Laserfiche WebLink
<br />SLW amounts can be converted to SLW flux (in acre-ft) by knowing the average wind speed <br />perpendicular to the mountain through the depth of the SLW and the horizontal distance over <br />which this flux of water could be converted to snow by seeding. Heggli (1986) calculated such <br />a value for the ARB (4220 km2) for a 3.5-mo period from mid-December through March 1984-85 <br />(using a 60-km width at the crest of the Sierra). He showed that the flux of SLW passing <br />unused from the ARB was about 12 pct of the total runoff or 190,000 acre-ft. Super and <br />Reynolds (1991) reviewed SLW flux measurements for smaller watersheds in Utah and Arizona <br />and found values of one to three times greater than the annual streamflow of the watersheds <br />in which these values were calculated (about 40,000 acre-ft/yr). <br /> <br />A consistent feature noted in all locations was that more than half the SLW flux in a season <br />comes in less than 20 pct of the storms. Observations show that the remainder of the seasonal <br />SLW flux comes in the many hours oflow amounts of SLW; a seeding program must treat both <br />situations to increase water production over a given area. These SLW flux measurements are <br />crude and should be thought of as an optimistic estimate of the seeding potential of a given <br />area. They do provide some perspective in interpreting the SLW data. <br /> <br />A third mechanism for determining SLW was installation of an icing detector near the crest of <br />the Sierra. This device was originally designed to deteet icing on aircraft. It has proven to be <br />a useful indicator of the presence of SLW at mountaintop level. An ice detector was installed <br />on SQP (Squaw Peak) at the 2642-m elevation within the Squaw Valley ski area during the <br />winters of 1983-84 through 1986-87. During the winter of 1986-87 (November through January), <br />three additional sites were established to determine the representativeness of the SQP data. <br />During the winter of 1986-87, 67 pct of all storm hours had icing (192 of 288 h) as observed at <br />SQP. Signal Peak, at 2390 m, located near Cisco Grove on 1-80, averaged 69 pct (175 h). For <br />all four sites the average was 55 pct. Altitude and location relative to the Sierra crest were <br />responsible for much of the differences observed between stations. Lower elevation sites or sites <br />east of the Sierra crest experienced less icing. <br /> <br />For comparison, following the SCPP, additional mountaintop icing data were collected at Dyer <br />Mountain (2290 m) near Lake Almanor, and at Red Hill (1950 m) in the Feather River drainage. <br />Determining a seeding strategy requires knowledge of the temperature during icing events. As <br />mentioned, different seeding agents have varying threshold temperatures at which they begin <br />to convert cloud droplets to ice crystals (fig. 2.1). :Figure 3.3 shows the cumulative frequency <br />of icing as a function of temperature for three sites, two in the Feather drainage and SQP in the <br />American. The data were consistent in showing that nearly 50 pct of the icing occurs at <br />temperatures warmer than -3 oC. For the two higher elevation stations, less than 20 pct of the <br />icing occurs at temperatures colder than -6 oC. This temperature is the threshold below which <br />AgI begins to convert water droplets to ice crystals. From November through April of 1987-88, <br />a critically dry year, icing varied from 419 h at Dyer Mountain to 265 h at Red Hill. <br /> <br />The Red Hill site has been maintained each winte,r since 1987. Data from the four- winters, <br />1987-1991, have been analyzed with respect to temperature, SLW amounts, hours of icing, and <br />hours of icing with respect to precipitation. These data were very consistent with results from <br />SCPP. They indicate that over 50 pet of the icing events occur at temperatures warmer than - <br />3 oC. Liquid water contents are generally below 0.2 g/m-3. Icing occurs between 250 and 350 <br />h each season (3 of the past 4 yr have been critically dry as determined from the Sacramento <br />4 river runoff index). During the 21 mo of observations, 62 pet of all precipitation hours had <br />icing. If one looks only at those storms having icing (storms where 0 oC level is below the <br />elevation of Red Hill), figure 3.4, about 50 pct of the storms have fewer hours of icing than <br /> <br />7 <br />