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1 Forests, Water, and People The connections among forests, water, and people are strong: forests cycle water from precipitation through soil and ultimately deliver it as streamflow that is used to supply nearly two-thirds of the clean water in the United States. This connection between forests and water is not always tension-free. In fact, in many areas across the United States, water-related tensions are growing. In one caseâthe North Platte River Basin in Colorado and the Rocky Mountain regionâthe tension is about headwater sources and how, if at all, ma- nipulations of land uses in the forested headwaters produce changes in the water supply. Water scarcity contributes additional strain to this situation because water demand in the North Platte River Basin exceeds the allotted water supply from the river most of the time. The U.S. Bureau of Reclamation is responsible for ensuring adequate supplies of water to other water managers and suppliers, irrigators and agriculturalists, hydropower generators and users, and municipali- ties. The U.S. Forest Service (USFS) and other landowners, recreationalists who utilize headwater areas, and those with concerns about the environment and endangered species, are pressed to change their land use and management prac- tices to increase the amount of water available downstream. These issues are not academic: both upstream and downstream stakeholders recognize the strength of the connections between forests and water and their access to how these connections affect water. In another case on the other side of the Mississippi River, a similar tension is felt. In West Virginia, an extreme weather event dropped more than 6.5 inches (165 mm) of rain in a single storm in July 2001. The resulting floods caused extensive property damage and worse, death. The impacted downstream residents filed a lawsuit that claimed timber harvesting, among other headwater land uses, caused or contributed to the devastating flood damage, and the state- appointed Flood Protection Task Force concluded that forest harvesting opera- tions may have affected flood flows, and the major flood risk was associated with logging roads and culvert designs. In both of these cases, as well as many others across the United States, the science of forest hydrology may provide valuable inputs in understanding and resolving these tensions. The science of forest hydrology investigates the rates and pathways of water movement through forests. In most parts of the country, as in the North Platte Basin and West Virginia, forested headwater areas are a primary source of water supply. In the United States in the twentieth century, per capita water use in- creased from less than 10 to more than 75 gallons per day, and water demand per acre of forest increased by five- to twentyfold. Societyâs growing demand 13
14 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE for clean water and healthy ecosystems, combined with tensions related to water supply or flooding risks, challenge forest hydrologists to predict how changes in a forest will affect the quantity and quality of water to help meet that demand. These challenges are becoming more acute as water demand increases simulta- neously with changes in climate, land use, and other processes in forest systems. This report discusses these challenges and provides the scientific basis and con- text for addressing them using a suite of recommendations for the scientist, the forest or water manager, and the citizenry. FORESTS Forests account for 33 percent of all U.S. land area, covering about 750 mil- lion acres (300 million hectares) (Powell et al., 1993; Smith et al., 2004). Of this, 57 percent (430 million hectares) are privately owned, and the remainder is public forest. The federal government owns or manages land in all 50 states, with its largest holdings concentrated in 13 western states. The forest products industry is an important element of the global economy, accounting for approximately $200 billion each year. In the United States, tim- ber harvesting operations produce nearly 400 million cubic meters of wood an- nually. Forests also provide recreational opportunities and aesthetic values, car- bon sequestration and mitigation of some air pollutants, and fish and wildlife habitat. Forest management plans and programs must address fire, drought, insect and diseases, habitat protection, wilderness areas, and recreation. All of these activities can have measurable influences on water supply and quality for municipalities, agriculture, and aquatic ecosystems from the channel to the wa- tershed and landscape scales. Forests are also efficient, low-maintenance, solar-powered living filters that provide high-quality water supplies that support aquatic ecosystems. Precipita- tion that comes as rain or snow in forested areas is cycled back to the atmos- phere or drains through the soil to streams and aquifers, thereby producing much of the nationâs water supply. In this way, forested areas provide water to 40 percent of all municipalities (Nulty, 2008) or about 180 million people in the United States (http://www.fs.fed.us). The Forest Reserves Act (1891), the Organic Act (1897), and the Weeks Act (1911) first designated and established management of national forests. Since then, the social, economic, and political changes of the twentieth century, especially after World War II, increased the number, scope, and complexity of laws and regulations that guide the management of public and private forests. In addition to favorable conditions of flow and a continuing supply of timber, the USFS today must manage national forests for multiple objectives. These man- agement responsibilities are sometimes supported and sometimes constrained by an increased understanding of forest- and water-related ecosystem services: natural filtration by vegetation and soils, provision of species habitat, groundwa- ter and streamflow regulation, erosion control, and channel stabilization.
FORESTS, WATER, AND PEOPLE 15 Forests that once provided high-quality runoff are becoming developed par- cels that can adversely affect runoff patterns and water quality. Many of these ownership and use conversions occur through discrete, small parcels, such that land use change is hard to detect and has been easy to underestimate. Piecemeal changes in forest land use produce cumulative watershed effects that may be considerable and challenging to mitigate. Climate change has potentially large but uncertain effects on forests and the water they process. Specific hydrologic effects of climate change on forests are complex and vary based on regional characteristics. The most important, wide- spread, and immediate effects of climate change are in the shift from snow to rain. In areas such the western United States that depend on snowpack for sea- sonal reservoir, the reduction of seasonal snow storage is expected to shift peak runoff earlier in the spring and reduce summer water availability to agriculture and cities. Climate change may increase favorable conditions for forest fires, outbreaks of insects and disease, and changes in forest structure and species composition, producing indirect hydrologic effects. MOVEMENT OF WATER THROUGH FORESTS A few basic principles form the foundation for the science of forest hydrol- ogy. Forest hydrologists use concepts of âbalancesâ or âbudgetsâ of water, en- ergy, sediment, and nutrients, to understand how forests affect water quantity and quality. The degree to which the effects of forest management modify water quantity and quality over the long term has been the subject of forest hydrology studies for the past century. The resultant literature of forest hydrology is large, with consensus on many topics. The water balance traces the transformation of precipitation (input) to run- off (output), which is of interest to the general public and water managers (Fig- ure 1-1). The amount of precipitation is the dominant control on the amount of runoff. The timing and type of precipitationârain, snow, or fog dripâalso af- fect the amount and timing of runoff. A second major control on runoff is the transfer of water to the atmosphere by evaporation and transpiration from vege- tation including trees (evapotranspiration, or ET), and a third control on runoff is the amount of water that can be infiltrated and stored (Figure 1-1). A third con- trol on runoff is the amount of water that is stored or flows as groundwater (re- ferred in this report as sub-surface flow), i.e., water that infiltrates into the soil surface; water that is stored in the soil profile, and water that moves laterally as groundwater flow (Figure 1-1). Although surface and groundwater hydrology are undoubtedly connected, forest hydrology and therefore, hydrologic effects of changes in forest cover, more strongly focus on surface flow, sub-surface flow within a few meters of the ground surface, infiltration, and overland flow. The amount and timing of runoff are controlled in part by the water used by vegetation, which in turn depends on the amount of heat gained and lost by the system (energy balance; Figure 1-2). The energy budget influences air, soil, and
16 10 1 11 1 Soil surface 12 1-2 Water table 9 2 time 4 3 Stream 5 8 9 6 7 7 FIGURE 1-1 Elements of the water balance in a forest: 1 = precipitation (rain, snow, cloudwater deposition); 2 = net precipita- tion; 1- 2 = interception; 3 = infiltration; 4 = surface runoff, or infiltration excess (Horton) overland flow; 5 = subsurface flow, or lateral subsurface flow; 6 = groundwater recharge; 7 = groundwater flow; 8 = saturation excess overland flow; 9 = discharge or streamflow; 10 = evapotranspiration; 11 = precipitation intensity; 12 = peak flow or peak discharge. Although it is not shown, understory vegetation also contributes to these processes.
17 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE 3 3 6 1 2 1 Soil surface 1 5 6 3 Isothermal 2 3 4 layer 7 2 Streamflow FIGURE 1-2 Elements of the energy balance in a forest. 1 = insolation (incoming short- wave radiation); 2 = reflection (of shortwave radiation due to albedo or reflectivity of vegeta- tion, soil, and water surfaces); 3 = longwave radiation emitted by the Earth; 4 = longwave radiation reflected back to Earth from greenhouse gases including water vapor and CO2; 1- 2-3 + 4 = net radiation. Radiation inputs into the forest may be transformed into sensible heat (5), resulting in warming of the environment, latent heat (6, the energy consumed in evapotranspiration), or metabolic heat (7, the energy stored in biochemical reactions). Although it is not shown, understory vegetation also contributes to these processes. water temperatures and drives key processes such as photosynthesis and transpi- ration. In snow-dominated systems, snowmelt is a primary hydrologic consid- eration, and energy exchange at the snowpack surface influences the rate and timing of runoff. Water quality from forests depends on the flowpaths and the budgets of water, sediment, and nutrients within ecosystems (Figures 1-3 and 1-4). An understanding of these flowpaths and constituents of water quality is needed to predict forest water quantity, quality, and delivery processes in the majestic red- wood and Douglas fir forests in the Pacific Northwest, the taiga forests in Alaska, the snow-dominated spruce and pine forests in the Rocky Mountains, or the broad-leafed, deciduous forests in the eastern United States. FOREST HYDROLOGY Forest hydrologists employ multiple approaches to study the pathways and fates of water, energy, sediment, and nutrients; these are called âprocess stud- ies.â Watershed studies examine (1) inputs and outputs of water, sediment, and nutrients, and (2) forest management activities and forest change. Modeling studies test process understanding and allow predictions.
18 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE 1 Soil surface 1 Water table 2 7 4 3 Stream 5 6 FIGURE 1-3 Pathways of the sediment and nutrient budgets in a forest: 1 = atmospheric deposition; 2 = net deposition; 1-2 = interception; 3 = immobilization in soil; 4 = surface erosion; 5 = shallow mass movements (soil creep, debris slides, slumps, etc.; see Figure 1- 4); 6 = deep-seated mass movements (earthflows, etc.; see Figure 1-4); 7 = nutrient up- take. Not shown in figure: volatilization, wind erosion, nitrogen fixation, denitrification. Al- though it is not shown, understory vegetation also contributes to these processes. This report discusses the implications of spatial scaling in forest hydrology and management. Spatial scale terms used in this report are defined in Box 1-1. Forest hydrology studies are conducted in plots, small experimental watersheds, and across landscapes and regions. Process studies and modeling are most commonly conducted at the small watershed spatial scale. At various scales, these process-based studies are used to examine the mechanisms of energy, wa- ter, sediment, and nutrient movement and transformations. Temporal scales of forest hydrology studies range from days to multiple decades, but many studies examine periods from a single storm event to a few years. The first paired watershed experiment in North America to quantify the hy- drologic effects of forest management was conducted by the USFS from 1909 to 1928 in southern central Colorado (Bates and Henry, 1928). By the 1960s, the USFS had established more than 100 experimental forests and experimental watersheds in the United States (USDA Forest Service GTR NE-321, 2004), and other public agencies, universities, and private companies established additional small watershed studies around the world (Ice and Stednick, 2004). Fifty years ago, more than 150 experimental watersheds were being studied in the United States, but only a handful of those are still active today (Ziemer, 2000). These small watershed studies are the foundation of our current understanding and predictive capabilities of the effects of forest harvest practices on runoff.
FORESTS, WATER, AND PEOPLE 19 FIGURE 1-4 Mass movement processes in the forest. SOURCE: USGS (2004).
20 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE BOX 1-1 Definition of Spatial Scales 0 2 Plot scale: areas of 10 to 10 m 2 Small experimental watershed: drainage area up to 5 km Large watershed: drainage area up to hundreds of square kilometers that drains to a res- ervoir or lake that is part of the water supply infrastructure Landscape: collections of several large watersheds Region: multiple municipal areas, each of which has its own water supply In the early twenty-first century, water and resource managers are asking questions that challenge forest hydrologists to go beyond general principles and study designs of the past to make predictions and respond to emerging issues. These include, for example, questions about cumulative watershed effects in large watersheds, legacy effects of roads on peak flows and sediment movement, or direct and indirect effects of climate change on forest hydrologic processes. The present body of knowledge provides a foundation for answering these ques- tions, but there are significant information gaps and research needs, described later in this report (see Chapters 3 and 4). These issues and questions are the centerpiece of the tensions in basins around the country. Scientists, managers, and the citizenry are looking for new approaches to more fully understand watersheds, make stronger connections between forests and water, and achieve multiple stakeholder goals. THE NRC STUDY OF HYDROLOGIC EFFECTS OF FOREST MAN- AGEMENT The Department of the Interior Assistant Secretary for Water and Science initiated discussions in 2005 with the Water Science and Technology Board (WSTB) of the National Academiesâ National Research Council (NRC) for an assessment of the science of forest hydrology and how it relates to hydrologic effects of forest management practices. The USFS joined these discussions at the end of that year. Together, the U.S. Bureau of Reclamation and the USFS requested that the WSTB convene a committee to produce a report on the com- prehensive understanding of forest hydrology, connections between forest man- agement and attendant quality and quantity of streamflow, and directions for future research and management needs. In early 2006, the WSTB formed the Committee on Hydrologic Impacts of Forest Management, a panel of 14 mem- bers with expertise in forest hydrology and ecology, fire ecology, watershed sciences, geomorphology, water quality, and forest management on public and private land ranging from small woodlots to extensive industrial holdings (see Appendix B). The overall charge to the NRC committee was to examine the effects of forest management on water resources (see Box 1-2). The committee held five meetings between March 2006 and April 2007 in open and closed sess-
FORESTS, WATER, AND PEOPLE 21 BOX 1-2 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 in- vasive species. 4. How well are forest hydrologic impacts understood over short- and long-temporal scales and small- and large-spatial scales? ions around the United States to gather information and examples for this report and to hear perspectives from forest managers, water supply system managers, and water users on key issues related to forests and water. Scope of the NRC Study The committee produced this report to have maximum application and util- ity for a diverse audience of scientists, forest and water managers, and citizens in the community. To best reach this broad audience, the committee clarifies three points in its interpretation of the statement of task. First, this report ex- pands the focus to be applicable to state and private forests, in addition to for- ested lands under federal management. Second, the report takes a national view of issues related to forests and water. The federal sponsors of this study have land holdings and jurisdiction primarily in the western United States, but issues and concerns about water and forests are evident in all 50 states. Finally, this report provides recommendations for scientists, managers, and citizens on ap- proaches that can begin to ease tensions over water resources. Given the wide interest in the array of issues associated with forests and their hydrologic effects, this report builds on decades of forest and forest hydrology research to present key findings and recommendations that advance the understanding of connec- tions among forests, water, and people and make that understanding accessible to scientists, managers, and citizens.
22 HYDROLOGIC EFFECTS OF A CHANGING FOREST LANDSCAPE Structure of the Report The following four chapters of the report describe the current understanding of forests and water, discuss information gaps and research needs in forest hy- drology and management; and present recommendations to address issues and challenges in the science, research, and management of forests and water. The descriptions and discussions of forests (Chapter 2) include the primary manage- ment objectives, ownership patterns, and historic and emerging issues in forests. The state of the science of forest hydrology and the understanding of how forest management activities affect streamflow quantity and quality are assessed and presented in Chapter 3, including general principles and basic processes that have been gleaned from the forest hydrology literature. Research needed for managing forests and water in response to contemporary challenges is discussed in Chapter 4, with an emphasis on moving from principles to prediction at larger spatial scales and longer time scales. The reportâs final chapter (Chapter 5) draws upon the state of the science (Chapter 3) and research needs (Chapter 4) to make recommendations for scientists, managers, and communities to meet forest and water needs in this and future generations.