Under the sort of capitalistic principles America had in the very early 1900's, if you could prove what you've laid out above, you could sue Roy for all the damages.
If you could prove that before Roy showed up you had PRIOR USE of groundwater at say 100 litres/day, and since Roy showed up and began pumping 90 litres/day your groundwater fell to only 10 litres/day, then you can sue him just fine.
The incentives this sort of system would create would be if you were about to engage in NEW activity that was going to involve groundwater, you need to either:
1) Buy PRIOR USE rights off others in area - this is easy if what you plan to do with the water is far more highly valued than what they currently do with the water / it's very difficult if what you plan to do with the water is very wasteful
2) Do your research on what amount of groundwater you think is "freely available" - if you correctly predict that there's say 100 litres/day flowing and yet only 20 litres/day is being used in productive processes, then if you had plans for say 40 litres/day that should be a piece of cake. To the extent you're "really pushing it" then you better do more homework as you're at risk of being sued for damages by trying to avoid #1
Side factoid: property law used to be dealt that way all the time in America. Where it started to change was when the railroads came along. Originally there was a problem where trains would pass by fields of craps and sparks would either set the fields on fire or large amounts of soot might ruin the crops on all the edges of the tracks. Farmers rightfully went to the courts and said, "WTF. I was here doing this first, then you chose to build those things so close, now you're damaging part of my crops and you need to pay for those damages". Instead though, courts began to slowly move away from the idea that a SMALL individual can still win his case if he goes up against a BIG company... he just needs to be in the right. What began happening is the courts essentially said, "Sorry. That railroad is such a benefit to the public good that I'm going to give him permission to keep doing as he does and YOU are the one who needs to adjust."
To me it's a shame we started going down that road, instead of forcing the railroads (in this case) to pay for their own fixes out of their own earnings (which SHOULD be easy if it really is the case this service they're providing is being widely asked for by many).