valley vs nullah

valley and nullah both are nouns.

Word	Noun	Adjective	Verb	Adverb
valley	Yes	No	No	No
nullah	Yes	No	No	No

As nouns, nullah is a hyponym of valley; that is, nullah is a word with a more specific, narrower meaning than valley:

valley: a long depression in the surface of the land that usually contains a river
nullah: a ravine or gully in southern Asia

Other hyponyms of valley include dale, glen, gully, holler, hollow, ravine, rift valley.

valley (noun)	nullah (noun)
a long depression in the surface of the land that usually contains a river	a ravine or gully in southern Asia