Unraveling the mysteries of the large watershed black box: Implications for the streamflow response to climate and landscape perturbations

Spatial and temporal trends in stream chemistry have been used to provide insights into the scale dependencies of streamflow generation processes in small catchments. However, these scale dependencies have not been thoroughly investigated at large watershed scales (defined as drainage areas greater than 1000 km²). Quantifying these scale dependencies is critical to understanding how large watersheds will respond to future perturbations; e.g., the long-term streamflow response to climate change and/or changes in land-cover and land-use. Here we investigate the spatial and temporal scaling relationships of all dominant streamflow generation processes in a large alpine watershed in the southern Rocky Mountains of Colorado. Observations in the watershed indicate that dominant streamflow processes are spatially and temporally variable. The relative strengths of dominant streamflow mechanisms vary as a function of internal watershed structure (i.e., spatial variability in topographic relief, soil development, groundwater flowpath development, and stream network structure) and external forcing such as timing and character of precipitation. This behavior coupled with previous observations that streamflow from the watershed contained a significant component of basin-scale groundwater, suggests that similar large watersheds may have internal buffering, at least initially, against rapid change.