| 19 Mar 2026
A non-explicit representation of macropores in the SVS land surface model improves streamflow simulations under frozen soil conditions
Abstract. Soil freezing is a major cold region process that influences hydrological response of northern catchments, in particular during winter rainfall and snowmelt events. 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 a challenging task. This is particularly true for operational models, where physical process integration must balance performance improvements against computational efficiency and complexity. In this study, we propose a conceptual approach to represent the effects of macropores on frozen soil infiltration into the Soil, Vegetation, and Snow (SVS) model used within the operational prediction systems of Environment and Climate Change Canada (ECCC). We assessed the effects of this new configuration (Fr-MP) on streamflow simulations at more than 580 hydrometric stations located in the Great-Lakes and Saint-Lawrence domain over a five-year period. The conceptual representation of macropores improves the Kling-Gupta Efficiency (KGE) at 88 % of the assessed stations, resulting in an increase in the median KGE of 0.28 compared to the configuration without macropores and soil freezing. Detailed analysis of a decomposed hydrograph shows that the Fr-MP configuration increases SVS soil drainage (slow response) and reduces surface runoff and lateral flow (quick response). To ensure that the proposed change is also acceptable in the context of operational numerical weather prediction, an evaluation of its impact on soil freezing depth as well as screen-level temperature and dew point temperature predictions is performed against in-situ observations. These results support the potential operational implementation of the Fr-MP configuration at ECCC for numerical weather and streamflow prediction.
The manuscript addresses a significant and timely topic in cold-region hydrology. The methodology is sound, and the results provide a clear path toward improving operational streamflow forecasting. However, the manuscript requires a more critical discussion regarding the physical basis of the macropore parameterization and a deeper analysis of the spatial variability in model performance. Addressing the major comments regarding the physical justification and the sensitivity of the alpha_MP parameter will significantly enhance the impact and scientific rigor of the work. I look forward to seeing a revised version of this study.
Major Comments：
1. The authors propose a non-explicit, threshold-based representation of macropores. While this is computationally efficient for operational forecasting, the manuscript lacks a robust discussion on the physical limitations of this simplification. Specifically, how does this approach account for the spatial heterogeneity of macropore networks at the grid-cell scale? A more thorough comparison with existing dual-domain or dual-permeability models (e.g., those cited in the introduction) is needed to justify why this simplified approach is sufficient for large-scale hydrological modeling.
2. The calibration of alpha_MP is a central component of the study. However, the manuscript does not sufficiently address the potential for equifinality or the transferability of this parameter across different hydro-climatic regions. Are the optimal values of alpha_MP consistent across the diverse landscapes of the Great Lakes and Saint-Lawrence domain? A more detailed analysis of the spatial variability of the optimal alpha_MP and its relationship with soil/land-cover characteristics would significantly strengthen the paper.
3. The results indicate that the Fr-MP configuration performs differently in agricultural areas compared to forested catchments. The authors attribute this to the interaction between tile drainage and frozen soil. I suggest a more in-depth analysis of this interaction. Does the current model structure adequately decouple the effects of macropores from those of anthropogenic drainage systems? This is crucial for the model's reliability in human-altered landscapes.
4. While the authors evaluate the impact on TT and TD, the improvements are described as "slight" or "neutral." Given that the primary goal is to improve streamflow, the authors should clarify whether these meteorological improvements are statistically significant and if they have any meaningful impact on the broader numerical weather prediction (NWP) performance at ECCC.
Minor Comments：
1. Page 2, Line 40: The term "discrepancy" is used well here; ensure this is consistently linked to the scale-dependency of the processes throughout the discussion.
2. Page 7, Eq. 9: Please clarify if alpha_MP is a static parameter or if it could be dynamic based on seasonal vegetation growth.
3. Page 8, Figure 1: The conceptual diagram is excellent. Please ensure the legend clearly distinguishes between the "matrix flow" and "macropore flow" pathways in panel (c).
4. Page 10, Section 2.4.1: The exclusion criteria for stations (NSE < 0 or KGE < -0.41) are reasonable, but please provide a brief justification for why these specific thresholds were chosen over others.
5. Page 13, Line 355: The mention of the "perched aquifer" in Michigan is interesting. Could the authors provide a bit more detail on how this specific geological feature might be better represented in future iterations of the model?
6. Page 16, Line 411: The observation that only 4 stations show a Delta KGE < -0.5 is a strong point. Please emphasize this in the abstract as a measure of the model's robustness.
7. Page 23, Line 495: The authors mention that wetlands and meanders are not fully represented in Watroute. It would be beneficial to include a brief sentence on how the transition to the "Raven" routing scheme might specifically address these limitations.
8. Clarity of Figures: In Figure 4 and Figure 5, the color scales are effective, but please ensure that the symbols for different drainage areas are easily distinguishable in the printed version.
9. References: Ensure that all cited preprints (e.g., Bauer et al., 2025) are updated to their final publication status if available by the time of revision.