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VII. 4 0.01211 Error of the same sort may occur if the reservoir-stage recorder is a manometer- or pressure-type gage set at a level below that of the reservoir surface. Here, the relationship between pressure and water level changes with varying temperature of the reservoir water. Here error can be minimized as in the case of the insulated stilling well just outlined, either by providing multiple sensing points so that the measured depth of water is held to a minimum, or by frequently resetting the recording gage to an accurate outside staff gage. An additional source of error may come from temperature effects on the gage mechanism, such as change in density of manometer mercury with change in air temperature. This source of error can be eliminated by a sui table temperature-compensating device, or by maintaining the gage chamber at a constant temperature. In another example, a float-actuated stage recorder was installed over a steel-pipe stilling well attached to the vertical upstream face of a high dam. During the first summer of operation, with the reservoir drawn down and the stilling well exposed in full sun much of the day, an erroneous diurnal fluctuation in stage was recorded as the bronze float cable elongated and shortened with variations in air temperature. With the installation unchanged, a substantial seasonal error would have been introduced, between the low reservoir stage and widely variable air temperature in summer and a high stage but less variable temperature in winter. In this instance the error was diminished to a tolerable mininn:ml by substituting a float cable of stainless steel and by periodically resetting the recorder in agreement with the outside staff gage. Wind-generated "tides" Most reservoirs and lakes have only one stage station, usually near the dam or outlet, on which the records of change in contents are based, strong winds may generate a "tide" or seiche so that, at this single station, the stage is transiently higher or lower than the mean over all the reservoir. If these false stages are applied to a capacity table computed for a horizontal water surface, indicated storage may be seriously in error. Errors of stage may be in the order of 1 foot; errors of content, in p~oportion to reservoir area. Usually such wind tides are random in magnitude and frequency. However, given a pronounced diurnal wind shift such as the down-canyon movement com- mon in mountainous areas, and given also once-a-day readings on a single staff gage, the records of stage and of contents may be systematically in error over days, weeks, or months at a time.