Determination of the aerodynamic roughness length of a bare soil field using Monin-Obukhov similarity theory Makito MORI * † , Hisashi Y OSHIKOSHI **, Tetsuo KOBAYASHI **, Weizhen W ANG***, and Daisuke Y ASUTAKE * *Faculty of Agriculture, Kochi University, Nankoku, Kochi, 783–8502, Japan **Faculty of Agriculture, Kyushu University, Fukuoka, 812–8581, Japan ***Cold and Arid Regions Environmental and Engineering Research Institute, CAS, Lanzhou, 730000, China Abstract 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 simple profile 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 (x ) were made in the field with a sonic anemometer-thermometer for two days in late April of 2008, when the field was not fro- zen but without vegetation. These measurements were substituted into the expression of the Obukhov length, and equations for H and x that were derived based on Monin-Obukhov similarity theory, and three kinds of estimates of z0 were obtained. The estimates of z0 made 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 z0 estimated 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. Key words: Aerodynamic roughness length, Evaporation, Monin-Obukhov similarity theory, Universal similarity function. Received; February 4, 2009. Accepted; February 9, 2010. † E-mail: morimaki@kochi-u.ac.jp 1. Introduction In late autumn after the harvest, fields in the upper Yellow River valley are irrigated with a large quantity of water diverted from the Yellow River. This post-harvest irrigation is called the “autumn irrigation” (Wang and Akae, 2003; Feng et al., 2005). Part of the irrigated water is carried over winter in frozen soil layers and aquifers, and is used by crops in the following spring (Kaneko et al., 2006). The rest of the irrigated water is lost due to evaporation and deep percolation. However, the amount of evaporation in winter from fields irrigated in late autumn has not been evaluated with a sufficient degree of confidence; although a few measurements of it have been made, they are not necessarily consistent with each other (Kaneko et al ., 2006). The evaluation is essential for a proper understanding of the water budget of the “autumn irrigation” To study water-saving and salinity-controlled ir- rigation techniques, the present authors established an experimental field at Pingbu (Jingyuan, Baiyin, Gansu) (hereafter Pingbu field) in the upper Yellow River valley in 2007 (Wang et al ., 2008). A weather station 117 Short Paper J. Agric. Meteorol. (農業気象) 66 (2): 117-124, 2010