Measurement and simulation of evaporation from a red earth. II. Simulation using different evaporation functions

Abstract

The performance of several alternative evaporation functions for simulating water loss from a bare red earth was assessed by including them in a computer model which was used to simulate evaporation both from red earth monoliths in a glasshouse, and from a study site in the field. The coefficients in the different evaporation functions were also optimized to minimize the root mean square discrepancy (RMSD) between simulated and observed soil water contents. RMSD values for the alternative evaporation functions before optimization of coefficients ranged from 3.2 to 7.0 mm for the glasshouse data and from 4.0 to 6.6 mm for the field data. Optimization reduced these values 3.0 to 6.4 mm (glasshouse) and 3.9 to 6.1 mm (field). The sensitivity of the model to errors in hydraulic conductivity estimates was assessed. Overestimating hydraulic conductivity by 2 and 10 times increased predicted cumulative evaporation by 8 and 28% respectively. Underestimating conductivity by the same factors produced similar reductions in predicted cumulative evaporation. The model was used to test the effect of basing the simulation of field evaporation on different thicknesses of surface compartment, for two alternative evaporation functions. Optimum thicknesses of surface compartment were 20 and 30 cm, and increasing these thicknesses to 60 cm resulted in only c. 20% increase in RMSD. This effect was considerably less than the increase caused by using inferior alternate types of evaporation function.