SNOW STORAGE ESTIMATION FOR THE OPUHA DAM CATCHMENT
Tim Kerr,1 Tony McCormick,2
1
Aqualinc Research Ltd.
2
Opuha Water Ltd.
Background
Snow that accumulates in a catchment is an uncontrolled reservoir of water. Quantifying the
amount of snow enables anticipation of how much snow melt will augment stream flows during
the spring and summer. The Opuha Dam has been identified as having a high contribution of
snow melt to its annual inflows (Kerr 2013). Knowledge of the amount of snow stored in the
catchment at the end of winter would assist with water management (McCormick 2015).
Aims
The aim of the research described in this presentation was to provide an automated daily snow
storage estimation system for the Opuha Dam catchment.
Method
Thirty years of daily snowfall for the catchment was estimated from rainfall and temperature
observations at a low elevation site near the catchment. This follows the methods of Fitzharris
and Garr (1995) but spatially distributed following McAlevey (1998) and Kerr (2005). The places
within the catchment where the precipitation was likely to fall as snow were identified through
consideration of how temperature and precipitation vary across the catchment. In a similar way,
areas where snow melt was likely to occur were identified by estimating where the temperature
was likely to be above 0 oC each day. Daily accounting of the snow fall and snow melt for each
grid square enabled an estimate of the snow remaining each day. The sum of all the snow within
the Opuha Dam’s catchment provided the total catchment estimate. This was compared to the
median value for the same day of the year for the entire thirty years to provide a relative daily
snow storage estimate. Automation was achieved through implementing the routines in R and
running as a scheduled task from the Aqualinc servers.
Results
The snow storage estimates are updated daily and presented online as a graph with the snow
quantity shown as a fraction of the median maximum (Figure 1). Three times a week, results are
automatically emailed directly to the Opuha Dam managers.
Figure 1 Opuha Dam snow storage estimate as at the beginning of September 2015. The 2014 snow
estimates are in pink showing that the snow that year was in the lowest 5 th percentile of the 30 years for
most of winter and spring. At the beginning of September the 2015 snow estimates were double the 2014
year, but about half of what could be expected in a median year.
Snow builds up in the catchment from April, with the maximum occurring in late August leading to
complete melt by the end of December. The greatest melt occurs in October. The 2014 end of
September snow storage was the lowest on record. The 1986 year was the highest. The 1998
year was near the median. The automated on-line snow storage estimates are used in
association with local knowledge to plan the management of the lake. In addition, the estimates
provide a crucial step in developing a hydrological model for the Dam.
References
Fitzharris, B., Garr, C.E., 1995. Simulation of past variability in seasonal snow in the Southern Alps, New
Zealand. Annals of Glaciology 21, 377–382.
Kerr, T., 2005. Snow storage modelling in the Lake Pukaki catchment, New Zealand: An investigation of
enhancements to the SnowSim model (M.Sc. thesis). University of Canterbury, Christchurch.
Kerr, T., 2013. The contribution of snowmelt to the rivers of the South Island, New Zealand. Journal of
Hydrology (NZ) 52, 61–82.
McAlevey, B., 1998. A distributed seasonal snow model for New Zealand (M.Sc. thesis). University of Otago,
Dunedin.
McCormick, T. (2015) Water Restrictions Update #11. Opuha Water Limited. [Online] Available from
http://opuhawater.co.nz/category/news/newsletters [Accessed 19th April 2014].