A preliminary isotope‐based evapotranspiration partitioning approach for tropical Costa Rica

Abstract

Spatially and seasonally distributed information on transpiration (T) fluxes is limited in the tropics. Here, we applied a coupled isotope mass balance model to separate water fluxes of T and evapotranspiration (ET) from precipitation (P). The mean annual T was estimated at a resolution of 100 m for Costa Rica (51,100 km2) and a partitioning of monthly T and evaporation (E) for the 2370‐km2 San Carlos catchment. The dominant flux in the forest ecosystems was T with a mean annual T of 1086 mm that ranged from 700 mm in Tropical Montane Very Humid Forest to 1400 mm in Subtropical and Tropical Low Montane Rainforests. We estimated an average 85% of ET was T, which is concurrent with expectations for forested tropical regions, but varied according to model parameterization and data sources. A model comparison exercise showed a range of mean annual T estimates from 988 to 1465 mm and a range of T/P from 0.35 to 0.5 with temperature and relative humidity exhibiting the highest impact on the model results. Across Costa Rica, we estimated an average loss of precipitation by T of 38% (1085 mm), whereas interception (I) constitutes 10% (230 mm) and direct evaporation (E) only 7% (192 mm). Similarly, the results at the catchment scale indicated that monthly T contributes 76% (85‐mm monthly average) to total ET and E corresponds to 24% (24‐mm monthly average). The T rates exhibited an opposite seasonality to rainfall with highest T over the drier months from December to April with a peak in March (101–144 mm) and the minimum T in September (53–71 mm). Around 17% (79–130 mm) of precipitation over the catchment area is lost to T, both E (10–35 mm) and I (15–38 mm) correspond to 5%. Despite the inherent uncertainties of the data assumptions and simplifications, including data interpolation errors, the coupled isotope mass balance model showed in comparison to other global products reasonable water partitioning for different ecosystems in Costa Rica and the San Carlos catchment area. These results can help to evaluate the impact of land cover conversion on the hydrological cycle in Costa Rica, and the simple isotope‐based model could be transferred to different biomes of the tropics.

Tracer‐aided ecohydrological modelling across climate, land cover, and topographical gradients in the tropics

“…However, their Transp/ET ratios up to 0.8 were also higher than the model results here. Despite differences and method‐dependent uncertainties surrounding the absolute values of ET partitioning, all estimates by Correa et al (2020) and Iraheta et al (2021) and measurements by Aparecido et al (2016) generally were consistent with our large‐scale STARRtropics results within the uncertainty ranges of the different models and measurements (Coenders‐Gerrits et al, 2014).…”

“…Correa et al (2020) with Penman-Monteith PET resulted in slightly higher Transp/ET ratios (avg 0.65 compared to 0.5 from the largescale model) with overall less than half ($350 mm/year) of the simulated annual transpiration flux from the large-scale model ($600 mm/ year, Table4). The large-scale transpiration flux estimates for the San Carlos ofIraheta et al (2021), based on a simple mean annual and monthly isotope mass balance model, were in the same range of around 600 mm/year compared to the STARRtropics simulations.However, their Transp/ET ratios up to 0.8 were also higher than the model results here. Despite differences and method-dependent uncertainties surrounding the absolute values of ET partitioning, all estimates byCorrea et al (2020) andIraheta et al (2021) and measurements byAparecido et al (2016) generally were consistent with our large-scale STARRtropics results within the uncertainty ranges of the different models and measurements(Coenders-Gerrits et al, 2014).…”

“…Available point‐scale measurements (soil and groundwater isotope composition) and modelled estimates (MRT, Transp/AET) were used for an independent model evaluation. The latter estimates were independently compared against measured groundwater isotopes from Sánchez‐Murillo and Birkel (2016), flux tracking transit time estimates by Correa et al (2020) and Birkel et al (2021), measured transpiration by Aparecido et al (2016), and independently modelled transpiration estimates by Iraheta et al (2021).…”

Tracer‐aided modelling reveals quick runoff generation and young streamflow ages in a tropical rainforest catchment

“…3. water partitioning under a changing climate (Correa et al, 2020;Iraheta et al, 2021;Sohel et al, 2021).…”

Evapotranspiration partitioning based on field‐stable oxygen isotope observations for an urban locust forest land

“…Among the various approaches mentioned above, the stable water isotope method distinguishes ET based on the concept that E and T have different isotopic compositions. (e.g., Aron et al, 2020;Dubbert et al, 2013Dubbert et al, , 2014Iraheta et al, 2021;Rothfuss et al, 2010;Wang et al, 2010;Williams et al, 2004;Wu et al, 2017;Wu et al, 2021). Soil evaporation is more fractionated in comparison to vegetation transpiration at equal initial isotopic boundary conditions (Rothfuss et al, 2020;Wu et al, 2017).…”