Sorry---long story but wanted to give all the details up front.

I have a 4" well, 90' deep with a submersible pump. I live on a 38' bluff overlooking a large reservoir. When the well was originally put in, we hit water at 40'. At the time, the well driller was only going to go another 15' and call it a day. I told him I had seen the reservoir very low during a drought which caused neighbors wells to go dry and to continue till he couldn't go any further. The area is all sand and we went down another 50' through sand till he hit clay---or as he called it "the fines". There, he installed a 4' stainless well point and I don't recall the size pump that was installed---maybe 10 GPM.

Several years later, I added an irrigation system to the yard. I pulled up the old pump and installed a Goulds 13 GPM 2 wire pump to better handle the load. The vertical rise is basically 40' above the lake level and add another 10 to make the math easy gives a 50' pumping level. The 195' of 1" poly pipe used gives a friction loss of 36 and the pressure is 60 for I believe about 225 TDH. (it's late---my brain is fried)

I am using an 86 gallon storage tank running at 40/60 with a 23 gallon draw. To my surprise, the pump actually delivered about 22 GPM to the house so it at least had a 60 second cool down. After a couple of years of listening to the pump cycling off and on when the irrigation system was in use, I bought a bunch of nozzles for the sprinkler heads and replaced all the nozzles with larger ones so that the pump would come on at 40, slowly rise to 50 and then stay there till that zone was finished. I did that to all 8 zones. This was the advice of the company that installed the irrigation system and it made sense at the time.

Now, 9 years since the pump was installed, I noticed the sprinkler heads looked a bit wimpier than I remembered. I checked the gauge and the pump comes on at 40 and stays there or only climbs back up to 41 or 42, depending on the zone. I tried just cycling the pump with no load and it recovers the 23 gallon drawdown of my tank in 63 seconds--no problem.

Is it a worn pump combined with the friction loss of the irrigation system causing the pump to stay at 40 psi?

I hope I gave you all you need to troubleshoot this. Thank you for the effort!