Snow hydrology of a mountainous rain-on-snow environment - The Waimakariri catchment, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy (PhD)
Studies were carried out in the Waimakariri catchment on the east side of the South Island, to contribute to the general understanding of the inter- and intra-regional variations of snow hydrological phenomena and to the practical needs of hydrologists in New Zealand. Turbulent exchange of sensible and often latent heat can produce rapid melt because of the exposed, alpine nature of the seasonal snow zone and the dominance of maritime air masses and strong westerly air flow. Regional air mass and circulation characteristics provide stronger controls on turbulent exchange than local advection and locally-generated winds at a site near the Main Divide. Estimates of turbulent exchange at a site 20 km in the lee of the Main Divide show weaker correlations with large scale indices than at the Main Divide site. Air flow-terrain interactions cause a variation in snowmelt climate across the South Island. The spatial patterns of snow surface lowering in a 71 ha catchment suggest that melt patterns vary with weather type, with exposure to wind being the main control during advective melt situations. Field observations, climatic considerations and theoretical calculations indicate that snow cover should not have an important impact upon water routing near the Main Divide, where mid-winter rain-on-snow is common. In the drier climate of the eastern ranges, ice layers may significantly impede percolation. Movement of water at the base of the pack is difficult to predict because of the variability in runoff mechanisms and lack of knowledge about superimposed and ground ice. A simple model using daily climate data satisfactorily reproduced snow accumulation at a snow course at 1750 m elevation when temperatures measured at 1550 m and low elevation precipitation were used. Simulation quality was unsatisfactory when low elevation temperatures were used, because of the deviations of the assumed from the actual lapse rates. A runoff model using a lumped transformation routine and a distributed snow routine performed poorly when applied to a 94 ha catchment ranging from 1020 to 1730 m elevation. The model overestimated runoff, possibly because of subsurface losses. Snow accumulation was reasonably reproduced above 1500 m, but below 1500 m many snow events were misclassified because of the inadequacy of daily temperature as a discriminator. A spring melt period was simulated using observed snow lines rather than simulated snow cover. A melt routine incorporating a regional air flow index reproduced runoff variations better than a simple temperature index. The lumped transformation routine is inadequate in situations where runoff sources and mechanisms vary between events. The magnitude of seasonal snow storage varies between years, but is great enough to require inclusion in streamflow models of mountainous catchments. The contribution of melt in the seasonal snow zone to flood runoff is difficult to assess given the available data, but may vary across the South Island because of systematic geographic variations in storm types producing heavy rain during rain-on-snow events. Melt in the transient snow zone is limited by wind speed, not air temperature, but can augment short term runoff during intense rain events.