Applicability of spectral analysis to determine hydraulic diffusivity

This study is to evaluate the applicability of estimating the one-dimension al horizontal hydraulic diffusivity of an unconfined aquifer with time-depe ndent fluctuation of lateral head and vertical recharge boundaries using ob served water level spectra. Different models of boundary condition are impo sed to evaluate the statistical significance between the calculated hydraul ic diffusivity (xi') with the given hydraulic diffusivity (xi). The auto-sp ectra of the water level in observation wells tapping the same aquifer are closely related to those at the disturbed boundaries. For an aquifer with a constant hydraulic diffusivity, the water level fluctuation in the monitor ing wells is linearly related to the water level spectra observed at the bo undaries. The spectral density function of aquifer hydraulic head varies in versely with specific yield (S-y) and directly with recharge. Given small v ariation in water level spectra at the disturbed boundaries, the water leve l fluctuation in the aquifer is affected by the recharge condition and the aquifer spectral density function is sensitive to S-y. Using an iterative t echnique to estimate xi from 1400 sets of given parameters, 99% of the xi'/ xi values deviated within only one order of magnitude with the model length (L) being equal to 1 km and 10 km. For L equal to 100 m, approximately 82% of the xi'/xi population falls within two orders of magnitude. Therefore, spectral analysis of aquifer hydraulic head response can be used to estimat e the hydraulic diffusivity of an unconfined aquifer which is affected by p eriodic variations in recharge and head at boundaries.