far and wide
洽 centers on something spreading or extending broadly, like water soaking through an area. The idea of "far and wide" captures this sense of thorough permeation or wide reach.
洽 combines 氵 (water) with 合, which likely contributes the sound and the idea of joining or fitting together, suggesting water spreading and soaking evenly.
Water 氵 joins 合 with the land, soaking it thoroughly and spreading far and wide.
For コウ, imagine a wide lake spreading out like a calm cove: cove -> コウ, and the water reaches everywhere.