Can I drill a well on my own land and who owns the water I extract?

Yes. The Civil Code (Decree-Law 106) is the legal basis for self-supply: art. 579(4) declares 'groundwater obtained by artificial means on private property' to be privately owned,… and art. 581 allows any owner to open wells on their land 'provided such works cannot diminish or divert public or private waters from their natural surface course' destined for public service or for a pre-existing use held under legitimate title; the authority may order the works suspended at the request of interested parties. Arts. 582-583 set minimum distances: 40 m from other people's buildings, a railway or a road; 100 m from another water source, spring, river, canal, irrigation ditch or public watering place, unless the owners or the municipality grant a license; 2 m between wells inside a settlement and 15 m in open country. Art. 584 assigns ownership of the water obtained to whoever brings it to the surface.