DESPITE REFILLING PRIOR TO DEP; A319 FLT CREW EXPERIENCES LOSS OF GREEN SYSTEM HYDRAULICS.

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AT THE GATE IN PHL; THE FO NOTICED A LOW GREEN HYD FLUID LEVEL AND CALLED MAINT TO FILL THE SYS BACK UP. HE HAD SOME TROUBLE PUTTING THE FLUID IN AND COMMENTED ON HOW MUCH FLUID IT TOOK. AFTER ABOUT 1 HR; HE GOT SOME HELP AND THE FLUID WAS PUT IN. NORMAL START TAXI AND TKOF OCCURRED. AFTER LEAVING FL200; I CHKED THE HYD PAGE AND NOTICED A SIGNIFICANT FLUID LOSS. WE LEVELED OFF AT ALT AND WERE CRUISING FOR A FEW MINS WHEN THE ECAM WARNING CAME ON WITH A GREEN SYS LOW LEVEL AND LOW PRESSURE. WE PROCEEDED TO DO ALL ECAM ACTIONS AND PROCS. WE ALSO TOLD ATC THAT WE WERE GOING TO DIVERT AND WANTED TO START GOING TO ZZZ. AN EMER WAS DECLARED. ATC STARTED US DOWN AND I COMMUNICATED WITH DISPATCH AND FLT ATTENDANTS. I MADE AN ANNOUNCEMENT TO THE PAX AND TOLD THEM THE SITUATION AND OUR PLAN. ATC COORD WITH ZZZ AND WAS HANDED OVER TO THEM. WE HAD COMMUNICATED WITH THE COMPANY; SO EVERYBODY WAS WAITING FOR US. WE HAD AN EXTENDED FINAL; SO A MANUAL GEAR EXTENSION WAS DONE AND ALL CHKLISTS WERE ACCOMPLISHED. LNDG WAS NORMAL WITH MANUAL BRAKING. SINCE WE DID NOT HAVE NOSEWHEEL STEERING WE STOPPED THE ACFT ON THE RWY. CFR EQUIP WAS STANDING BY AND DID NOT SEE ANY FLUIDS LEAKING. ENGS WERE SHUT DOWN AND WERE TOWED IN. THE MAINT PEOPLE SAID THEY USUALLY DON'T LOG THE AMOUNT OF HYD FLUID THAT IS PUT INTO A SYS BUT; IF THERE IS A LEAK; NOTING THE AMOUNT MIGHT DETECT A LEAK SOONER.