Li et al. (2026) Estimating the long-term daily evapotranspiration in the source region of the Yangtze River based on a universal trapezoid method

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-01-12
• Authors: Yan Li, Lin Zhao, Maoqi Lun, Feinan Xu, Lingxiao Wang, Chong Wang
• DOI: 10.1016/j.ejrh.2026.103119

Research Groups

• School of Geographical Sciences, Nanjing University of Information Science and Technology, Nanjing, China
• Cryosphere Research Station on Qinghai-Xizang Plateau, State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China
• The Academy of Digital China (Fujian), Fuzhou University, Fuzhou, China
• Key Laboratory of Cryospheric Science and Frozen Soil Engineering, Key Laboratory of Remote Sensing of Gansu Province, Heihe Remote Sensing Experimental Research Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China

Short Summary

This study developed a modified universal trapezoid method (UniTM_asc) to produce a reliable 1 km daily evapotranspiration (ET) dataset for the Source Region of the Yangtze River (SRYR) from 2001 to 2018. The research found a mean annual ET of 516 mm/yr, with a dominant decreasing trend in 71.1% of the SRYR primarily driven by reduced solar radiation, while increases in 28.9% are linked to vegetation greening and climate warming.

Objective

• To modify the Universal Trapezoid Method (UniTM) by integrating an aerodynamic resistance (ra) formulation with atmospheric stability corrections.
• To produce a daily evapotranspiration (ET) dataset at 1 km spatial resolution for the SRYR from 2001 to 2018 using the modified UniTM (UniTM_asc) with MODIS and ancillary meteorological data.
• To assess the accuracy of the produced ET dataset by comparing it with eddy-covariance system measurements, a catchment water balance model, and existing ET datasets (MOD16-STM and PML_V2).
• To analyze the spatiotemporal variability of ET and its dominant drivers in the SRYR.

Study Configuration

• Spatial Scale: Source Region of the Yangtze River (SRYR), approximately 1.4 × 10^5 km^2, spanning 90°33′–97°20′ E and 32°26′–35°43′ N. Data resolution: 1 km.
• Temporal Scale: Daily, from 2001 to 2018.

Methodology and Data

• Models used:
  • Modified Universal Trapezoid Method (UniTMasc) with atmospheric stability corrections.
  • Original Universal Trapezoid Method (UniTM) for comparison.
  • Catchment water balance model (for validation).
  • Penman-Monteith-Leuning Version 2 (PMLV2) (for cross-validation).
  • MOD16 soil texture model (MOD16-STM) (for cross-validation).
  • Theil-Sen estimator and Mann-Kendall (MK) test (for trend analysis).
  • Partial correlation analysis (for dominant factor analysis).
  • Extended Fourier Amplitude Sensitivity Test (EFAST) (for sensitivity analysis).
• Data sources:
  • Satellite: MODIS products (MOD06_L2 for cloud emissivity, cloud temperature, fractional cloud coverage, solar zenith angle; MCD43A3 for land surface albedo; MOD13A2 for Normalized Difference Vegetation Index (NDVI)). TRIMS LST-TP V2 (daily 1 km all-weather land surface temperature). GRACE mass concentration (mascon) solutions (monthly total water storage anomaly (TWSA)).
  • Observation/Reanalysis: China Meteorological Forcing Dataset (CMFD) (air temperature, wind speed, specific humidity, near surface pressure at 0.1° resolution). Eddy-covariance (EC) system flux and Automatic Weather Station (AWS) measurements (ET at Tangula (TGL) and Xidatan (XDT) sites). TPHiPr (long-term high-resolution precipitation dataset at 1/30° resolution). Zhimenda hydrological station (annual basin runoff depths).

Main Results

• The modified UniTM (UniTMasc) showed improved accuracy in ET estimation compared to the original UniTM, MOD16-STM, and PMLV2, validated against EC system measurements (e.g., RMSE ~0.75 mm/d at TGL site) and catchment water balance (basin-averaged annual ET of 515 mm/yr vs. 519 mm/yr for water balance ETwb).
• The multiyear (2001–2018) mean annual ET in the SRYR was 516 ± 67 mm, exhibiting a spatial decrease from southeast to northwest.
• ET showed clear seasonal variations, with the highest values in summer (247 ± 33 mm), followed by spring (135 ± 15 mm), autumn (100 ± 15 mm), and lowest in winter (31 ± 6 mm).
• Spatiotemporal analysis of annual ET trends (2001–2018) revealed that 71.1% of the SRYR experienced a decreasing trend (mean rate: -0.81 mm/yr), while 28.9% showed an increasing trend (mean rate: 0.61 mm/yr).
• Dominant drivers of ET variation were maximum vegetation fraction (fc,max) (42.8% of area), solar radiation (Rs) (23.8%), and precipitation (Prec) (14.3%).
• Decreased annual ET was primarily attributed to reductions in solar radiation, while increased annual ET was associated with vegetation greening and climate warming.

Contributions

• Developed and validated UniTM_asc, a modified universal trapezoid method that incorporates atmospheric stability corrections into aerodynamic resistance parameterization, leading to more accurate ET estimations in complex, sparsely-gauged high-mountain regions.
• Generated a high-resolution (1 km daily) and long-term (2001–2018) ET dataset for the SRYR, addressing existing inconsistencies in ET product magnitudes for the region.
• Provided a comprehensive spatiotemporal analysis of ET dynamics and identified the dominant climatic and biological drivers in the SRYR, attributing ET decreases to reduced solar radiation and increases to vegetation greening and warming.
• Demonstrated the robustness and transferability of UniTM_asc across diverse hydro-climatic and vegetation regimes, including validation in the Heihe River Basin.

Funding

• Second Tibetan Plateau Scientific Expedition and Research (STEP) program (No. 2019QZKK0201)
• Natural Science Foundation of the Jiangsu Province (No. BK20230429)
• National Natural Science Foundation of China (No. 42301157 and 42101411)

Citation

@article{Li2026Estimating,
  author = {Li, Yan and Zhao, Lin and Lun, Maoqi and Xu, Feinan and Wang, Lingxiao and Wang, Chong},
  title = {Estimating the long-term daily evapotranspiration in the source region of the Yangtze River based on a universal trapezoid method},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2026.103119},
  url = {https://doi.org/10.1016/j.ejrh.2026.103119}
}