What is Drawdown?
Drawdown is the difference between the static water level in a borehole (when no water is being pumped) and the pumping water level (when the borehole is being pumped). It is measured in metres and represents the depth to which the water level drops as a result of pumping.
Imagine a borehole with a static water level of 20 metres below the surface. When you start pumping at a rate of 10 m³/hr, the water level drops to 60 metres below the surface. The drawdown is therefore 40 metres (60m – 20m = 40m drawdown).
Drawdown is a dynamic measurement—it typically increases as pumping continues until it reaches a stabilised level (assuming the aquifer is being recharged at the same rate as extraction). The drawdown at any given moment depends on the pumping rate, the aquifer's permeability, the borehole's construction, and the duration of pumping.
In Kenya's volcanic and sedimentary aquifers, drawdown behaviour can vary significantly. A small drawdown for a given pumping rate suggests a productive aquifer and an efficient borehole, while a large drawdown indicates low permeability, poor borehole construction, or aquifer depletion.
What is Recovery?
Recovery is the rise of the water level in a borehole after pumping stops. It measures how quickly the aquifer replenishes the water that was extracted and is a critical indicator of aquifer productivity and borehole efficiency.
Using the same example, if you stop pumping and the water level rises from 60 metres back to 25 metres after one hour, the recovery is 35 metres. The rate at which recovery occurs—and whether the water level returns to the static level—tells you a great deal about the aquifer's characteristics.
A borehole that recovers quickly (within a few hours) indicates a highly transmissive aquifer with good connectivity to the borehole. Slow recovery, or partial recovery, may indicate low permeability, limited aquifer thickness, or poor borehole development.
Recovery is often expressed as a percentage of the total drawdown. For example, if drawdown was 40 metres and the recovery after 24 hours is 32 metres, the recovery is 80% of the drawdown.
Key Insight: The recovery test is often more reliable than the drawdown test for estimating aquifer properties because it is not affected by pumping-induced turbulence or well losses.
Why Drawdown and Recovery Matter
Drawdown and recovery are the most direct indicators of borehole and aquifer performance. They matter for several critical reasons:
- Determine sustainable yield – drawdown tells you how much the water level drops at a given pumping rate. If drawdown exceeds a safe limit (often 50–70% of the total depth), the borehole may run dry or experience excessive drawdown that could damage the pump or screen.
- Assess aquifer health – recovery rates indicate how quickly the aquifer can recharge. A declining recovery rate over time is a warning sign of aquifer depletion or screen clogging.
- Design the right pump – knowing the drawdown at the target pumping rate allows you to select a pump that can deliver the required flow from the pumping water level.
- Detect problems early – sudden changes in drawdown or recovery can reveal issues like screen clogging, pump wear, or aquifer damage.
- Support regulatory compliance – the Water Resources Authority (WRA) requires borehole owners to monitor abstraction and maintain records. Drawdown and recovery data are often part of this reporting.
- Plan for the future – understanding how your borehole responds to pumping helps you plan for expansion, irrigation, or future water needs.
How Drawdown and Recovery Are Measured
Drawdown and recovery are measured during a pumping test, which is typically conducted after drilling and before pump installation. The test follows a standardised procedure:
- Measure static water level – the water level is recorded before any pumping begins (the static level).
- Start pumping – the borehole is pumped at a constant rate (usually the target yield) for a defined period (typically 24–72 hours).
- Record water levels – water levels are measured at regular intervals (e.g., every minute initially, then every 5, 15, 30, and 60 minutes) to track drawdown.
- Stop pumping – after the pumping period, the pump is switched off.
- Measure recovery – water levels are measured at regular intervals as the water level rises back toward the static level.
- Analyse data – the drawdown and recovery data are plotted on graphs to evaluate the borehole's performance and calculate aquifer properties.
Modern pumping tests use electronic pressure transducers and data loggers that automatically record water levels at high frequency, providing precise data for analysis. In Kenya, WaterLink Limited uses state‑of‑the‑art equipment to ensure accurate measurements.
Factors That Influence Drawdown and Recovery
Several factors affect drawdown and recovery. Understanding these helps interpret test results and diagnose problems.
| Factor | Effect on Drawdown | Effect on Recovery |
|---|---|---|
| Aquifer permeability | Higher permeability → lower drawdown | Higher permeability → faster recovery |
| Aquifer thickness | Thicker aquifer → lower drawdown | Thicker aquifer → slower recovery (more storage) |
| Pumping rate | Higher rate → higher drawdown | Higher rate → slower recovery |
| Borehole diameter | Larger diameter → slightly lower drawdown | Larger diameter → slightly faster recovery |
| Screen length and placement | Proper screen placement → lower drawdown | Proper screen placement → faster recovery |
| Screen clogging | Clogging → higher drawdown | Clogging → slower recovery |
| Gravel pack condition | Poor gravel pack → higher drawdown | Poor gravel pack → slower recovery |
| Aquifer recharge | Good recharge → lower drawdown over time | Good recharge → faster recovery |
In practice, the interplay of these factors determines the unique drawdown and recovery behaviour of each borehole. This is why professional testing and interpretation are essential.
Interpreting Drawdown and Recovery Curves
The drawdown and recovery curves plotted from pumping test data are rich with information. Here is what to look for:
Drawdown Curve
- Stabilised drawdown – if the drawdown stabilises during pumping, the aquifer is being recharged at the same rate as extraction. This indicates a sustainable yield at that pumping rate.
- Continued increase – if drawdown continues to increase without stabilising, the pumping rate exceeds the aquifer's recharge capacity. This is unsustainable and will lead to eventual depletion.
- Step changes – sudden increases in drawdown may indicate encountering a low‑permeability zone or borehole damage.
Recovery Curve
- Rapid initial recovery – indicates that the borehole is well‑connected to the aquifer and that the drawdown was primarily due to pumping, not well losses.
- Slow recovery – may indicate low aquifer permeability, screen clogging, or poor borehole development.
- Incomplete recovery – if the water level does not return to the static level, it may indicate a long‑term decline in the water table or damage to the borehole.
- Recovery percentage – generally, recovery of at least 80% of the drawdown within 24 hours is considered good for most aquifers.
A professional hydrogeologist will also use the drawdown and recovery data to calculate aquifer properties such as transmissivity and storativity, which are essential for long‑term water resource planning.
Common Issues Revealed by Drawdown and Recovery
Monitoring drawdown and recovery over time can reveal a range of problems. Here are the most common:
- Screen clogging – gradual increase in drawdown for the same pumping rate, and slower recovery, often caused by mineral scale or biofilm buildup.
- Aquifer depletion – increasing static water level (indicating a falling water table) and declining recovery rates over time.
- Pump wear or mis‑sizing – a worn pump may not deliver the same flow, causing the water level to stabilise at a higher drawdown.
- Casing or screen damage – sudden changes in drawdown or recovery may indicate physical damage that allows sediment to enter or reduces flow.
- Gravel pack failure – if the gravel pack settles or becomes compacted, drawdown may increase and recovery may slow.
- Interference from nearby boreholes – if neighbours are pumping from the same aquifer, you may see increased drawdown and slower recovery.
Regular testing and monitoring are the best way to catch these issues early, before they become major failures.
How WaterLink Limited Uses Drawdown and Recovery Data
At WaterLink Limited, we place a strong emphasis on accurate pumping tests and thorough interpretation of drawdown and recovery data. Our approach includes:
- High‑precision instruments – we use electronic pressure transducers and data loggers to capture water level changes with millimetre accuracy.
- Customised test design – each pumping test is designed based on the specific borehole, aquifer, and target yield, ensuring the data is relevant and actionable.
- Expert interpretation – our hydrogeologists analyse the drawdown and recovery data to determine sustainable yield, aquifer properties, and any performance issues.
- Comprehensive reporting – you receive a detailed report with drawdown and recovery curves, calculated parameters, and clear recommendations for pump selection and long‑term management.
- Ongoing monitoring – we recommend periodic testing to track changes over time and detect developing problems early.
- WRA compliance – our test data and reports support your WRA permitting and abstraction reporting requirements.
By understanding the dynamics of your borehole, we help you make informed decisions that protect your investment and ensure a reliable water supply for years to come.
