Bruno et al. (2025) Imprints of increases in evapotranspiration on decreases in streamflow during dry periods, a large-sample analysis in Germany

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-09-17
• Authors: Giulia Bruno, Laurent Strohmenger, Doris Duethmann
• DOI: 10.5194/hess-29-4473-2025

Research Groups

• Department of Ecohydrology and Biogeochemistry, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany

Short Summary

This study quantifies the impact of increased evapotranspiration on decreased summer low flows and altered precipitation-streamflow relationships during dry periods in 363 small German catchments from 1970 to 2019, finding that increased evapotranspiration is a significant driver, particularly in more arid regions.

Objective

• To determine if summer low flows decreased in small catchments in Germany between 1970 and 2019.
• To quantify if increases in actual evapotranspiration (E) were a relevant driver of decreases in summer low flows in small catchments in Germany between 1970 and 2019.
• To assess whether, during the multi-year drought in Germany in the early 1990s, the occurrence of changes in the annual precipitation (P) – streamflow (Q) relationship was related to increases in E over past decades.

Study Configuration

• Spatial Scale: 363 small catchments in Germany, each with an area between 50 square kilometers and 1000 square kilometers (median area of 181 square kilometers), without substantial water management influences.
• Temporal Scale: Long-term analysis over 1970–2019. Specific analysis of a multi-year drought event between 1989 and 1993.

Methodology and Data

• Models used:
  • Data-based analysis.
  • Water balance equation (E = P - Q - ΔS) to derive annual evapotranspiration.
  • Hierarchical clustering algorithm with Ward’s criterion for catchment grouping.
  • Sen’s slope estimator and Mann–Kendall test for trend detection.
  • Multiple linear regression for trend attribution and change analysis of the P–Q relationship.
  • Pearson’s correlation coefficient for spatial coherence assessment.
  • Box–Cox transformation for streamflow data in P–Q relationship analysis.
• Data sources:
  • Streamflow (Q) and catchment boundaries: Environment Agencies of the German Federal States, Global Runoff Data Center (GRDC), and Global Streamflow Indices and Metadata Archive (Do et al., 2018a, b).
  • Precipitation (P): Gridded dataset over Germany at 0.11° resolution (SPHEREMAP method), corrected for gauge undercatch (Richter, 1995).
  • Air temperature: E-OBS dataset (v26.0e, Cornes et al., 2018).
  • Catchment attributes (topography, climate, land cover, soil): EU Digital Elevation Model (v1.1., EU-DEM, 2016), Corine 2000 Land Cover data (v20.1, European Environment Agency, 2019), SoilGrids250m dataset (Hengl et al., 2017).

Main Results

• Summer low flows (minimum 7-day streamflow during June-August, 7dQmin, JJA) widely decreased across the catchments from 1970–2019, with a median trend of -3.7 % per decade and significant negative trends in 31 % of catchments.
• Annual evapotranspiration (E) generally increased over the same period, with a median trend of 1.1 % per decade and significant increases in 27 % of catchments.
• Increases in E were a relevant driver of decreases in summer low flows, particularly in the more arid eastern and northern clusters (contributing 35 % to long-term dynamics in the eastern cluster, and showing a spatial correlation coefficient of -0.74 between trends in 7dQmin, JJA and E in the eastern cluster).
• During the multi-year drought between 1989 and 1993, 26 % of the affected catchments experienced a change in the annual P–Q relationship, resulting in a median decrease of 10 % in streamflow compared to the pre-drought relationship.
• Catchments exhibiting this change in the P–Q relationship were characterized by strong underlying increases in E (median trend of 6.1 % per decade), whereas catchments without change showed negligible E trends (median trend of -0.5 % per decade).

Contributions

• Provides an updated and comprehensive assessment of long-term changes in summer low flows (1970–2019) for small, minimally managed catchments in Germany.
• Quantifies the relative importance of long-term variations in evapotranspiration and seasonal precipitation as drivers of summer low flow decreases, highlighting regional differences.
• Demonstrates that increases in evapotranspiration are a significant driver of summer low flow decreases, especially in more arid regions.
• Establishes a link between long-term increases in evapotranspiration and changes in the precipitation-streamflow relationship during multi-year droughts, leading to lower-than-expected streamflow.
• Emphasizes the critical role of monitoring evapotranspiration changes for predicting future streamflow decreases during dry periods, informing water management and climate adaptation strategies.

Funding

No specific funding projects, programs, or reference codes were listed in the paper.

Citation

@article{Bruno2025Imprints,
  author = {Bruno, Giulia and Strohmenger, Laurent and Duethmann, Doris},
  title = {Imprints of increases in evapotranspiration on decreases in streamflow during dry periods, a large-sample analysis in Germany},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-4473-2025},
  url = {https://doi.org/10.5194/hess-29-4473-2025}
}