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

Abstract. Climatic variability can considerably affect the catchment-scale root zone storage capacity (S_umax) 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 S_umax 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 S_umax and how fluctuations in S_umax 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 S_umax 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) S_umax significantly changes over multiple decades reflecting vegetation adaptation to climate variability, (2) changes in S_umax are a dominant control on the evaporative index I_E = E_A/P and thus on the partitioning of water into drainage and evaporative fluxes as described by deviations ΔI_E from parametric Budyko curves over time, (3) changes in S_umax also affect short term hydrological response dynamics and a time-dynamic implementation of S_umax 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 I_A (i.e. E_P/P) ranging between ~ 0.9 and 1.1 over the study period was accompanied by deviations ΔI_E between -0.02 and 0.01 from the expected I_E inferred from the long-term parametric Budyko curve. Similarly, fluctuations in S_umax, 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 S_umax is controlled by the ratio of winter or summer precipitation (p < 0.05). In other words, S_umax in the study region does not depend on the overall wetness condition as for example expressed by I_A, but rather on how water supply by precipitation is distributed over the year. However, fluctuations in S_umax were found to be uncorrelated with observed changes in ΔI_E. Consequently, replacing a long-term average, time-invariant estimate of S_umax 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 I_E (and fluctuations ΔI_E thereof), nor in an improved representation of the shorter-term response dynamics.

Overall, this study provides quantitative mechanistic evidence that S_umax significantly changes over multiple decades reflecting vegetation adaptation to climatic variability. However, this temporal evolution of S_umax cannot explain long-term fluctuations in the partitioning of water (and thus latent heat) fluxes as expressed by deviations ΔI_E 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 S_umax 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.