Undoubtedly, water is the most important product from forests. This is especially significant since California is now in the third year of drought. Voluntary reductions in water use are being considered and rising temperatures will increase demand. About 60% of the state’s water comes from forested watersheds – mostly public lands at high elevation – so how these lands are managed is an important issue.
What is a watershed?
A watershed is an area of land that contributes water to a location such as a reservoir. It may be above tree line or covered by trees, shrubs, or grasses. Water derived from rain or snow flows through the soil profile to reach streams that flow into a reservoir, recharge ground aquifers, and eventually into the ocean. Healthy watersheds with intact native vegetation provide the important functions of water filtration, flood control, and nutrient recycling – all these are called ecosystem services1.
Role of Trees and Forests
Tree growth consumes large quantities of water through the process of evapotranspiration. However, trees can control water use by their capacity to close their stomata when available soil water is low. Some trees, such as pines, are more efficient in controlling water use than firs. However, tree cover remains a desirable cover on watersheds due to their providing shade and litter cover, stabilizing soils, and filtering water.
Forest Management plays an important role in ensuring healthy forest cover through controlling the density of trees and canopy cover. Where precipitation is in the form of snow, more open canopies are desirable to enable snow to accumulate on the ground under shade thus delaying melt and increasing water yields. On the California coast growth of forests and understory is enhanced by fog interception. Controlling the density of forests by thinning may increase water yields by reducing direct evaporation from tree crowns, but the amount of water transpired is a function of total leaf area rather than the number of trees per acre2.
Critical components of watershed management are minimizing surface erosion by creating water bars on skid trails and controlling water discharged from road culverts. Another critical issue is the protection of riparian borders adjacent to watercourses and shade over streams is protected to maintain low stream temperature favorable to fish and other stream biota. All these aspects of management are strictly controlled through state regulation and federal policies aimed at improving the management of California’s forests and watersheds3.
And of course wildfires (see Topic 6) are extremely detrimental to watersheds as they destroy vegetative cover, cause surface runoff and gullying, and reduce reservoir capacity through siltation. Currently, there are no financial incentives to remunerate forest owners for managing to enhance water yields. This is mostly because there are no suitable metrics for quantifying the relative effects of different treatments.
Recognizing the need to support watershed restoration, the National Fish and Wildlife Foundation in 2020 allocated $1.6 million in grants to improve watersheds throughout California, generating $2.5 million in matching contributions4. These projects were aimed at improving infrastructure such as roads, bridges, and drainage to reduce sedimentation, remove invasive species, and strengthen watershed resilience.
Sierra Nevada watersheds are critical to supplying California’s water needs. Forests at medium and lower elevations are commonly owned by industry and watershed protection is ensured by ownership goals and strict forest practice regulation. Forests at higher elevation are mostly public lands owned and managed by the USDA Forest Service and the National Park Service. Protecting and managing these lands are crucial for supplying water to meet California’s agricultural and domestic needs. The city of San Francisco obtains 85% of its water supply from the Tuolumne River watershed which flows by gravity for 167 miles to reach the city. Partnerships among the Public Utilities Commission, National Park Service, and US Fish and Wildlife Service ensure permanent watershed protection through proactive forest management. The goal is to use Best Management Practices under Federal and State environmental protection laws to maintain high quality water supply that reduces the need for costly filtration plants5.
Climate change effects on water supply are difficult to predict due to the diverse topography of the Sierra Nevada. It is expected that climate change will alter the proportion of precipitation that falls in the form of rain or snow as well as the frequency and severity of storms. In general, however, warmer temperatures are thought to increase the proportion of rain, which changes our past reliance on an accumulated snow pack to slowly release water into our reservoirs.
Forested watersheds are critically important to maintaining supplies of high-quality water to meet California’s need for water for domestic and agricultural uses. Highest priority is to ensure continued stewardship and management to maintain the health and sustainability of these lands. And, above all, to keep forests as forests and avoid conversion to other land uses.