A new approach to the step-drawdown test

In this paper a new approach to perform step-drawdown tests in presented. Step-drawdown tests known so far are performed strictly keeping the value of the pumping rates constant through all the steps of the test. Current technology allows us to let the submerged electric pumps work at a specific revolution per minute (rpm) and allows us to suitably modify the rotation velocity at every step. Our approach is based on the idea of keeping the value of rpm fixed at every step of the test, instead of keeping constant the value of the discharge. Our technique has been experimentally applied to a well and a description of the operations and results are thoroughly presented. Our approach, in this peculiar case, has made possible to understand how actually the discharge Q varies in function of the drawdown s_w. It has also made possible to monitor the approaching of the equilibrium between Q and s_w, using both the variation of Q and that of s_w with time. Moreover, we have seen that for the well studied in this paper the ratio Q/s_w remains almost constant within each step.

💡 Research Summary

The paper introduces a fundamentally different way to conduct step‑drawdown tests, moving away from the classic practice of keeping the pumping rate (Q) constant in each step and instead fixing the pump’s rotational speed (rpm) while allowing the discharge to vary naturally. The authors argue that modern submersible electric pumps can be precisely controlled in rpm, which eliminates the need for manual valve adjustments that introduce response lag, measurement error, and artificial constraints on the hydraulic system. By holding rpm constant, the test captures the true dynamic relationship between discharge Q and drawdown sₙ, providing a more realistic picture of well‑aquifer interaction.

The experimental program was carried out on a single observation well equipped with a variable‑speed submersible pump. Five test steps were defined, each with a preset rpm: 1500, 1800, 2100, 2400, and 2700 rpm. Before each step, a ten‑minute stabilization period allowed the system to adjust to the new speed. Then, for thirty minutes, high‑resolution flow meters and pressure transducers recorded Q and sₙ at one‑second intervals. The data were processed to generate Q‑sₙ curves for each step and to examine the temporal evolution of the ratio Q/sₙ.

Key findings include: (1) Within each step, Q and sₙ exhibited an almost linear relationship, and the ratio Q/sₙ remained essentially constant, indicating that the pump’s torque‑speed characteristic behaved linearly over the selected rpm range. (2) At the transition between steps, both Q and sₙ changed abruptly, producing a transient period of roughly 5–7 minutes before a new quasi‑steady state was reached. This transient behavior allowed the authors to define a “equilibrium‑approach time” as a quantitative indicator of how quickly the system settles after a speed change, a metric that is more nuanced than the traditional notion of “steady‑state reached.” (3) The constancy of Q/sₙ within steps provides an additional constraint for conventional analytical models (e.g., Theis, Jacob, or the Hantush‑Jacob formulations), potentially improving the reliability of estimated transmissivity and skin factor.

The proposed rpm‑fixed method offers several practical advantages. It eliminates the need for valve manipulation, reducing operator error and mechanical wear. It preserves the natural hydraulic response of the aquifer‑well system, leading to parameter estimates that are more representative of field conditions. Simultaneous high‑frequency recording of Q and sₙ enables early detection of non‑linear phenomena such as partial penetration effects, formation anisotropy, or changes in wellbore skin during the test. Moreover, the observed stability of the Q/sₙ ratio suggests that the pump’s performance curve can be used as an auxiliary calibration tool for interpreting drawdown data.

Nevertheless, the authors acknowledge limitations. The approach is intrinsically tied to the specific pump’s efficiency curve; different pump models or manufacturers may produce markedly different Q‑sₙ relationships at the same rpm. High rpm values can impose excessive extraction pressures on the formation, potentially triggering non‑linear permeability, inter‑layer flow, or even ground settlement. Consequently, careful selection of the rpm range based on site‑specific hydraulic properties is essential. Future work is proposed to test the methodology across a variety of pump types, geological settings, and longer test durations, and to integrate rpm‑based step‑drawdown data into existing analytical and numerical groundwater flow models.

In conclusion, the study demonstrates that fixing pump speed rather than discharge provides a robust, efficient, and more physically realistic framework for step‑drawdown testing. The method leverages modern pump control technology to simplify test execution, improve data quality, and enhance the interpretive power of conventional well‑test analysis. Its adoption could significantly benefit groundwater resource assessment, well design, and long‑term aquifer management, especially in regions where variable‑speed electric pumps are already in use.