Quantifying hydrological impacts of compacted sandy subsoils using soil water flow simulations: the importance of vegetation parameterization

Abstract. Numerical models can quantify subsoil compaction's hydrological impacts, useful to evaluate water management measures for climate change adaptations on compacted subsoils (e.g., augmenting groundwater recharge). Compaction also affects vegetation growth, which, however, is often parameterized using only limited field measurements or relations with other variables. This study shows that uncertainties in vegetation parameters linked to transpiration (leaf area index [LAI]) and water uptake (root depth distribution) can significantly affect hydrological modeling outcomes. The HYDRUS-1D soil water flow model was used to simulate the soil water balance of experimental grass plots on Belgian Campine Region's sandy soil. The compacted plot has the compact subsoil at 40–55 cm depths while the non-compacted plot underwent de-compaction. Using two year soil moisture sensor data at two depths, these models of these compacted and non-compacted plots were calibrated and validated under three different vegetation parameterizations, reflecting various canopy and root growth reactions to compaction. Water balances were then simulated under future climate scenarios. The experiments reveal that the compacted plots exhibited lower LAI while the non-compacted plots had deeper roots. Considering these vegetations' reactions in models, model simulations show that compaction will not always reduce deep percolation, compensated by the deep rooted non-compacted case model's higher evapotranspiration. Therefore, this affected vegetation growth can also further influence the water balance. Hence, hydrological modeling studies on (de-)compaction should dynamically incorporate vegetation growth above- and belowground, of which field evidence is vital.