The Snowy Mountains region in south-eastern New South Wales has an extensive area of snow cover during winter months within which are located the ski resorts of Thredbo, Perisher Valley, Smiggin Holes and Mount Selwyn. Also located in the Snowy Mountains is the massive Snowy Mountains hydroelectric scheme involving a complex system of reservoirs, aqueducts, tunnels and power stations. As part of the planning, design and operation of the scheme the Snowy Mountains Hydroelectric Authority (now Snowy Hydro) have taken regular observations since 1954 of snow depths on a course near Spencers Creek. The location is at an elevation of about 1,650 m.
Snowy Hydro (snowyhydro.com.au) have published plots of snow depth data observations at Spencers Creek for the 62 years since 1954 and these have been used as the data source for this article. A typical plot for a “good” year (2012) is:
My analysis involved the extraction for each of the 62 years of the following information from the Snowy Hydro plots:
– the peak depth of snow cover for each year,
– the dates of first and last snow cover for each year,
– the dates of the first and last snow cover exceeding 50 cm and 100 cm for each year,
From this data it was possible to compute the annual duration of snow cover of minimal, >50cm and >100cm depth. Also computed were the percentage of days in each month, over the past 20 years, of minimal, >50cm and >100cm snow cover.
PEAK SNOW DEPTH
ANY SNOW COVER
50cm AND OVER SNOW COVER
100cm AND OVER SNOW COVER
PROBABILITY OF SNOW COVER
This graph shows the percentage of days in each month with minimal, 50cm and 100cm snow cover. For instance, 90% of days in August had a snow cover of at least 100cm. This analysis was done only for the past 20 years, 1996-2015.
- Peak snow depth trends have fallen by about 20% (210cm to 170cm) since the 1950s. During the period since 1992 there have been no peak depths exceeding 300cm, whereas there were five such years between 1956 and 1992.
- Both the start and end of minimal snow cover have been decreasing since about 2000. Note that the trendline indicates that the first snow cover is now about 10 days earlier than in 2000 (from 26 May to 6 May). The combined effect is that the overall seasonal duration of any snow cover has remained fairly constant (average 160-170 days) for the past 62 years.
- The trend of the start date of at least 50cm snow cover has remained fairly constant (about 28 June) since 1970. However, the trend of the end date has been reducing steadily (from 1 November to 11 October) since 1954. The effect has been a significant decrease of almost 30% (from 140 days to 100 days) in the duration of snow cover exceeding 50cm since 1954. Note that in about 1970 there was a sudden reduction, of about 10 days, in the 50cm duration.
- The trend of the start date of at least 100cm snow cover has remained fairly constant (at about 18 July) since 1970. Prior to 1970 the start dates were about 10 days earlier. The trend of the end date has reduced by about 12 days (from 18 October to 6 October) since 1954. The overall effect on the duration of at least 100cm snow cover trend is a sudden 20% (from 100cm to 80cm) reduction in about 1970, with a fairly constant trend of about 80 days since then. Note that the peak snow depths were less than 100cm in two years, 1982 and 2006.
- Abnormally high snowfall years in the 1950s and 1960s have affected the overall trends of both depth and duration of snow cover. This effect reflects a period of wetter climate in southeastern Australia in the 1950s and 1960s.
- The probability graph shows that August and September are the months with most reliable snow cover of both 50cm and 100cm, with July and October being less reliable.
- There is evidence of a sudden change in the climate of the Snowy Mountains in about 1970, resulting in less heavy snowfall events after 1970.
- The trend has been for shorter durations of both 50cm and 100cm snow cover in recent years. Since 1975 (40 years ago) the duration of the 50cm snow cover trend has reduced by 18%, while that for 100cm snow cover has reduced by 8%.
- August and September are the most reliable months for snow cover exceeding both 50cm and 100cm, with July and October being less reliable.
The above “Observations” and “Conclusions” are my interpretation of the results of my analysis. However, the graphs presented are pure facts using the data published by Snowy Hydro.
If you have any comments I would love to hear from you. Please write your comments in the box below so that we can all learn from them. BG
This article was written by Brian Gunter of Narooma, NSW. In his previous life Brian lived in Cooma and was an engineering hydrologist involved over many years in the analysis of rainfall and river flow data for the planning of water resources projects in Australia, Asia and Africa. In recent years he has been one of the Marine Rescue NSW (previously Royal Volunteer Coastal Patrol) volunteer weather observers who operate the Narooma station for the Bureau of Meteorology.