Insights into evapotranspiration partitioning based on hydrological observations using the generalized proportionality hypothesis

Abstract. Evapotranspiration (ET) comprises transpiration, soil evaporation, and interception. The partitioning of ET is challenging due to the lack of direct measurements and uncertainty of existing ET partitioning methods. We propose a novel method to estimate long-term mean transpiration to evapotranspiration (T/ET) ratios based on the generalized proportionality hypothesis using long-term mean hydrological observations at the watershed scale. We tested the method using 648 watersheds in the United States classified into six vegetation types. We mitigated impacts of the variability associated with different PET data products by rescaling their original PET values using the product ET/PET ratios in combination with the observed ET calculated from watershed water balance. With PET thus rescaled, our method produced consistent T/ET across six widely used PET products. Shrubs (0.38) and grasslands (0.33) showed lower mean T/ET than croplands (0.46) and forests (respectively 0.73, 0.55, and 0.68 for evergreen needleleaf, deciduous broadleaf, and mixed forests). T/ET showed significant dependence on aridity, leaf area index, and other hydrological and environmental conditions. Using T/ET estimates, we calculated transpiration to precipitation ratios (T/P) ratios and revealed a bell-shaped curve at the watershed scale, which conformed to the bell-shaped relationship with the aridity index (AI) observed at the field scale (Good et al., 2017). This relationship peaked at a T/P between 0.5 and 0.6, corresponding to an AI between 2 and 3 depending on the PET dataset used. These results strengthen our understanding of the interactions between plants and water and provide a new perspective on a long-standing challenge for hydrology and ecosystem science.