Xu et al. (2025) Controls on actual evapotranspiration in wetland ecosystems across different climates

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2025
• Date: 2025-12-09
• Authors: Kang Xu, Qiong Han, Tiejun Wang
• DOI: 10.1016/j.ejrh.2025.103023

Research Groups

• Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin, PR China
• Tianjin Bohai Rim Coastal Earth Critical Zone National Observation and Research Station, Tianjin University, Tianjin, PR China
• Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Tianjin University, Tianjin, PR China

Short Summary

This study investigated the controls on actual evapotranspiration (ETa) in 40 wetland ecosystems across temperate, cold, and polar climates, revealing that while incoming shortwave radiation is a common driver, other controls like leaf area index and hydrological regimes vary significantly with climate type and timescale. The research identified four distinct response patterns of daily ETa to vegetation dynamics, enhancing understanding of wetland ETa responses to environmental changes.

Objective

• To investigate the spatial variations of wetland ETa under different climates.
• To compare the relative impacts of environmental variables on mean annual and daily ETa dynamics across the wetlands.
• To examine the roles of vegetation in regulating wetland ETa over diverse climates.

Study Configuration

• Spatial Scale: 40 wetland sites located in mid- to high-latitude regions of the Northern Hemisphere, classified into temperate, cold, and polar climate zones.
• Temporal Scale: Long-term observations, with data spanning at least one year per site (average of 623 days in growing seasons). Analysis conducted for both mean annual and daily timescales, with daily data primarily from the growing season (May to September) and mean annual data from the entire year.

Methodology and Data

• Models used:
  • Boosted Regression Tree (BRT) for quantifying relative influence of predictor variables on daily ETa.
  • Dual-source modification of the Penman-Monteith equation for ETa partitioning into evaporation (Ev) and transpiration (Tr).
  • Generalized Additive Models (GAMs) for analyzing responses of ETa and its partitioning to Leaf Area Index (LAI).
  • Budyko framework for characterizing energy and water constraints.
  • K¨oppen-Geiger climate classification system.
• Data sources:
  • FLUXNET-CH4 Community Product and FLUXNET 2015 Datasets (Tier 1) for daily and half-hourly eddy covariance (EC) data, including latent heat flux (LE), sensible heat flux (H), incoming shortwave radiation (SW_IN), gross primary productivity (GPP), net radiation (Rn), precipitation (P), air temperature (TA), relative humidity (RH), vapor pressure deficit (VPD), wind speed (WS), and water table depth (WTD).
  • MODIS MCD15A3H dataset (version 6, 4-day and 500-meter resolution) for Leaf Area Index (LAI).
  • ERA-Interim reanalysis data (used for gap-filling in FLUXNET data processing).

Main Results

• Mean annual ETa exhibited a significant climate gradient, with the highest rates in temperate wetlands (0.59–2.01 m/year) and the lowest in polar wetlands (0.08–0.33 m/year).
• Incoming shortwave radiation (SW_IN) was a common dominant driving force for ETa across all climates.
• Other controls on ETa varied by climate: LAI strongly contributed to ETa in cold and polar environments, while hydrological regimes (WTD and inundation duration) positively affected ETa in temperate wetlands. VPD also co-controlled ETa in polar wetlands.
• At the daily timescale, the effects of LAI in cold wetlands and WTD in temperate wetlands on ETa weakened, attributed to complex interactions between water regimes, vegetation, and water management.
• Boosted Regression Tree (BRT) analysis showed that SW_IN (average 47.1% contribution) and VPD (average 30.8% contribution) were the dominant drivers of daily ETa across the 40 wetlands.
• Four distinct response patterns of daily ETa to LAI were identified:
  1. A strong, approaching-linear positive relationship, common in polar wetlands without clear inundation.
  2. A significant nonlinear relationship (linear below a critical LAI threshold, then saturated or decreasing), mostly found in non-inundated cold wetlands and some inundated temperate sites.
  3. An initial decrease followed by leveling off or increase, observed in a few wetlands (e.g., polar SJ-Adv, cold FI-Sii).
  4. No significant response, prevalent in inundated cold and most temperate wetlands.
• In wetlands with no clear inundation or very high vegetation coverage, an increase in LAI was more likely to increase ETa.
• Long-term climatic factors (mean annual air temperature and precipitation) and LAI indirectly controlled site-level ETa by influencing the contributions of LAI to daily ETa, with vegetation's role being more pronounced in extreme environments (e.g., polar sites with constrained energy inputs and sparse vegetation).

Contributions

• Enhances the understanding of how wetland ETa responds to diverse environments across different climate types under climate change and human activities.
• Stresses the distinct roles of wetland vegetation in regulating ETa across various climates.
• Improves the consideration of wetland hydrological processes within land surface models for simulating surface-atmosphere interactions under future environmental changes.
• Provides insights into how climate change might alter daily LAI-ETa relationships.
• Addresses the long-standing debate on whether increased vegetation cover leads to increased wetland ETa, concluding it depends on inundation and vegetation coverage.

Funding

• National Natural Science Foundation of China (42494824)
• Tianjin Municipal Science and Technology Bureau (24ZYJDJC00340)
• Natural Science Foundation of Tianjin (24JCQNJC01760)
• Tianjin University (financial support for Qiong Han)

Citation

@article{Xu2025Controls,
  author = {Xu, Kang and Han, Qiong and Wang, Tiejun},
  title = {Controls on actual evapotranspiration in wetland ecosystems across different climates},
  journal = {Journal of Hydrology Regional Studies},
  year = {2025},
  doi = {10.1016/j.ejrh.2025.103023},
  url = {https://doi.org/10.1016/j.ejrh.2025.103023}
}