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Distributed modelling of snow water equivalent - coupling a snow accummulation and melt model and GIS


Catchment
The Jalovecky creek catchment (Fig. 4) is representative for the highest part of the Carpathian Mountains, the 1200 km long mountain chain situated in middle Europe. Early Quaternary glaciers intensively modeled catchment relief. Catchment area is 22.2 km2. Elevations in the catchment range from 800 to 2178 m a.s.l. (mean 1500). Mean slope is 30º. Catchment is generally south oriented - northern, north-eastern and eastern slopes represent only 15% of catchment area. Igneous (granodiorite), metamorphic (schist, gneiss, migmatite) and sedimentary (Quaternary loose sediments) build 21, 48 and 24% of the catchment, respectively. Rest 7% is made up by Mesosoic limestone and dolomite. Soils distribution is generaly characterised by vertical zonality (cambisols, podzols, ranker, lithosol). Spruce dominated forest cover 44% of catchment area. Dwarf pine covers 31% and meadows and bare rocks cover the rest 25% of catchment area. 
Catchment mean precipitation (1989-2000) is 1550 mm, runoff 1015 mm, air temperature at mean elevation 3.5 ºC. Seasonal snow cover typically occurs in the catchment between November and May. First short snowmelt events usually occur in March. The snowmelt is then interrupted for about two weeks. Continual snowmelt typically begins in April and the main phase of snowmelt starts in the last decade of April. 
Measurements of snow water equivalents in the last 30 years show that winter 1999/2000 was relatively cold and snow-rich (Fig. 5, Table 1). Time course of snow accumulation and melt was relatively simple without short-time melts during the accumulation period. Point UEB model provided successful simulation with different time-steps of the input data (Fig. 6). 


Fig. 4 The Jalovecky creek catchment.



Fig. 5 Catchment mean snow water equivalents in two mountain catchments in the research area in winters 1968/69 - 2000/2001.

Table 1 Climatic characteristics of winters (November-March) 1995/1996 - 1999/2000 in the Jalovecky creek catchment measured at catchment mean elevation; P-precipitation, Tave-mean air temperature, Tneg-sum of negative air temperatures, SWEmax- maximum snow water equivalent
   P[mm]   Tave [°C]  Tneg[°C]   SWEmax [mm]
1996  344  -5,6  -862,3  237
1997  282  -3,8  -688,3  280
1998  530  -2,3  -450,0  348
1999  536  -5,0  -804,5  461
2000  708  -4,6  -785,5  773



Fig. 6 Simulation of snow water equivalent at catchment mean elevation with the point version of the UEB model using different time steps of input meteorological data; 1-hourly, 2-daily, 3-input data measured at standard observation hours 7 a.m., 2 p.m. and 9 p.m.

Results
Simulations of spatial distribution of snow water equivalent were generally acceptable. Few examples from different sites are shown in Fig. 7. Good simulations were achieved for the forest sites. The simulations were slightly overestimated for forest sites at lower elevations. Snow water equivalents simulated for sites situated above forest which were not exposed to significant drift were also acceptable. Worse simulations were received for the wind exposed sites.
Spatial distributions of modelled snow water equivalents at the beginning of the winter and at the time of maximum snow accumulation are shown in Fig. 8. As can be seen that snow distribution at the beginning of winter was significantly affected by vegetation in terms of relatively small spatial differences in the forest and much larger differences above the forest. Slopes aspects played important role in the spatial distribution of snow cover at the beginning of the winter, too. At the time of maximum accumulation, elevation gradients seem to have dominant effect on spatial distribution of snow. 
Another hydrologically significant output of the UEB-EHZ model is the outflow from melting snow. It may help to identify the parts of the catchment which dominate in providing meltwater for overland runoff formation. High snowmelt output areas may also contribute to acid surges at the beginning of the snowmelt thus affecting water quality and environamental hazards. Figs 9 and 10 show modelled meltwater outputs in the Jalovecky creek catchment at certain dates in snowmelt season 2000. The dates were chosen as following: 

  • 9 March 2000 - first melt event 
  • 25 March 2000 - period without snowmelt short before the beginning of continual melt period 
  • 15 April 2000 - beginning of the main snowmelt phase 
  • 22 April 2000 - maximum melt 
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