Snow drought propagation and its impacts on streamflow drought in the Alps

Snow droughts, that is negative anomalies in snow water equivalent, impact society as well as natural ecosystems in winter and influence the hydrological cycle downstream in spring and summer. Thereby, pronounced snow drought conditions can lead to streamflow droughts, i.e., anomalously low discharges, during the following melt season. Under continued global warming, the frequency and intensity of snow droughts are expected to increase. However, we still know little about the rate at which snow droughts propagate to subsequent streamflow droughts, the spatial patterns of such events, or the influence of snow droughts on the occurrence, intensity or duration of subsequent streamflow droughts. To quantify the link between snow and streamflow drought, we developed a snow drought propagation scheme, which dynamically identifies pairs of snow and streamflow droughts from a high-resolution gridded snow product and streamflow observations, and applied it to 207 catchments in Switzerland and Austria. Between 1961 and 2021, we identified 147 propagating snow droughts, and found that 18 % of the snow droughts propagated to a streamflow drought and that 21 % of streamflow droughts during the melt season were preceded by a snow drought. Propagating snow droughts are most common in high-elevation catchments and among the most extreme snow droughts. Streamflow droughts are characterized by higher deficits, longer durations and earlier occurrences when preceded by a snow drought. We identify snow drought deficit as a good predictor for subsequent streamflow drought deficit and duration when the snow drought is intense and occurs in low-elevation catchments. We show that the presence of water resources management increases the chance of snow drought propagation. Finally, we find that the period 1990–2021 is characterized by an increase in the number of propagating snow droughts compared to 1961–1990. In conclusion, we unveil a non-negligible link between snow and streamflow droughts that could help improve early warning systems for spring and summer droughts.