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Basic concepts for the linear model of ground water level recession
1U.S. Geological Survey, 433 National Center, Reston, VA 20192, USA. rutledge@usgs.gov
Ground Water
|May 10, 2006
Summary
Groundwater level recession can be linearly plotted on a semilog graph using Rorabaugh's method when water levels are relative to an outflow boundary. This method estimates aquifer hydraulic diffusivity but can be inaccurate with complex boundaries or post-recharge instability.
Area of Science:
- Hydrogeology
- Geophysics
- Environmental Science
Background:
- Groundwater level recession analysis is crucial for understanding aquifer behavior.
- Rorabaugh's method provides a framework for analyzing exponential decay in groundwater levels.
- Definable outflow boundaries, like rivers or lakes, are key to applying linear recession models.
Purpose of the Study:
- To illustrate Rorabaugh's method for displaying groundwater level recession.
- To assess the accuracy of the linear model with varying outflow boundary conditions.
- To identify factors causing deviations from linear recession and impacting hydraulic diffusivity estimates.
Main Methods:
- Utilized three finite-difference simulations to model groundwater flow and recession.
- Simulated an ideal Rorabaugh case with a uniform outflow boundary.
- Simulated cases with sloping and complex outflow boundaries to test model accuracy.
Main Results:
- The linear recession model is accurate for ideal and simple sloping boundaries when water levels are referenced to the nearest boundary point.
- Complex boundary shapes and sloping boundaries introduce nonlinearities and errors in hydraulic diffusivity estimation.
- Early-time groundwater head profiles after recharge can exhibit nonlinear behavior, affecting recession analysis.
Conclusions:
- Rorabaugh's linear recession model is effective under specific conditions, particularly with uniform or simple sloping boundaries.
- Accurate application requires careful consideration of the outflow boundary's geometry and the reference point for water level measurements.
- Deviations from linearity highlight the complexity of groundwater systems and the need for advanced analysis in certain scenarios.
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