Impact of paddy irrigation levels on floods and water use in the Mekong River basin

During the Asian monsoon, water use is considered to be well integrated because of the sustainable use of the agricultural water supply. From the viewpoint of watershed management, paddy areas help regulate floods by serving as retarding basins (ponds), but they also ensure a rational supply of water resources by taking advantage of natural hydrologic conditions. Paddies commonly serve both agricultural and flood‐control functions in low‐lying areas in Japan, Cambodia, Vietnam, and other Asian countries. We discuss herein the impact of irrigation levels on floods and water use by analyzing typical examples of paddies harmoniously used for agriculture and flood regulation in and around Tonle Sap Lake and its environs in the Mekong River Basin (drainage area, 790 000 km 2 ; river length, 4400 km). Flood storage by the Tonle Sap involves a unique reverse flow from the river into the lake during the flood season. Rice cultivation starts after the floodwater recedes, so floods and agriculture are closely interrelated. A second example involves reservoirs created by dikes in paddy areas that protect the urban areas of Phnom Penh from floods and supply irrigation water. A third example involves the use of colmatage, in which a levee is created and water is led behind the levee from the Mekong and Bassac Rivers as the water levels in the rivers rise. Part of the water stored in this manner during the flood season can be used for rice production during the dry season. We then estimate the volume of flooded water on paddies in order to evaluate the impact of non‐irrigated paddies on floods and water use. We also evaluate the potential to harmonize the role of paddies in managing floodwaters with their agricultural role and propose policies, for Japan and the rest of the world, that could help resolve flooding problems.