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Summary The forests of the United States cover about one-third of the countryâs land area and are managed for a number of purposesâtimber harvesting, wilderness, habitat, and recreationâbut arguably their most important output is water. Pre- cipitation is cycled through forests and soil, and ultimately some is delivered as streamflow to receiving bodies of water. In this way, forests process nearly two- thirds of the freshwater supply in the United States. Demand for water in the United States is increasing, and forest managers today are asked to provide higher quantities and qualities of water. Water sup- ply managers question whether different land use management in forested headwaters can help meet downstream water quantity or quality demands. Meeting water supply needs is becoming more difficult because elevated water demand is occurring simultaneously with changes in climate, human population and development, land use, and ownership. How to manage forests and sustain water supplies will be a primary challenge in the twenty-first century. The science of forest hydrology investigates the rates and pathways of water movement through forests. Forest hydrology researchers have amassed a com- prehensive understanding of how water is connected to and moves through for- ests. A strong evidence base has emerged for understanding basic processes and principles of water movement through forests that can be used to predict the general directions and magnitudes of hydrologic effects of changes in forest cover, climate, and land use. As the demand for water increases in the United States, water managers in- creasingly draw upon this strong scientific foundation and seek input from the forest hydrology community to identify ways to ensure reliable supplies of wa- ter. The U.S. Department of the Interiorâs Bureau of Reclamation is the largest wholesaler of water in the United States, providing water for more than 31 mil- lion people and 10 million acres of irrigated farmland. The U.S. Forest Service (USFS) manages 193 million acres of land for a continuing supply of timber, favorable conditions for streamflow, recreation, wilderness areas, and other ob- jectives. These two agencies requested that the Water Science and Technology Board of the National Research Council (NRC) convene a committee to study and produce a report on the present understanding of forest hydrology, connec- tions between forest management and attendant hydrologic effects, and direc- tions for future research and management needs to sustain water resources from forested landscapes (see Statement of Task, Box S-1). In response, the NRC appointed the Committee on Hydrologic Effects of Forest Management, a group of 14 experts, to generate this report. 1
2 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE BOX S-1 Statement of Task This study will examine the effects of forest management on water quantity, quality, and timing. The report will reflect on the state of knowledge, relevant policy implications, and research needs that would advance understanding of connections among hydrology, science, and land management and policy in forested landscapes. 1. What is the state of knowledge of forest hydrology? 2. What are information and research needs regarding forest hydrology in forested lands? â Topics could include sediment-related watershed processes; surface and groundwater hydrology; biological and ecological aspects; and extrapolation of small- scale study results to large-scale management practices. 3. What are the new issues that need to be addressed to ensure clean and plentiful water? â Topics could include extreme weather events, climate change, fire, and invasive species. 4. How well are forest hydrologic impacts understood over short and long temporal scales and small and large spatial scales? STATE OF FOREST HYDROLOGY SCIENCE Forest hydrology is the study of water in forests: its distribution, storage, movement, and quality; hydrologic processes within forested areas; and the delivery of water from forested areas. Forest hydrology research uses field measurements, experiments, and modeling to characterize and predict hydro- logic processes and their responses to natural disturbance and management of forests. It draws upon disciplinary knowledge from several branches of hydro- logical sciences, water resources engineering, and forestry to address primary questions of forests and water: â¢ What are the flowpaths and storage reservoirs of water in forests and forest watersheds? â¢ How do modifications of forest vegetation influence water flowpaths and storage? â¢ How do changes in forests affect water quantity and quality? âPaired watershedâ studies have been a primary empirical approach in for- est hydrology. In paired watershed studies, two watersheds that are similar in size, initial land use or land cover, and other attributes are selected for study; both are monitoredâone is then left as âcontrol,â and the other is âtreatedâ (i.e., subject to manipulations such as forest cutting, road building, etc.). The meas- ured changes in the relationship of streamflow and water quality between the treated and the control watersheds quantify the effects of forest treatment and
SUMMARY 3 regrowth. Most paired watershed studies in forest hydrology were begun in the 1940s, 1950s, and 1960s, but many of these studies were discontinued in the 1980s. Paired watershed studies, process measurements, plot-scale studies, and hy- drologic modeling are important elements of forest hydrology science. Study plots and paired watershed experiments generally range in size from less than a square meter to 1-2 km2, and time scales for plot and process studies most com- monly span only a few growing seasons. However, some USFS Experimental Forests and Ranges have conducted watershed studies spanning several decades or longer, particularly those designated as Long-Term Ecological Research (LTER) sites, funded by the National Science Foundation. Forest Hydrology Processes and General Principles Forest hydrology studies show that changes in forest structure and composi- tion, and associated changes in forest soils and hillslopes, can alter the storage and flowpaths of water through soil and subsoil, which modifies water yield, peak flows, low flows, water chemistry, and water quality (see Figure S-1). The general principles of water movement in and through forests are understood with a high level of certainty (Table S-1). Using Forest Hydrology Science to Inform Management Decisions The current body of forest hydrology science supports forest and water management decisions in many ways. Forest hydrology science has led to a clear understanding of general principles (Figure S-1, Table S-1) of water movement through forests. These principles indicate the general magnitudes and directions of direct hydrologic responses to changes in forests over short time scales and in small areas. However, todayâs forest and water managers need forest hydrology science to predict or indicate the indirect and interacting hydrologic responses in forest landscapes that are changing over large areas or long time scales. A pressing question for forest hydrologists is whether cutting trees in for- ested headwaters will augment water yield downstream for agricultural, munici- pal, or other uses while maintaining desired ecological attributes associated with forested landscapes. Although it is possible to increase water yield by harvest- ing timber, the increases in water yield from vegetation removal are often small and unsustainable, and timber harvest of areas sufficiently large to augment wa- ter yield can reduce water quality. The potential for increasing water yield from forest management is low, which reflects that increases are less likely in seasons when water demand is high and increases tend to be much smaller in drier years.
4 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE Modifiers of forest hydrology Forest disturbance: Forest management: Wildfire Forest harvest & silviculture Insects & disease Road networks Species changes Grazing Hydrologic response: General principles 1. Changes in 2. Changes in 3. Changes in forest flowpaths in water, soil structure soil and subsoil chemistry Specific hydrologic responses Hydrologic responses within forests: Interception & transpiration Infiltration & overland flow Water flowpaths in soil and subsoil Changes in watershed outputs: Water yield Floods Lowflows Sediment Chemistry Temperature Managing forests for water FIGURE S-1 Forest hydrology examines the flowpaths and storage of water in forests and how forest disturbance and management modify hydrologic responses. Hydrologic re- sponses to changes in forests fall into three categories of general principles, as well as specific hydrologic responses, discussed in the text. The final section of this chapter evalu- ates the state of knowledge of forest hydrology and its implications for managing forests for water, including feedbacks to processes that modify forests.
SUMMARY 5 TABLE S-1 General Principles of Forest Hydrology Describing the Direct Effects on Hydro- logic Processes of Changes in Forest Structure, Changes in Water Flowpaths, and Applica- tion of Chemicals Principles of Hydrologic Response to Changes in Forest Structure 1 Partial or complete removal of the forest canopy decreases interception and in- creases net precipitation arriving at the soil surface 2 Partial or complete removal of the forest canopy reduces transpiration 3 Reductions in interception and transpiration increase soil moisture, water avail- ability to plants, and water yield 4 Increased soil moisture and loss of root strength reduce slope stability 5 Increases in water yield after forest harvesting are transitory and decrease over time as forests regrow 6 When young forests with higher annual transpiration losses replace older forests with lower transpiration losses, this change results in reduced water yield as the new forest grows to maturity Changes in Water Flowpaths in Soils and Subsoils 7 Impervious surfaces (roads and trails) and altered hillslope contours (cutslopes and fillslopes) modify water flowpaths, increase overland flow, and deliver over- land flow directly to stream channels 8 Impervious surfaces increase surface erosion. 9 Altered hillslope contours and modified water flowpaths along roads increase mass wasting Hydrologic Response to Application of Chemicals 10 Forest chemicals can adversely affect aquatic ecosystems especially if they are applied directly to water bodies or wet soils 11 Forest chemicals (fertilizers, herbicides, insecticides, fire retardants) affect water quality based on the type of chemical, its toxicity, rates of movement, and persis- tence in soil and water 12 Chronic applications of chemicals through atmospheric deposition of nitrogen and sulfur acidify forest soils, deplete soil nutrients, adversely affect forest health, and degrade water quality, with potentially toxic effects on aquatic organisms NOTE: These general principles are not predictions, so qualifying adjectives such as âmay,â âusually,â etc. are omitted. See Chapter 3 for factors that influence when, where, and to what extent these principles apply. RESEARCH NEEDS IN FOREST HYDROLOGY To meet the needs of the managers and users of forests and water, forest hydrology research has to move from principles to prediction. Predictions are needed to understand the indirect and interacting hydrologic responses to changes in forested landscapes associated with climate change, forest distur- bances, forest species composition and structure, and land development and ownership, and how these changes will affect water quantity and quality down- stream and over long time scales. A Landscape Approach to Forest Hydrology A landscape perspective on forest hydrology links scientific principles from plot, process, and small watershed scales with indirect and interacting hydro-
6 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE logic responses at larger spatial scales (i.e., within drainage basins and across large climatic and physiographic regions) in forest landscapes that are changing over long time scales. Within watersheds, forests are located in headwaters and downstream areas, on hillslopes and in riparian zones, and forests fulfill differ- ent water-related functions depending on their location. A key unresolved issue in forest hydrology is how to âscale upâ findings from one part of a watershed to larger areas or to the entire watershed. The temporal context for a landscape approach to forest hydrology involves expanding the temporal scale into the past to quantify the effects of antecedent forest management and disturbances and into the future to project and anticipate changes in land use and climate. For example, past forest harvest practices, road networks, fire suppression policies, grazing practices, and natural disturbances such as fire and wind have left legacies in forest structure and composition. These legacies affect hydrologic processes. The research needs for a landscape approach to forest hydrology sci- ence involve studies that determine the following: â¢ How general principles developed in small, homogeneous watersheds can be used to improve predictions of hydrologic responses across large, hetero- geneous watersheds and landscapes; â¢ How forests and forest management activities affect hydrologic proc- esses, runoff, and water quality as a result of their position within a watershed; â¢ How local effects of roads can be scaled up to quantify the effects of road networks on water quantity and quality in larger watersheds and regions, particularly during large storms; and â¢ How long-term legacies of forest disturbance and forest management practices affect forests, water quantity, and water quality. Forest Disturbance Forests are dynamic ecosystems subject to both incremental and episodic disturbances that vary in frequency, severity, and extent. Probable hydrologic responses to fire, insects and disease can be inferred from the general principles of forest hydrology (Table S-1). However, compared to the extensive literature on hydrologic responses to forest management, relatively few studies have ex- amined hydrologic responses to fire, insects, and disease in forests, especially at long time scales or in large watersheds. The research needs for understanding hydrologic effects of forest dis- turbances involve studies that determine â¢ Effects of high- versus low-severity forest fires on water quantity, qual- ity, and flooding, and how these effects vary over time and spatial scales; and
SUMMARY 7 â¢ Hydrologic responses to interacting and cumulative effects of forest disturbance (such as fire and insect outbreaks) and forest management (includ- ing thinning, salvage logging, roads, timber harvesting, and fire suppression). Forest Management Much of the forest hydrology literature focuses on the hydrologic effects of timber management practices and roads. Forest management practices evolve over time, resulting in new practices, such as thinning for fuel reduction, and best management practices (BMPs), such as managing wider riparian buffers for species protection. Moreover, recent increases in fire, insects, and disease in forests have spurred the adoption of forest management practices, such as thin- ning and salvage logging, whose effects on hydrology have received little study. The hydrologic effects of many of the new management practices and BMPs have not been studied, and dynamic forest conditions make it important to un- derstand how contemporary practices influence water resources. Research needs for understanding hydrologic responses to forest man- agement involve: â¢ Studies that determine how contemporary forest management on public and private lands affects water quantity and quality and â¢ Improved forest hydrology models that reliably simulate the hydrologic and water quality responses of watersheds in varied forest conditions. Cumulative Watershed Effects One of the biggest threats to forests, and the water that derives from them, is the permanent conversion of forested land to residential, industrial, commer- cial, and infrastructure uses. Cumulative watershed effects (CWEs) include the hydrologic effects resulting from multiple land use activities over time within a watershed. Assessing CWEs requires an understanding of the physical, chemi- cal, and biological processes that route water, sediment, nutrients, pollutants, and other materials from hillslopes and headwater streams to downstream areas. CWE research strives to establish cause-effect relationships among forests, wa- ter, and watersheds over large spatial and temporal scales. Research needs for CWEs involve the following: â¢ A landscape-scale approach to relate downstream conditions to changes in forest conditions and land use in the contributing watershed; and â¢ Spatially explicit models that identify, connect, and aggregate changes due to forest disturbance and management over time in large watersheds.
8 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE Climate Change Some effects of climate change on forests and water are already evident, and future climate changes are likely to have major effects on forest hydrology. Observed direct effects of climate warming on forests and hydrology include such as changes in the timing of snowmelt runoff and increases in wildfires. More research is needed to better predict indirect effects of climate change, in- cluding evaluations of how changes in forests and forest management influence hydrologic response. The research needs related to the hydrologic effects of climate change include: â¢ Direct effects of climate change on hydrologic processes in forests and on water yield and water quality from forests; â¢ Indirect effects of climate change on forest structure and species com- position and the consequences of these changes for water yield and water qual- ity; and â¢ Indirect effects of climate change on forest disturbance, including wild- fires, insects and diseases, and the consequences of these changes for water yield and water quality. RECOMMENDATIONS TO SUSTAIN WATER RESOURCES FROM FORESTS Scientists who study forest hydrology, forest and water managers, and citi- zens who use water can take actions to sustain water resources from forests. Each of these groups has important roles to play in applying the current under- standing, exploring research gaps and information needs, and pursuing recom- mended actions (Table S-2). Recommendations for Scientists Scientists are poised to advance forest hydrology science to address critical water issues. New research approaches should be pursued in addition to main- taining and expanding existing data. In doing so, scientists should: â¢ Continue current small watershed experiments; â¢ Reestablish small watershed experiments where research has been dis- continued; â¢ Centralize historical records from watershed studies in digital, well- documented, publicly accessible databases; â¢ Use the whole body of paired watershed data as a âmeta-experimentâ to
SUMMARY 9 better understand and improve utility for managers of hydrologic responses to forest disturbance and management over large spatial and temporal scales and a range of forest types; â¢ Expand the capability for visualization and increase the prediction ac- curacy of hydrologic response in large watersheds through geographic informa- tion systems (GIS), remote sensing, sensor networks, and advanced models; and â¢ Work with economists and social scientists to improve and communi- cate understanding of the value of sustaining water resources from forests. Recommendations for Managers Managers of forests and water play critical roles in providing water re- sources from forests. Because forests, forest management, and the climatic and social contexts of forests are dynamic, BMPs must be updated continually through an adaptive management approach. Forestry BMPs can mitigate the negative consequences of forest management activities (roads, timber harvest, etc.), but their effectiveness can be highly site- and storm-specific or difficult to quantify. Forest and water managers are well positioned to use rigorous moni- toring to assess the effectiveness of BMPs. In response to their assessments, managers can adapt management approaches and modify the current suite of BMPs to increase their effectiveness and test the results. To assist the evolution of BMPs, managers should: â¢ Catalogue individual or agency BMP use, design, and goals at the na- tional level and make this information available to the public; â¢ Monitor BMP activities for effectiveness, and coordinate analyses of monitoring data for use in an adaptive management framework; and â¢ Design adaptive management approaches for forested watersheds that coordinate management, research, monitoring, and modeling efforts. Recommendations for Citizens Cumulative watershed effects, changes in land ownership and management, changing population and development patterns, and water supply concerns have spurred activity to protect watersheds and water quality from the grass-roots, community level. New community-level watershed councils and forest groups are proactive in watershed-based restoration and management. Water research- ers and policy makers have long recognized the benefits of organizing land and water management around watersheds and taking an integrated approach to wa- tershed management. An integrated watershed management approach can help track the effects of various land uses on water supply and quality. Citizens and communities can influence forest and water management at the local, regional, or watershed level.
10 TABLE S-2 Current Understanding, Research Needs, and Recommendations for Sustaining Water Supplies from Forests Information Gaps and Research Current Understanding Recommended Actions Needs Science The body of forest hydrology science Hydrologic effects of past Enhance, maintain, and reestablish derives from almost 100 years of management, such as fire abandoned small watershed studies at small spatial and time suppression, clear-cutting, roads studies scales Ways to quantify hydrologic Combine existing data from the large Forest hydrology science has responses at larger spatial and body of small watershed studies established general principles that temporal scales and analyze them for large-scale are understood with a high degree Ways to scale up findings from small trends as a meta-experiment of certainty describing direct spatial and short time scales to Use new technologies, including hydrologic effects of forest larger spatial and longer time sensor networks and remote management and disturbance scales sensing, to improve understanding Effects can be understood through Use general principles to predict of forest hydrology in changing changes in indirect hydrologic responses to landscapes â¢ Forest structure changes in forest landscapes and Engage in adaptive management to â¢ Magnitudes, rates, and flowpaths interacting responses to forest help managers and community â¢ Erosion, nutrient cycling, and soil management and disturbance groups design monitoring chemistry strategies, develop and test models, and conduct studies Reduced forest cover results in relevant to management increased water yield that is â¢ Generally short-lived â¢ Greatest during times of water excess rather than water scarcity â¢ Small or undetectable in water- scarce areas â¢ May be associated with a decline in water quality Management Forests in the United States are Assessment of BMP effectiveness Advance BMP evolution by managed for a wide range of goals Principles and practices of adaptive rigorously assessing and and objectives: timber harvesting, management developing new BMPs and
road networks and road measuring their effectiveness construction, high-severity At the federal level, provide wildfires, and exurban sprawl sustained support for adaptive modify forest hydrology management activities, enabling Forest management practices are managers to partner with scientists evolving in response to to design and implement environmental change, social and monitoring, develop and test economic forces, and technological models, and conduct studies developments relevant to management issues BMPs are used to mitigate impacts Increase role of agency technical on water resources from forest expertise in watershed councils management activities Community Integrated watershed management How watershed councils and their Use watershed councils to meet is a viable vehicle for both stakeholders view and utilize forest multiple goals of integrated community groups and state and hydrology science and scientific watershed management at the federal agencies to help manage expertise from federal agencies community level water and forest resources at the How industry-sponsored green Expand the number and influence of community scale certification and federal forest watershed councils. Citizens groups can influence local stewardship contracts affect water Engage in adaptive management and integrated watershed quantity and quality from forests with scientists and managers management Community watershed groups benefit from state and federal agency technical expertise Existing laws can be used to strengthen the standing and influence of watershed councils New laws offer increased opportunities for community involvement 11
12 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE Watershed councils and citizen groups should work within communi- ties and with state and federal agencies to: â¢ Use watershed councils as vehicles to meet multiple goals of integrated watershed management at the community level; and â¢ Participate in watershed councils and help them grow in number and in- fluence over watershed uses at the community level. CLOSING Forest hydrology science has produced a solid foundation of general princi- ples that describe how water is connected to and moves through forests and how hydrologic processes respond to forest disturbance and forest management. The forest landscape is dynamic: it is continually changing in response to climate, natural disturbance, and forest management, as well as demographics and devel- opment patterns. Forest hydrology science and management are adapting as land use and ownership within forested watersheds become more heterogeneous, changes in climate and its effects are becoming more evident, and new tech- nologies provide improved capability to predict and visualize cumulative water- shed effects over larger spatial scales and longer periods of time. Building on the strong foundation of general principles of forest hydrology, new forest hy- drology research can fill information gaps in the coming decades (Table S-2). Forests are essential for the sustainable provision of water to the nation. It is incumbent upon scientists, policy makers, land and water managers, and citizens to use the lessons of the past and apply emerging research, technology, and part- nerships to protect and sustain water resources from forested landscapes.