Effects of Climatic Drought on Groundwater Level Based on Time Series Analysis in Time and Frequency Domains

Climatic drought, characterized by a decrease in precipitation and an increase in temperatures, significantly influences groundwater resources by reducing recharge and increasing abstraction. Interactions between climatic droughts and groundwater systems are complex, because of the varying hydrodynamic properties of aquifers, which influence their responses to surface stresses. Understanding these relationships is crucial for optimizing groundwater resource management and mitigating drought-induced crises. This study investigated the relationships between climatic droughts and groundwater level fluctuations in two climatically different basins in Iran: the semi-arid Mashad Basin (Khorasan Razavi province) and the arid Gowharkuh Basin (Sistan and Baluchestan province). We employed the Standardized Precipitation Index (SPI) to represent climatic conditions and the Standardized Water Table Index (SWTI) to show groundwater level fluctuations. Time series analyses were conducted in both time and frequency domains to assess the measure and quality of relationships between climatic conditions and water level variations. In the time domain, we calculated correlation coefficients and lag times between SPI and SWTI, using a modified cross-correlation function (MCCF). This innovative approach allowed for cross-correlation calculations between time series of unequal lengths. Using the Blackman-Tucky method, we computed spectral density, cross-spectrum amplitude, coherency, and phase functions in the frequency domain. Time domain results showed that the correlation coefficient and lag time between climatic variations and groundwater levels were higher in the Gowharkuh Plain (0.9 and 7 years) compared to the Mashhad Plain (0.7 and 5 years), highlighting the influence of interacting factors, including climatic, hydrological, and hydrogeological conditions, as well as human interventions, in shaping these relationships. Frequency domain analysis indicated that low-frequency fluctuations in SPI (long-term droughts) exert the most significant impact on groundwater resources.