Evaluation of Soil Moisture Dryness Using Land Surface Model in the Case of Forest Fires in Tohoku 2017

Tohoku Region is drier comparing other area in Japan and prone to have many forest fires in the spring when is just before rainy season. Especially in the spring of 2017, several forest fires had widely occurred in the region. Especially, in Kamaishi forest fire in Iwate prefecture, its burnt area was 413ha which is larger than the total burnt area recorded in 2016 for the whole Japan. It is necessary to clarify the factors to prevent future fire hazards. However, forest fires occur as a result of various factors such as strong wind speed, soil moisture dryness, atmospheric dryness and artificial factors. Therefore in this research, in order to clarify the meteorological factors of this fire, soil moisture dryness was evaluated by comparing with the average year. Soil moisture was calculated by land surface model which can simulate vertical water and heat balance and simple Biosphere including Urban Canopy (SiBUC) was utilized in this research. For meteorological input, the model needs precipitation, temperature, wind speed, short wave radiation, long wave radiation, specific humidity and air pressure which are not observed by general observation system. Therefore, reanalysis data and physical model simulated results were utilized in this research. Grid Point Value (GPV) data calculated by Mesoscale Model (MSM) under Japan Meteorological Agency (JMA) and Dynamical Regional Downscaling Using the JRA-55 Reanalysis (DSJRA55) were utilized. The analyzed results showed drier soil moisture in 2017 and 1987 known as dry year as well. And Tohoku region and Setouchi region were drier than other region. Drier soil moisture in 2017 in Tohoku region was due to lower precipitation in the winter, which was only 70% of averaged year. Drier region was widely spread in Northeast side of Japan, however forest fire were actually occurred only in limited region because wind speed was stronger only in the region. This mean drier soil moisture creates potentially dangerous region in fire and it can be serious only when wind speed is stronger. These results are valuable to find threshold to alarm dryness hazards and climate change projection.