Figure 7.8 



Highest Percentage Water Depletion (1975-2000) in a Mean Water Supply Year, by Water Resource Region 



ec» 



61-90% 



91 % and over 



supply. The Upper Arkansas (1102) has one of the 

 highest depletions in the Nation — 119 percent in a 

 dry year. 



The Rocky Mountain Region contains three of the 

 Rio Grande subregions, all of which have very high 

 depletion rates. Both the Rio Grande Headwaters 

 (1301) and Upper Pecos (1304) would now exceed 

 100 percent depletion in the dry year. The Gila (1503) 

 consumes 99 percent of the average supply, of which 

 66 percent is mined ground water. All of these sub- 

 regions are 60 to 70 percent forest and range, indi- 

 cating that resource management may offer at least 

 partial solution. 



Overall, the Great Basin drainages show very high 

 depletion rates. Water consumption in the Humboldt- 

 Tonopah (1603) would exceed the available supply 

 by 17 percent in a dry year, and will exceed 100 

 percent of supply by 1985 in the average supply situa- 

 tion. Because of heavy irrigation use in these basins, 



the seasonal distribution is a problem. Depletion 

 exceeds 90 percent supply several months every year. 



The Pacific Coast Region consists of the lower 

 portion of the Columbia River and all of the 

 California-South Pacific Water Resource Region. 

 Several subregions show significant potential prob- 

 lems, including the Oregon Closed Basin (1707), San 

 Joaquin-Tulare (1803), Central California (1805), 

 Southern California (1806), and the Lahontan-South 

 (1807). Several others show moderately high depletion. 



In all of these areas where water shortages are 

 expected, excluding the Great Lakes region, irrigated 

 agriculture is the major consumptive water use. Water 

 values for irrigation are among the lowest of all with- 

 drawal or consumptive uses. As water becomes scarce, 

 its use will ultimately decline for those purposes of 

 lower value. Thus, it is evident that the \vater short- 

 ages enumerated will ultimately have the greatest 

 direct impact on the agricultural and related economy. 



293 



