Effective Saturation as an Indicator of Hydraulic Performance in Urban Infiltration Systems Using Numerical Simulations

This study evaluates the use of effective saturation as a comparative indicator of the hydraulic response of urban infiltration systems, with particular emphasis on infiltration trench geometry. Effective saturation is an established variable in vadose zone hydrology; therefore, the contribution of this study lies in its application to the comparative assessment of alternative trench configurations through numerical simulation. Two-dimensional numerical models were developed in COMSOL Multiphysics using the Richards equation and the van Genuchten–Mualem formulation to simulate transient unsaturated flow conditions. Nine geometric configurations, resulting from the combination of three trench widths and three depths, were evaluated under identical hydraulic loading and boundary conditions. The numerical analysis was conceived as a synthetic comparative framework, and the hydraulic response of the configurations was assessed from their spatial and temporal evolution of effective saturation. The physical and hydraulic parameters used as model inputs were obtained from previous field work. In addition, field infiltration tests were conducted using an infiltration trench and a Pask-type permeameter to characterize the hydraulic response of the study-site soil and provide an independent field reference for interpreting the numerical results. The proposed Hydraulic Performance Index combines the simulated effective-saturation response, geometric aspect ratio, and an engineering feasibility coefficient because these components represent complementary aspects of the comparative assessment: hydraulic response, trench geometry, and practical engineering feasibility, respectively. The three components are assigned equal weighting within the index formulation to avoid assigning a priori preference to any of these dimensions. The results showed that trench geometry influenced the spatial and temporal evolution of effective saturation. Among the evaluated configurations, the 0.3 × 1.0 m geometry achieved the highest Hydraulic Performance Index (V = 0.67), followed by the 1.0 × 1.0 m configuration (V = 0.61), whereas the 1.0 × 4.0 m geometry exhibited the lowest value (V = 0.42). Field infiltration tests yielded hydraulic conductivity values in the 10−6–10−5 m/s range, providing an independent reference for interpreting the simulated hydraulic response. Under the imposed boundary conditions, the results indicate that shallower configurations maintained a more distributed moisture response, whereas deeper configurations exhibited greater moisture accumulation in the lower regions of the domain. Because the simulations assume a constant ponding head, no-flow boundaries at the lateral and bottom boundaries, and no explicitly represented water table, groundwater conditions in the Aguascalientes Valley were not explicitly incorporated as a phreatic boundary in the model; instead, the simulations used a synthetic initial pressure-head condition of −0.06 m to represent a wet antecedent state. The proposed index should be interpreted as a comparative decision-support metric under controlled conditions rather than as a direct predictor of field performance. The findings demonstrate the potential of effective saturation as a comparative variable for evaluating hydraulic responses among infiltration trench geometries and provide a framework that can be further validated using field-scale observations.