HYDROLOGICAL PROCESSES Hydrol. Process. 18, 2223–2234 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5535 Seasonal variation of energy partitioning in irrigated lands Yanjun Shen, 1,2 * Yongqiang Zhang, 1 Akihiko Kondoh, 3 Changyuan Tang, 2 Jianyao Chen, 2 Jieying Xiao, 2 Yasuo Sakura, 4 Changming Liu 1 and Hongyong Sun 1 1 Institute of Agricultural Modernization, CAS, 286 Huaizhong Rd., Shijiazhuang 050021, People’s Republic of China 2 Graduate School of Science and Technology, Chiba University 1-33 Yayoi, Inage, Chiba 263-8522, Japan 3 Center for Environmental Remote Sensing, Chiba University 1-33 Yayoi, Inage, Chiba 263-8522, Japan 4 Department of Earth Sciences, Chiba University 1-33 Yayoi, Inage, Chiba 263-8522, Japan Abstract: The energy balance components were measured above the surface of an irrigated wheat and maize field over three successive years using the Bowen ratio technique. The experiments were carried out at Luancheng Experimental Station of Agro-ecosystem (LESA), Heibei, China, from December 1999 through to September 2001. The seasonal course of energy balance over an irrigated field, the diurnal Bowen ratio patterns in different seasons, and the effect of leaf-area index (LAI) and soil moisture on the energy balance are discussed. Over 3 years, the net radiation R n varied from 31Ð6 to 668Ð1Wm 2 , and the soil heat flux G varied from 12Ð7 to 170Ð9Wm 2 . The latent heat flux LE also shows apparent correspondence with the development of phenology, e.g. LAI. The diurnal course of the Bowen ratio in different seasons can be categorized into three typical patterns: (1) a ‘wheat pattern’, characterized by a steep morning peak followed by a decrease with the daytime mean value of around 0Ð30; (2) a ‘maize pattern’, which is a relatively flat course with a daytime mean ˇ of around 0Ð20–0Ð25; and (3) a ‘winter pattern’, with a near-noon high peak with a daytime mean ˇ of more than 10 times that those for wheat or maize. There are linear correlations between evaporative fraction (EF) and LAI for both wheat and maize before senescence seasons. The correspondence of EF appears more dependent on LAI for maize than for wheat. The EF does not appear correlated to soil water status, whereas the Bowen ratio is affected by extractable soil water content for wheat to some extent. No correlation for maize is found. Copyright  2004 John Wiley & Sons, Ltd. KEY WORDS energy partitioning; Bowen ratio; evaporative fraction; LAI; extractable soil water content; wheat; maize INTRODUCTION During the past decades, increasing interest has been focused on evapotranspiration (ET) from the land surface as a key component of the water cycle. ET links energy partitioning, stomatal conductance, carbon exchange, and water-use efficiency in plant communities, and it serves as a key regulator of ecosystem processes (Woodward and Smith, 1994; Sellers et al., 1996). In particular, it is also considered as the interaction of vegetation with the atmosphere from the aspect of global climate change. In the context of the North China Plain (NCP), wheat and maize are the main crops cultivated in rotation and constitute a major part of the landscape. Their land surface radiation balance, energy partitioning and groundwater table change are therefore crucial for the regional climate and hydrology, as well as the attention focused on food problems (e.g. Brown and Halweil, 1998). Therefore, it is important to quantify the energy balance components, to understand the different factors and their influence on these terms, their rhythmic change and the monitoring of the evaporative processes over the region. *Correspondence to: Yanjun Shen, Oki Lab, IIS, University of Tokyo, Komaba 4-6-1, Megaro-ku, Tokyo 153-8505, Japan. E-mail: sheny@ceres.cr.chiba-u.ac.jp Received 20 December 2002 Copyright  2004 John Wiley & Sons, Ltd. Accepted 30 July 2003