Geneviève A

Hydrological connectivity often describes the ease with which stormwater moves through a landscape. Because the timing and magnitude of peak runoff are controlled by the degree of hydrological connectivity, highly connected systems should be characterized by short response times to precipitations, a steeper rising hydrograph limb and a higher peak discharge than disconnected systems. Hence, processing and analyzing event hydrographs should help us identify patterns in the hydrological behavior of a catchment and examine the variability of the response to precipitation for given antecedent moisture conditions. We have analyzed a three year long record of rainfall and streamflow data collected at a frequency of 15 minutes at the Hermine, a small headwater forested catchment located in the Laurentians near Montreal, Quebec. We have segmented the time series into isolated hydrological in order to derive the characteristics of storm hyetographs and hydrographs. A 10-day antecedent rainfall index was utilized as a surrogate for antecedent moisture conditions. Multivariate statistics techniques, such as redundancy analysis and variance partitioning, were used to assess a vast spectrum of catchment responses. Results indicate that time to peak and lag time values are very large when associated with low antecedent wetness. However, there is no clear tendency when antecedent wetness is high. Under these conditions, there is a set of variable responses illustrating that a highly connected state only occurs under very particular conditions that are not easily predicted by antecedent rainfall patterns. The threshold at which this switch happens is not easy to identify. These observations are confirmed by regression tree analysis that classified runoff events with respect to antecedent moisture conditions, baseflow, runoff coefficient and catchment response times. The model shows that the catchment features a fairly homogeneous dry-state reaction reflecting a disconnected state in the watershed while several wet-state responses are observed. These results show that storm runoff is a very episodic phenomenon triggered by various mechanisms. They also suggest that across a continuum of wetness conditions, the catchment dynamics is driven by a very quick saturation excess response under very wet conditions, as opposed to a longer-lasting "filling and spilling" of the bedrock depressions under dryer conditions. The time-dependent influence of these two mechanisms needs to be better assessed at various scales.

Chaire de recherche du Canada en dynamique fluviale (Canada Research Chair in Fluvial Dynamics)

Université de Montréal