1) An intermittent watercourse 2) Vale 3) Valley 
In Hong Kong, a nullah is typically a concrete-lined channel designed to allow rapid run off of storm water from high ground, to prevent flooding of urbanised coastal areas. ...
• (n.) A water course, esp. a dry one; a gully; a gorge; -- orig. an East Indian term.
small stream, ditch, drainage channel
Nul'lah noun [ Hind. nālā , from Sanskrit nāla tube.] A water course, esp. a dry one; a gully; a gorge; -- orig. an East Indian term. E. Arnold. 
Nullah was British army Second World War slang for a watercourse, gully or stream.
[n] - a ravine or gully in southern Asia
Dry or wet stream bed or ditch
noun a ravine or gully in southern Asia
an intermittent watercourse. · a gully or ravine.