dry up, mature
涸 centers on water disappearing or being used up: drying up, running dry, or reaching a state of depletion. The 'mature' sense is a rare extension, likely referring to something fully developed and then past its prime, like a dried-up fruit.
涸 combines 氵 (water) with 固, which likely contributes the sound and the idea of something solid or hardened, suggesting water that has solidified into dryness.
Water 氵 beside something solid 固: imagine a riverbed that has become hard and dry, with all the water gone.
For コ, picture a dry, empty lakebed that looks like a solid core: core -> コ, and the water has dried up.