running sprinkler wire over ground
Ensure that the sprinkler wire you select is rated for UV exposure and a suitable temperature range for your region. Buried wire is generally protected from these elements by virtue of being buried.

You may also consider placing a UV rated conduit the protect the wire above ground.

Either way, sprinkler system wiring is considered low voltage and there are very few codes governing the installation. Being above or below ground shouldn't be a safety concern.