During the winter months the ground is frozen, and any precipitation we receive is also frozen. So the amount of water entering the pool decreases. And the pool lowers. During the spring, the ground thaws, allowing winters melting precipitation to enter the pool. So it rises. The pool keps a pretty constant elevation through the spring, summer, and fall. But once winter returns the cycle repeats, and pool lowers. This rising and lowering cretes a " washing effect" on the pillars, allowing subsidences. Or if the water pool is close to an outcrop with little or no rock cover, a subsidence may occurr due to the same washing action. The elevation of the pool also effects the flow of air through a mine. When the pool is lower, more workings are exposed ( or if we have a drought) allowing more coal faces to give off black damp. When the pool is higher, the water may act as a " plug". Cutting down on the flow, and also the amount of exposed coal to give off black damp. Hope I diddnt confuse you.
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