Calculate aquifer drawdown from pumping rate and specific capacity. Estimate water level decline during irrigation pumping to plan operations.
When water is pumped from a well, the water level in the well drops below the static (non-pumping) level. This decline is called drawdown. The amount of drawdown depends on the pumping rate and the aquifer's specific capacity — the number of gallons per minute the well produces per foot of drawdown.
Excessive drawdown can expose the pump intake, reduce pump efficiency, and even damage the aquifer by compacting sediments. Monitoring and predicting drawdown helps you plan pumping schedules and avoid operating beyond safe limits.
This calculator estimates drawdown from the pumping rate and specific capacity, then computes the pumping water level for comparison with the pump intake depth. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process. This tool handles all the complex arithmetic so you can focus on interpreting results and making informed decisions based on accurate data.
Predicting drawdown prevents pump damage and ensures the pump stays submerged throughout the irrigation season. It also helps you decide if the well can handle the required flow rate. Having a precise figure at your fingertips empowers better planning and more confident decisions. Manual calculations are error-prone and time-consuming; this tool delivers verified results in seconds so you can focus on strategy.
Drawdown (ft) = Pumping Rate (GPM) / Specific Capacity (GPM/ft) Pumping Water Level = Static Water Level + Drawdown
Result: Drawdown = 40 ft; Pumping Level = 120 ft
Drawdown = 800 GPM / 20 GPM/ft = 40 ft. Pumping water level = 80 + 40 = 120 ft below surface. If the pump intake is at 200 ft, there is 80 ft of submergence — adequate.
When pumping begins, drawdown increases rapidly at first, then gradually stabilizes as the cone of depression reaches equilibrium with aquifer recharge. The time to stabilization depends on aquifer transmissivity and storativity. In highly transmissive sand-and-gravel aquifers, equilibrium may take hours; in low-permeability formations, it may take days.
If multiple wells pump from the same aquifer within 500–2,000 ft, their cones of depression overlap, increasing drawdown at each well. Account for interference by adding drawdowns from each pumping well or by reducing per-well pumping rates to stay within total aquifer capacity.
In many regions, aquifer levels decline 0.5–2 ft per year due to regional over-pumping. This raises the static water level measurement each year, increasing energy cost and drawdown. Tracking static levels over decades informs water policy and individual farm planning.
Specific capacity (SC) is the well's productivity ratio: GPM per foot of drawdown. A well yielding 800 GPM with 40 ft of drawdown has SC = 20 GPM/ft. Higher SC means a more productive well.
It is approximately linear for confined aquifers and low pumping rates. In unconfined aquifers at high rates, drawdown increases faster than linearly due to dewatering effects. This calculator uses the linear assumption; for precision, use a step-drawdown test.
Pumping faster than the aquifer recharges, well screen plugging (reducing specific capacity), or interference from neighboring wells. All three can be diagnosed with monitoring data.
Set the pump at least 20–30 ft below the expected pumping water level at peak season rates. This accounts for seasonal aquifer decline and prevents air entrainment.
If drawdown drops to the pump intake, the pump takes in air, loses prime, and can burn out. Modern pumps have low-water shutoffs, but damage can still occur. Prevent this by ensuring adequate submergence.
Use an electric water level indicator (e-tape) lowered into the well before the pump starts. Measure from the wellhead to the water surface. Record this before each irrigation season.