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<br />011123'1 <br /> <br />-39- <br /> <br /> <br />The California Department of Water R~sources publishes measureMents <br />for approximately 325 courses in California. Its publication, "Water <br />Conditions in California", is issued about the lOth of each month from <br />February through May. The base period for station averages ourrently <br />spans the 30 years ending with 1960. For stations not operated through- <br />out the base period, the record is adjusted to the base by correlation <br />with comparable long-term courses. ' <br /> <br />Still other courses, perhaps 1,000 altogether, have been measured <br />periodically for research or special-study PusPoses by several agencies. <br />Records from courses in this latter oategory are not generally included <br />in the more common serial publicatiO'ns. <br /> <br />Use of data <br /> <br />As has been stated, in a snow-course survey the ordinary objective is <br />to derive a consistent index to total quantity of the snowpack on an adja- <br />cent basin or area, usually in terms of water equivalent. Criteria for <br />selecting an advantageous site are suoh that very commonly the course indi- <br />cates a proportion to basin mean snowpaok but not an absolute measure of <br />that pack. Snow-course data should be used With appreciation of this <br />inherent limitation. <br /> <br />In practice, snow-course data in terms of water equivalent are <br />correlated with ensuing runoff as measured at a suitable stream-gaging <br />station, to derive a relationship from which seasonal water yield is <br />forecast. Such a correlatiO'n avoids uncertainty in total water volume <br />of the snowpack. AlsO', it may include some index of soil moisture and <br />an estimate of expected precipitation; inclusion of these variables <br />commonly improves the forecast of runoff. Obviously, the relationships <br />and the derived forecasts are most dependable for those basins that <br />derive most of their runoff from snowmelt. <br /> <br />Estimates of total volume ef water content in a snowpack, basin-Wide, <br />are best derived from maps or profiles that array and cO'rrelate all <br />applioable and available data--depth and water equivalent of snow at <br />climatologic stations and aerial markers, as well as from courses; <br />highway-patrol reports of snow depths along roads; aerial surveys of <br />snowline; and established relatiens among 'snow, : topography, arid vegetal <br />cover. <br /> <br />The unique schedule of snow surveys intreduces seme complexi ties intO' <br />comparisons of the survey data with conventional records of other clima- <br />tological variables such as precipitation. For example, if snow-course <br />data are used to' define monthly increments, not adjusted to actual dates <br />of the surveys, their time interVal might be as much as 20 percent too <br />long or too short for correlation with calendar-month values of the other <br />variables. <br />