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Development and Validation of a Numerical Model Simulating Evaporation from Short Cores¹

ABSTRACT

A theoretically-based assessment of the short core evaporimeter technique for measuring evaporation from the soil surface can be conducted using numerical simulation. As an initial step, a numerical model is developed that simulates evaporation from isothermal, homogeneous, finite soil columns with a specified initial water content profile. Features of this model include nonlinearized solution of the highly nonlinear surface boundary condition, automatic adjustment of time-step size according to a mass balance criterion, remaximization of program efficiency at each time-step, and the use of soil surface temperature as the primary forcing function. The validity of the model was established through its ability to realistically simulate the constant and falling rate stages of soil drying, including root-time behavior, and by its good mass conservation and stability characteristics. Drying of a clay soil with an initial uniform water content of 0.40 cm³_w cm^-3_pm for 12 000 min under a constant evaporative demand of 4.4 x 10^–4 cm³_w cm^-2_pm min^–1 could be simulated with a time-step size ranging from 10 to 13.7 min, a specific mass (i.e. flux) balance of < 3%, a cumulative mass balance of < 0.3%, and with generally < five iterations per time-step. A discretization analysis revealed that for the first centimeter below the soil surface a 0.2-cm nodal spacing was required to obtain accurate estimates of the evaporative flux and the near-surface water content profile. Below that depth, however, nodal spacing can be greatly increased without serious reduction of accuracy. Successful validation of the model supports extension to field conditions.

¹ Contribution from the Dep. of Land Resource Science, Univ. of Guelph, Guelph, Ontario, Canada. N1G 2W1.

² Graduate Student and Research Scientist, respectively.