At each of the stops on this tour a different component of the hydrologic system as it relates to Mirror Lake was discussed. When it was necessary to show ground-water flow directions, such as at Stop 9, a simple two-dimensional flow system was described. In reality, the ground-water flow system in the Mirror Lake watershed is a complex three-dimensional flow field. To gain an understanding of the flow field, a three-dimensional computer model was constructed of the ground-water system in the entire lower end of the Hubbard Brook valley, including the Mirror Lake watershed. This computer model indicated that the recharge area for the ground water that enters Mirror Lake includes not only the lake’s surface drainage area, but extends up-valley beyond Norris Brook. As a result, the ground-water watershed is 50 percent larger than the surface watershed. The model also indicated that the ground-water flow field extends deep into the bedrock. Thus, the hydrogeologic setting of Mirror Lake consists of (1) two large and one small inlet streams, (2) a ground-water contributing area that is 50 percent larger than its surface watershed, (3) occasional surface outflow over the dam, and (4) a large loss of lake water as seepage to ground water through the gravel-filled bedrock valley on its south side.

The hydrology of Mirror Lake is best summarized by examining its water budget. Data on all of the water fluxes to and from Mirror Lake have been collected since 1981, and monthly and annual water budgets have been calculated for the 20-year period, 1981-2000. On an average annual basis, about 625,000 cubic meters of water enter and leave the lake. Precipitation accounts for about 28 percent of the input, surface water contributes about 65 percent (38 percent from stream NW, 25 percent from stream W, and 2 percent from stream E), and ground water discharge accounts for only 7 percent of the input to the lake. The largest loss from the lake is seepage to ground water, which accounts for about 46 percent of the water lost. The next largest loss is by way of surface outflow over the dam, about 41 percent, and the smallest loss is by evaporation, about 13 percent.

Some of the water fluxes vary greatly on a seasonal basis and others are relatively constant. For example, precipitation usually is greatest in the spring and fall, and evaporation is greatest in mid-summer. Streamflow to and from the lake is greatest during spring snowmelt, especially if the melt period is accompanied by rainfall. Streamflow to and from the lake usually has a secondary peak in the fall. In contrast, ground water exchange with the lake is relatively constant throughout the year.