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<br />001458 <br /> <br />Exposure and spacing of gages <br /> <br />The amoUnt of precipitation occurring at any given point is controlled <br />primarily bW large-scale physiographic features (such as the oceans, plai~s, <br />and mountaili ranges) and by the location in relation to storm paths. In . <br />turn, the percentage of the true precipitation that is measured or observ~d <br />in a gage is dependent upon small-scale features such as the orientation and <br />steepness oj: canyons; upon nearness of the gage to trees, buildings, and <br />other obstl'/lctions; and upon the type of gage, its condition, and the carE! <br />exercised h,r the observer. <br /> <br />Many i~estigators have shown that strong wind or sudden changes in <br />wind direction. commonq diminish the precipitation catch in comparison <br />with actual or true precipitation. Windshields have been developed to <br />offset the adverse effects of wind, but these lose effectiveness if the <br />wind is strong, especially if the precipitation is in the fom of snOW'. <br />Thus, desi~ab+e gage sites are those that have natural protection from <br />strong wind,. . Best is a small forest clearing with angles from the lip <br />of the gage' to the nearest tree tops between 200 and 300 from the <br />horizontal., Under no circumstance should an obstruction be nearer to the, <br />gage than its OJ(Il height. Sites which might receive snow blowing from <br />nearby ridges also should be avoided. <br /> <br />Thus, the precipitation caught in any of the conventional gages is <br />basically only an index to the actual precipitation on the surrounding <br />area. The degree to which the precipitation catch by gages correlates <br />with the actual precipitation fall over an area depends upon physiographic <br />and other terrane features, and upon spacing of the stations. FOr the <br />same spacing the records of precipitation catch may reflect accurately <br />the yearly or seasonal basin-wide fall but may be much less reliable in <br />regard to monthly or storm amounts. Records from average-spaced stations <br />(20 to 25 miles apart) may be misleading in regard to isohyetals of a <br />single storm because the storm center may miss all stations. <br /> <br />Dependability of observations <br /> <br />Sources of error <br /> <br />Considerable information has been published on the possible sources <br />of error in, precipi tation measurements. Many of these are associated with <br />gage exposure while others depend upon the type of equipment and the <br />observer's llbility. Ordinarily, 11 ttle can be done by" the hydrologist to <br />ascertain and adjust for possible errors, except those associated with <br />exposure and onq on a current basis. Certain specific sources are <br />summarized below. <br /> <br /> <br />II.2 <br />