. It is calculated as the difference between the initial piezometric level (before pumping) and the level observed at a given distance from the pumping point, at a given time. Drawdown calculation is central to two types of applications: sizing pumping structures (construction site dewatering wells, remediation pumping of contaminated sites, water supply) and assessing impacts on nearby uses of the groundwater (other wells, wetlands, watercourses).
Two families of analytical equations dominate the practice of hydrogeological engineering firms: the Theis equation, suited to confined aquifers under transient conditions, and the Dupuit-Thiem equation, suited to unconfined aquifers under steady-state conditions.
Theis equation — confined aquifer, transient conditions
The Theis equation (1935) describes the evolution of drawdown as a function of time and distance from the pumping well, in a confined aquifer assumed to be homogeneous, isotropic, and of infinite extent:
s(r,t) = Q / (4πT) × W(u)
with u = r²S / (4Tt)
where s is the drawdown (m) at distance r (m) from the well and at time t (s), Q the pumping flow rate (m³/s), T the aquifer transmissivity (m²/s), S the storativity coefficient (dimensionless) and W(u) the Theis well function, a mathematical function that is tabulated or numerically approximated. This equation notably allows T and S to be estimated from a pumping test, by fitting the measured drawdown curve to the theoretical Theis curve.
Dupuit-Thiem equation — unconfined aquifer, steady-state conditions
The Dupuit-Thiem equation applies to unconfined aquifers under steady-state conditions (drawdown stabilized over time) and relies on the assumption of horizontal radial flow toward the well:
Q = πK(h2² − h1²) / ln(r2/r1)
where K is the permeability (or hydraulic conductivity, m/s), h1 and h2 the hydraulic heads (m) measured respectively at distances r1 and r2 from the pumping well. This formula makes it possible, from drawdown measurements in two observation piezometers, to determine the maximum exploitable flow rate for a target drawdown, or conversely to estimate the expected drawdown for a given pumping flow rate.
Required hydrogeological parameters
IOTA classification and item 2.1.3.0
Any groundwater withdrawal structure, including temporary drawdown pumping for a construction site or remediation of a contaminated site, may fall under the IOTA nomenclature (Installations, Structures, Works and Activities) of the French Environmental Code. The item historically designated R.214-1 — now codified as item 2.1.3.0 (water nomenclature, annex to Article R.214-1 of the Environmental Code) — governs permanent or temporary withdrawals from an aquifer system according to their depth and volume withdrawn.
Depending on the flow rate and duration of the pumping, the operation is subject to a declaration or authorization requirement from the relevant State authorities (DDT). Other items of the IOTA nomenclature may also apply depending on the context: discharge of pumped water to the natural environment, impact on a wetland, proximity to a drinking water supply well.
Preparing the Water Law filing
When groundwater drawdown is subject to a declaration or authorization requirement under the Water Law, the technical file submitted to the authorities must generally demonstrate:
- Hydrogeological characterization of the site (nature of the aquifer, parameters T, S, K)
- Sizing of the pumping operation (flow rate, duration, radius of influence estimated via Theis or Dupuit-Thiem)
- Impact assessment on sensitive uses nearby (other wells, wetlands, watercourses)
- Proposed monitoring measures (control piezometry, alert thresholds) and, where applicable, compensatory measures
Interpreting a pumping test
Reliable determination of transmissivity and storativity requires carrying out and interpreting a pumping test in the field. The typical process comprises several steps:
1. Prior piezometric monitoring
Before starting any pumping, a baseline piezometric survey establishes the initial level of the groundwater and identifies any natural trends (tidal influence, effects of rainfall or a nearby watercourse) that will need to be corrected for when interpreting the drawdown data.
2. Constant-rate or step-rate pumping
The test can be conducted at a constant flow rate (long-duration test, several hours to several days) to characterize the transient regime according to Theis, or in increasing flow-rate steps to assess head losses in the structure and determine its optimal operating capacity.
3. Measuring drawdown in observation piezometers
Drawdown is measured not only in the pumping structure but, ideally, in one or more observation piezometers located at increasing distances, which makes it possible to separate the head losses specific to the structure (well effects) from the actual hydrogeological response of the aquifer.
4. Fitting the analytical model
The measured drawdown curves are then fitted to the theoretical Theis curves (or to its logarithmic Jacob approximation for long time periods), which makes it possible to derive by regression the values of T and S most consistent with field observations. These calibrated parameters are then fed back into the predictive drawdown calculation for the intended operating flow rate.
Multi-layer drawdown
At many sites, the aquifer targeted by pumping is not homogeneous throughout its thickness but comprises several layers of differing permeability (multi-layer aquifer), separated or not by semi-permeable layers. Calculating multi-layer drawdown then requires an approach based on equivalent layers or on superposition, taking into account the transmissivity specific to each layer and vertical exchanges between layers (leakage). This configuration is common at older industrial sites, where several superimposed geological formations can each contribute to flow toward the pumping structure, with direct consequences on the actual radius of influence of the drawdown and on the sizing of treatment systems for the water pumped in the context of contaminated site remediation.
Applications in contaminated site remediation
Groundwater drawdown is frequently used in the context of contaminated land management, for several purposes: construction site drawdown to allow earthworks or excavation above the water table, remediation pumping (pump & treat) to capture a dissolved contamination plume, or test pumping to characterize the hydrodynamic parameters of the aquifer prior to soil-groundwater-well transfer modeling. In all these cases, sizing the flow rate and estimating the radius of influence directly determine the effectiveness of the system and the compliance of the regulatory filing submitted.
Calculate groundwater drawdown with the GMEP tool
GMEP's Groundwater drawdown tool applies the Theis and Dupuit-Thiem equations, handles multi-layer calculation, and automatically determines the applicable IOTA classification (item 2.1.3.0) for your project.
FAQ — Groundwater drawdown, flow rate calculation, and IOTA classification
What is the difference between the Theis equation and the Dupuit-Thiem equation?
The Theis equation applies to confined aquifers under transient conditions and accounts for the storativity coefficient. The Dupuit-Thiem equation applies to unconfined aquifers under steady-state conditions and relies on the assumption of horizontal radial flow toward the pumping well.
What are transmissivity and storativity?
Transmissivity (T) characterizes the aquifer's capacity to transmit water horizontally, expressed in m²/s. The storativity coefficient (S) represents the volume of water released or stored per unit surface area for a unit change in hydraulic head; it is dimensionless for a confined aquifer.
What is the IOTA classification and item 2.1.3.0?
The IOTA classification (Installations, Structures, Works and Activities) under the French Environmental Code subjects water withdrawals and discharges to declaration or authorization requirements depending on their scale. Item 2.1.3.0 (formerly 1.1.1.0, formerly R.214-1) governs groundwater withdrawal structures according to their depth and flow rate.
When is a Water Law filing required for groundwater drawdown?
A Water Law filing (declaration or authorization depending on the IOTA nomenclature thresholds) is required whenever groundwater drawdown involves a groundwater withdrawal exceeding the thresholds set by item 2.1.3.0 or other relevant items (discharge, impact on a wetland, etc.).
Does GMEP software calculate multi-layer drawdown?
Yes, GMEP's Drawdown V15.87 tool incorporates multi-layer calculation, the Theis and Dupuit-Thiem equations, an AI assistant to help with interpretation, and IOTA classification in accordance with item 2.1.3.0 of the Environmental Code.
Methodological references
Also see the pricing of GMEP software.
Written by Eric Azulay — SSP Engineer (contaminated land), Managing Director of SARL G.M.E.P