Retarding Capacity Estimation of Wetland Paddy Fields under Climate Change and Land Use Change

Wetland paddy fields are known to have a function of flood control. Recently in Japan, under rapid depopulation and aging, it will soon become difficult to maintain budget for traditional river improvement such as embankments and riverbed excavation. In such a situation, ecosystem-based disaster risk reduction (Eco-DRR), i.e., utilising ecosystems possessing flood storage capacity, such as paddy fields and mountainous forests, gradually come into the limelight as one of prospective flood alleviation schemes. However, flood storage capacity of those ecosystems is quite vulnerable to change of their surrounding conditions because of its nature-based function. Thus, as the effects of climate change and depopulation emerge on various social aspects in these days, it is worthwhile to explore whether these nature-based flood storage facilities can keep their function after changing the surrounding conditions in future.

In this paper, inundation processes and retarding capacity of wetland paddy fields were evaluated using a commercial inundation simulation software, AFREL, based on a 2-D shallow flow model. In addition to the present precipitation, predicted precipitations for the end of this century based on RCP8.5 were given to the model, and how the flood control function is affected was examined. Moreover, how land use change in the test field, including housing land development and cultivation abandonment, will change the flooding characteristics there is also of interest. A scenario of the housing land development was considered based on a prediction by an urban development model and the highway planning. Whereas cultivation abandonment scenario was considered based on depopulation estimation for the field. Financial advantages of keeping paddy fields as a retarding basin rather than applying new river improvement works were also examined.

After careful examination on future precipitation prediction including the down scaling and the bias correction, it was found that particularly for the test field total amount of precipitation within one rainfall event, from frequent to rare probabilities, do not change very much from the current condition. Currently change of hyetograph shape, and the worst possible scenario among them, is analysed then the results will be obtained before preparing the full paper. On the other hand, land use changes clearly influence the inundation process and retarding capacity. Among tested scenarios, the planned highway with raising the road surface level well prevents flood spreading, indicating that road level rising, setting surrounding a protective area, can be used effectively as an alternative of extensive river bank construction.