Multi-decadal fluctuations in root zone storage capacity through vegetation adaptation to hydro-climatic variability has minor effects on the hydrological response in the Neckar basin, Germany
Authors: Wang Siyuan , Hrachowitz Markus , Schoups Gerrit 
Published: March 2024

Abstract. Climatic variability can considerably affect the catchment-scale root zone storage capacity (Sumax) which is a critical factor regulating latent heat fluxes and thus the moisture exchange between land and atmosphere as well as the hydrological response and biogeochemical processes in terrestrial hydrological systems. However, direct quantification of changes in Sumax over long time periods and the mechanistic drivers thereof at the catchment-scale are missing so far. As a consequence, it remains unclear how climatic variability, such as precipitation regime or canopy water demand, affects Sumax and how fluctuations in Sumax may influence the partitioning of water fluxes and therefore, also affect the hydrological response at the catchment-scale. The objectives of this study in the Upper Neckar river basin in Germany are therefore to provide a detailed analysis of multi-decadal changes in Sumax that can be observed as a result of changing climatic conditions over a 70-year period and how this further affects hydrological dynamics. More specifically, we test the hypotheses that (1) Sumax significantly changes over multiple decades reflecting vegetation adaptation to climate variability, (2) changes in Sumax are a dominant control on the evaporative index IE = EA/P and thus on the partitioning of water into drainage and evaporative fluxes as described by deviations ΔIE from parametric Budyko curves over time, (3) changes in Sumax also affect short term hydrological response dynamics and a time-dynamic implementation of Sumax as parameter in a hydrological model can improve the performance of a hydrological model. In this study, based on long-term daily hydrological records (1953–2022) and a stepwise approach over multiple consecutive 20-year periods, we found that variability in hydroclimatic conditions, with aridity index IA (i.e. EP/P) ranging between ~ 0.9 and 1.1 over the study period was accompanied by deviations ΔIE between -0.02 and 0.01 from the expected IE inferred from the long-term parametric Budyko curve. Similarly, fluctuations in Sumax, ranging between ~ 95 and 115 mm or 20 %, were observed over the same time period. While uncorrelated with long-term mean precipitation and potential evaporation, it was shown that the magnitude of Sumax is controlled by the ratio of winter or summer precipitation (p < 0.05). In other words, Sumax in the study region does not depend on the overall wetness condition as for example expressed by IA, but rather on how water supply by precipitation is distributed over the year. However, fluctuations in Sumax were found to be uncorrelated with observed changes in ΔIE. Consequently, replacing a long-term average, time-invariant estimate of Sumax with a time-variable, dynamically changing formulation of that parameter in a hydrological model did not result in an improved representation of the long-term partitioning of water fluxes, as expressed by IE (and fluctuations ΔIE thereof), nor in an improved representation of the shorter-term response dynamics. Overall, this study provides quantitative mechanistic evidence that Sumax significantly changes over multiple decades reflecting vegetation adaptation to climatic variability. However, this temporal evolution of Sumax cannot explain long-term fluctuations in the partitioning of water (and thus latent heat) fluxes as expressed by deviations ΔIE from the parametric Budyko curve over multiple time periods with different climatic conditions. Similarly, it does not have any significant effects on shorter term hydrological response characteristics of the upper Neckar catchment. This further suggests that accounting for temporal evolution of Sumax with a time-variable formulation of that parameter in a hydrological model does not improve its ability to reproduce the hydrological response and may therefore be of minor importance to predict the effects of a changing climate on the hydrological response in the study region over the next decades to come.