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## Main Question or Discussion Point

Suppose we have monthly totals of observed data for last 35 years. That data is of inflow of a river in a reservoir and monthly demands from the reservoir. We are interested to check the effect of construction of a dam in the upstream. The effect is, whether the downstream reservoir will have enough water to meet its dependent demands after the upstream construction. We also have 12 estimated monthly demands for the proposed dam which are assumed to be fixed for future years.

For future prediction of inflow and demand, I primarily will use a simple time series model using regression for linear trend, without cyclic component. My query is, how many future years can I predict reasonably. I know that there is no hard and fast rule, but still can I find a logical length of forecast horizon from some sort of thumb rule?

For future prediction of inflow and demand, I primarily will use a simple time series model using regression for linear trend, without cyclic component. My query is, how many future years can I predict reasonably. I know that there is no hard and fast rule, but still can I find a logical length of forecast horizon from some sort of thumb rule?