Shifting Flood Regimes Under Contradictory Precipitation Trends

Global warming is driving an increase in extreme precipitation events across many regions worldwide, often leading to intensified flooding. However, other changing precipitation characteristics may counterbalance this effect. These include reductions in total event precipitation, precipitation coverage area, duration, and frequency. The interplay of these often-contradictory trends remains poorly understood, with limited mapping and quantification available.
Through a series of studies focusing on the eastern Mediterranean region, we identify this area as susceptible to these contrasting precipitation trends. Our research reveals a decline in average precipitation and the number of wet days, alongside an increase in extreme precipitation events for return periods ranging from 10 to 100 years. Furthermore, storm total precipitation, coverage area, and duration decrease while conditional precipitation intensities rise.
When these trends are incorporated into hydrological models to simulate catchment responses and flood impacts, the role of soil moisture emerges as a critical factor in flood regulation. Due to lower precipitation amounts and wet days number, average soil moisture decreases. Despite heightened precipitation intensity, this leads to diminished runoff in most cases. Additionally, smaller storm sizes reduce runoff-contributing areas, resulting in lower flow discharges within concentrating channels. However, urbanization amplifies these dynamics, as urban areas are more sensitive to increased precipitation intensities due to limited soil moisture regulation. Consequently, in future climate scenarios, the largest runoff events produce higher peak discharges and total runoff compared to historical conditions. In contrast, lower-intensity events exhibit reduced peak and total runoff. These effects are intensified as urban impervious surfaces expand, making precipitation intensity a dominant driver of urban runoff.
Our findings suggest that floods are not universally intensifying, even in the context of more extreme precipitation. The dampening effects of other precipitation properties can offset flood magnitudes, highlighting the complexity of flood behavior under changing climate conditions.