As heavy rain swept across Southern California on Tuesday, another rare water event was taking place. The Los Angeles-based Metropolitan Water District of Southern California agreed to buy thousands of acre-feet of water from the San Diego County Water Authority.

Usually, water sales go in the other direction.

The transaction is significant on a number of fronts. The water should help some of MWD’s member agencies that are in dire straights due to drought and severe cutbacks from the State Water Project. Also, the deal again underscores that the Water Authority, which also is a member under Metropolitan’s umbrella, has for years moved to diversify and stockpile water supplies to become more resilient during drought conditions.