Exploring the impact of average water content on wetting bulb expansion from a buried point source

Subsurface drip irrigation (SDI) represents a significant innovation in water-saving irrigation methods. However, accurately predicting the shape and expansion of the wetting bulb remains a challenge due to the complex interaction between soil hydraulic properties and emitter characteristics. This study investigates the influence of the average volumetric water content, θavg, in determining the wetting bulb expansion from a buried point source. The analysis is based on six years (2018–2023) of experimental data collected in a citrus orchard in Sicily, Italy, equipped with an SDI system and a soil moisture probe. The simplified analytical model proposed by Philip (1984) was applied, with calibration of θavg using a k parameter, which provides θavg varying between the initial (θ0) and the saturated (θs) volumetric soil water content. After characterising the soil hydraulic properties required by Philip’s model, results from 832 travel time (tt) and θ0 measurements showed that the simplified assumption of k equal to 0.5 often leads to significant overestimations of travel times. In contrast, calibrating the model substantially improved the accuracy of tt estimation, reducing the standard error of the estimate from 4 h to 26 min, which is moderate considering the temporal resolution of data acquisition (20 min). These findings highlight the impact of the average volumetric water content for accurately predicting the shape of the wetting bulb using Philip’s model, while also accounting for uncertainties associated with the simplified assumptions of the original model.