Silburys' moat has been dry for 2 years now.
reasons are,
1/ The Winterbourne riverbed was deepened so there is no overflow from the winterbourne river straying across the fields.
2/ The water board has put in a new pipeline on the Devizes road and is sucking more water out from the lower watertable.
The Beckhampton brook has not flowed ( which is the Only source for the Silbury moat now) for 2 years.
Only a prolonged spell of wet weather will get a moat back around the Hill in the future.