Pressure Reduced

I assume that you will control the pressure with the VFD, and not a throttling valve.

I can't see a problem.

i have changed the set point at 5 bar , but will i get the same flow at each point as it was , and will it cause to increase in velocity.

Exactly! fan law will apply, GA.

how it come, please can you give some references

flow rate ∞ pressure²

So if flow rate is doubled pressure will be quadrupled,

Typo--other way around--pressure ∞ (flow rate)².

Stupid mistake, Thank you for correcting,

The flowrate will go down. Less fluid will pass through the <...large irrigation network...> and it is possible that at its outer edged, no fluid will arrive at all.

How far down it goes depends upon the piping system characteristic curve, which is installation-specific and cannot be seen from here.

Assuming it is electrical, the current drawn by the pump(s) propelling the fluid will probably change, though in which direction is unpredictable without knowing where they are operating on their characteristic curves, which are a feature of each pump. It can be estimated by multiplying the flowrate by the pressure rise for each of the operating scenarios, which are a function of the piping system characteristic curve, which still cannot be seen from here, and comparing them.

The amount of noise generated by the installation will change. In which direction is unpredictable and an experiment will determine this.

In the limit, the only <...anyone...> who can answer the questions in any detail is the original poster.