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<br /> <br />..20~ <br /> <br />the true precipitation that i$measur~d orol;>s~!l;'Ved in a gage is dependent <br />upon small-scale features such as the qrient<ltion and steepness of <br />carryons; ,upon nearness of the gage, to' trees, buildings, and other .obstruc- <br />tions; and up .on the type .of gage, it~ condition, and the care exercised <br />by the observer. ' <br /> <br />, ' <br />Many investigators have shown that, strong 'wind .or sudden changes in <br />wind direction cormnonly diminish the, precipitation catch in c.omparis.on <br />with actual or true precipitati.on. Shields have'been developed to offset <br />the adverse effects of wind, but these l.os,e ef:t'ectiveness if the wind is <br />strong" especially if the precipitation is in the, form of snow. Thus, <br />desirable gage sites are th.ose that have natural protection fr.omstrong <br />wind. Best is a small forest Clearing W%th ang15s fr.om the lip of the <br />gage to the nearest tree tops betweeh, 20 and 30 from the horizontal. <br />In general an obstruction' should not: be nearer to the gage than its own <br />height. Sites which might receive show bl.owing from nearby ridges also <br />should be avoided. <br /> <br />Thus, the precipitation caught in any of the conventi.onal gages is <br />basically only an index to the actual precipitation on the surrounding <br />area. The degree to which the preoipitation catch by gages correlates <br />with the actual precipitati.on over an area Qep~nds[,\liponphysiographic <br />and other terrane features, and upon spa,cingof the st,ations. :For the <br />same spacing the reco~ds 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~spaceQ :steti.ons <br />(20 to 25 miles apart) may be miSleading in regard to isohyetals .of a <br />single storm because the storm centep:-may miss all stations. <br /> <br />Dependability .of observations <br /> <br />Sources of error <br /> <br />Considerable information has be,en published on the possible sources <br />of error in precipitation measurements. Many of these are associated <br />with gage exposure while others depend upon the type of equipment and the <br />observer1sability. Ordinarily, little ,<;lan be done by the hydr.ologist to <br />ascertain and adjust f.or possible errors, except those associated with <br />exposure and only on a current basis. Certain specifiQ s.ources are <br />summarized below. <br /> <br />Improper exposure.~-Criteria have been given for properly locating <br />a gage in relation to nearby obstructions. When such criteria are vio- <br />lated, both speed and directio.n of w;Lnd as :well as catch of the gage are <br />likely to be abnormal. The abn.ormality oan be identified and evaluated <br />by comparing the catches of adjacent: gages between windy and non-windy <br />periods, certain of the gages being properly exposed. <br /> <br />Catch of snow.--Snow may bridge and cap a preCipitation gage, so that <br />the apparent time of snowfall may be displaced (in the case of a recording <br />~age), or the catch may be grossly in err.or. Also, the gage may become <br />partially filled with unmelted snow,: so that its catch efficiency is <br />