A non-explicit representation of macropores in the SVS land surface scheme improves streamflow simulations under frozen soil conditions

Soil freezing is a major cold region process that influences the hydrological behavior of northern catchments during winter rainfall or snowmelt events. Growing ice within the soil matrix reduces the pore space available for water to infiltrate, while the presence of soil macropores in structured soils maintains rapid water percolation even in frozen conditions. Representing the complex effect of soil freezing on water infiltration in land surface models is therefore a challenging task. This is especially the case for operational systems where the integration of a physical process must improve or maintain reasonable model performance and minimize the increase in complexity and computational cost. In this study, we propose a conceptual approach to represent the effect of macropores on frozen soil infiltration into the Soil, Vegetation and Snow (SVS) land-surface scheme used within the operational prediction systems of Environment and Climate Change Canada (ECCC). In this approach, the macropores are activated when soil moisture exceeds 55% of the available pore space. We assessed the impact of this new approach on streamflow simulations at more than 580 hydrometric stations located in the Great Lakes-St. Lawrence domain over a five-year period. The conceptual representation of macropores results in a major upgrade to the soil freezing scheme of SVS with an improvement of the Kling-Gupta Efficiency (KGE) at 88% of the stations (KGE_med = 0.55; KGE_med = 0.28) as it better captures the timing and amplitude of peak flows. Detailed analysis of a decomposed hydrograph shows that the macropore configuration increases SVS soil drainage (slow response) and reduces surface runoff and lateral flow (quick response). The SVS experiment with macropores also results in accurate simulations of freezing depth and surface meteorological variables (i.e. air and dew point temperature) which paves the way for an operational implementation of this new configuration in the numerical weather and hydrologic prediction systems at ECCC.