Section 1.1 — Well Yield
How Much Water Can a Well Give?
Imagine poking a straw into a soaking wet sponge and starting to suck. The amount of water flowing up through the straw each minute is the well's yield. A well with high yield produces a lot of water quickly — enough to supply a whole neighborhood. A well with low yield might barely fill a bathtub in an hour.
A well begins as a hole drilled into the earth — sometimes hundreds of meters deep — by a drilling rig that cuts through soil, sediment, and rock until it reaches a water-bearing layer. Once the casing is installed and the well is completed, it can't simply be handed over to the owner and put to use. The underground formation has been disturbed by drilling, and nobody yet knows how much water the well can reliably produce or how the aquifer will respond to pumping. That's where a pumping test comes in.
During a pumping test, the well is pumped at a steady, constant rate for several hours — sometimes days — while instruments record the water level continuously. The goal is to push the aquifer hard enough and long enough to reveal its true productive capacity. That measured, constant pumping rate is Q.
▶ Go deeper
Well yield, denoted Q, is the volumetric flow rate of water extracted from a well under sustained pumping conditions, expressed in m³/day (or gallons per minute in U.S. practice). In the Wilkinson equation, Q is the constant-rate value measured during a standard pumping test — the pump runs at a single, fixed rate for the full duration of the test.
Other pumping methods exist and are used for different purposes. A step-drawdown test pumps the well at progressively increasing rates to evaluate well efficiency and turbulent head losses near the screen. A recovery test records water-level rebound after pumping stops to estimate transmissivity without sustained pumping. Variable-rate tests are used when a constant rate cannot be maintained in the field. Any of these can yield useful information — but the Wilkinson equation uses the constant-rate value measured during a standard pumping test, which represents the well's sustained productive capacity.
Section 1.2 — Drawdown
What Happens When You Pump?
When you start pumping a well, the water level inside the well declines — just like sucking on that sponge straw and watching the water level sink around it. This decline in water level is called drawdown. The more you pump, the lower it falls. If you pump too fast, the water can't keep up and the level declines sharply.
Drawdown is measured simply: the distance from the static water level (where the water sits before pumping starts) down to the stressed water level (where it sits while the pump is running). That distance, in meters, is s.
▶ Go deeper
Drawdown, s, is the difference between the static water level (SWL) prior to pumping and the pumping water level (PWL) measured in the well during steady pumping: s = SWL − PWL, expressed in meters.
In the Wilkinson equation, s is exactly this measured value — the simple vertical distance the water level has dropped. Well losses (head losses due to turbulent flow near the well screen) are already incorporated into the equation's empirical calibration, so no correction is needed. You use the drawdown you measure in the field.
The spatial pattern of drawdown around a pumping well forms a cone of depression — the water table (or potentiometric surface) dips symmetrically around the well, deepest at the casing and flattening out at distance. Any pumping of water from an aquifer is an act of dewatering, regardless of whether the aquifer is confined or unconfined — water is being physically extracted, and there is that much less water remaining in the system. The pressure head may respond differently depending on aquifer type, but the water budget is reduced either way. A complete evaluation of net gain or loss requires examining the total water budget: all inputs (recharge, lateral inflow) weighed against all outputs (pumping, discharge, evapotranspiration).
Section 1.3 — Specific Capacity
The Well's Report Card
Here's the clever part. If we divide the amount of water coming out (yield, Q) by how much the water level declined (drawdown, s), we get a single number called specific capacity. Think of it as the well's report card — it tells you how productive the well is per meter of water-level decline.
A well with high specific capacity is efficient: it delivers a lot of water without the level falling much. A well with low specific capacity has to "work hard" — the water level must decline substantially to produce even a modest yield. This single ratio, Q/s, turns out to carry surprising information about the sediments.
▶ Go deeper
Specific capacity, Q/s, is the pumping rate divided by the stabilized drawdown, with units of m²/day (m³/day per meter of drawdown reduces to m²/day). It is arguably the most field-accessible indicator of aquifer productivity and is universally computable from any standard pumping test — including records that already exist in well databases and driller reports around the world.
The Wilkinson equation draws primarily on data recorded at the time of drilling — before any construction or mechanical influences can affect water flow. These original driller's report values represent the aquifer in its undisturbed state and are what the equation uses in the vast majority of cases. Subsequent pump tests may show modest changes in specific capacity over time, but when compared against the original drilling data those differences are negligible. For a deeper look at how aquifer type affects Q/s behavior over time, the reader is encouraged to consult standard hydrogeology references such as Freeze & Cherry (1979) or Domenico & Schwartz (1998).
Field Example — Nuwala Eliya, Sri Lanka
Productive water sources are sometimes found in the most unlikely rock types. During a well-drilling program in the central highlands of Sri Lanka, a series of wells penetrated a regional metamorphic sequence. A layer of quartzite — dense, tightly cemented, and a poor water source on its own — proved invaluable as a marker bed: its presence in the drill cuttings signaled that a highly productive fractured and weathered zone lay just beneath it.
When the pump test was run on one of these wells, no flow meter was available. The field crew directed the discharge hose into a standard 55-gallon drum (≈ 0.208 m³) and timed how long it took to fill — approximately 7 to 8 seconds. That translates to a pumping rate Q of roughly 2,400 m³/day. Drawdown in the well was barely noticeable — measured at a fraction of a meter. The resulting specific capacity exceeded 8,000 m²/day.
Applying the Wilkinson equation places the effective porosity squarely at the top of the interpretation table — comparable to karst or clean gravel — consistent with a well-developed fracture network in the weathered metamorphic zone beneath the quartzite cap. It is a reminder that the geology of a drill site tells only part of the story; the pump test tells the rest.
Page 2: What's in the Rock? →