With much of the state’s water supply originating in the mountains as precipitation on the forested landscape, the health and management of the upper watersheds are critically important to California’s water quality and water supply.
In recent years, concerns about the impacts of devastating wildfires, disease and tree mortality, the impacts the extended drought, and the effects of climate change on water supply and hydropower generation have focused attention on the importance of forest management for protection and improvement of water resources.
How we manage our forests can affect both water quality and quantity. Forest management agencies have responsibilities for protecting water quality and beneficial uses, such as aquatic habitat, and most have implemented water quality management plans designed to prevent adverse impacts to water quality from forest management activities.
Recent devastating wildfires have been shown to have large implications to water resources, especially to water quality. Since the 1920s, a policy of active fire suppression has allowed the forests to become overgrown, greatly increasing the risk of catastrophic, high intensity fires that kill all vegetation and generate large volumes of soil and ash. Projects to reduce fuels through techniques such as prescribed fire, thinning, and mastication have been shown to reduce both the severity and frequency of wildfire.
While several studies have clearly demonstrated that the forests protect water quality by reducing erosion and removing pollutants, the effects of forest management on water supply are less clear; however, the potential for improvements in water supply availability through active forest management should not be overlooked. These include management of forest vegetation and meadow restoration, as well as preventing forest loss and fragmentation.
Efforts to manage forest vegetation to improve water supplies have been underway for some time, but have only met with limited success. Changes in water yields due to vegetation management are highly variable and difficult to measure, and indications are that at least 20% of the vegetation must be removed in order to have a measurable effect on streamflow. Innovative approaches such as selective thinning of younger trees have shown some promises for limited improvement, but more research is underway to evaluate such benefits.
Meadows may be just a small part of the forested landscape, but they provide substantial important ecological services by acting as natural reservoirs, retaining water in or near the surface during the long, dry summers. Many meadows throughout the Sierra have become eroded as a result of unrestricted livestock grazing, road building, railroad construction, and other causes, losing their capacity to store groundwater. Meadow restoration has been underway for several decades, but the effect of restoration on streamflow is not well documented, except in a few cases, but recent research does indicate that meadow restoration would benefit downstream flows during dry periods.
An often overlooked but important component of watershed health are the trees that are planted in our cities and communities. These trees add form, structure and breathing room to urban design, as well as reduce noise and separate incompatible uses. Urban forests also offer important benefits for water resources and climate change, and are an important means of mitigating heat and air pollution.
A healthy urban forest can help control stormwater runoff by intercepting rainwater on leaves, branches, and tree trunks, which in turn changes runoff quality, timing, and pollutant loads. The root systems of trees increases soil infiltration rate, and the interception of rainfall by the tree’s canopy reduces soil erosion from the impacts from raindrops.
In the hotter months, urban forests help to reduce energy use by reducing temperatures inside buildings and lowering energy use for air conditioning; they also offset greenhouse gas emissions and provide larger-scale climate benefits through their persistent sequestration of carbon in woody material.
FOR MORE INFORMATION
Helpful documents and websites …
- The State of the Sierra Nevada’s Forests Report, from the Sierra Nevada Conservancy
- Looking to the source: Watersheds of the Sierra Nevada, from the Water Education Foundation
- Sierra Nevada Water Facts, from the Sierra Nevada Conservancy
- Sierra Nevada Abandoned Mine Facts, from the Sierra Nevada Conservancy
- Native American use of fire (and its effect on water), Short report by Chuck Jachens, Regional Hydrologist, Bureau of Indian Affairs – Pacific Region
- Effects of meadow erosion and restoration on groundwater storage and baseflow in national forests in the Sierra Nevada, Draft report by the USFWS and DWR
- Improving the Resiliency of California’s Headwaters, policy framework from the Association of California Water Agencies
Some of the agencies and organizations working in California’s forests …
- Mountain Counties Water Resources Association
- Sierra Nevada Conservancy
- Sierra Nevada Research Institute
- Sierra Nevada Watershed Improvement Project
- University of California Forest Research and Outreach
- USDA Forest Service Region 5