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

Hydrological models play a vital role in evaluating future changes in streamflow. Despite the strong awareness of non-stationarity in hydrological system characteristics, model parameters are typically assumed to be stationary and derived through calibration on past conditions. Integrating the dynamics of system change in these models remains challenging due to uncertainties surrounding changes in future climate and ecosystems.
Nevertheless, studies show that ecosystems evolve in response to prevailing climate conditions. There is increasing evidence that vegetation adjusts its root zone storage capacity – considered a critical parameter in hydrological models – to prevailing hydroclimatic conditions. This adaptation of the root zone to moisture deficits is central to the water balance method. When combined with long-term water budget estimates from the Budyko framework, the water balance method offers a promising approach to describe future climate-vegetation interactions within process-based hydrological models

Our study provides an exploratory analysis of the role of non-stationary hydrological model parameters for six catchments in the Austrian Alps. More specifically, we investigate future changes in the root zone storage and their consequent impact on modeled streamflow. Using the water balance method, we derive climate-based parameter estimates of the root zone storage capacity under historic and projected future climate conditions. These climate-based estimates are then implemented in our hydrological model to assess their consequent impact on modeled past and future streamflow.
Our findings show that climate-based parameter estimations significantly narrow the parameter ranges linked to root zone storage capacity. This stands in contrast to the broader ranges obtained solely through calibration. Moreover, using projections from 14 climate models, our findings indicate a substantial increase in the root zone storage capacity parameters across all catchments in the future, ranging from +10% to +100%. Despite these alterations, the model performance remains relatively consistent when evaluating past streamflow, independent of using calibrated or climate-based estimations for the root zone storage capacity parameter. Additionally, no significant differences are found when modeling future streamflow when including future climate-induced adaptation of the root zone storage capacity in the hydrological model. Variations in annual mean, maximum, and minimum flows remain within a 5% range, with slight increases found for monthly streamflow and runoff coefficients.

In summary, our research shows that although climate-induced changes in root zone storage capacity occur, they do not notably affect future streamflow projections in the Alpine catchments under study. This suggests that incorporating a dynamic representation of the root zone storage capacity parameter may not be crucial for modeling streamflow in humid and energy-limited catchments. However, our observations indicate relatively larger changes in root zone storage capacity within the less humid catchments studied, corresponding to higher variations in modeled future streamflow. This points to a potential higher significance of dynamically representing root zone characteristics in arid regions and underscores the necessity for further research in these areas.