Projections of streamflow intermittence under climate change in European drying river networks

Abstract. Climate and land use changes, as well as human water use and flow alterations, are causing worldwide shifts in river flow dynamics. During the last decades, low-flows, flow intermittence, and drying have increased in many regions of the world, including Europe. This trend is projected to continue and exacerbate in the future, resulting in more frequent and intense hydrological droughts. However, due to a lack of data and studies on temporary rivers in the past, little is known about the processes governing the development of flow intermittence and drying, their timing and frequency, as well as their long-term evolution under climate change. Moreover, understanding the impact of climate change on the drying up of rivers is crucial to assess the impact of climate change on aquatic ecosystems, biodiversity and functional integrity of freshwater systems.

This study is one of the first to present future projections of drying in intermittent river networks, and to analyze future changes in the drying patterns at high resolution spatial and temporal scale. The flow intermittence projections were produced using a hybrid hydrological model forced with climate projection data from 1985 until 2100 under three climate scenarios in six European drying river networks. The watersheds areas are situated in different biogeographic regions, located in Spain, France, Croatia, Hungary, Czechia, and Finland, and their areas range from 150 km² to 350 km². Additionally, flow intermittence indicators were developed and calculated to assess changes in the characteristics of the drying spells at the reach scale, and changes in the spatial extent of drying in the river network at various time intervals.

The results show that drying patterns are projected to increase and expand in time and space in all three climate scenarios, despite differences in the amplitude of changes. Temporally, in addition to the average frequency of drying events, the duration also increases over the year. Seasonal changes are expected to result in an earlier onset and longer persistence of drying throughout the year. Summer drying maxima are likely to shift to earlier in the spring, with extended drying periods or additional maxima occurring in autumn, and in some regions extending into the winter season. A trend analysis of extreme events shows that the extreme dry spells observed in recent years could become regular by the end of the century. Additionally, we observe transitions from perennial to intermittent reaches in the future.