Splitting streamflow : understanding the contribution of snow and rainfall in Canterbury Headwaters.
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Abstract
Seasonal snowmelt from mountainous regions are important sources of freshwater for downstream uses. In order to effectively manage water resources, a better understanding of the sources contributing to streamflow and the hydrological processes occurring in mountain regions is necessary. This study aims to address this by evaluating the temporal variability in source water contributions to streamflow in a Canterbury headwater catchment. This study also aims to conceptualise the dominant runoff mechanisms and hydrological functioning of a mountain headwater catchment. Over the winter period high frequency (daily-monthly) sampling of stream, snow, precipitation, seep, and overland water samples were collected alongside hydrological and meteorological data. Samples were then analysed for their isotopic composition and used in a mixing model analysis to determine contributions of sources to streamflow. This was then compared with the hydrological and meteorological data to conceptualise runoff mechanisms and hydrological processes in the catchment.
Results of the mixing model analysis estimated that old water (snow, groundwater, and overland) contributed between 41.5% to 53.2%, while new water (precipitation) contributed between 46.8% to 58.5% during the study period.
The conceptualisation of runoff mechanisms and hydrological functioning results indicate that during dry conditions baseflow from the emptying of the groundwater storage is the primary contributor to streamflow. During snowmelt and wet conditions, the over-saturation of soils leads to an increase in overland flow during each event. Following overland runoff from snowmelt or precipitation events cause large rises in the hydrograph before a return to baseflow conditions at the completion of the study.
The present investigation is the first scientific work estimating rainfall, snow, seep, and overland flow contributions to a mountain catchment in New Zealand through the use of isotopic tracers.
This study also presents the importance of high-resolution data in mountain catchments.