A Wetting-Front Model for Vadose Zone Infiltration via Drywells

Drywell infiltration is a common approach to recharge groundwater and reduce load from drainage systems. Due to rapid acceleration in urban developments, as well as climate change that predicts an increase in frequencies and magnitude of extreme precipitation events in the Mediterranean area, it is critical to predict the drywell infiltration capacity, i.e., its response to anticipated precipitation events. The infiltration capacity of a drywell is mainly determined by the geometrical parameters, i.e., diameter and depth, and the soil hydraulic parameters, i.e., hydraulic conductivity, porosity, and water retention. Predictions of drywell infiltration capacity are commonly conducted using models that solve the unsaturated flow in the subsurface using complex and costly numerical schemes. This work proposes a different approach based on a semi-analytical model that relies on a sharp interface wetting front assumption. The proposed model can predict the water levels in the well and the subsurface wetting front location during and after an infiltration event. The semi-analytical model was tested and compared with numerical simulations of Richards' equation and with data from a field experiment and proved to be sufficiently accurate. The typical run times of the semi-analytical model are smaller than 1 second and about three orders of magnitude shorter than the numerical model of Richards' equation. The field data was further utilized to calibrate the soil hydraulic properties by matching the semi-analytical model's outcomes to the measured water levels in the well. A sensitivity analysis of the drywell response to variable hydraulic properties, climatic scenarios, and well configurations (depth and diameter) was conducted, demonstrating some practical applications for analysis, which may be used for adequately determining site-specific drywell design.