predicting change or predicting absolute values

we define
forecasting as testing model on data not utilised to develop model and
predicting as testing model on data which is obtained from observing the system in a future time.

in that case, the above graph shows a prediction of algal biomass one hour ahead of time. the system was developed to emulate the natural function relating (water quality parameters) and (chlorophyll one hour ahead in time) as observed in a period of 388 hrs, which is around 16 days and 4 hrs. it is validated over the next (approximately) 16 days and is tested over the next 32 days. The figure below summarises this.

what we find is that we follow trends well, but base value is lost. which suggests that we might as well try to predict 'change' in algal biomass. we could experiment by defining change as a vector value - with a magnitude and a direction.

One reason why this idea has not been persued yet is also because the above graph collapses to almost gibberish when time gap is increased further - see below.

To be fair, the above graph is not ALWAYS the case, the exact graph changes often enough - in one case it was tracing ok for a while and then turned into a straight line. but in all cases the regression coeff drops to something like .3 and the graph whatever be their flaw they all have this in common that they ARE NOT ACCURATE!