WMOD00597 - Laserfiche WebLink

Home Browse Search

.. early as October 23 and as late as February 13. Mean date for 50-percent precipitation receipt is January 27, but has varied from December 8 to March 16 [8]. Thus, snowpack data from snow courses cannot be relied upon for information about early season snowfall; nor can data collected once a month be relied upon for determination of snowpack water content during the 30-day periods between monthly measurements. Evaluation of data taken on the first of February, March, April, and May from seven snow courses2 located on an altitudinal transect from 1325 to 2590 m (4350 to 8500 ft) for the period of record (usually over 30 years) offers some insight into snowpack development within the basin. Seven snow courses were selected. The lowest elevations from which snow course data are available in the American River Basin and the Yuba River Basin are 1615 m (5300 ft) and 1585 m (5200 ft), respectively. Other snow courses are located at 1325 m (4350 ft) in the Mokelumne River Basin, to the south of the American, and further south in the Stanislaus River Basin at 1450 m (4750 ft). Antelope Springs at 1325 m (4350 ft) elevation has a mean snow water content on February, March, and April 1 of 7.6 to 10.2 cm (3 to 4 in) over a 28- to 36-year record, with a mean of less than 2.5 cm (1 in) recorded on May 1. On the first of February, March, and April, up to 12.7 cm (5 in) of water was measured for 9, 10, and 7 years, respectively. However, between 25.4 and 50.8 cm (10 and 20 in) of water was measured in the packs - three times on February 1, four times on March 1, and once on April 1 and May 1 (table 2). Dorrington snow course data at 1450 m (4750 ft) elevation shows similar measurements, except that average February to April 1 snow water measurements have increased to over 12.7 cm (5 in). May 1 shows just over 2.5 cm (1 in) (table 3). Ice House snow course data at 1615 m (5300 ft) elevation shows a dramatic difference. The pack contains from 22.9 cm (9 in) to over 27.9 em (11 in) of water for February to April at first- of-the-month measurement periods. Even on May 1, the pack holds 9.37 cm (3.69 in) of water. The site shows no snow on February 1 for only 4 out of 39 years. The course was bare only 3 years out of 36 on March 1, but in 10 and 22 years out of 36 and 35 years, respectively, the site was bare on May 1 (table 4). . If the "snow zone." is defined as the area in which a snowpack develops and lasts throughout an average snow season and is of sufficient magnitude to affect soil freezing, plant coverage, and the entire soil surface, then the snow zone lower boundary must be established at about 1680 m (5500 ft). As shown in the accompanying tables, however, in individual years it may range from about 1370 to 1980 m (4500 to 6500 ft) elevation. Thus, the weather modification study area in the American River Basin may be divided into three zones with shifting boundaries. Each zone is defined by its basic precipitation type and snowpack development. For this study, zone boundaries are idealized; they vary drastically from year to year. Rain is the predominant form of precipitation from sea level to about 1525-m (5000-ft) eleva- tion. Snow generally lasts from a few days to about 1 month. Soil freezing occurs routinely on ex- posed sites. A rain/snow zone occurs from about 1525 to about 1680 m (5000 to 5500 ft) elevation. Once snowpacks reach and remain at 30.5 to 45.7 cm (12 to 18 in), soil freezing is not a normal occur- rence. The snowpack holds about one-third of a normal year's precipitation through April 1. However, the snowpack does not increase with time to the "maximum pack" condition normally , . -. 2 A snow course is defined as a specific site from which snow depth density and water content data are gathered at predetermined intervals. These data are used to formulate forecasts of spring and summer streamflow from snow zones. IV-ll