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Show Waddell and Fields- Model for Evaluating the Effects of Dikes on the Water and Salt Balance of Great Salt Lake 0.20 JFMAMJJASOND 0.20 0.13 - 0.10- 0.05 - JFMAMJ JAS0N0 Figure 4. Graph showing average monthly distribution of precipitation and evaporation on Great Salt Lake, 1951- 60. year during 1931- 73. This monthly distribution was computed as a percentage ( Pmi)( 100) of the annual total. The monthly distribution had only a small variation from year to year during a selected test period ( 1951- 60) for 11 sites in the vicinity of Great Salt Lake. So an average monthly distribution was computed for the test period and assumed to be the same for each year of 1931- 73 ( fig. 4 and table 11). Thus, the average monthly precipitation for a given lake altitude becomes Pm = ( Pad) ( Pmi) The average annual inflow from precipitation on the lake surface during 1931- 73 was estimated to be 966,000 acre- ft ( 1,190 hm3) per year and ranged from 680,000 to 1,260,000 acre- ft ( 840 to 1,550 hm3) per year. In addition to precipitation on the surface of Great Salt Lake, precipitation on the wetland areas between long- term surface- inflow stations ( fig. 6) and the shoreline of the lake was computed. This was done in order to extrapolate surface- inflow data observed at the long- term stations downstream to those applicable at the shoreline at an altitude of 4,200 ft ( 1,280.2 m) ( table 2). The variance of precipitation ( and evaporation) for these areas was small, so a mean value of 13.81 in ( 351 mm) per year was used for all areas. The annual distribution factor ( Aj) and monthly distribution factor ( Pmi) computed for Great Salt Lake were also used for the wetland areas. Evaporation Evaporation from Great Salt Lake ( Oe) was developed as a function of latitude, longitude, water- surface altitude, pan coefficients, and salt content. To do this several intermediate steps were necessary. The first step involved the extension of short- term class A pan records at 49 sites to the period 1931 - 70. l Most of the stations have records only during June- September, so the June- September evaporation data for all the short- term stations were extended to 1931- 70 ( Est^^ g). This was done by using the ratio of the short- term data ( Est) to the concurrent record at a long- term site ( Elt), as a factor times the 1931- 70 data at the long- term site ( Ht1931.70) ( table 12). EstJune- Sept. 1931- 70 = ( Est/ Elt) ( Elt1931.70) The record at Utah Lake Lehi is complete for 1931- 70 and was used as the long- term site. The second step involved the extension of the June- September data to the entire year. The ratio of June- September data to that for the entire year was computed for those few sites where complete annual records were available. It was found that these ratios varied as a function of latitude. Using the multiple- regression technique, an equation describing the annual correction factor ( Acf) as a function of latitude was developed. This equation was then used to extend the June- September evaporation data to the entire year for the complete data set ( table 12). Very little evaporation occurs from November to February; thus, the extension of June- September evaporation to January- December evaporation essentially adds the months of March, April, May, and October. For each site, therefore, the January- December evaporation ( EstJan.- Dec. 1931- 70) is obtained by dividing the June- September estimates ( Estjune. gept 1931- 70) ^ Y the annual correction factor ( Acf) associated with a particular latitude ( table 12): E= E, Jan.- Dec. 1931- 70 ~ EJune- Sept. 1931- 70/ Acf 1 The period 1931- 70 was used because the records for 1971- 73 were not yet available. The small annual variance during this period indicated that a 1931- 70 base period for evaporation would be adequate even though 1931- 73 was the base period for the model. |