Modelling non‐stationary water ages in a tropical rainforest: A preliminary spatially distributed assessment

• Published in: Hydrological processes 2020-12, Vol.34 (25), p.4776-4793
• Main Authors: Correa, Alicia, Birkel, Christian, Gutierrez, Jason, Dehaspe, Joni, Durán‐Quesada, Ana María, Soulsby, Chris, Sánchez‐Murillo, Ricardo
• Format: Article
• Language: English
• Subjects: Age; Base flow; Catchment area; Climate system; Costa Rica; Drought; Drought conditions; Dynamics; Evaluation; Evaporation; Evapotranspiration; Fluxes; Global climate; Groundwater; humid tropics; Hydrologic models; Hydrology; Isotope composition; Isotopes; Leaf area; Leaf area index; Mass balance; Moisture content; Partitioning; Rain; Rainfall; Rainfall-runoff relationships; Rainforests; Rainy season; ReBAMB; Regional climates; Runoff; Slope gradients; Soil; Soil dynamics; Soil mixtures; Soil water; Soil water composition; Soil water storage; Stream discharge; Stream flow; Tracers; tracer‐aided modelling; Transient response; Transpiration; Tropical climate; Tropical environments; Tropical forests; Vegetation; water ages; Water content; water partitioning; Water storage; Wet season
• ISSN: 0885-6087; 1099-1085
• DOI: 10.1002/hyp.13925

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 km2) 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 time‐invariant 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. Transpiration represents 65% of actual ET and relates to LAI, slope, and soil properties. The stream water ages are dominated by young water. Higher soil mean water age (MA) are related to more developed soil sand MA of ground water increases t