Abstract
Soils of rainfed rice fields have unique characteristics. Inland valleys are extensively found in West African savannas, and their hydromorphic valley bottoms are intensively used for rainfed rice-culture during rainy seasons when become wetlands. Data of rainfall and soil moisture are perennially collected in a study site located in Northern Region of Ghana. The soil of the site is classified as Dystric Planosols. The soil moisture retention curve at a depth of the soil is identified from observed volumetric water content and matric head. Since the soil is variably saturated, it is important to clarify abruptly changing hydraulic properties of the soil near saturation. A bimodal model including eight empirical parameters turns out to be suitable for this purpose. The vertically one-dimensional Richards equation is considered to reproduce infiltration process of rainwater into the soil. The saturated water content, one of the empirical parameters, and saturated hydraulic conductivity are assumed to be functions of the depth of soil. Then, the finite element method is applied for solving the Richards equation to simulate the process. The simulation results are in good accordance with actually observed data. The physically-based model serves as a tool for comprehending formulation process of the wetlands at early stages of rainy seasons.
