Hydraulic performance and water balance modeling of biochar–sand capillary barrier systems for enhancing soil moisture conservation

Climate change–driven increases in drought frequency and rainfall variability have reduced soil water availability in rainfed agricultural systems, underscoring the importance of capillary barrier systems as a soil water conservation strategy for improving moisture retention and agricultural sustainability. This study evaluated the hydraulic performance and water balance of different capillary barrier systems for improving soil moisture conservation under simulated rainfall conditions. A laboratory-based completely randomized design was employed using transparent PVC soil columns containing four treatments: loam soil (control), fine sand capillary barrier, gravel capillary barrier, and biochar–sand capillary barrier, each with three replications. Rainfall was simulated at an intensity of 50 mm h^-1 for 60 min. This rainfall regime was selected based on rainfall characteristics reported for Philippine wet-season storm events and provided a controlled, repeatable infiltration event for evaluating the hydraulic performance and water balance of the biochar–sand capillary barrier system. Hydraulic responses, including infiltration characteristics, saturated hydraulic conductivity, soil moisture retention, runoff, deep percolation, evaporation, sediment yield, and water storage efficiency, were quantified. Soil water retention was described using the van Genuchten model, while infiltration behavior was evaluated using the Green–Ampt, Philip, and Kostiakov models. Based on the combined criteria of lower saturated hydraulic conductivity, greater available water capacity, and higher soil water storage, the biochar–sand capillary barrier significantly outperformed the control treatment. Saturated hydraulic conductivity decreased from 4.86 to 3.28 cm h^-1, available water capacity increased from 14.6% to 24.3%, and soil water storage improved by 52.2% (p < 0.05). Deep percolation, evaporation losses, and sediment yield were reduced by 52.2%, 30.2%, and 46.8%, respectively, while water storage efficiency increased to 84.6%.  Among the evaluated models, the Philip infiltration model showed the best fit to the experimental data (R² = 0.95, NSE = 0.91). Within the experimental dataset, the developed water balance model reproduced the observed hydraulic responses with prediction errors below 3%; however, its predictive capability requires independent validation under additional conditions.These findings indicate that biochar–sand capillary barrier systems effectively improve soil hydraulic performance and rainfall-use efficiency and represent a promising, low-cost, and climate-resilient strategy for sustainable soil water conservation in rainfed agriculture. Future field-scale studies are recommended to validate their long-term performance under diverse environmental conditions.