Figure 7.7 



Annual Mean Streamflow within the United States, 1930-1976, and the Moving 

 Average, 1920-1974 



% of Mean"" 

 150 



1920 1930 1940 1950 1960 1970 1980 



Water Year 



^Computed from base period 1931-1960. 



Water Quantity 



Table 7.11 presents tne water demand-supply data 

 used to evaluate water supply adequacy. The propor- 

 tion of each subregion that is currently in forest and 

 range is presented to indicate the relative importance 

 of forest and range management to each subregion. 

 In the analysis of water quantities, two levels of supply 

 are considered: (1) The mean supply, which is the 

 amount of water that would be expected in the average 

 water supply year, and (2) the dry year supply, which 

 is the minimum amount that is expected 80 years out 

 of 100. In effect, the expected water supply will be 

 less than the dry year supply 20 percent of the time. 



Consumptive water use is one of the more impor- 

 tant factors to consider in evaluating water adequacy. 

 Table 7. 1 1 shows the percentage depletion of supplies, 

 which is the proportion of the available supply that 

 will be consumptively depleted in the mean and dry 

 years. 



Figures 7.8 and 7.9 also present the 106 Water 

 Resource subregion by four water depletion categories 

 based upon the highest depletion rate over time in 



the mean water supply year (fig. 7.8) and the dry 

 supply year (fig. 7.9). 



Often, seasonal water supply problems are not 

 apparent from annual supply-use data. Therefore, 

 table 7.11 also presents the number of months each 

 year in which consumptive use would exceed the 90 

 percent supply in both the mean and dry supply years. 

 This indicates the importance of seasonal variations, 

 and also will have important implications for instream 

 uses which will be discussed later. 



It is important to note that in some subregions, 

 ground water mining is used to supplement surface 

 flows. Thus, the monthly data represent the situation 

 if ground water mining does or does not occur. For 

 example, table 7.11 shows that in the average supply 

 year for the San Joaquin-Tulare Subregion (1803), 

 consumptive use normally will exceed 90 percent of 

 streamflow for 4 months without mining ground 

 water, and only for 3 months if ground water mining 

 continues. Mining is only a temporary solution to 

 water supply problems and cannot continue indefi- 

 nitely. 



291 



