I want to use a set of daily water quality data including 3 parameters in a Copula model. But somebody told me this data does not have a condition of a random variable to use in copula, and I should do some work on it (my guess is that it involves detrending of the data). Can anybody help me in finding methods to prepare the data to use in a copula method. I would appreciate if you can refer me to good journal papers or books.

Thank you