Mechanistic assessment of runoff connectivity patterns: The hierarchy of space-time controls

Abstract:

The patterns and processes influencing runoff generation are many and complex. In the northern Great Plains, threshold-like activation of infiltration-excess overland flow during spring snowmelt is common. However, the multiple controls on, and their hierarchy in determining, hydrological connectivity are poorly understood in space and time. Here we present analysis of a 52-year data set from three 5 ha runoff plots in southern Saskatchewan, Canada. We found that the key controls on the activation of snowmelt runoff are firstly snow water equivalent, followed by soil moisture in the preceding fall, and finally maximum depth of the surface thawed layer during snowmelt. Building on this analysis, we conducted a field experiment during the spring 2014 snowmelt period to assess these controls in high spatial and temporal detail. We found that soil moisture and thawed layer depth show little spatial variability during snowmelt, while the spatial patterns of snow water equivalent and snow cover ablation show time-varying hot spots of snowmelt. Catchment connectivity is determined by the topography of the soil surface, specifically the local micro-scale depressional area, volume and slope. Finally, we analyzed the stable isotope composition of soil water, snowmelt water from the base of the snowpack (both exhibiting high spatial variability) and hillslope runoff (with high temporal variability). This further illustrates how these time-varying controls on runoff generation are sequenced in an individual melt season, where filling and spilling of surface depressions over frozen ground dominates.