Unformatted text preview: 111 he 11* :_r 1‘ Lab 1 Water Budget of Mono Lake: Precipitation and Evaporation 7 4. Construct an isohyetal map of the Mono Basin on Figure 1.4, making reference to the topographic map in
Figure 1.2. Compute the average annual precipitation on Mono Lake by this method. Compare your result with
precipitation determined by the other two methods (Nos. 1 and 2). EVAPORATION You will estimate the evaporation at Mono Lake in two ways: (1) by referring to a published map and (2) by
reducing data from an evaporation pan at Cain Ranch. 1. Estimate average annual evaporation, or lake evaporation (In), by viewing the evaporation map in Figure 1.5.
For conversion tables from American units to SI units, refer to your textbook (e.g., Fetter, 2001,
Appendices 7—9). The data for the lake evaporation map compiled by Kohler and others (1959) were derived from measurements of
water evaporated from a standardized pan: a 47ft-diarneter, galvanized pan known as the Class A land pan. These
measurements of pan evaporation are shown on the map in Figure 1.6. Because evaporation pans have greater evaporation rates than lakes, typically about 140 percent, they must be
corrected by applying an empirically derived pan coefﬁcient. The variation in pan coefﬁcients is shown on a similar
map in Figure 1.7. In effect, the lake evaporation map you used (Fig. 1.5) is a derivative of the other two maps—-
measured pan evaporation (Fig. 1.6) multiplied by a pan coefﬁcient (Fig. 1.7). 2. The following pan data were recorded at Cain Ranch. Cain Ranch Station
Elevation: 6880 ft Class A evaporation pan, water level held constant by ﬂoat valve; volume of water needed to recharge pan
(corrected for precipitation) is recorded. May 33.71 gal August 73.94 gal
June 51in gal September 49.41 gal
July 94.10 gal October 33.51 gal (3) Calculate the pan evaporation (or) for this period. (b) Data for the winter months are usually not recorded because in much of the United States, freezing prevents
measurement. Extrapolate for annual pan evaporation using the map in Figure 1.8. (c) Correct for the pan eﬂ’ect, using the map in Figure 1.7 to determine annual lake evaporation.
(d) How does this measurement compare with the reading you took directly from the map (No. I)?
3. Calculate the annual evaporation (m3) from Mono Lake. LAB REPORT Prepare a lab report summarizing the analysis to date, showing all calculations. For all quantitative values, ensure that you report signiﬁcant ﬁgures (signiﬁcant digits) only (e.g., see Fetter, 2001,
p. 19). ...
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