Recent Developments in Water, Energy, and Biogeochemical Budgets Research

Harry F. Lins

U.S. Geological Survey, 415 National Center, Reston, VA 20192, hlins@usgs.gov

Understanding and predicting global change is a major scientific focus of the late 20th century. Although atmospheric scientists have made substantial progress in developing models that account for many components of the climate system, significant progress is needed in understanding processes associated with the exchange of water, energy, and carbon between terrestrial systems and the atmosphere.

To strengthen terrestrial process research, especially research associated with the interactions of water, energy, gases, nutrients, and vegetation, the U.S. Geological Survey initiated an intensive study of Water, Energy, and Biogeochemical Budgets (WEBB). WEBB is aimed at improving understanding of processes controlling terrestrial water, energy, and biogeochemical fluxes, their interactions, and their relations to climatic variables; and the ability to predict continental water, energy, and biogeochemical budgets over a range of spatial and temporal scales.

WEBB research watersheds form a geographically and ecologically diverse set of environments for investigating the interactive effects of changes in CO², climate, and biogeochemistry on the terrestrial carbon cycle; how global change will affect biogeochemical interactions with the hydrologic cycle and surface energy balance; and how global change will affect biogeochemical controls over the transport of water, nutrients, and materials from land to freshwater ecosystems.

Each of these topics was identified this year by the National Research Council as a research need critical for reducing uncertainty about the response of terrestrial ecosystems to global changes in climate and land use, and the effect that terrestrial responses may have on global climate. USGS hopes to help reduce such uncertainty by enhancing its WEBB activities through interaction with complementary programs of other federal agencies, as well as through expanded collaboration with the academic community.

Use of Existing Sites Fosters Research

WEBB process studies are being implemented as a systematic program of intensive, long-term field investigations. Study sites are selected on the basis of geographical and environmental diversity and their history of related research. Priority is given to established multidisciplinary research locations such as the National Science Foundation-sponsored Long-Term Ecological Research (LTER) sites, the U.S. Forest Service Experimental Forests, and UNESCO-designated International Biosphere Reserves, as well as other similar research sites.

Existing research sites are important to change-related research because only observations spanning several decades can be used to distinguish between a climate change induced signal and natural variations in the hydrologic and other biogeochemical cycles [Loaiciga et al., in press]. By building on an existing observational record, it may be possible to begin making such distinctions within several years, rather than waiting several decades. Even so, the USGS currently envisions a decades-long research commitment at WEBB sites.

WEBB investigations are also designed to develop and maintain strong collaborative research relationships with scientists in other federal agencies and the academic community. In fiscal year 1991, WEBB investigations were initiated at the Luquillo Experimental Forest in eastern Puerto Rico; Panola Mountain Watershed near Atlanta, Ga.; Sleepers River Watershed in northeastern Vermont; Trout Lake Watershed in the North Temperate Lakes region of Wisconsin; and in the Loch Vale Watershed in Rocky Mountain National Park, Colo. Brief descriptions of the activities at the sites follow.

Biochemistry of Weathering and Erosion and Effects of Agricultural Development

Luquillo Experimental Forest, Puerto Rico. The 11,300-hectare Luquillo Experimental Forest, in the Luquillo mountains of eastern Puerto Rico, is a tropical rainforest administered by the U.S. Forest Service. In addition to being a WEBB research site, the forest has been designated an LTER site by NSF and an International Biosphere Reserve by UNESCO.

WEBB activities at this site are closely coordinated with the Forest Service, with USGS research focusing on the biogeochemical and geomorphic processes that control the movement and transformation of water, energy, bedrock weathering products, and nutrients in the humid tropics [Larsen et al., 1993]. WEBB research is also being conducted in the Río Grande de Loiza basin, an urbanized and agriculturally developed 60,000-hectare watershed just west of the Luquillo Experimental Forest.

The USGS strategy at Luquillo is to compare and contrast biogeochemical and geomorphic processes in a pair of small natural watersheds in the rainforest (the Río Icacos and the Río Mameyes) with matched agricultural watersheds in the Loiza basin (the Río Cayaguas and Río Canóvanas).

Two major research problems are being addressed. The first is biogeochemistry of weathering and erosion, with a focus on how mass wasting and other processes control rates of erosion, the composition of solid and dissolved erosion products, and the rate and nature of nutrient and carbon accumulation in soils. This task has several elements, including determination of nutrient and elemental budgets based on regular and event sampling, hillslope geomorphic and hydrologic processes, and biogeochemical processes in soils.

Also being investigated are the effects of agricultural development, with a focus on evapotranspiration, nutrient budgets, and gas budgets. A comparison of developed and undeveloped watersheds is being made that involves water, sediment, and nutrient fluxes in streams, soil gas fluxes, and gas fluxes from artificial ponds and lakes.

Collaborative research is being conducted at Luquillo by investigators from USGS, the U.S. Forest Service, and several other organizations and universities.

Streamflow, Water Quality, and Soil-Solution Chemistry

Panola Mountain Watershed, Georgia. The Panola Mountain Research Watershed (PMRW) is in the Panola Mountain State Conservation Park, Stockbridge, Ga., a 41-hectare forested watershed 25 km southeast of Atlanta. WEBB research here focuses on three critical areas: streamflow generation and water-quality genesis, weathering and geochemical evolution, and regulation of soil-solution chemistry [Huntington et al., 1993]. To couple land-atmosphere-vegetation interactions more effectively, better definition and quantification of the processes controlling subsurface hydrologic flowpaths is necessary. Because watersheds are composed of chemically distinct environments, including soil horizons and riparian zones, a mechanistic determination of streamwater chemistry requires an understanding of the hydrologic flowpaths through the watershed as well as the interactions among soil, vegetation, and water. Plot-scale questions concern the movement of rain and solutes through the unsaturated zone and the interaction between the unsaturated and saturated zones, and, in general, removal of solutes by vegetation.

Research on weathering and geochemical evolution will help identify the sources of cations observed in the streamwater at Panola Mountain and quantify the changes in cation source during storm events. This research is specifically concerned with the use of stable and radiogenic isotopes of heavy minerals. The approach involves determining the isotopic composition of Sr, Ca, Pb, and Li and rare Earth element (REE) concentrations of potential cation sources, such as wet and dry atmospheric deposition, vegetation, bedrock lithologies, soil horizons, and saprolite along a hillslope transect.

A major need for predicting biogeochemical responses to climatic change is a better understanding of the processes controlling soil solution and groundwater chemistry. Many recently developed models used to predict terrestrial and aquatic responses to air quality and climatic change rely on principles of thermodynamics governing solubility, ion exchange, adsorption, and kinetics, which are difficult to apply in hydrologically complex natural systems. The research approach being used at PMRW focuses on testing model assumptions and evaluating the critical soil properties that are associated with departure from predicted response.

Movement of Water from Atmosphere to Stream Channel

Sleepers River Watershed, Vermont. The Sleepers River Watershed, in Danville, Vt., has been the site of intensive hydrologic investigations for more than 3 decades. The watershed was administered by the Department of Agriculture's Agricultural Research Service from 1957 to 1966; by the National Weather Service's Office of Hydrology from 1966 to 1979; and by the U.S. Army's Cold Regions Research and Engineering Laboratory (CRREL) since 1979. WEBB operations here are conducted in close conjunction with CRREL activities.

Research activities at Sleepers focus on the study of how water moves from the atmosphere as snow or rain down a hillslope and into a stream channel [Shanley et al., 1995]. A number of basic questions about the mechanisms of water movement, and the manner by which the chemistry of that water changes during its journey, remain unanswered. Answering these questions will improve our ability to predict how the hydrologic cycle, or the cycling of certain plant nutrients, may change in response to global change.

Another aspect of the WEBB research at Sleepers River focuses on the question of scale in hydrologic processes, the important processes by which water appears as streamflow change with the size of the basin. Understanding the mechanisms of streamflow generation at various scales is important because the effect of a perturbation to one mechanism can be projected to assess the effect on the hydrology of an entire basin. For example, global warming may cause less snowfall and more winter rainstorms. If the streamflow generation mechanisms are known, the hydrologic effect of less snowfall can be predicted.

Another area of research at Sleepers River deals with fluxes of the greenhouse gases CO², N2O, and CH4. Measurements of CO² fluxes and concentrations in soil gas have provided information on seasonal variations in soil respiration, sources of the respiration and their relative contributions as possibly reflected in diurnal cycles, and transport of CO² through snow.

It is often assumed that CO² production in the soil shuts down in winter, and thus its evolution from the snowpack into the atmosphere is negligible. Preliminary measurements at Sleepers River indicate, however, that CO² continues to be emitted from the soil through the snowpack in amounts that can be as high as 25% of the seasonal peak in soil respiration. Experimental results also show that natural channels such as ablation rings, and snowpack features like crusts, ice lenses, and density variations exert considerable influence on the transport of CO² to the atmosphere.

The principal collaborator at the site is CREEL, which has strongly supported the development of close data collection and research linkages with USGS. Close collaboration has also developed with the USDA Forest Service. A "research exchange" between Sleepers River and Forest Service watersheds at Hubbard Brook and Cone Pond, N.H., is yielding interesting insights on the effect of differing bedrock and till lithology on streamwater chemistry.

Transport of Water and Chemicals from Pristine Watersheds

Trout Lake, Wisconsin. The Trout Lake WEBB site, in the northern highlands area of northcentral Wisconsin, is operated in conjunction with the North Temperate Lakes LTER through the University of Wisconsin's Center for Limnology. USGS research activities at this site focus on the processes that control the transport of water and chemicals through the Trout Lake watershed [Elder et al., 1992].

Four streams in the watershed are being monitored to quantify the overall water and chemical yields of the Trout Lake system. Each stream travels through different terrestrial settings that are expected to exert a controlling influence on respective flow rates and stream chemistries. Large-scale investigations are aimed at identifying the important sub-basin characteristics that control water and chemical fluxes.

This research will provide a better understanding of the processes that control the transport of water and chemicals from relatively pristine forested watersheds. The approaches used and knowledge gained will transfer to other similar areas and will aid scientists in predicting watershed responses to external influences such as climatic change.

Specific questions to be answered include: What are the effects of surface water dynamics on the hydrologic budget and chemistry of area lakes? What kinds of interactions take place among the surface water system, groundwater system, and wetlands? What are the roles of atmospheric inputs and losses in the hydrologic and chemical budgets? And, what controls influence the transport and distribution of solutes flowing from the watersheds to the lakes and streams?

The North Temperate Lakes area has been the focus of limnological data collection and research by various universities and research institutions for more than 6 decades. Most of this data collection effort has been focused on the lakes themselves, rather than the tributary streams and other terrestrial systems.

WEBB data collection will complement and expand on these research efforts by providing a more complete picture of the entire ecosystem. In this regard, the USGS conducts operations in conjunction with the University of Wisconsin's Center for Limnology, which is responsible for the overall operations of the North Temperate Lakes LTER project.

Because this area has an extended historical data base, many long-term hydrologic and geochemical trends have been documented. The potential importance of influences from the terrestrial environment are largely unknown, however, and information from the WEBB study should aid in interpreting past and current limnological investigations.

Flowpaths and Their Impact on Climate

Loch Vale Watershed, Colorado. The interaction of climatic change with water, energy, and biogeochemical budgets will be most profound in watersheds having biotic communities that are at the limit of their tolerance to climatic conditions. One such class of watershed is the alpine/subalpine/montane basins typical of the Rocky Mountains.

Research at the Loch Vale watershed focuses on three topics that are poorly understood in alpine/subalpine/montane watersheds: controls on mineral weathering and biogeochemical budgets; controls on the energy balance and chemistry of snowpacks; and controls on the flowpath and flux of water [Turk et al., 1993].

Lack of understanding of weathering and biogeochemical budgets seriously affects the calculation of the CO² budget. Twice the amount of atmospheric CO² is converted to HCO3 and transported from a watershed if feldspar weathering dominates over carbonate weathering. The common assumption is that feldspar weathering controls HCO3 formation because feldspars are the most common weatherable mineral in the granitic bedrock of the Rocky Mountains. However, recent work indicates that carbonate weathering may be the controlling process throughout the Rocky Mountains. Stable strontium isotopes are being used as tracers of the minerals being weathered. Initial results indicate that eolian carbonates may be an important control on chemical weathering.

Incomplete understanding of the energy balance and chemistry of snowpacks limits the ability to predict runoff generation, the chemistry of runoff, and sediment transport and storage in response to external forcing factors. Also, changing albedo caused by changing snowmelt characterisitics, may represent an important feedback to the climatic system. Rocky Mountain snowpacks are colder than most of the better studied snowpacks. The Loch Vale WEBB study is attempting to determine, in detail, the metamorphism of Rocky Mountain snowpacks and to model the interaction of energy, snowpack accumulation and melt, and runoff generation. Initial results indicate that nitrate release is an important process in Rocky Mountain snowpacks.

Poor resolution of the flowpath and flux of water within the watershed constrains the ability to predict which geologic materials and biotic processes alter water flow and composition as it moves through the watershed. Quantification of the importance of hydrologic processes such as infiltration of unfrozen soil by snowmelt and evapotranspiration is, therefore, important. This study focuses on determining the flowpaths and relative amounts of water in various important landforms within the watershed and the interaction of common climatic variables with the routing of water along selected flowpaths, particularly talus and wetlands.

Loch Vale watershed lies within the Rocky Mountain National Park and is administered by the National Park Service. The watershed has been a key site for process-level study of acidification in alpine areas as part of the interagency National Acid Precipitation Assessment Program (NAPAP). It is also a UNESCO- designated International Biosphere Reserve.

USGS scientists at the Loch Vale WEBB site are conducting their research in collaboration with colleagues from the National Biological Survey, the National Park Service, Colorado State University, the University of Colorado, the University of Wyoming, and the U.S. Forest Service.

References

Elder, J. F., D. P. Krabbenhoft, and J. F. Walker, Water, energy, and biogeochemical budgets (WEBB) program: Data availability and research at the Northern Temperate Lakes site, Wisconsin, U.S. Geol. Surv. Open File Rep. 92-48, 15 pp., 1992.

Huntington, T. G., R. P. Hooper, N. E. Peters, T. D. Bullen, and C. Kendall, Water, energy, and biogeochemical budgets investigation at Panola Mountain research watershed, Stockbridge, Georgia--a research plan, U.S. Geol. Surv. Open File Rep., 93-55, 39 pp., 1993.

Larsen, M. C., P. D. Collar, and R. F. Stallard, Research plan for the investigation of water, energy, and biogeochemical budgets in the Luquillo Mountains, Puerto Rico, U.S. Geol. Surv. Open File Rep., 92-150, 19 pp., 1993.

Loaiciga, H. A., J. B. Valdes, D. Entekabi, R. Vogel, J. Garvey, and H. Schwarz, Global warming and the hydrologic cycle, J. Water Resour. Plann. Manage. Div. Am. Soc. Civ. Eng., in press.

National Research Council, The Role of Terrestrial Ecosystems in Global Change: A Plan for Action, 50 pp., National Academy Press, Washington, D.C., 1994.

Shanley, J. B., E. T. Sundquist, and C. Kendall, Water, energy, and biogeochemical budget research at Sleepers River research watershed, Vermont, U.S. Geol. Surv. Open File Rep., 94-475, 22 p. 1995.

EOS, Transactions, American Geophysical Union, v. 75, no. 38, p. 433-439, 1994

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