_________________________________________________________________________________ a Democritus University of Thrace, Greece. *Corresponding author: E-mail: cpapadp@civil.duth.gr; Chapter 9 Print ISBN: 978-81-971665-8-7, eBook ISBN: 978-81-971665-3-2 Investigating the Impact of Meteorological Drought on Groundwater Dynamics in a Heterogeneous Aquifer System Using Fuzzy Regression: A Case Study in Southeastern Xanthi, NE Greece Christopher A. Papadopoulos a* , Mike Spiliotis a , Basil Papadopoulos a and Fotios-Konstantinos Pliakas a DOI: 10.9734/bpi/rumcs/v3/7310C Peer-Review History: This chapter was reviewed by following the Advanced Open Peer Review policy. This chapter was thoroughly checked to prevent plagiarism. As per editorial policy, a minimum of two peer-reviewers reviewed the manuscript. After review and revision of the manuscript, the Book Editor approved the manuscript for final publication. Peer review comments, comments of the editor(s), etc. are available here: https://peerreviewarchive.com/review-history/7310C Abstract This chapter deals with the impact of meteorological drought on groundwater in a heterogeneous aquifer system located in the plain southeast of Xanthi, northeastern Greece, using fuzzy set theory. The research aims to investigate the linear relationship between meteorological drought and groundwater levels of shallow wells, as well as the key role of geology in drought propagation, in terms of time, to groundwater. The study area is bounded to the east by the Lake Vistonida, to the north by the River Kosynthos, to the west by the settlements of Vafeika and Genisea, and to the south by the hill of Avdira and the settlement of Nea Kessani. It is irrigated by a substantial number of wells (800 or more), predominantly shallow. The area exhibits strong heterogeneity in soil composition, consisting of coarse-grained fluvial deposits in the northwest and fine- grained lake sediments in the southeast. To achieve the study's objective, fuzzy regression analysis based on Tanaka's approach is implemented. Tanaka's fuzzy linear regression model is reduced to a linear programming problem with inclusion constraints, ensuring that all observed data is included within the produced fuzzy band. The Standardized Reconnaissance Drought Index (RDI St ) and Standardized Precipitation Index (SPI) serve as inputs (independent variables) of the multiple fuzzy linear regression model. These drought indices are used to estimate drought intensity and are calculated for several reference periods. Water table serves the dependent variable, recorded by two water table loggers installed in different geological formation areas. The multiple fuzzy linear regression model produces a water table