Step Drawdown Tests: Optimizing Pump Selection

What Is a Step Drawdown Test?

A step drawdown test is a controlled pumping test in which the discharge rate is increased in a series of discrete steps (e.g., 25%, 50%, 75%, and 100% of the anticipated design rate). At each step, the pump is operated at a constant rate until the water level stabilises, and the drawdown is recorded. The test is then typically followed by a recovery period.

Unlike a constant-rate test, which provides only a single point of performance, the step test generates a relationship between pumping rate and drawdown across a range of flows. This relationship is crucial for understanding the hydraulic behaviour of the well and the aquifer, and it forms the basis for selecting the most efficient and sustainable pump for your borehole.

The test is named for the "steps" of increasing flow. Each step typically lasts 60–120 minutes, depending on the time required to achieve stabilisation. The data collected—flow rate and corresponding drawdown—are then plotted and analysed using established hydraulic equations.

Why Step Drawdown Tests Are Essential

Step drawdown tests are invaluable for several reasons:

• Accurate pump selection — By knowing the drawdown at various pumping rates, you can choose a pump that operates within the allowable drawdown (often 50–70% of the maximum available drawdown to avoid pump intake exposure).
• Well efficiency assessment — The test quantifies well losses, allowing you to calculate well efficiency. Low efficiency indicates poor construction, screen clogging, or the need for rehabilitation.
• Predicting performance — With the aquifer and well loss coefficients determined, you can predict drawdown for any future pumping rate, helping with long-term water management.
• Economic optimisation — Over-sized pumps waste energy and increase costs; under-sized pumps may not meet demand. The step test data ensure you select the right pump for both performance and energy efficiency.
• Diagnosing well problems — A step test can reveal issues like partial penetration, excessive turbulence near the screen, or an improperly sized filter pack.
• Regulatory compliance — Many water authorities require step test data for abstraction permits, ensuring that pumping does not cause undue interference with neighbouring wells.

How to Conduct a Step Drawdown Test

Conducting a step drawdown test requires careful planning and execution. Follow these steps to ensure reliable results:

1. Pre-test preparation — Ensure the pump is operational and the discharge is routed away from the well. Install a calibrated flow meter (or use a weir tank) and an electronic water level recorder (pressure transducer) with data logging capability.
2. Measure static water level — Record the water level before pumping starts. Allow the well to recover fully (at least 24 hours of idle time) to ensure the static level is representative.
3. Select step rates — Choose 3–5 steps, typically 25%, 50%, 75%, and 100% of the planned design flow. The highest step should not exceed the maximum expected pumping rate. If the aquifer is sensitive, use smaller increments.
4. Start the first step — Pump at the lowest rate and maintain constant discharge. Monitor the water level; it will initially drop rapidly, then slow down. Continue until the water level stabilises (drawdown change < 0.1 m over 30 minutes).
5. Record data — At stabilisation, record the flow rate and the drawdown. Then increase the pump speed or open the valve to the next step and repeat the process.
6. Continue through all steps — Repeat for each step, ensuring each step reaches stabilisation. The duration of each step may vary; it typically ranges from 60 to 120 minutes.
7. Recovery monitoring — After the final step, stop pumping and monitor the water level recovery. Recovery data provide additional information about the aquifer's hydraulic properties.

Throughout the test, maintain a log of time, flow rate, and water level. Use a data logger for automatic recording to minimise human error.

Note: These are illustrative values; actual figures depend on aquifer characteristics and well design.

Data Analysis: Separating Aquifer and Well Losses

The total drawdown (s) during pumping consists of two components:

• Aquifer losses (BQ) — Linear losses due to the resistance of the aquifer to flow, proportional to the pumping rate (Q). The coefficient B is the aquifer loss coefficient.
• Well losses (CQ²) — Non-linear losses caused by turbulence, screen entry, and friction within the well. These increase with the square of the pumping rate. The coefficient C is the well loss coefficient.

The relationship is expressed as:

To determine B and C, the step test data (Q vs s) are plotted. The most common method is the Jacob method, which involves dividing both sides by Q:

If s/Q is plotted against Q, the data should fall on a straight line. The intercept on the y-axis gives B, and the slope gives C. In practice, a best-fit line is drawn through the points. This method assumes that the relationship is quadratic, which is valid for most wells.

An alternative method is the Rorabaugh method, which fits a power-law model: s = BQ + CQ^n, where n is typically between 1.5 and 3. The Jacob method is simpler and widely used in practice.

Once B and C are known, you can predict drawdown at any pumping rate Q using the equation s = BQ + CQ².

Note that the units must be consistent; we recommend converting to m³/d for compatibility with standard hydrogeological formulas.

Calculating Well Efficiency

Well efficiency is a measure of how much of the total drawdown is due to aquifer losses versus well losses. It is defined as:

At a given pumping rate, a high efficiency (typically > 80%) indicates a well with minimal well losses — i.e., the screen and filter pack are well-designed and the well is properly developed. Efficiency below 50% suggests significant well losses, which may be caused by:

• Incorrect screen slot size
• Poorly graded or compacted filter pack
• Screen clogging from mineral deposits or biofouling
• Incomplete well development
• Excessive turbulence due to high velocity through the screen

Well efficiency can vary with pumping rate; typically it decreases at higher rates due to the quadratic term. Therefore, it is important to calculate efficiency at the design pumping rate to ensure it is acceptable.

For example, if BQ = 15 m and CQ² = 4 m, then total drawdown = 19 m, and efficiency = (15/19)×100 ≈ 79%. This would be considered fair; values above 80% are desirable.

Using Results for Pump Selection

The primary practical application of step drawdown test results is pump selection. The data allow you to:

• Determine the maximum sustainable pumping rate — Based on the allowable drawdown (often limited by the pump intake depth or available drawdown), you can solve for Q using the equation s = BQ + CQ². This gives the maximum flow that can be pumped without dewatering the well.
• Select the appropriate pump size — With the target flow rate known, you can choose a pump whose performance curve matches the required head (total dynamic head = drawdown + friction losses + discharge head). The step test provides the drawdown component at that rate.
• Optimise energy efficiency — Selecting a pump that operates near its best efficiency point (BEP) at the design flow reduces energy consumption and extends pump life.
• Plan for future expansion — If you anticipate increased water demand, the step test data can be used to predict drawdown at higher rates, helping you assess whether the well can support future pumping rates.

For example, if your borehole has an available drawdown of 30 m, and the equation s = 0.02Q + 0.0001Q² (Q in m³/d), you can solve for Q: 30 = 0.02Q + 0.0001Q², which yields Q ≈ 1100 m³/d (approx. 764 L/min). This becomes your recommended maximum pumping rate.

In this example, the optimal pumping rate for the highest efficiency is around 300–450 L/min. If demand exceeds that, you may need to consider a larger pump but be aware of the efficiency trade-off.

Common Mistakes and How to Avoid Them

Step drawdown tests are straightforward, but errors can occur. Avoid these common pitfalls:

• Not achieving stabilisation — If you move to the next step before the water level has stabilised, the drawdown recorded will be too high, skewing the analysis. Always wait for stabilisation (criterion: < 0.1 m change in 30 minutes).
• Inconsistent pumping rates — The discharge must remain constant during each step. Use a flow control valve and monitor the rate continuously.
• Incorrect static water level — If the static level is measured incorrectly, all drawdown calculations will be wrong. Measure static level after at least 24 hours of no pumping.
• Ignoring temperature and density effects — For high-accuracy work, consider water temperature, which affects density and transducer readings.
• Using too few steps — At least three steps are needed to define the curve; four or five are better. Use steps that span the expected operating range.
• Extrapolating beyond test range — The equation s = BQ + CQ² is valid within the range of Q tested. Extrapolating far beyond the highest step can be unreliable.
• Failing to check for well interference — If there are nearby pumping wells, the drawdown may be affected. Monitor background water levels before and during the test.

Step Test vs Constant-Rate Test: Key Differences

Both step and constant-rate tests are used in borehole evaluation, but they serve different purposes:

In practice, both tests are often performed together. The step test is typically conducted first to guide the selection of the constant-rate test pumping rate and to provide efficiency data. The constant-rate test then provides the aquifer parameters and long-term yield information.

WaterLink Limited's Approach to Step Testing

WaterLink Limited has extensive experience in conducting step drawdown tests across Kenya's diverse hydrogeological settings. Our approach ensures accurate, actionable results:

• Customised test design — We design the test protocol based on the borehole depth, anticipated yield, and aquifer type, ensuring the steps are appropriate for your specific well.
• State-of-the-art equipment — We use calibrated electromagnetic flow meters, high-accuracy pressure transducers, and data loggers to capture precise data.
• Real-time monitoring — Our technicians monitor the test remotely, ensuring stable pumping rates and collecting high-resolution data.
• Comprehensive analysis — We apply the Jacob method and, where needed, more advanced analysis to determine B and C, calculate well efficiency, and generate performance curves.
• Pump selection guidance — Based on the results, we provide detailed pump recommendations, including head calculations, motor sizing, and efficiency analysis.
• Documentation and support — We deliver a full report with graphs, tables, and interpretations, and we are available to discuss the findings and assist with implementation.

Whether you are drilling a new borehole or optimising an existing one, our step drawdown testing service provides the data you need to make informed decisions and ensure your water supply is reliable and cost-effective.