1. INTRODUCTION
The hydrological cycle explains water distribution and its availability across the globe. Processes such as evaporation, transpiration, and precipitation connect the terrestrial and atmospheric components of the hydrological cycle through water and energy exchange. Atmospheric circulation allows regional-to-global water redistribution, establishing teleconnections between remote areas. These teleconnections are vital for the sustainability of ecosystems and biodiversity as well as for water security, and socioeconomic development (Wagener et al., 2010; Martinez & Dominguez, 2014; Swann, Longo, Knox, Lee, & Moorcroft, 2015; Molina, Salazar, Martínez, Villegas, & Arias, 2019).  Understanding moisture origin and the mechanisms driving atmospheric transport is key for defining the interdependence of a territory with its surroundings, and also for the definition of proper spatio-temporal dynamics that determine regional atmospheric processes.
The conventional methods to identify moisture source regions, and to estimate the proportion of incoming atmospheric moisture associated with each source include i) analytical or box models, ii) numerical water vapor tracers, and iii) physical water vapor tracers (Durán-Quesada, Gimeno, Amador, & Nieto, 2010). The first two are theoretical-computational models based on the Eulerian and Lagrangian notion of trajectory, respectively. These models often use input information from gauge stations and reanalysis data (Wang et al., 2004; Fuka et al., 2014; Liu et al., 2020). The third method is based on physical water vapor tracers in the isotopic composition of precipitation. The interpretation of moisture tracers is useful to infer the sources and the processes inducing fractionation in water isotopic composition throughout the movement of air masses (Simpson & Herczeg, 1991; Martinelli, Victoria, Sternberg, Ribeiro, & Moreira, 1996; Clark & Fritz, 1999). More specifically, the isotopic composition of rainwater allows the interpretation of prevailing meteorological conditions in the formation of air masses (temperature, humidity, wind speed) and the origin of moisture (evaporation and/or transpiration) (Gat & Carmi, 1970; Clark & Fritz, 1999; Duran-Quesada et al., 2010; Gimeno, Drumond, Nieto, Trigo, & Stohl, 2010; Van der Ent, Savenije, Schaefli, & Steele-Dunne, 2010; Gimeno et al., 2012). The study of moisture sources through stable isotopes has been widely used in understanding long-term changes in the water cycle and the dynamics of climate (Gat & Carmi, 1970; Salati, 1979; Gat & Gonfiantini, 1981; Clark & Fritz, 1999; Aggarwal, Froehlich, & Gat, 2005; Van der Ent et al., 2010; Gimeno et al., 2012; Négrel, Petelet-Giraud, & Millot, 2016; Sánchez-Murillo et al., 2016; Alexandre, 2020). However, the lack of isotopic data in many locations around the world is still a disadvantage for proper long-term analyses (Benjamin et al., 2005).
A current challenge for land and water resource managers is the definition and understanding of the potential implications of environmental change on the availability of water resources (Newman et al., 2006; Gain et al., 2020; Rivadeneira et al., 2020). This challenge has been generally addressed with local-scale management plans and strategies. However, in a region like Northern South America (particularly in Colombia), water, food and energy security (which largely supports the country’s economy) depend, almost exclusively, on surface water, which in turn is associated with short-term rainfall generation processes (Álvarez-Villa, Vélez, & Poveda, 2011; Díaz, Saurral, & Vera, 2020; Mesa, Urrea, & Ochoa, 2021). In addition, Colombia is one of the most biodiverse regions in the world (Myers, Mittermeier, Mittermeier, Da Fonseca, & Kent, 2000; Churchill, 2009; Bruijnzeel, Scatena, & Hamilton, 2011; Herzog & Kattan, 2011; Ehrendorfer, 2013; Hutter, Lambert, & Wiens, 2017; Hoorn, Perrigo, & Antonelli, 2018; Bax & Francesconi, 2019). The connection with the Pacific and Atlantic oceans, the Amazon-Andes interactions, and the orographic barrier of the regional Andes are the major drivers of atmospheric circulation and ecological diversity in the country (Sakamoto, 2011; Hoyos, 2017; Poveda, Jaramillo, & Vallejo, 2014; Espinoza et al., 2020). For biodiversity conservation, ecosystem health, and socioeconomic development, the availability of water is an important basis that depends on hydrologic functioning (Pringle, 2001). Therefore, understanding the origin and dynamics of moisture that becomes rainfall in the two most populated regions of the country is fundamental to maintaining water security. Yet, these analyses have only been performed with models that, to date, have not been contrasted with actual field measurements such as those from environmental tracers.
In this study, we explore the hydroclimatic features underlying the composition of Colombian atmospheric moisture by establishing the isotopic baseline for the regional precipitation on a seasonal time scale. We include data from 33 stations distributed along the inter-Andean mountain region and the Caribbean region in Colombia, available in the Global Network of Isotopes in Precipitation (GNIP) project. We analyze the monthly variation of δ18O and δ2H values, and the spatio-temporal reconstruction of D-excess during 1971-2016. Comparing the Local Meteoric Water Line (LMWL) with the Global Meteoric Water Line (GMWL) provides criteria of depletion or enrichment of hydrogen and oxygen isotopic composition of precipitation that, in turn, allow the identification of the oceanic or terrestrial origin of air-water masses that effectively precipitates over the target area. We use the results from the Lagrangian FLEXPART model to contrast the information inferred from isotopic composition with the regional moisture contributions structure based on the air masses trajectories, providing a more comprehensive understanding of moisture sources to the country and the potential implications of alterations in these dynamics associated with land use and vegetation cover change.