  Citation: Mezger, G.; De Stefano, L.; González del Tánago, M. Analysis of the Evolution of Climatic and Hydrological Variables in the Tagus River Basin, Spain. Water 2022, 14, 818. https://doi.org/10.3390/ w14050818 Academic Editor: Aris Psilovikos Received: 31 January 2022 Accepted: 2 March 2022 Published: 5 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Analysis of the Evolution of Climatic and Hydrological Variables in the Tagus River Basin, Spain Gabriel Mezger 1,2, * , Lucia De Stefano 1,2 and Marta González del Tánago 3 1 Departamento de Geodinámica, Estratigrafía y Paleontología, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain; luciads@geo.ucm.es 2 Observatorio del Agua, Fundación Botín, 39003 Santander, Spain 3 Departamento de Sistemas y Recursos Naturales, E. T. S. de Ingeniería de Montes, Forestal y del Medio Natural, Universidad Politécnica de Madrid, 28040 Madrid, Spain; marta.gtanago@upm.es * Correspondence: gmezger@ucm.es; Tel.: +34-662-432-558 Abstract: During the second half of the 20th century, several Spanish rivers experienced a decrease in the availability of water resources which coincided with an increase in human water demands. This situation is expected to be exacerbated by climate change. This study analyses the evolution of annual streamflow in 16 sub-basins of the Tagus River basin (Spain) during the 1950–2010 period and its relationship with selected variables. Our main objective is to characterize changes in in-stream flows and to identify what factors could have contributed to them. First, we used non-parametric tests to detect trends in the hydro-climatic series. Then, we analyzed changes in the runoff coefficient and applied regression-based techniques to detect anthropic drivers that could have influenced the observed trends. The analysis revealed a general decreasing trend in streamflow and an increasing trend in air temperature, while trends in precipitation are less clear. Residuals from regression models indicate that the evolution of several non-climatic factors is likely to have influenced the decline in streamflow. Our results suggest that the combination of the expansion of forested areas (a 60% increase from 1950 to 2010) and irrigated land (a 400% increase since 1950) could have played an important role in the reduction of streamflow in the Tagus basin. Keywords: streamflow; precipitation; temperature; trends; anthropic factors 1. Introduction Many river basins in the Mediterranean region have experienced a decline in the availability of water resources since the 1980s [1]. Several studies have reported a gener- alized decrease in annual precipitation in this region during the second half of the last century [2,3] while average temperatures have risen [4,5]. These climatic trends have led to a reduction in streamflow in many river basins in the Mediterranean area over the past decades [1,6]. In addition to the reduction in the availability of water resources as a result of climatic changes, intensive water use has led to water stress in many regions across Europe [7]. At least 17% of the European Union (EU) territory experiences water scarcity and countries in the Mediterranean region are especially affected by this problem [8]. The expected rise in the demand for water in the near future [9] will undoubtedly contribute to exacerbating water stress in many European countries, especially in those with pre-existing water scarcity. In Spain, the temporal evolution of climatic factors since the 1950s has varied sig- nificantly depending on the region and the variable considered. Temperature shows a significant increasing trend in most territories, while temporal patterns in precipitation are less clear [10,11]. However, it is widely acknowledged that changes in climatic variables have given rise to hydrological changes in most river basins during the second half of the 20th century [12]. Studies on a national scale have reported hydrological decline in both highly regulated [13] and non-regulated rivers [14]. Water 2022, 14, 818. https://doi.org/10.3390/w14050818 https://www.mdpi.com/journal/water