The effects of future climate-induced adaptation of root zone storage capacity on modelled streamflow dynamics

Abstract

Hydrological models are often used to evaluate future changes in streamflow. Despite the strong aware- ness of non-stationarity in hydrological system characteristics, model parameters are often assumed stationary and obtained through calibration on past conditions. The representation of system change in hydrological models is challenging, as a lot of uncertainty abounds on changes in future climate and ecosystems. However, it is shown that ecosystems co-evolve with the prevailing climate conditions. There is increasing evidence that vegetation adapts its root zone storage capacity - considered as a key parameter in any hydrological model - corresponding to moisture deficits in the root zone. This is the main assumption underlying the water balance method. In combination with long-term water budget estimates from the Budyko framework, this method has the potential to meaningfully describe future climate-vegetation interactions within the context of process-based hydrological models. Accordingly, this study provides an exploratory analysis for six catchments in the Austrian Alps to in- vestigate future changes in root zone storage capacity and their impact on modelled streamflow in the past and under two emission scenarios in the future. Our findings show that, although parameter ranges of the root zone storage capacity significantly narrow-down for climate-based estimates, modelling performance on past streamflow is similar when using the calibrated and climate-based parameter sets. Following climate projections from 14 cli- mate models, adaptive climate-based parameter estimates are predicted to increase by 10-100% in all catchments in the future. However, little to no dissimilarity in modelled future streamflow is found when adaptation in root zone storage capacity is included in the hydrological model. Modelled differences in annual mean, maximum, and minimum flows remain within 5%, with slight increases for monthly streamflow and runoff coefficients. Thereby, little to no evidence is found that time-dynamic representation of root zone storage capacity significantly alters modelled future streamflow and suggests limited necessity for its inclusion in hydrological models to obtain rea- sonable descriptions of future streamflow in the investigated Alpine catchments.