Large-scale climate variability footprint in water levels of alluvial aquifers across Iran

Abstract

The ability to predict the future variability of groundwater resources in time and space is of critical importance in society’s adaptation to climate variability and change. Periodic control of large-scale ocean-atmospheric circulations on groundwater levels serves as a potentially effective source for relatively long-term forecasting. In this study, as a first national-scale assessment, we use the continuous wavelet transform, global power spectrum, and wavelet coherence analyses to quantify the linkage between the Atlantic Multidecadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), and El Niño Southern Oscillation (ENSO) and the representative groundwater levels of the 24 principal aquifers, scattered across 14 different climate zones of Iran. Aquifer storage variations are found to be partially derived from annual to interdecadal climate variability and not solely a function of pumping variability. Moreover, teleconnections are observed to be both frequency and time-specific. The significant coherence patterns between the climate indices and groundwater levels are concentrated at five frequency bands of the annual (~ 1 year), short-interannual (2–4 years), medium-interannual (4–6 years), decadal (8–12 years), and interdecadal (14–18 years), consistent with the dominant oscillations of climate indices. AMO’s strong linkage to groundwater variability is found to be at interdecadal and annual modes of groundwater levels while PDO’s highest imprint is concentrated in interannual, decadal, and interdecadal periodicities. Unlike ENSO for which the highest modulating influence is found to be across the decadal and interannual modes, the NAO’s footprint in aquifers is marked at annual and interdecadal frequency bands. Findings further show that the groundwater variability is driven primarily by a combination of multiple large-scale atmospheric circulations rather than a single atmospheric circulation index. The decadal and interdecadal oscillation modes are the dominant modes in Iranian aquifers. Findings also mark the unsaturated zone contribution in damping and lagging of the climate variability modes, particularly for the higher-frequency indices of ENSO and NAO so that groundwater variability is more correlated with those climate circulations that have lower frequency such as PDO and AMO. Finally, the data length is found to have a significant effect on the teleconnections if the time series are not contemporaneous and for each particular series, only one value of coherence/correlation is computed instead of separate computations for different frequency bands and different time spans.