Mid-latitude regions in the North Pacific are generally vulnerable to climatological disasters and are possibly more sensitive to future climate changes. Severe flood disasters struck Hokkaido in August 2016 because of the multiple, continuous typhoons that struck the island. We evaluated the effect of these typhoons on floods and changes in future floods using a distributed hydrological model in a watershed located in eastern Hokkaido. We conducted two numerical examinations: a simulation with a major typhoon only (which caused flood disasters) without other preceding typhoons, and a simulation with a simple assumed future climate (in which we employed higher precipitation). The result of the former simulation demonstrated that the impact of the preceding typhoons on the highest flood peak was significant during the early stage of the major typhoon but weaker after the middle stage of the major typhoon. The result of the latter simulation indicated that flood peaks potentially increased with an increase in precipitation. Based on the water level distributions in the surface layer, the impact of multiple typhoons and future weather conditions on potential flood peaks depends on the degree of soil saturation over our target watershed.
Secondary salinization of irrigated lands in drylands is often caused by rising groundwater levels. Open drainage is widely employed to control groundwater. However, salinity levels tend to remain high under malfunctioning drainage conditions. Shallow subsurface drainage may be a possible solution to prevent salt accumulation, although it is difficult for farmers to apply conventional tile drainage systems owing to construction costs. In this regard, we proposed a low-cost shallow subsurface drainage system used in combination with a new mole-drain drilling technology (cut-drain) developed in Japan, whose drainage capacity is similar to tile drain. The aim of this study is to evaluate the effect of the proposed system. The system was installed in a farmland, Uzbekistan. The experimental field was set with/without the system to observe the differences in the balance of water and salt. The results revealed that the remaining infiltrated water in the field decreased by approximately 26% and the removed net mass of salt was 14 Mg ha−1. The direction of salt movement changed from the deeper zone or surrounding field to the open drainage. Therefore, the proposed system can enhance salt removal from fields.
Recent extreme weather events like the August 2016 flood disaster have significantly affected farmland in mid-latitude regions like the Tokachi River (TR) watershed, the most productive farmland in Japan. The August 2016 flood disaster was caused by multiple typhoons that occurred in the span of two weeks and dealt catastrophic damage to agricultural land. This disaster was the focus of our flood model simulations. For the hydrological model input, the rainfall data with 0.04° grid space and an hourly interval were provided by a regional climate model (RCM) during the period of multiple typhoon occurrences. The high-resolution data can take account of the geographic effects, hardly reproduced by ordinary RCMs. The rainfall data drove a conceptual, distributed rainfall–runoff model, embedded in the integrated flood analysis system. The rainfall–runoff model provided discharges along rivers over the TR watershed. The RCM also provided future rainfall data with pseudo-global warming climate, assuming that the August 2016 disaster could reoccur again in the late 21st century. The future rainfall data were used to conduct a future flood simulation. With bias corrections, current and future flood simulations showed the potential inundated areas along riverbanks based on flood risk levels. The crop field-based agricultural losses in both simulations were estimated. The future cost may be two to three times higher as indicated by slightly higher simulated future discharge peaks in tributaries.
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