Unveiling the Impact of Potential Evapotranspiration Method Selection on Trends in Hydrological Cycle Components Across Europe

Abstract. Hydrological models are essential tools for assessing and predicting changes in the hydrological cycle, offering detailed quantification of components like runoff (Q), total water storage (TWS), and actual evapotranspiration (AET). Precipitation (PRE) and potential evapotranspiration (PET) are the major required drivers for modeling these components. In modeling, the linkage of PRE to changes in these cycle components is well understood compared to PET. Here, we focus on the changes in PET and their influence on hydrological cycle components (AET, Q, and TWS). We consider 12 distinct PET methods from three different categories (temperature-based, radiation-based, and combination type) across 553 European catchments. The mesoscale Hydrological Model (mHM) was used to simulate 40 years of hydrological components, with a total of 6 636 mHM runs. Comprehensive trend analysis and data concurrence index (DCI) based on trend direction were applied to three different catchment categories (energy-limited, water-limited and mixed depending on PET method) to assess changes in PET and its influence on AET, Q, and TWS. PET methods exhibit diverse annual and seasonal trends across catchment categories for PET, AET, Q, and TWS. While PET demonstrate strong agreement in trend directions, the trend magnitudes vary depending on the choice of PET method. The findings reveal that the Jensen-Haise method produces the highest trends for PET on both annual and seasonal scales (summer, spring, and autumn), whereas no single PET method consistently represents the lowest trend. AET trends are similar to those of PET but are lower in trend magnitude at annual scale, while seasonally, only energy-limited catchments show a trend pattern similar to PET. Across all PET methods, there is strong agreement in trend direction, except during the winter season. For the majority of European catchments, Q and TWS show strong agreement among different methods, either positive or negative. In the annual trend, the summer season largely contributes to PET. For AET, summer season largely contributes to the annual trend only in energy-limited and water-limited catchments. Overall, studies focusing on the directional changes in the hydrological cycle or its components indicate that PET methods have a limited impact. However, when quantifying changes in hydrological cycle components, the choice of PET method becomes crucial. Therefore, selecting the appropriate PET method is crucial for studies on AET, Q, and TWS.