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2 05 Measurements of the volumes of water moving to, stored in, and moving from a lake are subject to some degree of uncertainty. The uncertainties in measuring water flows result from less than optimal placement of field instruments,the limited accuracy and reliability of field instruments,missing data, and the challenge of areally interpreting point data from field measurements .The methods used to quantify the water budget components of Mirror Lake are described briefly in chapter 2.This chapter presents more detail on how those values were derived, including the assumptions that went into the calculations and the uncertainties inherent in the values. WATER STORAGE IN THE LAKE Mirror Lake is a relatively small lake that has a simple geometry; the lakebed slopes sharply downward from the near-shore littoral zone to a gently sloping abyssal plain (Fig. 1-2). As a result of the many depth measurements made to define the shape of the lakebed in this detail, the volume of the lake can be determined relatively accurately if the lake stage is measured accurately. During this study, the lake stage was measured continuously using an analog strip-chart recorder and float system, where the charts could easily be read to within 2 mm. However, the trace of the lake stage on the chart can be highly variable under 4 EVALUATION OF METHODS AND UNCERTAINTIES IN THE WATER BUDGET thomas c. winter and donald o. rosenberry evaluation of methods in the wate r budget 206 windy conditions making it difficult to determine daily average stage for some days.Therefore, we estimate the error in changes in lake storage to be about 10 percent. PRECIPITATION Uncertainties in precipitation values can result from improper placement of the gages, limits in the precision of the gages, and limits in the density of gages used to extrapolate precipitation over the study area. With respect to placement, precipitation gages located in forested areas are subject to encroaching vegetation, especially during long-term studies.A cone of open air should surround a precipitation gage so that the height of nearby objects is preferably less than, but no more than equal to, the distance of that object from the rain gage (i.e., the angle of an opening of the collector to the sky should be at least 45 degrees from horizontal). As trees continue to grow over time, it is important to trim vegetation to maintain the proper opening to the sky to minimize any temporal bias associated with reduced precipitation catch by the gage. For the 20 years of this study,the openings in the forest where the gages were placed were sufficiently large to avoid this issue. To overcome the problem of catch efficiency of the gages related to wind, the precipitation gages used in this study were equipped with alter shields.These shields encircle the gage and are designed to disrupt the wind flow passing over the gage,causing the rain and snow to fall more vertically into the gage (Fig. 2-9) and thereby increasing their accuracy. Based on a review of the literature related to rain gage errors (Winter 1981), it is estimated that uncertainty in monthly measurements of precipitation related to the gages themselves probably is about 2 to 4 percent. Averaging the precipitation measured at two gages some distance from the lake is a potential source of uncertainty for the water budget of Mirror Lake.Precipitation was not measured at the lake;one gage is located about 0.4 km west of the lake, and the other is about 0.5 km southeast of the lake. For the 20-year period, the average difference between the two gages was 1.3 mm, and the standard deviation was 2.0 mm. Because of this small difference, it is believed that the average values of the precipitation measured by these two gages adequately represents the precipitation falling on the lake. Considering error associated with the gages themselves and with evaluation of methods in the wate r budget 2 07 the method used to areally distribute those point data, the overall error for monthly precipitation input to Mirror Lake is estimated to be about 5 percent. EVAPORATION One of the goals of the water budget study of Mirror Lake was to measure each component of the hydrologic system interacting with the lake as accurately as possible. During the late 1970s, when hydrologic studies of Mirror Lake began, the Bowen ratio energy budget method (Harbeck et al. 1958) was considered to...

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