Paddy Water Environ (2005) 3: 177–186 DOI 10.1007/s10333-005-0015-5 TECHNICAL REPORT Hiroaki Somura · Hajime Tanji · Koshi Yoshida · Osamu Toda · Katsuhiro Higuchi Estimation of supplementary water to paddy fields in the lower Mekong River basin during the dry season Received: 4 August 2004 / Accepted: 1 June 2005 / Published online: 13 September 2005 C  Springer-Verlag 2005 Abstract Efficient management of water resources in paddy fields requires an understanding of the volume of supplementary water used. However, quantifying the vol- ume is laborious due to the large amount of data that must be collected and analyzed. The purpose of our study was to estimate the volume of supplementary water used in paddy fields, based on several years of available statistical data, and to provide information on how much water can be sup- plied to paddy fields in each target area. In this study, the lower Mekong River basin of northeast Thailand, Laos, and Cambodia was selected as the study area. In the first step, we used agricultural statistics for each country, rainfall data acquired from the Mekong River Commission Secretariat (MRCS), and the value of virtual water required per unit of rice production. Because several years of data were used for dry season harvested areas and rice production in each country, the supplementary water to paddy fields in each province was calculated using virtual water and rainfall. This method made it possible to estimate changes in sup- plementary water in each province. Through this study, the H. Somura () Shimane University, Faculty of Life and Environmental Science, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan e-mail: som-hiroaki@life.shimane-u.ac.jp H. Tanji · K. Higuchi National Institute for Rural Engineering, River and Coast Laboratory, 2-1-6 Kan-non dai, Tsukuba, Ibaraki 305-8609, Japan K. Yoshida Japan Science and Technology Agency, River and Coast Laboratory, 2-1-6 Kan-non dai, Tsukuba, Ibaraki 305-8609, Japan O. Toda Graduate School of Agricultural and Life Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan supplementary water to paddy fields during the dry season in three countries was approximated from the minimum number of data sets. Moreover, for cases in which it is not possible to procure agricultural water use data for a hydrological model simulation, an alternative solution is proposed. Keywords Water use . Virtual water . Monsoon Asia . Rice production . Thailand . Laos . Cambodia Introduction Numerous studies have been performed on paddy manage- ment in countries located in the lower Mekong River basin. For example, Suzuki et al. (2001) and Kono et al. (2001) studied rice production in rainfed paddy fields in northeast Thailand; Linquist and Sengxua (2001) and Sipaseuth et al. (2001) studied nitrogen management for rainfed lowland rice and rice yield improvement in Laos; and Rickman et al. (2001) and Makara et al. (2001) studied rice production methods in Cambodia. However, few studies have examined water supplied to paddy fields in the basin. Although hydrological models are generally employed to estimate the supplementary water used in paddy fields, it is very difficult to construct a model in this case due to the scarcity of hydrological data and related information in the lower Mekong River basin. Number of pump sets as one type of related information is needed to create a function for estimating the volume of supplementary water in the model’s target areas. Gathering such data and information in local areas, however, is time-consuming and sometimes impossible to obtain for all the target areas. To assess efficient water use and management, the volume of water supplied must be quantified, at least approximately. Thus, the objectives of this study were to represent the amount of water supplied to paddy fields over several years by a quick and easy method, and to provide information on how much water can be supplied to paddy fields in each target area as input data to the model, instead of creating a function from data such as the number of pump sets.