Improvements in simulating liquid water flow in snow using Richards Equation

Abstract:

The runoff from a snow cover during spring snowmelt or rain-on-snow events is an important factor in the hydrological cycle. Over a decade of snow lysimeter measurements at two alpine sites with a seasonal snow cover (Weissfluhjoch and Col de Porte), but with different climatological conditions, are compared to snowpack simulations with the 1 dimensional physics-based snowpack model SNOWPACK. The implementation of a solver for Richards equation (RE) in the model is presented. It provides an improvement of the simulations of snowpack runoff, especially on sub-daily time scales, compared to a simple bucket-type approach. Nash-Sutcliffe efficiency coefficients for hourly runoff increase from 0.12 to 0.59 and r² values increase from 0.47 to 0.76 for the site Weissfluhjoch. An analysis of the runoff dynamics over the snow season showed that the bucket scheme releases meltwater later in the melt season than in the measurements, whereas RE provides a better agreement. The shallower and less stratified snowpack at Col de Porte reduces the differences between both water transport schemes. It can be concluded that solving RE for the snow cover improves several aspects of modeling snow cover runoff, especially for deep, sub-freezing snow covers and in particular on the sub-daily time scales. Also indications were found that RE improves the simulation of the internal snowpack. For example, a comparison with snow temperature measurement series at different depths showed a higher agreement in most years at the timing when the snowpack gets isothermal, which is related to infiltrating and refreezing melt water. The r² value for the depth to which the snowpack is isothermal between simulations and observations in traditional snow pits increased. RE is also able to simulate the accumulation of liquid water on strong inhomogeneities in snowpack stratification, which is considered to be a major factor in wet snow avalanche formation. This may contribute to the potential of SNOWPACK for assessing wet snow avalanche risks.