Potential gains in water yield from watershed-scale treatments of ponderosa pine forests in the Salt and Verde Watersheds, Arizona

Background/Question/Methods

Are there sustainable models of forest and water management in arid regions that address the threats of declining water supply and declining forest health? Infrastructure for water delivery in the southwestern US is currently over-allocated and proposals for new infrastructure must compete in an era of dwindling public funding. The current US federal forest management approach, where Congress pays most of the costs, is not economically viable and has not adequately addressed the impacts of amplified fire activity and tree mortality associated with climate change. Forest treatments may increase the resilience of forests to climate change and yield additional water to downstream communities, but have not been evaluated on par with other water infrastructure alternatives. In this paper, we adapt the Baker-Kovner regression equation to estimate additional water that may result from two levels of forest treatment: one, a planned landscape-scale restoration project, the Four Forest Restoration Initiative; and second, a hypothetical project that would thin all ponderosa pine forests in the Salt and Verde watersheds. We construct scenarios that explore how variations in winter precipitation (reconstructed from 20th century records) affect  increases in water supply from forest management.

Results/Conclusions

Treatment of approximately 150,000 acres of ponderosa pine forest in the Four Forest Restoration Initiative may yield an additional 1,279 to 2,621 acre-feet/year in the Verde watershed over a 15-year period. In a hypothetical scenario where 600,000 acres of ponderosa pine are treated over 15-, 25-, and 35-year periods, additional acre-feet of potential water yield projected on an annual basis are 4,540 to 9,327, 2,959 to 5,148, and 2,243 to 3,442, respectively. The low yields assume dry periods that are similar to the 1950s and 2000s droughts; high yields assume pluvial periods similar to the 1900s and 1970s. Initial forest treatment costs – $4,000/acre-foot – are on the high end when compared to other alternatives, whereas follow up maintenance costs are in the mid-range. Our study suggests that additional water yield to Phoenix Metropolitan Area accrued from watershed scale forest treatments are relatively modest, < 2% of current demand met from the Salt and Verde watersheds. However, this additional water supply compares more favorably to smaller sources of water – 10 to 43% of water from effluent in Phoenix Metropolitan Area – and for smaller cities and towns – 11 to 46% of the projected shortfall in water supplies for cities and towns in the Verde watershed by 2050.