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Tracing isotope precipitation patterns across Mexico [1]

['Ricardo Sánchez-Murillo', 'Department Of Earth', 'Environmental Sciences', 'University Of Texas', 'Arlington', 'Texas', 'United States Of America', 'Luis González-Hita', 'Department Of Hydrology', 'Mexican Institute Of Water Technology']

Date: 2023-11 

Abstract Mexico encompasses a large spectrum of landscapes with topographic, geographic, and climatic factors interacting in a complex ecohydrological setting. For decades, isotope hydrogeological tools have been applied in Mexico using short-term or seasonal local meteoric water lines as valid input functions. Yet, a systematic evaluation of meteoric isotope characteristics is still lacking. Here we report on the spatial and temporal isotope variations of 21 precipitation monitoring stations across Mexico. Our database includes 608 monthly samples collected from 2018 to 2021 over four regions (between 5 and 2,365 m asl): the Pacific coast, the Gulf of Mexico/Caribbean Sea region, and the Central and Northern plateaus. Precipitation δ18O seasonality from the dry (winter) to the wet season (summer) was characterized by a notable W-shaped variability. Monthly precipitation amounts and δ18O compositions exhibited poor to strong linear regressions (Adj. r2<0.01 to 0.75), with inverse (positive) relationships over the northern monsoon-affected region. Low d-excess (5.1 to 9.7‰) corresponded with greater terrestrial moisture contributions (20.5%) over the arid northern regions. Moisture inputs from the Gulf of Mexico/Caribbean Sea and the Pacific Ocean were associated with near-equilibrium or greater d-excess values (8.8 to 14.3‰), respectively. The best-fit linear models for δ18O (Adj. r2 = 0.85) and δ2H (Adj. r2 = 0.88) were determined for topographic and geographical predictors, resulting in an updated high-resolution precipitation isoscape (100 m2 grid) for Mexico. Orographic barriers (-2.10‰ in δ18O/km) coupled with the interaction of tropical cyclones and cold fronts, the evolution of the North American Monsoon system, and the passage of easterly trade winds play a remarkable role in controlling the spatial isotope rainfall variability. Our findings provide a robust baseline for ecohydrological, climatic, forensic, archeological, and paleoclimate studies in North America.

Citation: Sánchez-Murillo R, González-Hita L, Mejía-González MA, Carteño-Martinez B, Aparicio-González JC, Mañón-Flores D, et al. (2023) Tracing isotope precipitation patterns across Mexico. PLOS Water 2(10): e0000136. https://doi.org/10.1371/journal.pwat.0000136 Editor: Soni M. Pradhanang, University of Rhode Island, UNITED STATES Received: October 24, 2022; Accepted: September 7, 2023; Published: October 11, 2023 Copyright: © 2023 Sánchez-Murillo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All data is available within the supplementary information and from Hydroshare at https://www.hydroshare.org/resource/909aaa5edf1040b6a6244a0ca7f58890/. Funding: The National Monitoring Network of Isotopes in Precipitation (known as RENIP) across Mexico is the result of technical cooperation with the International Atomic Energy Agency (IAEA) through the Regional Project ‘Integration of Isotope Hydrology in National Assessments of Water Resources’ (Project Number RLA/7/024 to LGH). The authors acknowledge the support from the STARs Program (Project Number AR911486 to RSM), the Office of the Provost funds at the University of Texas-Arlington (Project Number 314075 to RSM), and the NASEM Gulf Research Program Early Career Fellowship (Project Number 12668055350 to RSM). The Xunta of Galicia supported MS under postdoctoral grant number ED481B-2021/134. EphysLab, Uvigo, received support from the Xunta de Galicia under project number ED431C 2021/44 to LG and RN (Programa de Consolidación e Estructuración de Unidades de Investigación Competitivas (Grupos de Referencia Competitiva and Consellería de Cultura, Educación e Universidade). The authors also recognized the crucial partnership between the National Meteorological Service and the Mexico City Water System. The authors thank the multiple helping hands that contribute to precipitation sampling across Mexico. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

1. Introduction Historically, the use of water isotopes (δ2H, δ18O) in hydrogeological studies has been site-specific across Mexico, where 1–2 years or even seasonal local meteoric water lines (LMWLs) have often been used as valid input functions in central Mexico [1–6], in the northern arid landscapes [7–9], and in the southern coastal wet regions [10–12] to underpin rainfall to groundwater and surface water connectivity. For decades, however, most studies have relied on data archives of two stations from the Global Network of Isotopes in Precipitation [13] operated in Mexico from 1962 to 1988, one in the city of Chihuahua (northern arid region) and a second one in the city of Veracruz (southeastern wet region) [14]. The latter represents a relatively low number of monitoring stations for such a large and heterogeneous country, with abundant rain in the south and scarce precipitation in the north, with rain forests but also with vast deserts, tropical and mid-latitude climates, with coastlines facing the Pacific Ocean, the Gulf of Mexico and the Gulf of California, and the Caribbean Sea, and with an enormous plateau bounded by two mountain ranges where ice caps are still present [15]. Similarly, Mexico is affected by multiple climatic features, such as the influence of cold fronts [16], atmospheric rivers [17], easterly waves [18], tropical cyclones [19], and northerly trade winds. All these topographic, geographic, and weather characteristics are represented in a large spectrum of climatic regions across Mexico [20]. While numerous catchment-to-basin-scale studies have been conducted across Mexico, only four isotopic geospatial efforts [21] have been reported. Wassenaar et al. [22] interpolated groundwater isotopic values based on the premise that the water isotope ratios of shallow groundwater (N = 234 at near 50 km latitudinal spacing) can be used as a proxy of the stable isotopic composition of long-term seasonally weighted precipitation. This product was later used to construct a feather hydrogen isoscape (i.e., migrant bird isoscape based on feather deuterium compositions) for Mexico [23]. Ammer et al. [24] used 158 tap water samples from 51 towns and cities collected throughout Mexico for six weeks from June to July 2018 to infer the region of origin of unidentified border crossers between Mexico and the United States. Lately, Fan et al. [25] studied δ18O, δ2H, and d-excess (N = 205) in surface water collected in wet and dry seasons from the west to the east coast of Mexico between 22°N and 26°N to understand the controlling factors of isotopic patterns and lapse rates. Nevertheless, a systematic and regional evaluation of isotope meteoric characteristics across Mexico is still lacking. Here we report on the spatial and temporal isotope variations of 21 precipitation monitoring stations across different physiographic units of Mexico (https://en.www.inegi.org.mx/temas/fisiografia/). These stations are part of the National Network of Isotopes in Precipitation (RENIP), operated by the Department of Hydrology of the Mexican Institute of Water Technology (IMTA). Our database includes 608 monthly samples collected from 2018 to 2021 over four regions: the Pacific coast (PC), the Gulf of Mexico and the Caribbean Sea coast (GCS), and the Central (CP) and Northern plateaus (NP). Monitoring sites are located between 5 and 2,365 m asl with distinct topographic, geographic, and weather characteristics. The main goal of our study is to evaluate the spatial and temporal isotopic variability in precipitation across Mexico. This will be complemented with an atmospheric analysis of the trajectories that reach the region using the FLEXPART Lagrangian model [as in 26]. This analysis differentiates between the monthly contribution of moisture sources from oceanic and terrestrial domains. Emergent relationships resulted in updated precipitation isoscapes (δ18O, δ2H, d-excess) for Mexico. Our spatial and temporal analysis should also serve as a baseline for ecohydrological, climatic, forensic, archeological, and paleoclimate studies in North America.

2. Topographic features Mexico is part of the northern hemisphere, ranging from 14°30’N to 32°43’N (Fig 1). A fraction of the territory is located within the subtropical zone (north of the Tropic of Cancer), while the southern portion is part of the tropics [15]. A notable orographic feature is an enormous plateau called the Mexican Plateau, bounded to the west by the Sierra Madre Occidental and to the east by the Sierra Madre Oriental [27]. This plateau intercepts the westerly winds that are characteristic of the middle latitudes. Due to their low humidity, these winds produce the characteristic dryness that prevails during the cold season. PPT PowerPoint slide

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TIFF original image Download: Fig 1. Map of Mexico including mean annual precipitation (in mm) (1981–2019; based on CHIRPS data) [ Map of Mexico including mean annual precipitation (in mm) (1981–2019; based on CHIRPS data) [ 46 ], isotope monitoring sites (monthly sampling frequency; pink circles, dotted), and physiographic regions (dashed-line polygons; obtained from https://en.www.inegi.org.mx/temas/fisiografia/ ). The number of monitoring sites per physiographic region ranged from 1 to 5. https://doi.org/10.1371/journal.pwat.0000136.g001 The Sierra Madre Oriental is a mountain range approximately 1,350 km long and 80 to 100 km wide, extending from the south of the Rio Bravo, parallel to the Gulf of Mexico, to the Neovolcanic Axis, which separates North America from Central America [28]. Headwater elevation oscillates between 2,000 and 3,000 m asl in this mountain range. Under favorable synoptic-scale flow [29], cold winter surges propagate from North America deep into the tropics bounded by the eastern side of the Sierra Madre Oriental [30–32]. The Isthmus of Tehuantepec, located in southeastern Mexico, is a narrow region that separates the Gulf of Mexico from the Pacific Ocean. The Sierra Madre del Sur range has an average altitude of 2,000 m asl, but on the Isthmus of Tehuantepec, the elevation drops to 250 m, forming a gap approximately 40 km wide and 220 km long, known as ‘Paso Chivela.’ It is through this gap that cold fronts sometimes leak into the Pacific Ocean. Another important mountain range is the Sierra Madre Occidental, which extends over 1,400 km from the northern border to the central part of the country, bordering the Pacific Ocean [28]. With an average altitude of 2,000 m asl, it reaches maximum altitudes of 3,300 m in Chihuahua [28]. When atmospheric rivers from the western Pacific Ocean influence the northwestern region, water vapor rises over the Sierra Madre Occidental, generating abundant precipitation [33, 34]. The main physiographic component of central Mexico is the ‘Mesa Central’ or Central Plateau, a region that encompasses the Trans-Mexican Volcanic Belt. This region is bordered to the south by the Balsas Depression, to the east by the Sierra Madre Oriental, to the west by the Sierra Madre Occidental, and to the north by Zacatecas. This region comprises several volcanoes such as Pico de Orizaba (5,363 m asl), Popocatepetl (5,426 m asl), Nevado de Toluca (4,690 m asl), Colima (4,330 m asl), and Cofre de Perote (4,282 m asl). The high relief is one of the main factors influencing the distribution of precipitation in this region, from values greater than 2,500 mm (windward) to 500 mm (leeward) [35] (Fig 1). The southeast region includes the Yucatan Peninsula with an elevation below 300 m asl [15] surrounded by the Gulf of Mexico and the Tabasqueña lowlands. On the Pacific slope of this region, there is a low area of 600 to 900 m asl, and to the north of this depression, the Central Plateau of Chiapas runs to the coastal plain of the Gulf of Mexico [15].

3. Climatic and meteorological characteristics Among the climatic and meteorological components that influence the annual precipitation cycle in Mexico are the North American high (NAH), the North Atlantic subtropical high (NASH), the North American Monsoon System (NAMS), cold fronts, atmospheric rivers, easterly waves, and tropical cyclones. The NAH is a center of high atmospheric pressure across most of North America during the boreal winter. Cold and dry air masses flow from the NAH, causing cold fronts in the region (September to May) [16]. Depending on the atmospheric conditions, the convergence of the cold fronts with moisture advected from the Gulf of Mexico and the Pacific Ocean could result in rainfall events. High vertically integrated water vapor bands are formed at the convergence zone of the cold fronts and the warm conveyor belt of extratropical systems [36]. Such moisture bands, called atmospheric rivers (ARs), are formed over the oceans and are typically associated with extratropical cyclones, covering long (thousands of kilometers) and narrow (hundreds of kilometers) transient water vapor corridors [37]. The influence of ARs is relevant in northwestern Mexico, whereby rainfall is produced via this mechanism during the winter [33, 34]. During the summer, the air over the continent warms up more than the air over the ocean, producing a pressure gradient between the continent (low pressure) and the ocean (high pressure). The pressure gradient induces wind currents transporting moisture from the ocean to the continent. The moisture warmed by the terrestrial heat flux is lifted in a strong upward air motion to high altitudes, producing convective rain (i.e., NAMS) [38]. Convective rain provides a substantial fraction of the total annual precipitation in northwestern Mexico (60%-80%) [39]. The NASH is a center of high atmospheric pressure in the Atlantic Ocean, located around 30°N, which varies in extension and intensity during the year [40]. During the winter, it weakens and contracts, and by early May, it begins to strengthen and expand, continuing until the end of July [41]. The winds produced by the NASH, called northerly trade winds, transport moisture from the Caribbean Sea and the Gulf of Mexico towards Central America, Mexico, and the southeastern United States [40]. During the summer, the northerly trade winds converge with the southerly trade winds in the tropical portion of Mexico [42], generating updrafts that condense the moisture and produce abundant rainfall [43]. Another important source of precipitation in the tropics of Mexico is the easterly waves (EWs). They are inverted troughs of low pressure, propagating from east to west (i.e., across the Atlantic Ocean and originated near the western coast of Africa) from May to November, with lengths of 2,000–2,500 km, and within latitudes 5o and 20o N. Easterly waves are highly convective systems, producing at least 20% of southern Mexico’s precipitation [18]. In addition, Mexico is located between the cyclogenetic regions of the northern Atlantic Ocean and the northeastern Pacific Ocean, receiving abundant rains from tropical cyclones (TCs) during the summer. On average, five tropical cyclones enter the national territory yearly [44]. Mexico receives up to 20% of its total annual rainfall from TCs [19].

6. Discussion 6.1. Spatial isotopic variations in precipitation across Mexico Isotope variations in meteoric water are a product of the interactions between topography (e.g., mountain ranges, depressions, inter-mountainous valleys), vapor transport (e.g., Pacific Ocean, Gulf of Mexico, Caribbean Sea, and terrestrial sources), sub-cloud evaporation, and the influence of recycled moisture (i.e., evapotranspiration) (Fig 2) [90–92]. These features interplay in a complex array of physiographic units across Mexico, resulting in complicated spatial isotope patterns (Figs 8 and 9). Tropical cyclones contribute up to 40% to the annual rainfall in the coastal regions of northwestern Mexico (e.g., Loreto and Hermosillo; Fig 1) [19], whereas annual contributions range between 20–30% along the Gulf of Mexico coast. During active hurricane seasons, these storms can result in abundant and enriched rainfall in the Gulf of Mexico and Caribbean Sea coast and in depleted compositions inland (i.e., orographic distillation) and along the northwestern Pacific coast [8, 93]. Cold fronts and atmospheric rivers are also responsible for depleted isotopic compositions across the northwestern region, with δ18O down to -15‰ during the boreal winter in the Northern Plateau (Fig 3B). The NAMS activation commonly results in large precipitation events, highly localized, and enriched across the northwestern and north central regions of Mexico [22, 80, 81]. The latter is clearly depicted by the inverse (positive) precipitation amount relationships (Fig 4). In contrast, the northerly trade winds and easterly waves result in enriched compositions along the Gulf of Mexico and Yucatan peninsula [10, 12]. The convergence of the northerly trade winds with the southerly trade winds across orographic barriers results in depleted compositions during the summer over central Mexico (Fig 3B). The stronger rainout effect (i.e., a progressive isotope depletion in precipitation with increasing distance from the ocean) observed over the GCS domain is most likely related to the direct influence of the trade winds and nearby moisture transport from the Gulf of Mexico and the Caribbean Sea basins (Fig 6). In the intra-mountainous plateaus and the Pacific slope, the combination of the rain shadow effect (i.e., an area of significantly reduced precipitation behind a mountainous region), more complex and rugged topography, recycled evapotranspiration, deep convective activity (June-September), and indirect influence of tropical cyclones results in weaker and more complex spatial trends [71]. In this regard, the isotopic bias introduced in low-elevation coastal stations affected by tropical storms within the Mesoamerican region should be considered when determining isotopic lapse rates for hydrogeological applications (e.g., mean recharge elevations). For instance, low-elevation sites (i.e., Loreto, Culiacan, and Hermosillo; Fig 1) across the northern Pacific coast were affected by strong hurricane landfalls and passages during the monitoring period (e.g., hurricanes Rosa-2018 and Sergio-2018, Lorena-2019, Nora-2021, and Pamela-2021, among other tropical storms). Remarkably, [8] reported a relatively weak isotope lapse rate within the Sonora River basin (PC to NP) during the wet season (-1.1‰ in δ18O per km of elevation) but noted that the isotope lapse rate increased (intra-seasonally) due to tropical cyclone events (-2.6‰ in δ18O per km of elevation; Adj. r2 = 0.86; p<0.001). In Mexico, the nationwide isotopic lapse rate (based on arithmetic means) can be defined as -1.75‰ in δ18O per km of elevation (Adj. r2 = 0.65, p<0.001; Fig 5), whereas the precipitation-weighted lapse rate can be described as -2.10‰ in δ18O per km of elevation (72% of the total variance explained, p<0.001; Fig 5). The lapse rate obtained in the isoscape modeling resulted in -1.72 δ18O/km. Overall, the reported isotope lapse rates agree with previous studies (-2.3‰/km) based on a combined approach of regressing the mean δ18O from GNIP stations against geographical and climatic regressors and applying the resulting function onto gridded climate data [50]. For instance, [71] reported a pantropical spectrum of isotopic lapse rates ranging from -3.5 to -0.5‰/km, with a pantropical mean of -2.2‰/km. Similarly, [92] reported a global isotopic lapse rate of -2.8‰/km. A previous groundwater isoscape effort in Mexico [22] evaluated 19 linear regression models, from a single variable model (elevation) to more complex iterations including drainage or slope (Pacific versus Gulf of Mexico and the Caribbean Sea basins), distance to coast, latitude, annual precipitation, and interaction among these variables. In the best model, elevation explained only 44% of the total variance in groundwater samples, while precipitation amount (10%), slope (Pacific versus GCS) (12%), and latitude (10%) explained a third of the total variance. Landscape damping effects in transitional regions from tropical to arid sub-tropical biomes can explain the relatively weaker elevation relationship in groundwater (e.g., secondary soil evaporation). In Costa Rica and Guatemala/Belize, [54, 94] reported similar lapse rates in precipitation, groundwater, and surface water (-2.5% in δ18O per km) across the wet Caribbean slope, whereas weaker relationships have been reported across the dry Corridor of Central America (-1.0% in δ18O per km) [50]. The updated spatial and temporal coverage of the Mexican isotope network (i.e., RENIP) facilitated a more robust representation of the meteoric isotope variability. In our model, 85% (δ18O) and 88% (δ2H) of the total variance is explained by topographic (elevation) and geographical regressors (latitude and longitude), with δ18O and δ2H residuals varying from +2.0‰ (Ciudad Victoria; northern region) to -1.2‰ (Tulancingo; central region). On average, the predicted mean annual δ18O compositions were more depleted across the central (0.04‰) and northern (0.27‰) plateaus, except for Hermosillo (coastal site affected by tropical storms) (more enriched by 0.58‰) and Pachuca (high elevation site; 2,365 m asl) (more enriched by 1.06‰) sites. The larger residual variability was observed across the Gulf of Mexico and Caribbean Sea sites with residuals ranging from 2.0‰ (Ciudad Victoria; northern coast) to -0.90‰ (Mérida; southern coast). In contrast, the predicted mean annual δ18O compositions across the Pacific coast sites were consistently under-predicted on average by -0.48‰, indicating the tropical cyclone bias affecting this region.

7. Conclusions This synthesis of stable isotope compositions in precipitation across distinct landscapes of Mexico shows how complex atmospheric processes and moisture contributions from terrestrial and maritime (the Pacific Ocean, Gulf of Mexico, and the Caribbean Sea basin) domains interplay with topographic features, resulting in isotopic spatial and temporal variations throughout the hydrological year. In this regard, water vapor sampling or high-frequency discrete samples [95, 96] are still needed to clearly separate the role of terrestrial (e.g., local and remote transpiration, ground evaporation, and canopy evaporation) [89] versus oceanic moisture in controlling the observed spatial isotope variability, with particular interest across the north Pacific coast and Northern Plateau region. The inherent complexity of observed monthly precipitation relationships across Mexico invokes the need to underpin the local (e.g., precipitable water column) and regional weather processes governing the spatial trends. The latter also precludes using the classical amount effect as a valid regressor. In the northern regions, terrestrial and maritime moisture sources resulted in enriched isotope compositions during the monsoon season. The latter is further evidenced by inverse (positive) precipitation amount and δ18O relationship in Torreon, Hermosillo, Piedras Negras, Durango, and Chihuahua (+0.4 to +2.3‰ in δ18O/100 mm). Windward (Gulf of Mexico coast) and high mountainous regions exhibited moderate to strong effects (-0.2 to -2.2‰ in δ18O/100 mm). Lowland arid regions across the north Pacific resulted in poor relationships (-0.8 to +0.01‰ in δ18O/100 mm) mostly affected by the indirect effect of tropical cyclones. In the central, southern Gulf coast, and south Pacific regions oceanic moisture sources were dominant (Gulf of Mexico>Pacific Ocean>Caribbean Sea). On the northeastern coast, moisture from the Gulf of Mexico predominated as the main contributor. The first attempt at building a high-resolution (100 m2 grid) nationwide rainfall isoscape highlighted areas with significant spatial variability such as the Pacific coast and the northern arid regions, which highlights the need for additional monitoring efforts (per region) focused on the temporal bias produced by the indirect or direct effect of tropical cyclones, monsoon activity, and evapotranspiration moisture inputs. Our results fill a recognized historical gap in the precipitation isotope monitoring in North America and will provide a baseline to pursue more detailed ecohydrological, climatic, forensic, archeological, and paleoclimate studies across Mexico.

Acknowledgments The authors thank the helping hands contributing to precipitation sampling across Mexico.

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