>Think about it - where is the water going to be drawn from?<
Pete G knows more about this than I do but from info he's posted in the past there are several wells within the Avebury circle and a streamlet that runs alongside Green Street, down Avebury High Street (past the post office and Henge Shop) and presumably on towards the Winterbourne. I'm not saying these wells and stream were enough to supply the Avebury ditch with water (for a moat) but simply that there is/was a constant and fairly abundant supply of water <i>within</i> the Avebury circle. As you say, a seasonal moat perhaps is a possibility - such as the seasonal lake around Silbury.