Roles of floods for agricultural production in and around Tonle Sap Lake

摘要

In monsoon Asia, agricultural water use is well integrated with other water uses. For watershed management, not only do paddy areas help regulate flooding by acting as a retarding basin (pond), but the rational supply of water resources is ensured by using the natural hydrologic conditions. This function is commonly found in low-lying paddy areas such as paddies in Japan, Cambodia, Vietnam and other countries. Typical examples for this are found in the Mekong River Basin (drainage area: 790,000km~2; river length: 4400km). This paper discusses the role of floods for rice production in Tonle Sap Lake and its environs by analysing the following examples. The first is flood storage by Tonle Sap Lake, where a unique reverse flow from the river to the lake occurs during flood seasons. Paddies are in flooding areas and rice cultivation starts after the flood water recedes, so floods and agriculture are closely interrelated. The second one is the presence of reservoirs formed by dikes in paddy areas. These not only protect the urban area of Phnom Penh from floods, but also supply irrigation water. The third is the use of warp soil dressing by colmatage. Colmatage is a facility for digging out a natural levee and leading water from the Mekong and Bassac Rivers into areas behind the levees as the water levels in the rivers increase. Part of the water stored through colmatage during the flood season remains in the area even after the water levels in both rivers decrease, and this water is used for rice production during the dry season.