The goal of the Sierra Nevada Watershed Ecosystem Enhancement Project is to quantify the potential for thinning of Sierra mixed conifer forests to enhance the ecosystem services they produce including fire resiliency, carbon storage in trees, and water in streams. Much of the Sierra Nevada is covered with forests that are dramatically denser than before fire suppression policies led to extinguishing all wildfires over a hundred years ago. The forest is denser because smaller trees that would have been killed by fires have grown to mature size. Forest restoration involves thinning to reduce the density of trees and shift species composition to a more sparse and fire resilient historic level.

Today’s denser forests are more prone to experiencing high severity fire in which most trees are killed and forest litter is consumed. This can lead to soil erosion, reduced ability of forests to absorb precipitation, and increased risk of flooding.

Compared to historic forests, dense forests store more of their carbon in smaller trees that historically would have been killed by frequent low and moderate severity fires. There is a higher risk in the Sierra Nevada of losing much of this stored carbon to high severity fires. Climate change also means that the risk of wildfire is increasing as the snowpack melts faster and the annual fire season is lengthening.

Dense forests also use more water than less dense forests and physically interfere with the ability of snow and rain to reach the forest floor. This decreases the amount of precipitation that is eventually discharged into streams and available for use by downstream water users. Dense shrubs will also transpire much more water than grassy meadows or bare ground. 

Thinning reduces the number of smaller trees and removes ladder fuels which reduces wildfire risk and severity, meaning it is more likely that trees and ground cover will survive fires that do occur. This reduces the soil erosion potential created by wildfires. Biomass stored on site is more likely to maintained into the future. If forest products are captured from thinning, these products also store biomass. Thinning also reduces the amount of vegetation using water and so results in more precipitation, particularly snowmelt, infiltrating into the soil and eventually running off as stream water. In west-side Sierra forests, the snowpack provides an important seasonal storage of water that, when melted, is transmitted downstream and used throughout California.

SWEEP will use scientific methods to learn how forest thinning in the Sierra Nevada affects the fire risk, carbon storage and water yield provided. Water research will measure a number of hydrological parameters and will leverage instrumentation and results from other sites to rigorously quantify the effect of thinning on stream flow. We will then use economics to identify the value of increases in fire resiliency, carbon storage, and water yield in collaboration with forest stakeholders. Multiple research sites will be used to allow for analysis of how tradeoffs between these ecosystem services differ in different locations.

The full report can be downloaded at the link below:

PDF iconSWEEP report 2011.pdf

Principal Research Organizations: 

University of California, Berkeley and Merced 

Collaborating Organizations: 

Funding Sources: 

SWEEP has been funded in part by:

  • University of California Competitive Grants Program 2012-2015
  • Sierra Nevada Conservancy
  • Bella Vista Fund
  • Environmental Defense Fund

SWEEP People: 

Project Location: 

The Phase I SWEEP report identified catchments in the American River basin as potential research sites. Use of this watershed for SWEEP would allow for leverage of other ongoing research, and a committed and active group of stakeholders who recognize the need for the research. Further evaluation of project sites, involving additional measurements, and discussions and collaboration with local water and land managers will be done in the coming year. New research sites need to have a potential to undergo thinning treatments in the future and be suitable for stream and forest instrumentation.

Current research sites that are collaborating with SWEEP include:

  • Last Chance Project Study site: Tahoe National Forest near Foresthill, Placer County, California, part of the Sierra Nevada Adaptive Management Project
  • King’s River Experimental Watershed: Sierra National Forest, part of the Critical Zone Observatory project

SWEEP Parameters to be Measured:

The field measurement program will collect data on high-density and intermediate-density forests, as well as on recently burned forests for another end-member point of comparison. This will inform analysis and modeling of how water yield will respond to both thinning and climate change over larger areas. This will allow us to develop a measurement framework to value ecosystem services to support potential future public and private investments. The overarching goal is to use the American River area as an informative case study for measuring and valuing water-based ecosystem services, assessing the impact and importance of those services to local stakeholders, and determining competition between services. 

Parameters to be measured: 

  • Leaf Area Index (LAI) is the sum of all the leaf surface areas projected downward per unit area of ground beneath the canopy. LAI will be calculated by felling about 10 trees (of varying sizes) per tree species and measuring all the area on the leaves allowing construction of a curve relating tree size to LAI
  • Soil moisture is measured by of soil moisture sensors at multiple depths.
  • Snowpack accumulation and melting is measured by acoustic depth
  • Stream runoff is measured using stage recorders at natural control sections, and weirs or flumes.
  • Evapotranspiration is the conversion of water (in the soil or within plants) into water vapor released to the atmosphere. This will be measured by sap flux, and estimated using meteorological methods and through basin-scale water balances.

Additional measurements on meteorology, snowcover, soils and forest structure are available from other research programs


PDF iconJune 2012 SWEEP Newsletter.pdf

August 15th, 2015 - More than 10,000 acres in Sierra Nevada protected in deal that aims to boost water supply, reduce fires with Dr. Roger Bales

October 14th, 2014 - "How Too Many Trees Contribute to California's Drought" KQED radio with Dr. Roger Bales and Dr. Scott Stephens

September 23, 2014 - "UC Berkeley Professor Says Wildfires Create Opportunities for California to Increase Water Supply"; Bay Area Drought Watch with Dr. Scott Stephens

September 15, 2014 - "Why more trees in the Sierra mean less water for California"; KQED Science 

March 24, 2014 -  "Overgrown Sierra forests gulping water that could flow to valley"; an article in the Modesto Bee with comments by Dr. Roger Bales.

February 8, 2014 - " California drought exposes Sierra's weakness"; an article in the Fresno Bee with comments by Dr. Roger Bales. 

January 28, 2014 - "Water-Forestry Forum charts a new course for California"; an article in the Calaveras Enterprise discussing the forum attended by Dr. Roger Bales and Dr. Kevin O'Hara. 

May 21, 2013 - FileCalaveras County Water District hosts Sierra watershed talk led by Dr. Roger Bales

November 1st, 2012 - Onward California short video on Dr. Roger Bales' water research

October 2, 2012 - Dr. Rob York, Blodgett Research Station Manager, is interviewed by Capital Public Radio.  Click HERE

September 21, 2012 - Drs. Battles and Bales are featured in California Almuni magazine, click HERE for the article.

September 21, 2012 - Forest scientists bet on the trifecta SWEEP in the Sierra Nevada, click HERE to read

August 17, 2012 - Mountain Counties talks about connecting water and forest, Mountain Democrat, click HERE for the article.