The static water level (SWL) is a measurement indicating the health of a water well and its surrounding underground water source, or aquifer. The SWL is the depth of the water surface inside the well casing when the pump is not running and the water has fully stabilized. This measurement represents the natural, undisturbed elevation of the groundwater available to the well. Understanding the SWL is essential for effective well management and ensuring a reliable long-term water supply.
Defining Static Water Level
The static water level represents the height of the water table at the specific location of the well when the water is entirely at rest. It is a measurement taken after a period of no pumping, allowing the water column to fully recover and reflect the natural conditions of the aquifer. This undisturbed state is a direct reflection of the hydrostatic pressure exerted by the water in the saturated underground rock or soil formation.
The measurement is typically expressed in feet, indicating the distance from a fixed reference point, usually the top of the well casing, down to the water surface. Establishing a consistent and clearly marked reference point on the wellhead is important for collecting reliable data over time. The SWL acts as the “full” mark for the well, showing the maximum natural water level before any withdrawal begins.
Measuring and Monitoring the Static Water Level
To obtain an accurate static water level, the well must be completely at rest. This means no pumping has occurred for a specified recovery period, often ranging from 12 to 24 hours. Allowing this time ensures the water level has stabilized and is not artificially lowered by recent use or nearby pumping activity. Measuring too soon after pumping will give a false, temporarily depressed reading.
Professionals often use specialized tools like electronic sounders or electric depth gauges for the measurement process. An electric sounder features a weighted probe on a marked cable; when the probe contacts the water, it completes an electrical circuit, triggering a signal or light at the surface. For very deep wells, an air line method that uses air pressure to determine the depth to water may be employed instead.
Regular monitoring of the SWL helps track seasonal variations, which are common due to changes in rainfall and snowmelt. It also provides long-term insight into the health and sustainability of the aquifer.
The Critical Distinction from Dynamic Water Level
The static water level is fundamentally different from the dynamic water level (DWL), which is the water surface elevation inside the well while the pump is actively running. When water is withdrawn, the level inside the casing drops because the pump removes water faster than the aquifer can supply the well bore. Consequently, the DWL is always lower than the SWL.
The difference between these two measurements is known as “drawdown,” which indicates the well’s capacity. Drawdown measures the vertical distance the water level falls from the SWL to the DWL during pumping. A small drawdown suggests a highly productive well and an aquifer that can quickly recharge the well bore. Conversely, a large drawdown suggests the aquifer is slower to yield water.
Impact on Well Performance and Pump Sizing
Knowing the static water level is the starting point for effective well management and is used when selecting the appropriate submersible pump. The pump must be powerful enough to lift the water from the well’s depth to the surface and deliver it at sufficient pressure. The SWL is the baseline for calculating the Total Dynamic Head (TDH), which is the total energy the pump needs to overcome friction, pressure loss, and the vertical lift required.
A consistently low or falling SWL over multiple years is a warning sign, potentially indicating stress on the aquifer due to drought or increased regional water usage. To prevent the pump from running dry, the pump intake must be strategically placed below the lowest expected dynamic water level. The SWL and the expected drawdown are therefore used to determine the overall depth required for the pump setting, ensuring the equipment operates efficiently and reliably.