The response of water level in a well to a time series of atmospheric loading under confined conditions

The water level in a well that penetrates a confined aquifer can fluctuate in response to changes in atmospheric pressure. The response varies with the well casing and screen dimensions, the transmissivity and compressibility of the aquifer, and to a small extent, its storativity. Recently this loading‐response problem has been solved in terms of a frequency response function that characterizes how attenuation and phase shifts in the response signal vary with frequency. The counterpart of this solution in the time domain is an impulse response function. This solution has the immediate appeal that it can be used directly to filter raw loading and water level records via serial convolution, where the response at any time is a weighted aggregate of the previous loading signal. The impulse response function derives from well‐known solutions to the slug test problem. When based on the Cooper‐Bredehoeft‐Papadopulos solution, the response function is precise but computationally intensive. Based on the Hvorslev solution, the response function is computationally simple, but applicable only to aquifers with small storativity. Simulations and a field example clearly illustrate the dominating influence of transmissivity in dampening and lagging the response of a confined aquifer to a loading signal, and how both effects increase with higher frequency. The results also illustrate the weak influence of storativity in modulating the response signal.