Geneviève A

The existence of several preferential states associated with various antecedent moisture conditions is often advanced to explain the connection or disconnection of runoff sources to the stream network. The issue of hydrological connectivity was addressed here by studying the temporal dynamics of precipitation, soil moisture and streamflow during storm events. A small headwater forested catchment, the Hermine, located in the Laurentians near Montreal, Quebec, was monitored across a 3-month period covering a gradient of meteorological conditions (dry, wet, very wet). The objective of the study is to understand how runoff at the catchment outlet is generated under various conditions of soil moisture and of connectivity between the hillslopes and the stream. The temporal and spatial patterns of soil moisture were monitored on two facing hillslopes on a North-South transect crossing the stream using a set of three equally-spaced automatic TDT (Time Domain Transmission) probes on each hillslope. The instruments measured moisture of the soil developed on a glacial till at a depth of 30 cm and a frequency of 15 minutes. Discharge data was automatically recorded every 15 minutes at a small V-notched weir located 100 meters downstream from the hillslopes. The relationships between precipitation, spatially-variable soil moisture and streamflow were assessed using cross-correlation techniques for each hydrological event identified. Regression trees were also used to partition the discharge data into dissimilar subgroups on the basis of soil moisture threshold values. Regardless of the hillslope and of the location of the TDT probe, soil water contents above 40 % were needed for significant peak discharges to be observed at the catchment outlet. A flashier response to precipitation was observed on the south-facing, flatter slope while the drainage of soil water was much longer on the north-facing, steeper slope, also characterized by a more organic soil matrix. Soil saturation did not always occur from the valley bottom upwards, especially under low antecedent wetness conditions, thus causing a disruption of hydrological connectivity. The various shapes of the soil water-streamflow hysteretic relationships can also be seen as an emergent property of hillslope-scale connectivity. This work sheds some light on the time-varying nature of hydrological connection between various points on a hillslope and the controls exerted by local landscape features.

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

Université de Montréal