Pristine tropical forests play a critical role in regional and global climate systems. For a better understanding of the eco-hydrology of tropical "evergreen" vegetation, it is essential to know the partitioning of water into transpiration and evaporation, runoff and associated water ages. For this purpose, we evaluated how topography and vegetation influence water flux and age dynamics at high temporal (hourly) and spatial (10 m) resolution using the Spatially Distributed Tracer-Aided Rainfall-Runoff model for the tropics (STARRtropics). The model was applied in a tropical rainforest catchment (3.2 km 2) where data were collected biweekly to monthly and during intensive monitoring campaigns from January 2013 to July 2018. The STARRtropics model was further developed, incorporating an isotope mass balance for evapotranspiration partitioning into transpiration and evaporation. Results exhibited a rapid streamflow response to rainfall inputs (water and isotopes) with limited mixing and a largely timeinvariant baseflow isotope composition. Simulated soil water storage showed a transient response to rainfall inputs with a seasonal component directly resembling the streamflow dynamics which was independently evaluated using soil water content measurements. High transpiration fluxes (max 7 mm/day) were linked to lower slope gradients, deeper soils and greater leaf area index. Overall water partitioning resulted in 65% of the actual evapotranspiration being driven by vegetation with high transpiration rates over the drier months compared to the wet season. Time scales of water age were highly variable, ranging from hours to a few years. Stream water ages were conceptualized as a mixture of younger soil water and slightly older, deeper soil water and shallow groundwater with a maximum age of roughly 2 years during drought conditions (722 days). The simulated soil water ages ranged from hours to 162 days and for shallow groundwater up to 1,200 days. Despite the model assumptions, experimental challenges and data limitation, this preliminary spatially distributed model study enhances knowledge about the water ages and overall young water dominance in a tropical rainforest with little influence of deeper and older groundwater.

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<p>The headwater catchments in the humid tropical forests are of major hydrological importance for regional and global climate systems and provide essential ecosystem services such as water supply for other ecosystems and industrial use in the lowlands. Anthropogenic pressure together with global environmental changes critically alter the hydrological functioning of these catchments. However, limited knowledge jeopardizes a proper water resources management of such water towers.</p><p>To contribute to filling this gap, we conducted a field monitoring of hydro-climatic and isotopic data (01.2013 – 07.2018) in a pristine tropical rainforest catchment (3.2 km<sup>2</sup>) in Costa Rica and used this data to test hypotheses about water age dynamics. The Spatially-Distributed Tracer-Aided Rainfall-Runoff model for the tropics (STARRtropics) was applied in high temporal (hourly) and spatial (10m) resolution. The best-obtained model simulations reflected a highly variable range and distribution of water ages. Nevertheless, superficial flow paths with young water contributions (40 months at most) dominate the streamflow generation entirely. The maximum water age was independently evaluated calculating the tritium-derived baseflow mean transit time. The highest simulated ages of transpiration flux varied between 12 days and 5.5 months depending on the soil depths where the water was uptake. Soil water age peaked at 5.4 months and groundwater at 40 months. The oldest stream water age, integrating all catchment processes, reached 24 months. Overall, the water age increased during dry conditions. The frequency of water ages reflected high occurrences of young water for transpiration flux and streamflow in their respective ranges. Maximum occurrences were reported for transpiration with 10 hours and streamflow with 2.8 months. The soil water age presented a bimodal distribution with peaks of 2.8 and 4.4 months and groundwater age occurrences peaked at 32 and 37 months. Spatially, high age dynamics of transpiration flux were associated with a higher leaf area index on the northern hillside in relation to the southern hillside. The oldest soil water was related to more developed soils and the groundwater age increased towards the bottom of the catchment. In the context of the tropics, our study is one of the first that quantitatively evaluated water age dynamics and distributions, and globally using such a high spatial and temporal resolution with a non-stationary perspective. These findings will support decision-makers to manage the water resources and ecosystem in the humid tropics and reduce the research gap regarding hydrological processes of tropical headwater towers under environmental changes.</p>

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