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<br />001450 <br /> <br />Even after all reasonable adjustments have been applied, many <br />long-term streamflow records may retain a progressive, man-caused <br />bias. As a result, the later part of the record may, in comparison <br />with the earlier part and with the natural stream regimen, be abno~al <br />in daily, seasonal, or yearly runoff; in extremes of flow; or in any <br />combination of these. In some instances, magnitude of the abnormality <br />will be suggested by double-mass plotting against records from adjacent <br />stations presumably not so biased. In other instances, there is no <br />ready means for demonstrating magnitude. As a consequence, long-term <br />correlations between streamflow and other hydrologic variables must <br />be interpreted with caution. <br /> <br />Long-term records from climatologic stations in and near metro- <br />politan areas well may embody substantial man-caused bias. In such <br />areas, large and generally increasing amounts of waste heat have been <br />and are discharged into the atmosphere; industrial smoke and duets " inject <br />abundant nuclei that may induce precipitation, and that probably modify <br />the natural regimen of thermal radiation; also, ever-changing structures <br />interpose variable barriers to the movement of low-level winds and m$y <br />affect the "exposure" of climatologic instruments. Currently, atmospherie <br />physics is.not understood sufficiently well to appraise all the potential' <br />influences on' climatic records. In some instances, the amount of influence <br />will be su~gested by double-mass plottings against records from outlying <br />stations. <br /> <br />Such bias is not restricted to metropolitan areas--witness the <br />familiar change in humidity regimen, and probably in air-temperature <br />regimen, that has been induced in areas of extensive irrigation. <br />Exposure of climatologic stations may have been changed by the growth <br />or felling of trees, construction or demolition of buildings, and the <br />like. The hydrologist must ever be alert to the possibility of such bias <br />in any of the longer records with which he deals. <br /> <br />SHORT-TERM ABNORMALITIES AND "STEPS" IN RECORDS <br /> <br />Both man's activities and natural events may cause short-term <br />abnormalities or lasting "steps" in hydrologic records--more commonly <br />in streamflow records. Among numerous examples are the effects of <br />temporar,r ~offerdams and by-pass channels, initial filling of reservoirs, <br />and emergency sluicing; the perennially repeated abnormal :flow regimen <br />downstream from a "peaking" hydroelectric plant is common. The larger <br />of such events may be well known and may be documented in the "remarks" <br />section of:the station records in Water-Supply Papers. However, many <br />will not be documented in the usual sources of information. If of <br />consequence in a particular hydrologic study, the possibility of such <br />an event can be tested by plotting records against one another, and <br />searChing out an explanation for data that deviate markedly from the <br />general regimen. <br /> <br />5 <br /> <br />