Understanding the key drivers controlling rainfall stable isotope variations in inland tropical regions remains a global challenge. We present novel high-frequency isotope data (5-30 minute intervals) to disentangle the evolution of six stratiform rainfall events (N=112) during the passage of convective systems in inland Brazil (September 2019-June 2020). These systems produced stratiform rainfall of variable cloud features. Depleted stratiform events (δ18Oinitial ≤ -4.2 ‰ and δ18Omean ≤ -6.1 ‰) were characterized by cooler cloud-top temperatures (≤-38 °C), larger areas (≥ 48 km2), higher liquid-ice ratios (≥ 3.1), and higher melting layer heights (≥3.8 km), compared to enriched stratiform events (δ18Oinitial ≥ -3.8 ‰ and δ18Omean ≥ -5.1 ‰). Cloud vertical structure variability was reflected in a wide range of δ18O temporal patterns and abrupt shifts in d-excess. Our findings provide a new perspective to the ongoing debate about isotopic variability and the partitioning of rainfall types across the tropics.

Groundwater-surface water interactions are important in controlling lake water residence time, biogeochemistry, and water availability for downstream communities in tropical volcanic catchments. To better understand these complex seasonal interactions, a multi-tracer approach including water and inorganic carbon stable isotopes (δ2H, δ18O, δ13CDIC), hydrochemistry, and 222Rn was applied in Lake Hule, northern Costa Rica. Seasonal isotope mass balance calculations using lake, stream, precipitation, and groundwater isotope compositions were supplemented with local hydrometeorological information. Evaporation to inflow ratios (E/I) revealed a small variability between the dry (December-April) and wet seasons (May-November), with relatively low evaporation losses, 2.9±1.0 % and 3.2±1.8 %, respectively. Bayesian end-member analysis indicated that annual inputs from groundwater, precipitation, and runoff represented 61.3±8.1%, 24.4±8.4, and 14.3±5.9% of total inflow, respectively. Temporal variations of δ13CDIC also confirmed the key role carbonate buffering plays in this lake and indicated greater CO2 degassing from groundwater sources in the wet season. This first tracer-aided assessment in a volcanic lake maar of northern Costa Rica provides evidence of previously unknown groundwater-surface water interactions and poses a promising tool for estimating seasonal variability of groundwater discharge into natural lakes across the volcanic front of Central America.

A document by Fabian Quichimbo. Click on the document to view its contents.

The spatial variation of soil water isotopes (SWI) - representing the baseline for investigating root water uptake (RWU) depths with water stable isotope techniques - has rarely been investigated. Here, we use spatial SWI depth profile sampling in combination with unmanned aerial vehicle (UAV) based land surface temperature estimates and vegetation indices (VI) in order to improving process understanding of the relationships between soil water content and isotope patterns with canopy status. We carried out a spatial sampling of ten SWI depth profiles in a tropical dry forest. UAV data were collected and analyzed to obtain detailed characterization of soil temperature and canopy status. We then performed a statistical analysis between the VI and land surface temperatures with soil water content and SWI values at different spatial resolutions (3 cm to 5 m). Best relationships were used for generating soil water isoscapes for the entire study area. Results suggest that soil water content and SWI values are strongly mediated by canopy parameters (VI). Various VI correlate strongly with soil water content and SWI values across all depths. SWI at the surface depend on land surface temperature (R² of 0.65 for δ18O and 0.57 for δ2H). Strongest overall correlations were found at a spatial resolution of 0.5 m. We speculate that this might be the ideal resolution for spatially characterizing SWI patterns and investigate RWU. Supporting spatial analyses of SWI with UAV-based approaches might be a future avenue for improving the spatial representation and credibility of such studies.

The El Niño-Southern Oscillation (ENSO) phenomena, originating in the tropical Pacific region, is an interannual climate variability driven by sea surface temperature and atmospheric pressure changes that affect weather patterns globally. In Mesoamerica, ENSO can cause significant changes in rainfall patterns with major impacts on water resources. This commentary presents results from a nearly 10-yr hydrometric and tracer monitoring network across north-central Costa Rica, a region known as a headwater-dependent system. This monitoring system has recorded different El Niño and La Niña events, as well as the direct/indirect effects of several hurricane and tropical storm passages. Our results show that ENSO exerts a significant but predictable impact on rainfall anomalies, groundwater recharge, and spring discharge, as evidenced by second-order water isotope parameters (e.g., line conditioned-excess or LC-excess). The Oceanic Niño Index (ONI) is correlated with a reduction in mean annual and cold front rainfall across the headwaters of north-central Costa Rica. During El Niño conditions, rainfall is substantially reduced (by up to 69.2%) during the critical cold fronts period, subsequently limiting groundwater recharge and promoting an early onset of baseflow conditions. In contrast, La Niña is associated with increased rainfall and groundwater recharge (by up to 94.7% during active cold front periods). During La Niña, the long-term mean spring discharge (39 Ls -1) is exceeded 63-80% of the time, whereas, during El Niño, the exceedance time ranges between 26% and 44%. These stark shifts in regional hydroclimatic variability are imprinted on the hydrogen and oxygen isotopic compositions of meteoric waters. Drier conditions favored lower LC-excess in rainfall (-17.3‰) and spring water (-6.5‰), whereas wetter conditions resulted in greater values (rainfall=+17.5‰; spring water=+10.7‰). The lower and higher LC-excess values in rainfall corresponded to the very strong 2014-16 El Niño and 2018 La Niña, respectively. During the recent triple-dip 2021-23 La Niña, LC-excess exhibited a significant and consistently increasing trend. These findings highlight the importance of combining hydrometric, synoptic, and isotopic monitoring as ENSO sentinels to advance our current understanding of ENSO impacts on hydrological systems across the humid Tropics. Such information is critical to constraining 21 st century projections of future water stress across this fragile region.

Tracer-aided studies to understand source water partitioning in tropical ecosystems are limited. Here we report dry season source water partitioning in five unique ecosystems distributed across Costa Rica in altitudinal (<150-3,400 m asl) and latitudinal (Caribbean and Pacific slopes) gradients: evergreen and seasonal rainforests, cloud forest, Páramo, and dry forest. Soil and plant samples were collected during the dry season (2021). Plant and soil water extractions (triplicates) were conducted using controlled centrifugation. Stem water extraction efficiency and stem water content were calculated via gravimetric measurements. Water source contributions were estimated using a Bayesian mixing model. Isotope ratios in soil and stems exhibited a strong meteoric origin. Enrichment trends were detected mainly in stems and cactus samples within the dry forest ecosystem. Soil profiles revealed nearly uniform isotopic profiles; however, a depletion trend was observed in the Páramo ecosystem below 25 cm depth. More enriched compositions were reported in cactus samples for extracted water volumes above ~20% ( Adj. r2=0.34, p<0.01). The most prominent dry season water source in the evergreen rainforest (74.0%), seasonal rainforest (86.4%), and cloud forest (66.0%) corresponded with soil water. In the Páramo ecosystem, recent rainfall produced by trade wind incursions resulted in the most significant water source (61.9%), whereas in the dry forest, mean annual precipitation (38.6%) and baseflow (33.1%) were the dominant sources. The latter highlights the prevalence of distinct water uptake sources between recent cold front’s rainfall to more well-mixed soil moisture during the dry season.