I am analyzing time series concerning daily water temperatures in rivers and I stubmled upon a series with autocorrelation like this:
Moreover the series is stationary (I checked it with the Augmented Dickey-Fuller test). Partial autocorrelation plots looks like this (and I guess it indicates that it is AR(2) process):
So my problem is: what does this weird autocorrelation plot indicates? It seems to be telling me something, but I can not understand what it is exactly, so any help will be much appreciated.