Adam Clarke on Genesis 47:18

When that year was ended - The sixth year of the famine, they came unto him the second year, which was the last or seventh year of the famine, in which it was necessary to sow the land that there might be a crop the succeeding year; for Joseph, on whose prediction they relied, had foretold that the famine should continue only seven years, and consequently they expected the eighth year to be a fruitful year provided the land was sowed, without which, though the inundation of the land by the Nile might amount to the sixteen requisite cubits, there could be no crop.