* IOC, MRPL term volumes to drop by 10-17% - sources * BPCL, HPCL volumes expected to fall - sources * Reliance switches to Basra Medium from Basra Light - sources NEW DELHI, Jan 21 (Reuters) - Iraq has reduced annual supplies of Basra crude oil to several Indian refiners by up to 20% for 2021, industry sources said, in a rare move by OPEC’s second-largest producer which is trying to meet its obligations under the group’s production deal. Iraq was the top oil supplier to India in 2020 and a reduction in long-term Basra crude supplies could erode Baghdad’s market share in the world’s t...