A Comprehensive Model for Single Ring Infiltration I: Initial Water Content and Soil Hydraulic Properties

Single ring infiltration tests are commonly used to understand soil hydraulic properties. While a number of models have been developed to describe three-dimensional infiltration from single ring sources, these expressions are often restricted to cases where the soil is initially dry and the source pressure head is negligible. These conditions may restrict the use of these expressions in real field settings. In this study we modified a set of infiltration models to explicitly account for variations in soil hydraulic properties, initial conditions, and experimental setups. The resultant expressions allowed us to explore how soil capillary length (a measure of the capillary force acting through the wetted zone) and three-dimensional wetting profiles vary under different initial matric heads. Specifically, as initial matric head increases, infiltration becomes increasingly one-dimensional due to a decrease of the soil capillary force. However, for most realistic situations (i.e., moderately wet to very dry initial conditions) the model can be simplified by assuming a constant capillary length. The resulting expressions compared well with numerical results from HYDRUS-3D for five different soils, including both dry and wet initial conditions, two ring insertion depths, and two ponding depths. Overall, the model provides further insight on the variations in soil capillarity (as sorptivity) and infiltration across a range of soil types and initial conditions, and can be used to describe single ring infiltration under nearly every scenario.