sorry Anthony, but the pressure has sod all to do with friction ... you're talking flow rates.
The pressure will be uniform throughout the entire pipe .... assuming they are both on the level .... as Jordy says the pressure will diminish as it has to climb in altitude ... the lower the pressure the lower the flow rate .... granted there will be a miniscule loss due to friction and it would be calculable given the stiction rate of water passing over the MDPE or whatever surface ..... and the bigger the pipe, the greater the surface area, so there would be more stiction / friction generated ...... but indiscernible .... but the pressure would remain constant, on the level