Spatiotemporal and cross-scale interactions in hydroclimate variability: a case study in France

Understanding how water resources vary in response to climate at different temporal and spatial scales is crucial to inform long-term management. Climate change impacts and induced trends may indeed be substantially modulated by low-frequency (multi-year) variations, whose strength varies in time and space, with large consequences on risk forecasting systems. In this study, we present a spatial classification of precipitation, temperature and discharge variability in France, based on a fuzzy clustering and 5 wavelet spectra of 152 near natural watersheds between 1958 and 2008. We also explore phase-phase and phase-amplitude causal interactions between time scales of each homogeneous region. Three significant time scales of variability are found in precipitation, temperature and discharge: 1 year, 2-4 years and 5-8 years. The magnitude of these time scales of variability is however not constant over the different regions. For instance, Southern regions are markedly different from other regions, with much lower 5-8 years variability 10 and much larger 2-4 years variability. Several temporal changes in precipitation, temperature and discharge variability are identified during the 1980s and 1990s. Notably, in the Southern regions of France, we note a decrease in annual temperature variability in the mid 1990s. Investigating cross-scale interactions, our study reveals causal and bi-directional relationships between higher and lower-frequency variability, which may feature interactions within the coupled land-ocean-atmosphere systems. Interestingly, however, even though 15 time-frequency patterns (occurrence and timing of time scales of variability) were similar between regions, cross-scale interactions are far much complex, differ between regions, and are not systematically transferred 1 from climate (precipitation and temperature) to hydrological variability (discharge). Phase-amplitude interactions are indeed absent in discharge variability, although significant phase-amplitude interactions are found in precipitation and temperature. This suggests that watershed characteristics cancel the negative feedback 20 systems found in precipitation and temperature. This study allows for a multi-time scale representation of hydro-climate variability in France, and provides unique insight into the complex non-linear dynamics of this variability, and its predictability.