Abstract: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.
A hydrodynamic model, which covers the floodplains of the Mekong River from Kratie in Cambodia to near the Vietnamese border, was developed using a finite element method (FEM) with 2D shallow-water equations. The model was applied to typical flows of recent largest (2000) and smallest (2003) flood years. Main roads, dikes, colmatages, and road-opening works in the study area, which may influence flow regimes, were introduced into the simulation. Moving boundary problems were accommodated by applying a threshold technique in which nodes having a thin water depth were reset to dry ones in all moving boundary elements at every time increment. Simulated results and available observed water levels and discharges were compared to validate the model. The simulated results produced a lot of hydrologic data for the basin areas, without gauging, and these data can be used to evaluate the effects of basin management on flooding and agricultural water use.
In order to assess the effects of climate change on flood disasters in urban areas, we applied a two dimensional finite element hydrodynamic model (2D-FEM) to simulate flood processes for the case analysis of levee breach caused by Kathleen Typhoon on 16 September 1947 in Kurihashi reach of Tone River, upstream of Tokyo area. The purpose is to use the model to simulate flood inundation processes under the present topography and land-use conditions with impending extreme flood scenarios due to climate change for mega-urban areas like Tokyo. Simulation used 100 m resolution topographic data (in PWRI), which was derived from original LiDAR (Light Detection and Ranging) data, and levee breach hydrographic data in 1947. In this paper, we will describe the application of the model with calibration approach and techniques when applying for such fine spatial resolution in urban environments. The fine unstructured triangular FEM mesh of the model appeared to be the most capable of introducing of constructions like roads/levees in simulations. Model results can be used to generate flood mapping, subsequently uploaded to Google Earth interface, making the modeling and presentation process much comprehensible to the general public.
Reducing methane (CH 4 ) emissions from paddy fields that contribute to the greenhouse effect has been addressed recently through the application of the alternate wetting and drying irrigation method. However, in poorly drained areas, such as the Red River Delta in Vietnam, the soil cannot be dried immediately, and so CH 4 can continue to be produced unintentionally. Therefore, the purpose of this case study was to identify the optimal ponding water management schedule to reduce CH 4 emissions by using the measured data of ponding depth, soil redox potential (Eh), and CH 4 fluxes from field experiments and to show its effects on CH 4 emission and water conservation. Observations in the winter-spring cropping season showed that the non-flooding period of 3-8 days suppressed CH 4 emission, and the continuous flooding period of 14-22 days caused CH 4 re-emission. Information regarding the non-flooding period to be maintained and the flooding period to be avoided to suppress CH 4 emission was not obtained for the summer-autumn cropping season due to abundant rainfall. The proposed schedule could suppress CH 4 emission by 27%-85% and increase the amount of conserved water by up to 18% compared with traditional flooding protocols, but it may increase irrigation water due to the frequency and the amount of re-flooding. K E Y W O R D S greenhouse gas, paddy irrigation, soil redox potential, Southeast Asia, water conservation Résumé La réduction des émissions de méthane (CH 4 ) provenant des rizières qui contribuent à l'effet de serre a été abordée récemment grâce à l'application d'une méthode d'irrigation alternée par mouillage et séchage. Cependant, dans les zones mal drainées, comme le delta du fleuve Rouge au Vietnam, le sol n'a pas pu être séché immédiatement et du CH 4 pourrait continuer à être produit involontairement. Par conséquent, le but de cette étude de cas était d'identifier * Gestion de l'eau des mares de riz pour réduire les émissions de méthane sur la base d'observations des flux de méthane et du potentiel redox du sol dans le delta du fleuve Rouge, Vietnam
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