Water 2021, 13, 250. https://doi.org/10.3390/w13030250 www.mdpi.com/journal/water Article Relationships of Hydrological Seasons in Rivers and Groundwaters in Selected Catchments in Poland Przemysław Tomalski 1 , Edmund Tomaszewski 1, *, Dariusz Wrzesiński 2 and Leszek Sobkowiak 2 1 Department of Hydrology and Water Management, Institute of Climatology and Hydrology, Faculty of Geographical Sciences, University of Łódź, 88 Narutowicza Str., 90‐139 Łódź, Poland; przemyslaw.tomalski@geo.uni.lodz.pl 2 Department of Hydrology and Water Management, Faculty of Geographical and Geological Sciences, Adam Mickiewicz University, 10 Krygowskiego Str., 61‐680 Poznań, Poland; darwrze@amu.edu.pl (D.W.); lesob@amu.edu.pl (L.S.) * Correspondence: edmund.tomaszewski@geo.uni.lodz.pl Abstract: The study applied the method of hydrological season identification in a time series of river total and base flows and in groundwater levels. The analysis covered a series of daily measurements from the period 2008–2017 in nine catchments located in different geographical regions of Poland. The basis of the classification of hydrological seasons, previously applied for river discharges only, was the transformation of the original variables into a series reflecting three statistical features esti‐ mated for single‐name days of a year from a multiyear: average value, variation coefficient, and autocorrelation coefficient. New variables were standardized and after hierarchical clustering, every day of a year had a defined type, valorizing three features which refer to quantity, variability, and the stochastic nature of total and base river flow as well as groundwater stage. Finally, sequences of days were grouped into basic (homogenous) seasons of different types and transitional seasons in‐ cluding mixed types of days. Analysis indicated determinants of types, length, and frequency of identified hydrological seasons especially related to river regime, hydrogeological and hydromete‐ orological conditions as well as physiographical background were directly influenced by geograph‐ ical location. Analysis of the co‐occurrence of the same types of hydrological seasons allowed, in some catchments, periods of synchronic alimentation (groundwater and base flow, mainly in the cold half‐year) and water shortages (all three components, mainly in the warm half‐year) to be iden‐ tified. Keywords: Central Europe; hydrological seasons; hierarchical clustering; river flow components; seasonal synchronicity of flow 1. Introduction River discharge and groundwater stage are characterized by seasonal variability which is determined by the cyclic changeability of precipitation and evapotranspiration during a year. The multiannual and seasonal variability of flow determinants results in wet or dry years (very often sequences of years) as well as wet or dry summers, etc. [1–3]. As a result, during a year, more predictable periods of particular flow phases occurrence appear (e.g., summer floods, autumn low‐flows, spring snowmelt). Identification of such periods in a catchment allows hydrological seasons to be defined. Progress in terms of methodology, quality, and availability of hydrometrical data produces a huge set of measures and procedures used to describe the seasonality of river flows. This set is constantly enriched with new proposals. Very common parameters, widely applied to various components of the hydrological cycle, refer to seasonality index and seasonal time of concentration [4], seasonality coefficient [5], or central of mass data Citation: Tomalski, P.; Tomaszewski, E.; Wrzesiński, D.; Sobkowiak, L. Relationships of Hydrological Seasons in Rivers and Groundwaters in Selected Catchments in Poland. Water 2021, 13, 250. https://doi.org/10.3390/ w13030250 Received: 17 December 2020 Accepted: 18 January 2021 Published: 20 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (http://crea‐ tivecommons.org/licenses/by/4.0/).