Water Yield from Ponderosa Pine Forests

Introduction

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The Salt River Watershed spans 2.74 million acres across central Arizona in the southwestern United States, and is dominated by ponderosa pine and mixed conifer forests. Managed by the White Mountain Apache Tribe, San Carlos Apache Tribe, and U.S. Forest Service (USFS), the Salt River and its tributary, the Verde River, supply freshwater to the Phoenix metropolitan area. Recent studies showing snowpack declines across the western United States and large wildfires in the Salt River watershed have heightened concerns of the long-term sustainability of the region’s water supply. However, much less attention has been paid to the effects of unnaturally dense forests--a result of fire suppression--on water supply. Changes in forest density are important because approximately 90% of incoming precipitation in the Salt River watershed is lost to evaporation or water use by vegetation (transpiration). While there is little that managers can reasonably do to influence drought and precipitation patterns, they can have a direct influence on forest density.

Key Issues Addressed

Streamflow in central Arizona is greatly influenced by inter-annual variability in precipitation but also may be impacted by a legacy of fire suppression that has resulted in high-density forests. Several large-scale forest restoration programs are underway across the Southwest, including the Four Forest Restoration Initiative. These restoration programs have diverse goals, including re-establishing natural forest densities and fire regimes, decreasing the risk of catastrophic wildfire, and improving habitat conditions for wildlife. Water yield from forest may also increase due to reduced water use by trees. Research is needed to assess the potential for large-scale forest restoration to recover streamflow that supports wildlife as well as human communities downstream.

Project Goals

• Evaluate trends and relationships among forest density, climate, and streamflow in the Salt River Watershed in the 20^th century
• Inform forest restoration and water management in the region

Project Highlights

Flow Recovery: Increasing the pace and scale of prescribed fire and mechanical thinning aimed at reducing forest density may help recover streamflows lost during the early to mid-20^th century.

• Forest Condition Assessments: Using various data sources, researchers qualitatively assessed changes in forest conditions and fire during the 20^th century. Historical measurements of ponderosa pine canopy cover, density and basal cover were obtained from sites in northern Arizona and the Southwest. Historical logging estimates were derived from timber inventory and sales from the White Mountain Apache Reservation, USFS Alpine Ranger District, and Pleasant Valley Ranger District. Current estimates of forest basal area, canopy cover, and density were summarized from plots from the USFS Inventory and Analysis program within the Salt River watershed. Records of prescribed fires and wildfires were also gathered for the watershed.
• Climate and Streamflow Data: Monthly total precipitation and monthly mean temperature values were obtained from the McNary cooperative weather station. Monthly streamflow data from 1914-2012 were gathered from the U.S. Geological Survey Salt River stream gage in Roosevelt, Arizona. To evaluate snowpack trends, snow water equivalent data were gathered from multiple snow course stations near the study area.
• Flow Trends: To evaluate how the timing of spring flows has changed, time trends in the date at which 50% of the cumulative annual streamflow occurred were calculated. This is an estimation of peak spring snowmelt. This center of timing metric (CT) was calculated from January-June as opposed to the traditional October-September water year to avoid effects of tropical storms and the summer monsoon since these can lead to heavy rainfall but do not produce snow.
• Analyzing Relationships: Trends in monthly precipitation, temperature, snowpack, annual and monthly streamflow, and the timing of peak spring flows were evaluated for the century. Models were developed to estimate the influence of climate and forest conditions on streamflow.

Lessons Learned

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In summarizing a century of forest and climate data, this work showed a coinciding relationship between increasing forest density and decreasing streamflows. This relationship seems to be driven by a combination of climate conditions and management practices.

Ponderosa pine forest densities increased at least 10-fold in the 20^th century, with some evidence that most of this change occurred in the 1st half of the century. This increase is attributed to suppression of wildfires that would have naturally thinned the forests. 

The analysis indicated that annual and monthly flows in the Salt declined by 8-29% from 1914-1963 as ponderosa pine forest densities increased. During this period, streamflow volumes declined by 37-56% in summer and fall months. These results are consistent with other studies that have found that increasing forest density can decrease flows by increasing water consumption by vegetation. 

Declines in flows reversed at mid-century when spring and annual flows increased by 10-31% from 1964-2012, possibly due to the impacts of large-scale wildfires and/or large rain-on-snow winter events. The CT of spring flows occurred about 12 days earlier in this period, which coincided with winter and spring temperatures increasing by 1-2 Celsius.

Restoration practices include prescribed burning and mechanical thinning treatments designed to reduce wildfire risks, but when conducted at large-scales could also potentially recover seasonal flows that were lost due to historic increases in forest density.

Next Steps

• Complete a follow-up study to evaluate the impact that the two largest wildfires in state history, the Rodeo-Chediski in 2002 and the Wallow in 2011, had on annual streamflow in the Salt River Watershed
• Compare the influence of snowmelt versus large rain-on-snow events on annual streamflow in the Salt River Watershed
• Use results of this and other studies to provide information to land and water managers on how to sustain water supplies and forest health in a changing climate
• Declines in streamflow associated with increased forest densities did not persist in the 2^nd half of the 20^th century. Researchers are exploring two alternative hypotheses including the influence of large-scale wildfires and large rain-on-snow winter events played in flow alteration in the last 50 years.

Resources

• Robles et al. (2014) “Effects of climate variability and accelerated forest thinning on watershed-scale runoff in southwestern USA ponderosa pine forests.” PLoS ONE 9(10): e111092.
• Robles M., D., Turner D., S., and Haney J.A. (2017) “A century of changing flows: Forest management changed flow magnitudes and warming advanced the timing of flow in a southwestern US river.” PLoS ONE 12(11):e0187875.
• Related Case Study on Modeling the Relationship between Forest Thinning and Water Yield

Contacts

• Marcos Robles, Conservation Science Specialist, The Nature Conservancy: mrobles@tnc.org
• Dale Turner, Conservation Specialist, The Nature Conservancy: dturner@tnc.org
• Jeanmaries Haney, Hydrologist, The Nature Conservancy: jhaney@tnc.org
• Blakemore Thomas, USGS Arizona Water Science Center, retired

Case Study Lead Author

Briana Becerra, CART Graduate Research Assistant, The University of New Mexico

Suggested Citation

Becerra, B. (2020). “Climate Change and Management Practice Effects on Water Yield from Ponderosa Pine Forests.” CART. Retrieved from https://www.fws.gov/project/water-yield-ponderosa-pine-forests.