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Show 6 Utah Geological and Mineral Survey Water- Resources Bulletin 21, 1976 Table 1. Average annual precipitation and freshwater evaporation for various lake altitudes and diked areas. Area ( tigs. 3 and 5) Altitude above mean sea level ( ft) Precipitation ( Pa) ( in) Evaporation ( in) South part 4,205 4,199 4,196 4,195 12.98 13.46 13.70 13.74 55.98 56.25 56.39 56.4 1 7 4,205 4,199 4,196 4,195 10.66 10.80 11.08 11.13 62.72 62.09 61.48 61.32 1 4,205 4,199 4,196 13.09 12.93 12.95 52.56 52.56 54.18 2 4.205 4,199 4,196 12.89 12.89 12.89 54.42 54.35 54.47 4 4,205 4,199 4,196 13.42 13.38 13.34 52.88 53.26 53.44 5 4,205 4,199 4,196 13.86 13.81 13.71 51.42 51.50 51.94 2 + 4 + 8 4.205 4,199 4,196 13.34 13.81 13.71 53.36 53.52 53.84 Average for diked areas 13.33 53.25 Precipitation The inflow to Great Salt Lake from precipitation on the water surface ( Ip) was calculated in the following manner. The average annual precipitation ( Pa) during 1931- 73 was computed for 68 sites in a large area surrounding the lake. A multiple- regression analysis of the data was made to derive an equation describing mean annual precipitation as a function of latitude, longitude, and altitude. Using the equation, lines of equal average annual precipitation during 1931- 73 were drawn for the area around the lake for a water- surface altitude of 4,200 ft ( 1,280.2 m) ( fig. 3). The surface area of the lake varies with water- surface altitude, and because precipitation varies areally across the lake, the average precipitation on any part of the lake is dependent upon the area inundated at a given water- surface altitude. Thus the lake was separated into seven different areas- the north and south parts separated by the Southern Pacific Transportation Co. causeway and the bay areas east of a line joining the Promontory Mountains, Fremont Island, and Antelope Island ( fig. 3). Average precipitation values were computed for inundated areas at water- surface altitudes of 4,195, 4,196, 4,199, and 4,205 ft ( 1,278.6, 1,278.9, 1,279.8, and 1,281.7 m) ( table 1). Thus, by knowing Pa for various altitudes, the average precipitation for any lake altitude can be interpolated. For example, if the lake altitude of concern is 4,200 ft ( 1,280.2 m), then average annual precipitation would be Pa4200= [( 4200- 4199)/( 4205- 4199)] • ( Pa4205' Pa4199) + Pa4199 The ratio of annual precipitation for individual years to the 1931- 73 average ( Pa) ranges from 0.67 to 1.43 ( Aj) ( table 10). 1 To compensate for this variation, the 1931- 73 average was adjusted by the factor Aj. So, the adjusted annual precipitation is now Pad = ( Pa)( Aj), where Aj is the annual correction factor for any year, j. The next step was to compute the monthly distribution of precipitation for each month of each 1 The ratio Aj was determined by obtaining the ratio of annual precipitation of 10 stations near Great Salt Lake to the 1931- 73 average for the same 10 stations. |
