A numerical model to incorporate the influence of shrinkage cracks on the transient moisture movement through unsaturated soil

Most natural soil deposits develop a complex network of relatively coarse voids or macropores. In swelling soil, the macropores consist primarily of shrinkage cracks that develop when the soil is subjected to prolonged dry weather. The macropores are known to allow water to move faster and deeper into the soil profile. In addition, while migrating downward through the macropores, some of the water is absorbed by the micropores in the surrounding soil matrix. The conductivity through the macropore are several orders of magnitude larger than the conductivity of the intact matrix. The macropore conductivity is controlled by the size of the macropores, and the size of the macropores is dependent on the moisture contents within the soil mass. Both the flow through the macropores and the flow through the micropores have to be considered in the analysis of the flow through porous media which consists of both macropores and micropores.

In this study, a numerical model was developed to simulate the flow within the two pore systems. The conductivity of the macropores was estimated from an empirical equation. The flow in the macropores was modeled according to a simple water balance scheme. The resulting lateral flows were then coupled into the source term in the Richards equation. Finally, the Richards equation was solved using the finite element method to give the changes in the suction due to both the input fluxes at the surface and the lateral flows from the macropores.

The model was designed in such a manner that the simulation can be performed in 2-dimensions with minimum soil information. The model was also equipped with a weather data processor and weather generator.

The model was calibrated with field data from two sites. The result of the calibration showed that the model can fit the field data fairly well. Furthermore, several simulation runs were performed with the numerical model for several macroporous field conditions. The results of these simulations showed that macropores play an important role in water infiltration and recharg in macroporous soil.