restricted access 7. Summary and Conclusions

From: Mirror Lake

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32 9 Being a small lake in a mountainous setting, Mirror Lake has a highly dynamic interaction with the hydrologic cycle.The streams tributary to the lake drain steep mountainsides, so discharge into the lake responds rapidly to precipitation, especially rain, falling on the watershed. At the same time, when the elevation of the lake surface is near the elevation of the outlet dam, many episodes of large tributary inflow result in equally large surface outflow over the dam.The lake level declines below the top of the dam because of losses to evaporation and seepage to ground water. However, in most years, the decline is 30 cm or less. The three tributaries contribute a long-term average of about 59 percent of the inflow to the lake, precipitation directly on the lake contributes about 26 percent, and ground water contributes the remaining 16 percent of the inflow.Even though surface outflow over the dam can be substantial at times of snowmelt and heavy rains, it is episodic and represents a longterm average of about 38 percent of the water lost from the lake. The largest loss of water from the lake is by way of seepage to ground water, which represents a long-term average of about 51 percent of the losses.The remaining 11 percent of the water lost is by evaporation. In general, the largest surface inflows and outflows occur during the spring at times of snowmelt and spring rains.A secondary peak of surface inflows and outflows commonly occurs as a result of fall rains.Precipitation 7 SUMMARY AND CONCLUSIONS thomas c. winter and gene e. likens summary and conclusions 330 generally is higher during these seasons. At these times, surface outflow is the largest loss of water from the lake. However, because seepage to ground water is a fairly consistent and steady process year round, it is larger than surface outflow for most of the year, resulting in its being the largest long-term average loss of water from the lake.These dynamic exchanges of water result in the lake having an average residence time of only about a year. The water-flow dynamics of Mirror Lake had a substantial effect on the nutrient dynamics of the lake. Solute transport at Mirror Lake using the simple mass balance approach revealed that the lake was generally a flow-through system for many watershed-derived solutes, where inputs approximately equaled outputs. However, it was also a sink for chemicals such as hydrogen ion, ammonium, nitrate, phosphate, and dissolved silica, which were largely retained in the system by internal limnological processes.This uptake, or neutralization, was proportionally very similar to the retention measured in long-term studies of terrestrial watersheds in the Hubbard BrookValley. Changes in the chemical inputs to Mirror Lake by way of precipitation and tributaries usually resulted in observable changes in the lake itself.Although the total nutrient inputs and outputs varied according to the quantities of water exchange, the actual mass balances were relatively insensitive to the hydrologic conditions observed over the 20 years.Wet years resulted in more total flux, but inputs and outputs were balanced. That the lake chemistry was a mixture of the three tributaries and groundwater chemistries was anticipated, but the degree to which direct precipitation contributed so dominantly for the most biologically reactive solutes, such ammonium, nitrate, and phosphate, was not anticipated. Estimates of groundwater inputs,based on a simple premise of uniform water transport characteristics and constant solute concentration, appears to have given reasonable closure to most of the budgets. In the one exception, where chloride appears to have been routinely released from the lake, we suspect that there are unmeasured areas of ground water having high chloride concentrations that could balance the chloride budget. An unmeasured cation (possibly reduced iron) would need to be involved to provide the necessary charge to balance the unsupported release of chloride from the lake, and this solution would need to enter the lake through a narrow point-source area undetected by the broader summary and conclu si on s 331 network of groundwater sample sites.The contribution of base cations and ANC in ground water was very effective in helping to neutralize acidic precipitation falling directly on the lake. Groundwater ANC provided 63 percent of the annual ANC influx, compared to 37 percent for all three tributaries.The largest individual solute flux was by way of seepage to ground water. For the purpose of estimating mass balances, it was critical to...