Assessment of a snow storage gradient across a maritime mountain environment: a ground penetrating radar investigation

The Southern Alps of New Zealand experience some of the highest precipitation rates globally, and dramatic west to east climatic gradients. Our current knowledge of this precipitation distribution is based on weather station data and river discharge measurements, but there is a clear data gap in the high elevation, central Southern Alps. Here, estimates of precipitation strongly diverge. This problem exists because of the difficulties of quantifying the depth and distribution of snow in a remote, high-altitude mountainous region.  In order to improve our knowledge of snow distribution within this data-poor region, snow depths of (< 10m) were assessed parallel to the prevailing westerly wind direction at five locations across the mountain range, between the névé of Franz Josef Glacier, Waiho catchment, to the west and Jollie Valley, Pukaki catchment, in the east. The geophysical method of Ground Penetrating Radar (GPR) was used because of its ability to image the deep snow packs experienced in the study region.  Comparison of measurement techniques over the (< 3km) surveyed transects showed that ground-based GPR gave the best sample size (41000 samples) and accuracy due to the high spatial resolution. Airborne GPR (8571 samples) overestimated snow depth by 8 % in low-gradient homogenous terrain, and 24% in steep heterogeneous terrain. The difference is ascribed to the larger view area of the GPR in the airborne survey. Direct probing of snow depth also performed poorly in comparison to ground-based GPR when generalising snow distribution over an area.  Across-mountain precipitation peaked ~5 km west of the main divide, between 1700 and 2000 m a.s.l, providing the first empirical support to existing estimates of the location of peak precipitation. Results show decreasing precipitation from 12 ma-1 at Franz Josef Glacier, in the Waiho catchment, to 1.8 ma-1 at Jollie River valley, in the Lake Pukaki catchment, 25 km to the south-east.  Internal reflection horizons in snow-pack radargrams allowed snowfall events to be tracked, and a relationship lowland and mountain precipitation to be established. Snowfall accumulation 'factors' were derived for different atmospheric circulation indices, and these will enable improved accuracy in modelling of snow accumulation processes. Further research is required to refine the relationship between synoptic-classed accumulation rates and inter-annual variations in climatic circulation.  These refinements of measurement techniques and quantification of and snow distribution and depth allow for better estimation of river discharge and timing estimates for, hydroelectric power generation, and glacier mass balance.