Precipitation is the original source of all water to Mirror Lake, because all stream flow and all ground water flow also have their origin in precipitation. However, for purposes of this tour, having Mirror Lake’s water budget in mind, the precipitation we are referring to is the amount that falls directly on the lake surface. Precipitation is measured using a gauge that has a round opening at the top measuring only 20.3 cm in diameter. As a result, the gauge catches the precipitation falling in a very small area; that is, it samples only a small fraction of the precipitation falling on the lake and watershed surface. With this in mind, ideally it would be best to place the precipitation gauge in the middle of the lake and assume precipitation falls uniformly over the lake surface. However, it is impractical, and would be unsightly, to construct a fixed tower or platform in the middle of the lake, so precipitation gauges need to be placed on land. Furthermore, precipitation gauges need to be placed in open areas so there is no interference in the raindrop (or snowflake) trajectories, such as interception by trees or buildings. Fortunately, two official precipitation gauging stations are within half a kilometer of Mirror Lake.

	Rain gauge stations at Forest Servcie headquarters (left) and Pleasant View Farm (right)

One gauge is located west of the lake at the U.S. Forest Service Hubbard Brook headquarters and the other gauge is located east of the lake at Pleasant View farm. The precipitation input to Mirror Lake is determined by calculating the average of the precipitation amounts measured at these two gauges.