Li et al. (2025) Magnitude and impacts of non-rainfall water inputs and nocturnal evapotranspiration on temperate grassland ecosystems

Identification

• Journal: Journal of Hydrology
• Year: 2025
• Date: 2025-11-20
• Authors: Yafei Li, Andreas Riedl, Nina Buchmann, Werner Eugster
• DOI: 10.1016/j.jhydrol.2025.134632

Research Groups

• Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland
• Agroecology and Environment, Agroscope, Zürich, Switzerland

Short Summary

This study investigated the spatial variability and environmental drivers of non-rainfall water (NRW) inputs and nocturnal evapotranspiration (ETnight) across nine temperate grasslands, revealing that while NRW inputs were low and did not benefit early morning CO2 exchange, increasing ETnight losses pose additional challenges for grasslands in the future.

Objective

• To quantify high-temporal resolution non-rainfall water (NRW) inputs and nocturnal evapotranspiration (ETnight) over wide spatial and elevational gradients across Switzerland and the Italian-Swiss border during rain-free periods.
• To identify environmental drivers of NRW inputs or ETnight.
• To assess the effects of NRW inputs and ETnight losses on net ecosystem CO2 exchange (NEE) of temperate grasslands.

Study Configuration

• Spatial Scale: Nine grassland sites across Switzerland and the Italian-Swiss border, covering an elevational range from 400 meters to 2000 meters above sea level (m a.s.l.). Sites were located in the Swiss Plateau, Southern Alps, Pre-Alps, and Alps geographical regions.
• Temporal Scale: Measurements were carried out between May 2019 and December 2020, with varying study durations across sites. A specific comparison period from 28 August 2019 to 1 November 2019 was used for cross-site analysis.

Methodology and Data

• Models used:
  • Random Forest (RF) regressor models for identifying environmental drivers.
  • Ordinary least square regressions for assessing relationships between NEE, air temperature, and photosynthetic photon flux density.
  • Pairwise t-test for comparing NEE differences.
  • Stefan–Boltzmann’s law for calculating vegetation surface temperature.
  • Magnus equation for determining dewpoint temperature.
• Data sources:
  • Observation:
    • Highly accurate micro-lysimeters (25 cm diameter × 25 cm depth, ±0.005 mm accuracy) for quantifying NRW inputs and ETnight losses.
    • Agrometeorological measurements: air temperature (°C), relative humidity (%), wind speed (m s⁻¹) at 2 m a.g.l., visibility sensor (<1000 m for fog), leaf wetness sensor, soil temperature and moisture sensors (5TM) at 15 cm depth. Data recorded at 1-minute resolution.
    • Eddy-covariance (EC) measurements at three sites (CH-Cha, CH-Fru, CH-Aws) for net ecosystem CO2 exchange (NEE in µmol m⁻² s⁻¹), longwave incoming radiation (LWin in W m⁻²), longwave outgoing radiation (LWout in W m⁻²), and photosynthetic photon flux density (PPFD in µmol m⁻² s⁻¹). EC data measured at 20 Hz, processed to 30-minute averages.
  • Reanalysis/External:
    • Long-term (1981–2020) average air temperature and annual precipitation from nearby MeteoSwiss stations.
    • Total evapotranspiration (ETtotal) during the comparison period derived from nearby MeteoSwiss stations.

Main Results

• Changes in NRW inputs and ETnight were independent of elevation but strongly affected by terrain.
• Average NRW water gains were 0.14 ± 0.10 mm per event, with an average duration of approximately 9 hours per event.
• Average nocturnal water loss by ETnight was 0.24 ± 0.16 mm per night.
• During the comparison period, total NRW gains ranged from 0.12 mm to 6.98 mm, while cumulative ETnight losses ranged from 0.09 mm to 13.39 mm.
• ETnight contributed between 0.1 % and 11.6 % of total evapotranspiration during the comparison period.
• Sites with higher NRW input frequency and amounts generally had lower ETnight occurrence and water loss, and vice versa.
• Environmental drivers for NRW inputs were mainly air temperature changes (ΔTair) and event duration.
• Environmental drivers for ETnight losses were mainly the temperature difference between vegetation surface and dew point (ΔT), soil moisture (SWC), and wind speed (U).
• Net ecosystem CO2 exchange (NEE) in the early morning hours (within the first two hours after sunrise) was significantly lower (indicating less CO2 loss) following NRW input events compared to ETnight nights (e.g., 4.75 µmol m⁻² s⁻¹ lower within the first hour).
• Daytime NEE within two hours after ETnight nights increased with temperature, suggesting higher net ecosystem CO2 losses, whereas NEE after NRW inputs showed no clear relationship with air temperature.

Contributions

• Provided the first high-temporal resolution quantification of non-rainfall water (NRW) inputs and nocturnal evapotranspiration (ETnight) across a wide range of spatial and elevational gradients in central European temperate grasslands using highly accurate micro-lysimeters.
• Identified distinct sets of environmental drivers controlling NRW inputs (air temperature changes, event duration) and ETnight losses (surface-dewpoint temperature difference, soil moisture, wind speed).
• Demonstrated that NRW inputs did not lead to a net benefit for early morning ecosystem CO2 uptake in temperate grasslands, challenging assumptions about their ecological significance in these regions.
• Highlighted the low relevance of NRW inputs in terms of water amounts for temperate grasslands and underscored the increasing future risks posed by higher rates of ETnight due to climate change.

Funding

• No specific projects, programs, or reference codes were listed in the paper text. Funding acquisition was mentioned for Nina Buchmann and Werner Eugster.

Citation

@article{Li2025Magnitude,
  author = {Li, Yafei and Riedl, Andreas and Buchmann, Nina and Eugster, Werner},
  title = {Magnitude and impacts of non-rainfall water inputs and nocturnal evapotranspiration on temperate grassland ecosystems},
  journal = {Journal of Hydrology},
  year = {2025},
  doi = {10.1016/j.jhydrol.2025.134632},
  url = {https://doi.org/10.1016/j.jhydrol.2025.134632}
}