RELATIONSHIP BETWEEN INUNDATION AREA AND IRRIGATION AREA ON FLOOD CONTROL IN THE LOWER MEKONG TERUMICHI HAGIWARA Department of Civil Engineering, Tohoku University SO KAZAMA and MASAKI SAWAMOTO Department of Civil Engineering, Tohoku University hagiwara@kaigan.civil.tohoku.ac.jp Abstract-Flood and inundation are annual event, which is utilized for agriculture in the Lower Mekong basin. Inundation area can be associated with agricultural land. A numerical simulation was carried out to understand change of inundation area by flood control, and irrigation area estimated from reserved water. The decrease of inundation area caused by flood control is much smaller compared with potential irrigation area. Although decrease of inundating area influence agricultural land, that can be made up for by irrigation. However it is hard to prevent inundation of floodplain even if there is large-scale water control system. I. INTRODUCTION Now development of the Mekong river basin is one of the most notable planning all over the world. The Mekong river is an international river and its basin spreads six countries. Its development planning has various problems. For some decades of conflict and disruption over the basin countries, development in the Mekong is the lowest level in the world. The flood in rainy season and salinity intrusion from South China Sea in dry season are the most important annual events in the Mekong River. Flood and inundation analysis and assessment of salinity intrusion in the Mekong delta have been made with a numerical method ([1] and [2]). The subjects of those studies are not whole Mekong Delta but a part included in Vietnam. The Cambodian part studies are hardly made for the reason of difficulty in getting available data. Although annual flood and inundation have negative point such as disaster, that also has some merits. Inundating water is traditionally used as the water for agricultural use in this region. Inundation can be regarded as natural irrigation area, fish habitant, and grand water recharge. Water resource development in the Mekong basin may control annual flood and reduce the inundation area in the Mekong Delta. However it has not researched how the change of inundation area influences society and economy in term of water resource. Particularly agricultural area may be decided by decrease of inundation area. It is an important problem how much area can be irrigated with the water stored in the upstream dams or reservoirs by controlling a flood. That is useful in discussing the validity of development in this region. This paper estimates the change of inundation area under the various flood-controlled situation by using numerical simulation and irrigation area from stored water in upstream dams and from inundated water for natural irrigation in downstream. II. THE STUDIED REGION The Mekong River has its source in the south- eastern Himalayan mountains and flows 4200 kilometers to its mouth at the South China Sea, making it the world’s twelfth longest river. It ranks tenth in terms of discharge. The studied region of this study is a part of the Lower Mekong basin (Fig. 1). Annual precipitation in the Lower Mekong is strongly seasonal, as is the annual flow of the Mekong. About 85% of the precipitation falls during the rainy season. Annual precipitation averages about 1680 mm across the basin. The Mekong flood season lasts from July through December with an average discharge of 25,000 m 3 /s. The low flow season lasts from January to June with an average discharge of 6,000 m 3 /s. In this region Mekong has two large tributaries, Bassak and Tonle Sap, and these rivers joins at Phnom Penh, which is capital city of Cambodia. The end of Tonle Sap river is the Great lake(the Tonle Sap lake), which has a retarding function. Therefore the complicated flow is formed around Phnom Penh. Here spread low and flat land called Mekong Delta and floodplain cover a wide area. III. DATA SET As the boundary conditions in the flood simulation of rivers, water level data at Kampong Cham, upstream of the Mekong, at Prek Kdam of Tonle Sap, at Tan Chau, downstream of the Mekong and at Chau Doc of Bassak are used (Fig. 2). Water level data at Phnom Penh is used for validation of the simulation. Those data were periodically obtained by the Mekong river Commission (MRC). The simulation period is from July 6 to October 12.