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14 Estimating Economic Impacts of Salinity... t\:) CP. (0 ~J.> In addition, 250 mgiL TDS represents the approximate average TDS of California State Project water, the most likely available alterna- tive source of water for the majority of Lower Basin Colorado River water users. Finally, while it is recognized that some negative effects of salinity damages can start at lower TDS levels, in fact significant impacts do not occur below the range of 200-300 mgiL TDS. Thus 250 mgiL could be considered as a baseline representing a practical value for "ideal water." The 295 mgiL baseline corresponded with historical average Colorado River salinity near Grand J unction. Finally, the 349 mgiL baseline was the most recently (mid-1986) reported level of the mainstem near Cisco, Utah, and was selected to provide a mid-point balance to the baseline range. One other potential baseline discussed was that of 539 mgiL. This number was derived from the CRSS program by the staff of the Bureau of Reclamation. It represents a "modern" natural TDS level inasmuch as it is the salinity level of the Colorado River (measured at Yuma, Arizona) prior to construction/closure of any of the large storage reservoirs (1926 to 1934). This 539 figure does include diversions for irrigation and M&I use, and the subsequent return flows. While it in no way represents the "virgin" TDS of the Colorado River, it does represent river flow that is not particularly well controlled - thus assuming a larger proportion of natural TDS runoff than could be expected under the con- trolled conditions instituted by the storage reservoirs. Selection of Current Values for TDS During the course of this study, reviewers and advisors became as concerned about the selection of "current" TDS values as about the baseline. This appeared to be due in part to the fact that in 1986 the Colorado River had experi- enced an extendcd period of excess flows - thus reflecting abnormally low salinity levels. It also appeared to be partly due to a fear of misstating any salinity damage estimates, a concern about the lag in the reporting of salinity levels at gaging stations, and a recognition of the difficulty in estimating the actual TDS level of the usually blended water delivered to consumers in the metropolitan areas covered by this study. The Colorado River Basin Salinity Control Forum Work Group and staff of the USBR Colorado River Water Quality Office, at a meet- ing in October 1987, decided to select two values to be used as "current" TDS values at each of the gaging stations used in this study. One repre- sents the most recently available 1986 average at each station (provisional data). The other is a selected 10-year average at each station, based on CRSS data. They are shown in table 2. Table 2. - "Current" TDS values used In the study In mglL. 1 (1976-1985) 1986 Flow- Ten-year Flow- weighted Actual weighted ms. Average TDS Hoover Parker Imperial 652 678 767 542 542 579 tprovlslonal data In this report, the 1986 flow-weighted salinity level is used as the "current value" together with the two selected baselines to estimate salinity damages. However, for comparison, the 10-year flow-weighted average also is used in conjunc- tion with baseline values to produce another range of salinity damage estimates. The lO-year average is believed more representative of past and future river salinity levels than the 1986 actual TDS level. The salinity levels of the Colorado River reservoirs behind Parker and Imperial Dams are essentially the same as that of irrigation water delivered to the major agricultural areas, i.e., 542 and 678 mgiL TDS for Riverside and La Paz Counties and the future Central Arizona Project, and 579 and 767 mgiL for Imperial and Yuma Counties. However, the values arc blended for each of the metropolitan areas to represent the mix of Colorado River water and local ground or surface water supplied to water consumers.