Ring dike system to harness floodwater from the Mekong River for paddy rice cultivation in the Tonle Sap Lake floodplain in Cambodia P. Someth a,d , N. Kubo b, *, H. Tanji c , S. Ly d a United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509, Japan b Department of International Environmental and Agricultural Science, Tokyo University of Agriculture and Technology, 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509, Japan c Department of Geotechnical and Hydraulic Engineering, National Institute for Rural Engineering, 2-1-6 Kannondai, Tsukuba, Ibaraki 305-8609, Japan d Department of Rural Engineering, Institute of Technology of Cambodia, BP 86, Pochentong Boulevard, Phnom Penh, Cambodia 1. Introduction Cambodia is located in South-East Asia. About 36% of its population is living below the poverty line (ADB, 2005). Seventy-nine percent of manpower is working in the agriculture sector (AFD, 2006), which was responsible for 23% of the country’s gross domestic product (GDP) in 2000 (McKenney and Tola, 2002). Cambodia’s population is expected to reach 19 million in 2020 (NIS, 2004), at its current growth rate of 2.4%. High population growth indicates the need for more food supply. Development of the agriculture sector in this country is thus a fundamental and essential factor to boost the economy as a whole. The Tonle Sap Lake of Cambodia has a huge floodplain and potential water resources, suitable for large amount of rice production. The lake rises and falls according to the seasons. In June, the start of the rainy season, the lake receives water from the Mekong River. As a result, the lake area gets six times larger, from 2500 km 2 in the dry season to 15,000 km 2 during the rainy season (Fig. 1) with the water volume varying from agricultural water management 96 (2009) 100–110 article info Article history: Received 15 June 2007 Accepted 16 July 2008 Published on line 8 October 2008 Keywords: Floodplain development Floodwater Mekong River Rice cultivation Ring dike Tonle Sap Lake Water balance abstract Located in the floodplain of the Mekong River and the Tonle Sap River, Batheay irrigation system and its reservoir directly receive floodwater from the Mekong. The Batheay reservoir formed by a ring dike functions as both a reservoir and a paddy field. In the wet season, the ring dike prevents floodwater from entering the reservoir and rainy season rice is grown inside the dike. After harvesting, the gates on the ring dike are opened to receive floodwater. The water is stored inside the dike for cultivating dry season rice outside the dike. In this paper, the irrigation system is studied as a model site for future development of the floodplain of the Tonle Sap Lake of Cambodia. Specifically, this paper is concerned with the study of water balance and analysis of the hydrologic components of the Batheay irrigation system, and the effectiveness of the ring dike system. The study found that floodwater of the Mekong River contributed about 74% to the total inflow to the Batheay reservoir. Contributions to the total water supply of reservoir water, floodwater remaining in the fields, and precipitation were 73, 12, and 15%, respectively. The efficiency of the system was found to be 92%. The dike system is expected to be a paradigm for the floodplain of the Tonle Sap Lake. # 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +81 42 367 5757; fax: +81 42 367 5757. E-mail address: kubo@cc.tuat.ac.jp (N. Kubo). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2008.07.010