Seasonal variability in evapotranspiration partitioning in Syngonium podophyllum plant using dual stable isotopes (δ²H and δ¹⁸O) at Roorkee, Uttarakhand

This study investigated the seasonal partitioning of evapotranspiration (ET) using dual stable isotopes (δ²H and δ¹⁸O) for Syngonium podophyllum cultivated at the NIH Roorkee campus, Uttarakhand, India, during 2024. A total of 416 water samples were collected, comprising 93 soil evaporation, 82 leaf transpiration, 205 air moisture, and 36 rainfall samples, covering pre-monsoon, monsoon, post-monsoon, and winter seasons. Sampling was conducted every alternate day between 12:00–14:00 h to capture peak ET activity. The isotopic compositions of evapotranspiration ( $$\:{{\updelta\:}}_{\text{E}\text{T}}$$ ), evaporation ( $$\:{{\updelta\:}}_{\text{E}}$$ ), and transpiration ( $$\:{{\updelta\:}}_{\text{T}}$$ ) were estimated using the Keeling Plot, Craig–Gordon, and steady-state models, respectively, and partitioned through a two-endmember isotopic mass balance approach. Results revealed pronounced seasonal variability in the transpiration fraction (T/ET) for Syngonium podophyllum, with mean values ranging from 0.52 during monsoon to 0.95 in post-monsoon, and an annual mean of 0.76 ± 0.15. Transpiration dominated total ET during the pre-monsoon (T/ET = 0.65–0.69), post-monsoon (0.93–0.95), and winter (0.84–0.89) seasons, while evaporation contributions increased substantially during the monsoon (0.51–0.54), coinciding with high soil moisture and rainfall events that suppressed transpiration. These findings demonstrate that transpiration by Syngonium podophyllum was the major component of ET for most of the year, except during the monsoon when direct soil evaporation prevailed. The results highlight how hydroclimatic variability regulates plant-scale water fluxes, and emphasize that irrigation management in humid subtropical regions should aim to minimize soil evaporation losses during wet periods and enhance transpiration efficiency during dry seasons. The isotope-based framework also provides a robust, non-invasive tool for quantifying plant–soil–atmosphere water exchanges.