How does streamflow response vary with spatial scale? Analysis of controls in three nested Alpine catchments

Understanding how catchment streamflow response varies in time and at different spatial scales is critical to assess runoff production and water availability in mountain areas. Very little work on comparing runoff generation processes in catchments of increasing size has been done in Dolomitic areas. In this research, we adopted a nested catchment (0.14–109 km2) approach to i) assess whether and how streamflow response at the seasonal and the rainfall-runoff event time scale varies with increasing spatial scale, and ii) identify the main factors controlling the temporal variability of streamflow response in three mountain catchments in the Italian Dolomites. We used precipitation and streamflow data from June to September of 13 consecutive years (2005–2017) available for all three catchments, and computed runoff coefficients, stormflow volumes, streamflow peaks and lag times between the rainfall centroid and the streamflow peak for 203 rainfall-runoff events occurred during the study period. Results show that, overall, specific streamflow and runoff coefficients decreased with the increasing spatial scale. However, this pattern was disrupted at the end of spring-beginning of summer due to the relevant snowmelt contribution to streamflow in the intermediate catchment characterized by the largest fraction of high-elevation areas, where deep snowpack accumulated during the winter. The probability distribution of runoff coefficients was very well represented by a Beta-distribution for all three catchments, suggesting that analogous factors controlled the variability of runoff coefficients. This was confirmed by multiple regression analysis that revealed that event rainfall amount, storm duration and 3–12 days antecedent precipitation (assumed as a proxy for antecedent wetness conditions) explained up to 0.81 of the variability of streamflow response, with a minor role of rainfall intensity. Further research is needed to assess whether detailed measurements of hydrological variables collected at the small catchment scale (<1 km2) could be deemed as representative for the hydrological response of larger catchments.