Interpolation and Extrapolation of data without using Trend line.

Hello all,

I have attached a sheet below, which contains a table of values ( time v/s reduced water level) from this table I want to extract the data or extrapolate the data which should be logical and should match with the trend of the taken values.I have tried trend line in this table but trend line does not always work because sometimes the trend goes like a sin curve over the whole period of time and there is no such trend line equation which I can use to write a sin or cos equation to logically represent these values over a whole range of time abscissa.