Spring 1994

Vegetation Management for Water Supply in Perspective

K.M. Turner
Watershed Management Specialist, California Department of Water Resources, Sacramento

The concept of managing forests for "securing favorable conditions of water flows" (1897, 30 Stat 34) is of long standing. However, implementation has been slow. The California Water Plan (1957) regarded watershed management as a promising future technology "to improve the regimen of stream flow." However, several decades of chaparral to grass type conversion field experiments are being rendered irrelevant by multipurpose use policies and biological diversity concepts. The U.S. Bureau of Reclamation (Burgy, et al., 1975) and the California Dept. of Water Resources (CDWR, 1983) have studied augmenting the yield of major reservoirs through vegetation management.

In addition, National Forest management planning addresses increased water yield but treats it as an incidental benefit of timber, chaparral and range management. The sum of the historic (pre-1982) annual water yields shown in the draft/final environmental impact statements (as of 6/92) for management of National Forests in California (excluding Siskiyou and Rogue River) is 34.1 million acre-feet. The sum of the estimates of increase in annual water yield as an incidental benefit of preferred or selected plans is 362,000 acre-feet. Elimination of yields of watersheds not tributary to major reservoirs or ground water storage, and further reduction for variability results in a potential increase in water yield of 95,000 acre-feet that might augment municipal and irrigation water supplies when regulated by storage. This amount is subject to further reduction for spills due to lack of storage space determined with reservoir operation model studies.

To have value for municipal and irrigation purposes, water must be available at the place of use when needed. In the drought year of 1992, the State Drought Water Bank sold water for $175 per acre-foot to buyers able to transport it to a place of use. Water without such availability would only have value to water systems able to transpose the water from wet season or year to the dry time of need, and the value would depend on the system's alternative options.

Water salvaged by vegetation management in a Mediterranean-type climate is generally available at locations remote from a place of use, and has 4 major hydrologic disabilities identified by Turner (1991 a&b, 1993):

2. Most of the salvaged water occurs during the wet season;

3. Maximum evapotranspiration is reached at a shrub-tree cover density of about 65%; and

4. Without maintenance, an area cleared of shrubs and trees will return to full evapotranspiration in about 5 to 8 years.

The former two disabilities make surface or ground water storage essential for time availability. The latter two mean that rotational treatment (e.g., logging and prescribed burning for fuel management) will salvage little water. Prescribed burning of chaparral with a 30-year rotation and 5-year recovery would reduce the shrub cover by 10 percent. A 100-year patch or clearcut logging rotation with 8-year recover would reduce the tree cover by 4.5 percent. Natural openings and wildfires may bring shrub and tree cover into the 60-70 percent range, at which a small increase in water yield may be expected. However, if the effect of such rotational treatments is to reduce the magnitude and frequency of wildfires, there may be no long-term net increase in water yield.

Coordination of vegetation management to provide a more even year to year reservoir inflow would be quite valuable to a water agency, independent of increased yield. The ideal coordination objective would be least annual variation in mathematical sum of the products of acreage cleared and mean annual precipitation on such areas. Such management would increase the probability of salvaged water being available in a dry year, and reduce the chance of major contributions of salvaged water to flood flows.

Removal of brush understory in hardwood forests and conversion of chaparral to grass-oak woodland could supplement water salvage by rotational treatment to make the amount of water salvaged more attractive to water agencies. Implementation of these practices will depend on whether they are found environmentally benign and can be funded.

CDWR (1983) found vegetation management of 33,000 acres in the foothill portion of the Feather River watershed to be economically attractive. However, the increase in firm annual reservoir yield of 2,500 acre-feet was deemed insufficient to justify starting such a program. No funding scheme was proposed.

Vegetation management to augment water supplies will not be easy or bountiful. To get more than passing notive by the water industry, water salvage will have to be advanced from an incidental benefit to a primary management purpose and receive planning efforts similar to those directed towards any other major management purpose.

REFERENCES
Burgy, R.H., L.A. Jackson, M.B. Jones, A.W. Knight, W.M. Longhurst, and C.A. Siegfried. 1975. Summary report: evaluation of water yield potential in East Putah watershed under multiple use management. Dept. of Water Science and Engineering, Univ. of California, Davis.

California Dept. of Water Resources. 1983. Potential for salvaging water through vegetation management in the lower Feather River watershed. Central District report. 92 p.

Turner, K.M. 1991a. Annual evapotranspiration of native vegetation in a Mediterranean-type climate. Water Resources Bulletin 27:1-6.

Turner, K.M. 1991b. Water salvage from Mediterranean-type ecosystems. In: H.E. Bailey, C. Forrest and L. Snow, eds., Symposium Proceedings-Water Supply and Reuse: 1991 and beyond. (San Diego CA, June 2-6, 1991). Amer. Water Res. Assn. pp. 83-90.

Turner, K.M. 1993. Water harvesting/salvage: coming of age. In: Weeds and people, putting weed management in perspective. 45th annual weed conference. (Costa Mesa CA, January 18-20, 1993). 4pp.