Testing the EPIC Richards submodel for simulating soil water dynamics under different bottom boundary conditions

Abstract Most biogeochemical models simulate water dynamics using the tipping bucket approach, which has been often found to be too simplistic represent vadose zone adequately under shallow groundwater conditions. Recently, a solution Richards equation Mualem–van Genuchten model (Rich‐vGM) added into EPIC (Environmental Policy Integrated Climate) address this shortfall. Its performance was tested lysimeters operating free drainage (FD) and at table (60‐ [WT60] 120‐cm depth [WT120]). Model accuracy also compared with upgraded bucket‐based method implemented (the variable saturation hydraulic conductivity [VSHC]). Soil content (SWC) data were split calibration validation subsets. evaluation included annual evapotranspiration (ET), percolation (PRK), upward movements assess underlying soil balance factors. The submodels provided accurate similar results upon comparison SWC measures FD (Nash–Sutcliffe coefficient [NSE] = 0.26 0.61 VSHC Rich‐vGM, respectively). Rich‐vGM accurately reproduced observed ET (e.g., NSE 0.70 percentage bias [PBIAS] −3.7% for WT120, respectively) although it slightly overestimated PRK (PBIAS 47.8%, on average). Instead, proved unable correctly conditions −1.85 WT60 SWC). Under conditions, is recommended, despite additional required need define bottom boundary according fluctuations. In conclusion, solver introduced in improved model's ability complex biophysical processes terrestrial ecosystems associated hydrological balance.