Morava–Dyje floodplain forests: A comprehensive analysis of climate-driven changes in current and future water balance

The confluence of the Dyje (Thaya) and Morava rivers hosts one of the largest floodplain forest complexes in the Danube basin (~15,000 ha). This area has been intensively studied over past decades, and its ecological importance was further recognised by the designation of a UNESCO Biosphere Reserve and the establishment of a protected landscape area in 2025. The vitality and resilience of the forests as a system are closely linked to water availability in the Dyje headwater catchments, as well as to water management at the Nové Mlýny reservoir system. These shallow reservoirs, located upstream, play a key role in controlled flooding and groundwater replenishment within the floodplain.

This contribution builds upon our previous research conducted across the broader Dyje RB, which identified a statistically significant decreasing trend in annual runoff and a concurrent increasing trend in actual evapotranspiration over the period 1980–2020. No significant long-term change in total annual precipitation was detected. Instead, changes in runoff were primarily attributed to rising air temperatures, altered snow accumulation and melt dynamics, and a shift in seasonal water availability. In particular, reduced snow storage and earlier snowmelt during winter, combined with increased temperatures in spring (MAM) and autumn (SON), resulted in a prolongation of the vegetation period. This extended growing season was identified as a major driver of increased evapotranspiration and, consequently, declining runoff.

To better constrain evapotranspiration as the dominant outgoing flux of the basin water balance, additional monitoring was conducted on neighbouring water bodies. A custom floating evaporimeter platform was equipped with Li-COR Li-710 infrared gas analysers, custom (Class A derived) evaporation pans, and meteorological stations. The model input meteorological variables were derived from a gridded high resolution national data set (~500 m) and accompanied with bore hole water level measurements at eight locations. Climate change scenarios were derived using the Advanced Delta Change (ADC) method, which modifies observed time series such that changes in the mean and variability correspond to those simulated by climate models. At the daily time step, ADC explicitly accounts for changes in variability, allowing extremes to evolve differently from average conditions. For precipitation, the method also corrects systematic model biases, whereas temperature is adjusted linearly, ensuring consistency in projected warming signals. A representative ensemble of global climate models (GCMs) was used to propagate climate uncertainty into hydrological projections.

A spatially distributed multimodel framework of varying complexity and process representation was developed to assess hydrological response under present and future climate conditions. Building on this framework, a numerical groundwater flow model has been developed in MODFLOW in order to get a detailed representation of hydraulic conditions and interactions between surface water and groundwater. The modeling framework is designed to allow subsequent coupling with heat transport and solute transport modules in the future, enabling comprehensive assessments of thermal dynamics and contaminant migration and their impacts on the hydrological system and associated ecosystems.

Acknowledgement: This study was supported by the DALIA project n. 101094070, under the call HORIZON-MISS-2021-OCEAN-02 funded by the European Union.