Perched groundwater simulation using the picard iteration‐based always active cell method

Abstract Aquitards within the vadose zone impede vertical infiltration, leading to the formation of saturated perched groundwater. Perched groundwater flows laterally along aquitards, resulting in a heterogeneous spatial distribution of groundwater recharge that significantly influences groundwater formation and evolution. Because the evolution of perched groundwater involves both saturated and unsaturated flow, conventional saturated flow models cannot accurately simulate it. Moreover, the drying‐rewetting processes of grid cells in saturated flow models often cause convergence difficulties. This study evaluates the accuracy, convergence, and computational efficiency of the recently developed Picard iteration‐based always active cell (PAAC) drying‐rewetting method for perched groundwater modeling. Two representative perched groundwater scenarios were used to compare the performance of the saturated‐unsaturated flow model VS2DT, the saturated flow model MODFLOW‐NWT, and the saturated flow model COMUS‐PAAC, which incorporates the PAAC method. Results show that both COMUS‐PAAC and MODFLOW‐NWT completed the simulations within 2 s, exhibiting substantially higher computational efficiency than VS2DT. COMUS‐PAAC and MODFLOW‐NWT tended to overestimate the rate of perched groundwater formation and evolution; however, as recharge increased and the degree of unsaturation in VS2DT decreased, their simulated evolution converged toward that of VS2DT. Because the PAAC method implicitly accounts for unsaturated flow mechanisms, the perched groundwater thickness simulated by COMUS‐PAAC differed from that of VS2DT by less than 3%, and its mean and maximum deviations were approximately one‐seventeenth and one‐twenty‐first, respectively, of those produced by MODFLOW‐NWT. Consequently, COMUS‐PAAC achieved markedly higher simulation accuracy than MODFLOW‐NWT. Moreover, under extreme and highly nonlinear perched groundwater conditions, COMUS‐PAAC exhibited convergence comparable to that of MODFLOW‐NWT, a model well recognized for its robust convergence. In conclusion, the PAAC method offers an efficient, reasonably accurate, and robust approach for simulating perched groundwater within saturated flow models, and demonstrates potential for large‐scale, long‐term, and complex real‐world perched groundwater simulations.