Complex sentence: The biggest lake of the south is the Songkhla lake (1,040 km² altogether), the largest artificial lake is the Chiao Lan (Ratchaprapha dam) with 165 km² within the Khao Sok national park in Surat Thani.
Split into simpler sentences: The biggest lake of the south is Songkhla Lake (1,040 km² altogether). The largest artificial lake is the Chiao Lan (Ratchaprapha Dam), occupying 165 km² of Khao Sok National Park in Surat Thani.