SIMULATION OF THE WATER BALANCE OF SOIL COLUMNS AND FALLOW SOILS

A model is described that predicts the evaporation of water from, and the distribution of water in, a soil column evaporating into a constant environment. It is based on a numerical solution of the flow equation and requires only the initial water distribution in the column, the equilibrium (air-dry) water content at the soil surface and the relationship between volumetric water content and diffusivity. The model predictions show good agreement with a published analytical solution and with experimental results. Modifications to the model that allow for rewetting of the sod by rainfall, and changes in atmospheric conditions above the soil, enable predictions to be made of the water balance of a fallow field. In general, good agreement was obtained with the measured distribution of water deficits in the soil profile, although the predicted water content of the surface 2.5 cm of soil showed systematic differences from the measured values. The reasons for this are discussed.