The spuds will be deeper in the soil where there is probably still plenty of moisture - it's when the tubers are bulking up they need plenty of water.
Onions much more shallow rooted, so would probably benefit from a drink.
We've had just one millimetre of rain here in the last 3 weeks or so, and I've already had to water the shallots and blueberries as I will other crops if we don't get any rain soon (none forecast for the next week.)