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II j . , ,. - I In caltulatin~ total system water demand¡¡, we factor in a 4% increase to reflect Wlaccounted for watef in the distribution system; it equals the di1'l'erepoe between the water put Into the water dislributicn system andt!\tal billed water use. For the calendar year 2000, tbis mass balance accountîng shows unaccountcid fQr water use equals 4.4%. For the future ye¡II'S, the forecast cal(;Ulatioll assumes unaccounted water use win be 4.0%. Having an unaccounted for ~tor of about 4.0010 is relJltively good. Comparable water systems typically experience ooaccounted water Û'om 5 tö I()%. 5. Sensitivity Analysis This section explores how variations in .assumptions can impact the base case point estimates generated in Appendix D. Specifically, we looked at water use intensification and weather variation. 5.1 Water Use Intensification Increasing residential and commercial space costs can lead to intensification of use at an existing site. Apartments, fur example, ean see more persons per unit as higher rent costs cause more people to livetogelher to make the rent. Similarly, business OWt1.mare financially motivated to PJ1t more employees per squate foot to manage costs. We tested the bypothesis ofintensificatîon by comparing FY .1997198 water use to FY 20.00101 water use. We foood that water use did increase by about 8 percent on a water use per bi!! basis for both multíple family and commercial customers. This held true for winter as well as annual water use (weather is lIot likely to cause this change). Althougb this evidence supporlSwater use intensification over the 4-year period, it does not necessarily mean it wiJJcontÏnue in the future. In fact, prices in the real esfatemmet have decreased in ~nt months. Therefore, for the pllIposes of forecasting, we assume that the intel¡SificatÎon levels e¡¡perienced in 2000 win remain constant over our 20-year planning how-en. 5.2 Weather Variation RWC's annual water demand only varies by about +/- 2 pen:ent asaresuJt Ofweather. This conclusion is drawn wm análysís of$latistical regression models of monthly water purchases as a fu¡¡ctjon ofwc.a!hC¡' between 1975 IUId 2001. Note weather can significantly impact RWC's water supplies via the Hetch Hetchy water system; sbortages in supply can colI$equently require RWC to take actioll$ 10 cut water demands (e.g.. 1991). Absent supply shortages, however, RWC's annual water demand does not vary greatly beçause of local wc.ather. This lack of variability in annual water use is largely a fu¡¡c:tiónof the lack of variability in annual temperatures. Over the 1948 to 2001 period, about 70 percent of the time Page 7 .. .""",..,..~,,~,~..~,~ .. .._....'-"~-""'~~"""'--""""""""