Chou Collection, Middle Volume
Radical: Earth (tǔ)
坳
Kangxi stroke count: 8
Page 226, Entry 33
Broad Rhymes (Guangyun), Collected Rhymes (Jiyun), Rhyme Meetings (Yunhui), and Correct Rhymes (Zhengyun) state the pronunciation is ao. A low-lying area.
Book of Zhuangzi (Zhuangzi), Chapter: Carefree Wandering (Xiaoyao You): If one pours a cup of water into a low-lying place in the hall, then a blade of grass can serve as a boat.
Also, Collected Rhymes (Jiyun) states the pronunciation is ao. The meaning is the same.