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annual average temperature did not vary by mom than I°F from its average of 70.3OF and <br />never had a difference greater than 3°F. Although greater differences are experienced on <br />a month-to-munth basis, they tend to average out over the year. This stability creates <br />stability in annual water demand. Exhibit 5 plots temperature against monthly water use <br />over 1975 to 2001. Water use and t~nperatur~ are highly correlated. <br /> <br />Annual rainfall, in contrast, is much more variable. However, rain tends to fall in the <br />winter and early spring raonths when temperatures and irrigation demands are relatively <br />low. Hence, rainfall variability does not tend to greatly impact annual water demands. <br />Rainfall has a much bigger impact on water supply (Hetch Hetchy) than demand. <br /> <br />Our model correlating water use with weather was specified to measure the deviations in <br />water use (as shown in Exhibit 5) from deviations in normal temperature arid rainfall as <br />follows: <br /> <br /> W.,I TERt = ~_, ai x MONTHi, t + ,~1 x TEMPDEVt + j~2 x RdlNDEVt <br /> <br />where, <br /> <br />ff'ATERt = ratio of water use in month t to a l2.month moving average of fft,~TERt <br />MONTtti, t -- binary variable creating one unique a intercept for each calendar month <br />TEMPDEIZt -- temperature deviation in month t from the average for that month (°F) <br />RAIIgDEIZt -- rainfall deviation in month t from the average for that month (inches) <br />ai, pl, fi2 = coefficients estimated using least squares regression <br /> <br />A~er estimating the raodel, we used actual weather data from 1949 to 2001 to simulate <br />the maximum impacts on annual water use. From this exercise, we determine that water <br />use can be expected to vary +/- 2 percent from weather alone on annual basis. <br /> <br /> Page 8 <br /> <br /> <br />