Testing the H60 concept in the Interior Watershed Assessment Procedure by process hydrology studies.

The H60 is a concept used in the Interior Watershed Assessment Procedure (IWAP; MoF 1999) by which the upper altitudinal zone within a watershed is weighted for its assumed contribution to peak flows. The approach assumes that there is a direct and immediate link between snowmelt and surface flow, there being little lag in delivery of water from the snowpack to the stream. However, work with stable isotopic tracers in water, namely deuterium and oxygen-18 (18O) in other part of the world, have demonstrated that there may be a considerable storage capacity within catchments, and long lags between the entry of water into the soil and its emergence as part of streamflow. This project uses tracers and water chemistry to test some of the assumptions in the H60 concept. It aims to determine the location in the watershed that makes the dominant contribution to the water in the peak hydrograph by using chemical means. The key uncertainty being studied is the lag between snow melt and water entering the stream channel. In the first two years of the study the snowpack in four small watersheds was marked with two selected chemical tracers, namely deuterium and chloride, that are (i) detectable in very small quantities and (ii) move conservatively through the soil (i.e. are not adsorbed onto soil particles but move through inertly with the snowmelt water). Streamwater was sampled continuously during the snowmelt season in order to determine the time at which water from particular locations in the catchment actually contribute to the peak-flow hydrograph. By this means a picture is beginning to emerge of the travel times of water in the watersheds and the contributions of different zones and altitudinal belts to the important peak flows. The results to date indicate that both tracers work for the intended purpose, though there are problems with using chloride in research catchments as it interferes with other water quality studies