Determination of the Aerodynamic Roughness Length of a Bare Soil Field Using Monin-Obukhov Similarity Theory

In order to measure the evaporation from a field in the upper Yellow River valley in winter, which is essential for a proper understanding of the water budget of the “autumn irrigation”, by a simplprofile method using routine measurements of wind speed and direction made at one height and those of air temperature and humidity at two heights, an intensive observation was conducted to determine the aerodynamic roughness length (z₀) of the field bare of vegetation. Measurements of turbulent fluxes of sensible heat (H) and momentum (τ) were made in the field with a sonic anemometer-thermometer for two days in late April of 2008, when the field was not frozen but without vegetation. These measurements were substituted into the expression of the Obukhov length, and equations for H and τ that were derived based on Monin-Obukhov similarity theory, and three kinds of estimates of z₀were obtained. The estimates of z₀made using the H equation were smaller than those made using the τ equations for neutral and non-neutral stratification and had a great dependence on the wind direction. Daily evaporation calculated from the equation for the turbulent flux of water vapor, which was also derived based on Monin-Obukhov similarity theory, using the z₀estimated using the H equation and an iteration technique were 1.1~1.3 times as large as the measurements made by the Bowen ratio method. Based on this conclusion, the evaporation from this field in winter was analyzed, which will be shown in the following paper.