( Pk ( ( is ( ( the ( wave period ) ) ( in ( years ( of ( an ( annual ( flow frequency ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Pk)) (VP (VBZ is) (NP (NP (DT the) (NN wave) (NN period)) (PP (IN in) (NP (NP (NNS years)) (PP (IN of) (NP (DT an) (JJ annual) (NN flow) (NN frequency))))))) (. .)))
Pk is the wave period in years of an annual flow frequency.
(ROOT (S (NP (NP (DT The) (NN frequency)) (PP (IN of) (NP (DT a) (NN wave)))) (VP (VBZ is) (NP (NP (DT the) (NN reciprocal)) (PP (IN of) (NP (NP (DT the) (NN period)) (PP (IN of) (NP (DT the) (NN wave))))))) (. .)))