WSP02037 - Laserfiche WebLink

Home Browse Search

UUUI n The 1974 estimales bv Ffolliotl and Thorud are based on low and high trealment options in three vegetation types. Treatment in the mixed conifer forests would convert to grass one-third to h.\'o- thirds of Ihe area next lo stream channels. The low option projection of 1.2 inches of increased water yield from converting one-third of the area appears consistent \vith the current estimate of 3 to 4 inches from complete conversion, since response in this vegetation Iype appears proportional to area treated. However. the high option projection of 6 inches from converting only two-thirds of the forest appears overly optimistic. Treatment op- tions in the chaparral would convert 40% and 60% of the area to grass for increases of 1.2 and 2.4 inches. These projections fall within the range of current estimates of 1 to 5 inches for an area fullv treated (the average on favorable sites is 3.8 inch';s adinsted to 2.4 inches for onsite and offsile losses). In ponderosa pine. the low option would clear one-third in strip cuts for 1.2 inches increase, and the high option would clear Iwo-thirds in strip cuts for 2.4 inches increase. Th"se are slightly higher estimates but not necessarily incompatible with ClIffent unadjusted estimates. However. since the ClIffent adjusted estimate of 0.1 to 1 inch is con- sidered more realistic from a management standpoint. Ihe water yield potential in ponderosa pine is believed to be much less Ihan estimated for the Lower Basin by the FfoIliott- Thorud study. Further. the acreages considered suitable for treatment in the F'foIliott-Thorud study may not be available. since much of the ponderusi::t pine is in poorly stocked condition. Assessment Amount of Additional Water The amount of additional water that can be pro- duced within a multiple use management concept needs to be delermined. II can be theorized that the maximum increase possible could be eslimaled by multiplying tolal acreage times the unadjusted av- erage increase for each vegetation type (tables 1 and 2). If this were done in this study. some 4 million acre-feel would be indicated for the Upper Basin and about 2 million acre-feet for Ihe Lower Basin. However. this type of estimate ignores several basic problems that cause aclual yield increases to fall short of Ihe maximum potential. 1. Only a portion of each vegelation type could be treated economically for water yield in- creases. The amount depends on the demand for water and ils value in the markel place. 2. Consideration of other resource values and desires of the public tend to reduce both the area that can be treated and the effectiveness of treatments below that which could be ob- tained if water yield was the main objective of management. 3. The time frame required for water yield im- provement practices to become fu lIy opera- tional is probably in the order of several dec- ades for commercial forests with timber har- vest rotation ages of up to 120 years. However, less time would be required for noncommer- cial types. such as chaparral. Thus. the amount of water that can be produced hy vegelation management can best be quantified in terms of a range of alternative management op- tions that would include consideration of all fac- tors above. Another approach 10 assessing water yield po- tential is to determine how much treatment would be required to meet certain water augmentation goals. For example. the proposed Yuma Desalting Plant' on the lower Colorado River will produce up to 42.000 acre-feet per year of highly concentrated reject water. which would not be suitable for agri- cultural. municipal. or industrial uses. The Lower Basin states are unwilling to lose any water through a desalting plant reject stream because of anticipated future shortages. Therefore. the need arises to augment the existing supply by 42.UUO acre-feet to make up the difference. Although veg- etation management may not be the appropriate way to get the extra water. it is used here as an example to illustrate the role that vegelation man- agement could play in augmentation of the surface waler supply. The 42.000 acre-feet could be obtained by plac- ing 250.000 acres of subalpine forests in Ihe Upper Basin under management designed for water yield improvement as a major objective. Or. the 42.000 acre-feet could be obtained by converling 210.000 acres of brush to grass in the Lower Basin. Larger acreages in each case might need to be treated. if water yield improvement practices should require modification to adequately accommodate other re- source values. As a second example, suppose it became neces- sary to augment the water supply of the Colorado River by 750.000 acre-feet per year on the average. with no restriction on where in the basin the water should be generated. Since the Upper Basin yields 7Re/eet stream replacement study status report, January 1978, 102 P USDf Bureau of Reclamation. Lower Colorado Region, Boulder City. Nevada. 22