( ( A drought ) ( ( is ( ( ( a period ) ( of time ) ) ( when ( ( a region ) ( ( has ( ( much less ) rain ) ) ( than usual ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN drought)) (VP (VBZ is) (NP (NP (DT a) (NN period)) (PP (IN of) (NP (NN time))) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN region)) (VP (VBZ has) (NP (ADJP (RB much) (JJR less)) (NN rain)) (PP (IN than) (ADJP (JJ usual)))))))) (. .)))
A drought is a period of time when a region has much less rain than usual.
(ROOT (S (NP (NNS Droughts)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (EX there)) (VP (VBZ is) (ADJP (JJ little) (PP (TO to) (NP (NP (DT no) (NN rain)) (PP (IN in) (NP (NP (DT a) (NN period)) (PP (IN of) (NP (NN time)))))))))))) (. .)))