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to decrease (in size or number); to diminish; to abate
夏になると川の水量が減る。
The water level in the river decreases when summer comes.
今年は交通事故の件数が大幅に減った。
The number of traffic accidents decreased significantly this year.
高齢化で地方の人口が年々減っている。
The population in rural areas is decreasing year by year due to aging.