WSP10386 - Laserfiche WebLink

Home Browse Search

00125'7 -31- SNOW MEASUREMENTS* Types of conventional observations Snow courses The most common practice in measuring "snow on the ground" in the western United States is to determine the water equivalent of samples taken at a number of points along an established line called a snow course. Because accessibility often determines their location, such courses extend over only a small fraction of the snow fields. In most instances their purpose is to obtain a consistent index to the quantity ,of runoff that the snow will generate. To this end it is necessary only that measured water equivalent of the snow correlate well with runoff as measured at some key station. For other purposes, a course may be located to represent the accumulation of snow over a fairly extensive adjacent area. Each course comprises from 10 to 15 sample points, usually in a, straight line, 50 to lOO feet apart. Successive samples are taken within a 3-foot radius of the marked sample points. Each year, samples are taken monthly beginning usually about January 1 and extending through the period of major accumulation and melt. On most courses, the latest measurement in each yearly series is made about May 1 although a few courses are meas- ured as late as mid-June. Mid-month surveys are made in some areas or for special purposes. Most snow-course surveys in the western States are made with the so- called Federal or Mt. Rose sampler which consists of 30-inch-long sections of aluminum tube, coupled together in the desired total length. The bottom section is tipped with an annular sawtobth steel cutter ,for pene- trating hard snow or icy crusts as the sampler is forced vertically through the snow to the land surface beneath. The depth of snow is indi- cated by one-inch graduations stamped on the outside of the tube. Three- inch-long by 1/8-inch-wide slots in the tube, arranged en echelon, permit the surveyor to observe the length of snow core. The inside diameter of the cutter shoe is such, theoretically 1.484 inches, that one ounce of snow core is equivalent to one inch of water depth; accordingly, water equivalent of the snowpack, in inches, is determined immediately by weighing the sampler and snow core on a spring scale. Aerial markers and surveys In remote mountainous areas where surface travel is impractical either by foot or by over-snow machine, depth of accumulated snow commonly is ob- served from aerial markers. Although they differ in detail, fundamentally these markers are vertical poles or posts with crossbars at either 1- or 2-foot intervals. The crossbars may be all horizontal, of equal length or alternately long and short, or they may combine horizontal and diagonal *Another chapter, on "Precipita,tion", treats some general aspects of snowfall and its measurement. This treatment is concerned primarily with "snow on the ground", in relation to runoff. -'....