ARGOS

WELL DRAWDOWN

Cone of depression, drawdown and well interference, explained

What a cone of depression is, how drawdown spreads from a pumping well, why neighboring wells interfere, and how the Theis equation is used to predict it.

By Argos Tellus · Updated

KEY TAKEAWAYS

  • Pumping a well lowers the water level in and around it. Seen in three dimensions, that lowered surface is a cone of depression (USGS).
  • Drawdown is the difference between the static (non-pumping) water level and the pumping water level at a point. It is largest at the well and decreases with distance.
  • When the cones of nearby wells overlap, their drawdowns add together — well interference. Neighboring wells lose water level, yield and pump submergence.
  • Cone size and shape depend mainly on the aquifer’s transmissivity and storativity, the pumping rate and time. In confined aquifers the cone is shallower but spreads much farther than in unconfined aquifers.
  • The Theis (1935) equation is the classic way to predict drawdown over time and distance from a pumping well; numerical models extend it to real wellfields.

What is a cone of depression?

When a well pumps, water flows toward it from the surrounding aquifer. To drive that flow, the water level (or pressure head, in a confined aquifer) must be lower at the well than farther away. The result is a depression in the water table centered on the well that is deepest at the well and flattens out with distance. In three dimensions it looks like an inverted cone — the cone of depression.

The outer edge of the cone is the radius of influence: the distance at which pumping no longer measurably lowers the water level. The cone keeps expanding as long as pumping continues and the well is drawing water from storage, until it captures enough recharge or other inflow to balance the pumping rate.

Drawdown: the key measurement

Drawdown is the decline in water level at a point caused by pumping, measured from the static level before pumping began. At the pumping well itself, drawdown also includes well losses from flow through the screen and gravel pack, so the level inside the well is lower than in the aquifer just outside it.

Drawdown matters because it controls cost and reliability. More drawdown means more lift (higher energy cost) and less water above the pump intake. If drawdown pulls the water level close to the pump, the pump can cavitate or break suction, and the well’s usable yield falls.

What controls the size and shape of the cone

  • Pumping rate — more water pumped means a deeper, wider cone.
  • Pumping duration — the cone expands with time while water comes from storage.
  • Transmissivity — in highly transmissive aquifers, water reaches the well easily, so the cone is shallow and broad; in low-transmissivity aquifers it is deep and narrow.
  • Storativity (storage coefficient) — confined aquifers release very little water per foot of head decline, so their cones spread much farther and faster than in unconfined aquifers, which drain pore space.
  • Boundaries — recharge boundaries (rivers, lakes) limit the cone; barrier boundaries (faults, aquifer edges) make it deeper.

Well interference: when cones overlap

Drawdowns from multiple pumping wells superimpose: the drawdown at any point is approximately the sum of the drawdowns each well would cause on its own. Two wells close together therefore each experience more drawdown than either would alone, and a new high-capacity well can lower the water level in neighboring wells that were working fine.

Interference is the mechanism behind many groundwater disputes — a new irrigation, municipal or industrial wellfield lowering water levels in nearby domestic wells. It is also why wellfields for large users such as data centers need well spacing and pumping schedules designed to limit self-interference.

Predicting drawdown: the Theis equation and beyond

In 1935 C.V. Theis published the first solution for transient drawdown around a well pumping a confined aquifer, built on an analogy to heat flow. The Theis equation predicts drawdown s at distance r and time t as s = (Q / 4πT) · W(u), with u = r²S / 4Tt, where Q is the pumping rate, T is transmissivity, S is storativity and W(u) is the well function.

The same equation, run backward on pumping-test data, is how hydrogeologists estimate T and S. Corrections (Cooper–Jacob, Neuman for unconfined aquifers, image wells for boundaries) and numerical models such as MODFLOW extend the approach to multi-well fields, layered aquifers and long-term forecasts.

From drawdown to sustainable pumping

The practical goal of a drawdown analysis is a pumping plan: how many wells, where, at what rates and on what schedule, so that water levels stay above pump intakes and minimum saturated thickness for the life of the project — without unacceptable impact on neighbors. Over longer horizons, drawdown merges with aquifer-wide groundwater depletion, so the analysis should project both.

Frequently asked questions

What is a cone of depression in a well?

The cone-shaped lowering of the water table or pressure surface around a pumping well. It is deepest at the well and decreases with distance out to the radius of influence.

What is drawdown?

The decline in water level at a point caused by pumping, measured from the static water level before pumping started.

Can my neighbor’s well affect my well?

Yes. If your well is inside the cone of depression of a neighboring pumping well, its water level drops. Overlapping cones add their drawdowns together, which is called well interference.

How far does a cone of depression extend?

It depends on pumping rate, time and aquifer properties. In unconfined aquifers it typically extends hundreds to a few thousand feet; in confined aquifers, which release little water from storage, it can spread for miles.

What is the Theis equation used for?

Predicting drawdown at any distance and time from a well pumping at a constant rate, and estimating aquifer transmissivity and storativity from pumping-test data.

HOW ARGOS HELPS

Phreatic — groundwater availability

Physics-based cone-of-depression cross-sections, interference radius maps and interference-aware pump scheduling.

CARI — aquifer reasoning agent

Plain-English answers about drawdown, interference and well placement for a site.

SOURCES

  1. USGS Water Science School — Groundwater wells
  2. Heath, R.C., 1983, Basic Ground-Water Hydrology, USGS Water-Supply Paper 2220
  3. USGS Circular 1186 — Ground water and surface water: a single resource, Box A
  4. Theis, C.V., 1935, The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage

Figures are taken from the primary sources above and dated as of the update shown. Concentrations are expressed in mg/L. Spot an error? Email support@argostellus.com.

RELATED GUIDES

Need a site-specific answer?

Argos models groundwater availability, drawdown and contamination for individual sites and portfolios.

CONTACT ARGOS