( ( ( The ( primary route ) ) ( for ( liquid ( water ( to ( return ( to ( ( the ( Earth 's ) ) surface ) ) ) ) ) ) ) ) ( ( is ( through precipitation ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ primary) (NN route)) (PP (IN for) (NP (JJ liquid) (NN water) (S (VP (TO to) (VP (VB return) (PP (TO to) (NP (NP (DT the) (NN Earth) (POS 's)) (NN surface))))))))) (VP (VBZ is) (PP (IN through) (NP (NN precipitation)))) (. .)))
The primary route for liquid water to return to the Earth's surface is through precipitation.
(ROOT (S (NP (NNP Precipitation)) (VP (VBP are) (NP (NP (DT a) (JJ natural) (NN process)) (SBAR (WHNP (WDT that)) (S (VP (VBZ allows) (NP (NN water)) (S (VP (TO to) (VP (VB return) (ADVP (RB again)) (PP (TO to) (NP (NP (DT the) (NN earth) (POS 's)) (NN surface))))))))))) (. .)))