Solving Richards Equation for snow improves snowpack meltwater runoff estimations in detailed multi-layer snowpack model

Abstract

Abstract. The runoff from a snow cover during spring snowmelt or rain-on-snow events is an important factor in the hydrological cycle. In this study, three water balance schemes for the 1 dimensional physically-based snowpack model SNOWPACK are compared to lysimeter measurements at two alpine sites with a seasonal snow cover, but with different climatological conditions: Weissfluhjoch (WFJ) and Col de Porte (CDP). The studied period consists of 14 and 17 yr, respectively. The schemes include a simple buckettype approach, an approximation of Richards Equation (RE), and the full RE. The results show that daily sums of snowpack runoff are strongly related to a positive energy balance of the snow cover and therefore, all water balance schemes show very similar performance in terms of Nash-Sutcliffe efficiency (NSE) coefficients (around 0.63 and 0.72 for WFJ and CDP, respectively) and r 2 values (around 0.83 and 0.72 for WFJ and CDP, respectively). An analysis of the runoff dynamics over the season showed that the bucket-type and approximated RE scheme release meltwater slower than in the measurements, whereas RE provides a better agreement. Overall, solving RE for the snow cover yields the best agreement between modelled and measured snowpack runoff, but differences between the schemes are small. On sub-daily time scales, the water balance schemes behave very differently. In that case, solving RE provides the highest agreement between modelled and measured snowpack runoff in terms of NSE coefficient (around 0.48 at both sites). At WFJ, the other water balance schemes loose most predictive power, whereas at CDP, the bucket-type scheme has an NSE coefficient of 0.39. The shallower and less stratified snowpack at CDP likely reduces the differences between the water balance schemes. Accordingly, it can be concluded that solving RE for the snow cover improves several aspects of modelling snow cover runoff, especially for deep, sub-freezing snow covers and in particular on the sub-daily time scales. The additional computational cost was found to be in the order of a factor of 1.5-2.

Simulating the influence of snow surface processes on soil moisture dynamics and streamflow generation in an alpine catchment

“…The onset of wetting of the soil due to snowmelt is well predicted. It illustrates that using the bucket scheme for water flow in snow is justified on daily and seasonal timescales (Wever et al, 2014b). The diurnal cycle of snowmelt is also visible as a diurnal cycle on soil moisture levels, well reproduced by the simulations.…”

“…The Richards equation (Richards, 1931) is used to describe soil moisture dynamics and is numerically solved using finite differences scheme over the model layers (elements). Water flow in snow is solved by the bucket scheme, which provides accurate snowpack runoff estimations on daily and seasonal timescales (Wever et al, 2014b), and has noticeably lower computational costs (on the order of a factor of 2-3) than using the full Richards equation for snow. The liquid water outflow from the snowpack is prescribed as the upper boundary condition for the Richards equation for the soil (Wever et al, 2014b).…”

“…Water flow in snow is solved by the bucket scheme, which provides accurate snowpack runoff estimations on daily and seasonal timescales (Wever et al, 2014b), and has noticeably lower computational costs (on the order of a factor of 2-3) than using the full Richards equation for snow. The liquid water outflow from the snowpack is prescribed as the upper boundary condition for the Richards equation for the soil (Wever et al, 2014b). In snow-free conditions, the upper boundary condition is defined by rainfall; evaporation and deposition result from the latent heat flux.…”

Experimental measurements for improved understanding and simulation of snowmelt events in the Western Tatra Mountains

“…Snow wetness is not commonly measured in snow hydrology research although several recent papers presented the development of some new devices and modeling of liquid water content of snowpack (Avanzi et al, 2014(Avanzi et al, , 2015. Examples of other new devices are Heilig et al (2015), or Kinar and Pomeroy (2015), while Hirashima et al (2014) or Wever et al (2014) discuss the new modeling methods. In this study we have used the Snow Fork in snow pits.…”

Rainwater propagation through snowpack during rain-on-snow sprinkling experiments under different snow conditions

“…Later, introduced a 2-D model describing water transport in subfreezing and layered snow including capillary forces. With the implementation of the full Richard's equation (RE) described by Wever et al (2014b), the influence of capillary forces on the water flow was finally represented in an operationally used 1-D SNOWPACK model. A multi-dimensional water transport model, which allows for the explicit simulation of preferential flow paths, has been introduced by Hirashima et al (2014).…”