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TOPMELT 1.0: a topography-based distribution function approach to snowmelt simulation for hydrological modelling at basin scale

Abstract. Enhanced
temperature-index distributed models for snowpack simulation, incorporating
air temperature and a term for clear sky potential solar radiation, are
increasingly used to simulate the spatial variability of the snow water
equivalent. This paper presents a new snowpack model (termed TOPMELT) which
integrates an enhanced temperature-index model into the ICHYMOD
semi-distributed basin-scale hydrological model by exploiting a statistical
representation of the distribution of clear sky potential solar radiation.
This is obtained by discretizing the full spatial distribution of clear sky
potential solar radiation into a number of radiation classes. The computation
required to generate a spatially distributed water equivalent reduces to a
single calculation for each radiation class. This turns into a potentially
significant advantage when parameter sensitivity and uncertainty estimation
procedures are carried out. The radiation index may be also averaged in time
over given time periods. Thus, the model resembles a classical
temperature-index model when only one radiation class for each elevation band
and a temporal aggregation of 1 year is used, whereas it approximates a fully
distributed model by increasing the number of the radiation classes and
decreasing the temporal aggregation. TOPMELT is integrated within the
semi-distributed ICHYMOD model and is applied at an hourly time step over the
Aurino Basin (also known as the Ahr River) at San Giorgio (San Giorgio Aurino), a
614 km 2 catchment in the Upper Adige River basin (eastern Alps, Italy) to
examine the sensitivity of the snowpack and runoff model results to the
spatial and temporal aggregation of the radiation fluxes. It is shown that
the spatial simulation of the snow water equivalent is strongly affected by
the aggregation scales. However, limited degradation of the snow simulations
is achieved when using 10 radiation classes and 4 weeks as spatial and
temporal aggregation scales respectively. Results highlight that the effects
of space–time aggregation of the solar radiation patterns on the runoff
response are scale dependent. They are minimal at the scale of the whole
Aurino Basin, while considerable impact is seen at a basin scale of
5 km 2 .