This year, the rainy season in the Kanto-Koshin region started a week ago, on June 14, one week later than normal and the latest start in the last 10 years. Okinawa, on the other hand, entered the rainy season five days earlier than usual.

According to the British Met Office, the rainy season (wet season) in English is the time of year when most of the annual rainfall occurs in a country or region. In many countries around the world, seasons are defined by temperature, and the rainy season in Japan is described as the rainy season (a time of heavy rainfall). On the other hand, in tropical countries close to the equator, while there is little change in temperature throughout the year, seasons are defined according to the amount of rainfall and are defined as rainy and dry seasons. The rainy and dry seasons vary widely around the world, with monsoons, savannahs, tropical regions, etc., varying greatly from region to region.

Monsoon originally means a seasonal reversing wind accompanied by corresponding changes in precipitation. In particular, the area from the east to the south of Asia, which is affected by the monsoon and is relatively wet and rainy, is called "Monsoon Asia," and Japan is considered to be a part of it. The monsoon creates a climate with a dry season and a rainy season, but the overall climate is humid, resulting in a tropical monsoon climate, a temperate summer rainy climate, and a warm humid climate, making it an ideal place for rice cultivation.　

In contrast, the savannah-type rainy season, which is also widely observed in African countries, is hot all year round, with a dry season that lasts for several months without any rainfall, and a short rainy season (sometimes multiple times a year, depending on the region) with heavy rainfall. The rainy season is quite different from the rainy season in Japan, as most of the time there is no need for umbrellas during the day, as the heavy rains, like evening showers, fall in the middle of the night.

Rainfall patterns have a great impact on the state of agriculture in many parts of the world. According to the International Water Management Institute (IWMI) of the CGIAR, an international agricultural research network, most farmers in developing countries rely on rainfed agriculture, with 95% of farmers in sub-Saharan Africa, 90% in Latin America, 75% in Central and Northeast Africa, 65% in East Asia, 65% in South Asia, and 25% in South America. Farmers who rely on rainfed agriculture face a variety of uncertainties and risks. For example, farmers who depend on rainfed agriculture with savannah-type rainfall patterns have to cope with multiple problems caused by water scarcity and intensity, such as drought due to delayed rainy season, and conversely, soil erosion caused by intense and concentrated rainfall during the rainy season.

JIRCAS conducts research activities in developing countries in Asia, Africa, Latin America, etc. Understanding the issues and needs of agriculture, forestry, and fisheries as defined by the rainy season and rainy season patterns of each country and region is the first step in developing technologies. It is feared that climate change will change the pattern of rainy seasons and bring about abnormal events. We are working on technological development to contribute to the establishment of agricultural systems that are resilient to climate change by mobilizing appropriate water management, breeding, and cultivation technologies, based on the assumption that rainy and dry season patterns will change in each region of developing countries.
 

Contributors: HAYASHI Keiichi (Director, Environment Program), NAKASHIMA Kazuo (Director, Food Program), IIYAMA Miyuki (Director, Information Program)