Have you tried talking to the neighbor's directly or talking to the neighbor's manager? If that doesn't work, you can have an attorney write a letter as this would be a potential nuisance claim.
Write a letter and document it to the neighbor and neighbor's landlord. Not manager but the landlord. Talking often does not get attended to. If there are losses then small claims court would seem to be the venue.
What do you do when it starts raining? Not that we have had much of that lately!
In addition to AVVO's disclaimer, please note that by this answer no attorney client relationship is intended mor entered into and unless there is a signed retainer agreement in place, neither me nor anyone in our office has intended to solicit clients nor reprints them. The answers are general in nature and without weighing specifics of particular query. No answer should be relied on in whole or in part, directly or otherwise to act or not to act in pursue of any of your potential claims in law or equity. You should consult with and obtain advise or representation of an attorney to protect your rights regarding your case or matter.
I agree with the previous answers and send notice in writing to the property owner of record. It is technically a trespass, but in reality it is a waste of water to water cars. If you do not get a response take the owner to small claims court. However you will have to prove damages , and that will require an expert and some bids for repair.
Please be sure to indicate the best answer. If this answer was helpful, please mark as helpful below. Only. If and until you and I sign an Agreement for Legal Services, I am not your attorney. These answers are provided for informational and/or novelty purposes