Influence of soil hydraulic parameters on bulb size for surface and buried emitters

Predicting the size expansion of wetting bulbs during surface and subsurface drip irrigation is compulsory for water saving and helps drive irrigation design and scheduling. To solve these issues, various numerical and analytical models, which take into account for the soil hydraulic parameters have been suggested in the literature. The model introduced by Philip (1984) is based on closed-form dimensionless solutions to determine the vertical and horizontal dimensions of the bulb expansion for both buried and surface point sources, under the assumption of homogeneous soil hydraulic properties. In this paper, the solutions provided by Philip (1984) were reformulated in dimensional terms and applied to an already available large data set of soil hydraulic parameters measured in a citrus orchard to evaluate the impact of soil hydraulic properties variability on the development of the wetting front. The results showed that the range of variability of the bulb geometric variables is similar for both buried and surface sources, with a slightly smaller boundary width observed for surface versus buried emitters. More importantly, the geometric bulb variables corresponding to the soil dataset, which characterizes the same soil, turned out to be very different, demonstrating that the soil hydraulic parameters have a strong control over the bulb size. In particular, the soil hydraulic properties have an important effect on the downward vertical expansion of the bulb for both surface (CV = 15.5 %) and buried (CV = 18.5 %) point sources. While the horizontal expansion of the bulb from the surface source (CV = 10.6 %) and the upward vertical expansion from the buried source (CV = 12.7 %) are a bit less affected. Therefore, the risk of an inadequate soil hydraulic characterization could be an incorrect estimate of the irrigation volume to be imposed, thus underwatering or overwatering the root zone.