Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2026
Publication Date: 7/7/2026
Citation: Wagle, P., Raghav, P., Kumar, M., Scanlon, T., Northup, B.K., Moffet, C., Xiao, X., Gunter, S.A. 2026. Partitioning evapotranspiration in C3-C4 mixed tallgrass prairies: An inter-comparison of five methods and evaluation of seasonal parameterization. Agricultural Water Management. 333. Article 110616. https://doi.org/10.1016/j.agwat.2026.110616.
DOI: https://doi.org/10.1016/j.agwat.2026.110616

Interpretive Summary: Various evapotranspiration (ET) partitioning approaches have been proposed in recent decades. Given the similarity between stomatal and non-stomatal exchanges, different methods are applied to partition ET and CO2 fluxes using high-frequency eddy covariance (EC) turbulence data. This study evaluated transpiration (T):ET dynamics for five partitioning methods, namely Flux Variance Similarity (FVS), Modified Relaxed Eddy Accumulation (MREA), Conditional Eddy Covariance (CEC), Conditional Eddy Accumulation (CEA), and a water- use efficiency- integrated modification of CEC (CECw), using 10 Hz EC data from two differently managed (grazing and hay harvest) C3- C4 mixed tallgrass prairies in Central Oklahoma, USA. Furthermore, three water-use efficiency algorithms (constant value, constant ratio, and optimum) for the FVS method were evaluated, along with sensitivity analysis and temporally dynamic (based on C3: C4 ratios) parameterization of intercellular CO2 concentration (ci) for the constant value and constant ratio methods. The study also tested whether ci parameterization based on temporally varying C3: C4 ratios, as compared to the C4 parameterization throughout the year for the constant value and constant ratio methods, yielded similar results. MREA and CEC generally produced higher T:ET ratios than the other methods. Numerous half- hourly T:ET ratios were equal to one (i. e., evaporation, E = 0) for CEC and MREA, whereas the minimum E was around 10-15% of ET for FVS. This resulted in higher T estimates (by 10-20%) for MREA and CEC at seasonal and annual scales. The results suggest that a C4 parameterization can be used year- round to derive T:ET ratios for the constant value and constant ratio approaches at both annual and seasonal scales in C4-dominated grasslands, as this simple, data-parsimonious approach produces results similar to those obtained using a more complex dynamic C3- C4 parameterization. However, using a C4 parameterization led to 6-7% larger T:ET ratios during the non- growing season. We observed significant variations in T:ET ratios across the methods when stomatal ('qc <- 0. 0.8) and non-stomatal ('qc > 0. 0.8) fluxes were dominant. When there was a balance between stomatal and non-stomatal exchange ('qc between -0.1 and 0.1), these methods agreed more closely (with an average T estimate of 48%). Validation against lysimeter-measured E showed that FVS and CECw optimum approaches performed best (r = 0.84–0.85; RMSE = 0.23–0.25 mm d-1), while other methods deviated substantially. This study provides new insights into the performance of different high-frequency EC data-based ET partitioning methods in mixed C3-C4 systems.

Technical Abstract: Accurate estimates of transpiration (T) remain particularly challenging in mixed C3-C4 ecosystems, where the relative dominance of species and their physiological characteristics shifts dynamically throughout the growing season. Some methods for evapotranspiration (ET) partitioning, such as Flux Variance Similarity (FVS), typically require an explicit specification of either C3 or C4 photosynthetic pathways to parameterize intercellular carbon dioxide concentrations (ci). Other methods, such as Modified Relaxed Eddy Accumulation (MREA), Conditional Eddy Covariance (CEC), Conditional Eddy Accumulation (CEA), and the FVS optimum approach, do not require prior ci approximations or designation of C3 and C4 vegetation. This study evaluated and inter-compared five ET partitioning methods: FVS, MREA, CEA, CEC, and a water-use efficiency-integrated modification of CEC (CECw) within two differently managed (grazing and hay harvest) C3-C4 mixed tallgrass prairies in Central Oklahoma, USA. Specifically, we investigated how partitioning outputs varied when using a static, year-round C4 parameterization compared to a seasonally dynamic C3-C4 framework. Results indicated that T:ET ratios from FVS methods using constant ci values or ci/ca ratios were similar across seasonal and annual scales, regardless of whether a year-round C4 or temporally dynamic C3-C4 parameterization was applied. During peak growth, MREA and CEC produced the highest T:ET ratios (0.93–0.95), whereas FVS and CEA yielded more moderate values (<0.75), and CECw provided the lowest estimates (0.56–0.62). Consequently, MREA and CEC estimated T 10–20% higher than the biophysically reasonable estimates from FVS and CEA. Methodological discrepancies were most pronounced during periods when either stomatal or non-stomatal fluxes dominated, with closer agreement occurring when these exchanges were nearly equivalent. Validation against lysimeter measurements showed that FVS and CECw optimum approaches performed best (r = 0.84–0.85; RMSE = 0.23–0.25 mm d-1), while other methods deviated substantially. As the first intercomparison of five partitioning methods in mixed C3-C4 Tallgrass prairies, this study demonstrates that the choice of method significantly influences the resulting ecosystem water budget.