Hydraulic Model of Transition of Transient to Steady Flows in the Vadose Zone

摘要

A simple hydraulic model is proposed to illustrate the transition mechanism of transient to steady flows in the vadose zone. This model consists of a set of cylinders that are vertically aligned, and each of the cylinders is open at the top and has a narrow outlet at the bottom. An unsaturated soil unit in the vadose zone is considered one of the cylinders, and the entire soil column is modeled as cylinders arranged in series. The cylinder is representative of the soil skeleton of the corresponding soil unit, the water distributed in the cylinder is representative of the water in the soil, and the water flow rate out of the outlet is representative of the soil permeability. Physical tests and theoretical analyses revealed that the water flow in the cylinders can also be obviously divided into transient and steady flow, which resembles the water flow in unsaturated soils. Moreover, the transition process of water flow from the active zone to the steady zone in unsaturated soils can be well captured by the model. The influence of water influx rates, soil permeability, and initial water contents on unsaturated infiltration can also be investigated using this model. The results show that the steady water content of the unsaturated soil depends on the water influx rate and soil permeability and is independent of the initial water contents. As a result, water flow in the vadose zone, especially in the steady zone, which is difficult to detect and monitor in the field, can be better understood with the proposed model. (C) 2019 American Society of Civil Engineers.