One inch of draft（吃水） in a single barge is about 17 tons of cargo,and that's almost enough to fill a semitruck（半挂车）.Comb

One inch of draft（吃水） in a single barge is about 17 tons of cargo,and that's almost enough to fill a semitruck（半挂车）.Combine that with carrying about 30 barges instead of the more typical 45,and the drought is decreasing the cargo carried per tow by more than 500 semitrucks worth of goods.
combine为什么用动词原型?carry about这里是考虑的意思吗,字典查查没这个用法.