Complex sentence: In times of severe drought, Lake Burley Griffin's water level can fall unacceptably low and when this happens, a release of water from Googong Dam located upstream could be scheduled to top up and restore the lake water level.
Split into simpler sentences: In times of severe drought, Lake Burley Griffin's water level can fall unacceptably low. When this happens, a release of water from Googong Dam located upstream could be scheduled to top up and restore the lake water level.