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

Compacted subsoils affect vegetation growth and soil water balance. Numerical models help quantifying the hydrological impacts of subsoil compaction. These models are useful to evaluate measures that augment groundwater recharge in compacted soils and are important to guide proper water resource management under climate change in these soils. However, vegetation in these models is often parameterized using only limited field measurements or using relations between vegetation parameters and other variables. In this study, we show that uncertainties in vegetation parameters linked to transpiration (leaf area index [LAI]) and water uptake (root depth distribution) can significantly affect modeling outcomes. We used the HYDRUS-1D soil water flow model to simulate the water balance of experimental grass plots on the sandy soil of Belgium’s Campine Region. The compacted case has the compact subsoil at 40- to 55-cm depths while the non-compacted case underwent artificial decompaction. The models for each case were calibrated using soil moisture sensor data at two depths. We calibrated the soil water flow model for the compacted and non-compacted case considering three different vegetation scenarios that represent various reactions of canopy and root growth. Subsequently, we simulated soil water flow for different future climate scenarios. 

Our experiments reveal generally higher soil moisture content on the compacted case, suggesting subsoil compact layer’s role of promoting soil water accumulation above it. Moreover, the compacted case had lower LAI while the non-compacted case had deeper roots. Considering these canopy and root growths’ reactions in our models, results show that compaction does not always reduce deep percolation because of enhanced water uptake from the non-compacted case’s deeper roots. Therefore, while soil compaction affects both vegetation growth and soil water balance, this affected vegetation growth can further influence the water balance. Hydrological studies on (de-)compaction should dynamically incorporate vegetation growth above- and belowground under cases with compaction being present or absent. Thus, field evidence of vegetation growth and yield, often far lacking in compaction studies, is vital.