Groundwater recharge estimation and uncertainty analysis for various soils in Austria

Groundwater recharge is a key component of the hydrologic cycle, yet its direct measurement is difficult. An alternative is its inverse estimation with a combination of physically based numerical models and soil water observations. However, simulated water fluxes are affected by model predictive uncertainty which are often not considered when simulating and predicting groundwater recharge rates. Therefore, the objective of this study was to use of long-term soil water content measurements at 14 locations from the Austrian soil water monitoring program to quantify and compare local, potential groundwater recharge rates, their temporal variability, and predict future changes in potential groundwater recharge for different climate scenarios. Observations were coupled with a Bayesian probabilistic framework to calibrate the model HYDRUS-1D and assess the effect of model predictive uncertainty on simulated recharge fluxes. Estimated annual potential recharge rates ranged from 44 mm/a to 1319 mm/a with a relative uncertainty (95% interquantile range/median) in the estimation between 1-39%. Recharge rates decreased longitudinally, with high rates and lower seasonality at western sites and low rates with high seasonality and extended periods without recharge at the southeastern and eastern sites of Austria.  Higher recharge rates and lower actual evapotranspiration were related to sandy soils. Future recharge predictions at the median remained close to past rates, except for sites in the East, where they increased. In general, predictions varied drastically between different climate models and emission scenarios, especially for the summer months. Across all projections, an increase in winter recharge at the western sites was predicted, due to higher temperatures with less snow accumulation and/or higher amounts of winter precipitation, followed by decreasing recharge rates in spring. Decreasing tendencies in groundwater recharge were stronger at western sites and at higher altitudes, with longer drought periods lasting until later within the calendar year. Uncertainty in recharge prediction was largely dominated by the difference in climate projections, only at the dry sites in the East and for shorter time periods, soil hydraulic parameter uncertainties played a role.