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Sustainable Society:  A society that balances the environment, other life forms, and human interactions over an indefinite time period.

A Plan for a New Science Initiative on the
Global Water Cycle
 

Chapter 4: Determining Links between Water, Carbon, Nitrogen, and Other Nutrient Cycles in Terrestrial and Freshwater Ecosystems

Report to the USGCRP from the Water Cycle Study Group, 2001*
Updated October 12, 2003

Synopsis
Background
     Box 4-1 Recent Notable Oxygen Losses from Important Coastal Waters
Goals
    Goal 1: Develop observations and experiments to characterize the coupling and feedbacks of water, C, and N cycles
    Goal 2: Develop a quantitative predictive framework for water, C, and N fluxes coupled to ecosystem responses
    Goal 3: Distinguish human-induced and natural variations in the coupling of water, carbon, and nitrogen cycles
Program Elements
    Program Element 1: (Goal 1): Observations and Integrated Database
    Program Element 2: (Goals 1, 2, and 3): Field Studies, Natural Experiments, Manipulative Experiments
    Program Element 3: (Goals 2 and 3): Better Representation of Physical and Biological Processes in Land-Surface Models
    Box 4-2 Links Between Vegetation and Climate
    Program Element 4: (Goals 2 and 3): Better Basin-Scale Models of Nutrient Sources and Transport
    Program Element 5: (Goal 3): A Knowledge Transfer Program to Study Human Influences
Initiatives
    Initiative: Establish a network of in situ monitoring stations near the mouths of major U.S. rivers, along with a program to encourage international partners to establish comparable networks globally
    Initiative: Develop a suite of aircraft- and satellite-based sensors to monitor parameters (e.g., turbidity, color, pigments) related to carbon and nitrogen concentrations in freshwater ecosystems
    Initiative: Strategically expand and augment 200 to 300 existing streamflow and water quality monitoring stations to provide long-term, high-frequency records of C and N as well as water, to characterize fluxes, and to compare with and calibrate remotely sensed data
    Initiative: Build cooperative worldwide programs, including volunteer efforts, to monitor water quality parameters of interest.
    Initiative: Establish nested basin studies in three to five river systems with varying land cover and levels of human disturbance and regulation (examples of possible basins include the Mississippi, the Potomac, and at least one high-latitude river)
    Initiative: Develop process models of coupled water, carbon, and nitrogen transport and transformation in aquatic ecosystems and other terrestrial components of the hydrologic cycle (e.g., soil and groundwater) that can be tested against data from integrated databases and results of field studies
 

Synopsis

• Integrated remote and ground-based observation programs, where observations are conducted at a hierarchy of spatial and temporal scales and recorded in a sustainable data archive and retrieval system

• Field studies to establish quantitative descriptions of processes involved in coupled C-N-water cycling

• Conjoining observations and models to understand and quantify slower feedback mechanisms of vegetation structural dynamics on coupled C-N-water cycling

• A knowledge transfer program for collaboration and communication among researchers, decision makers, and stakeholders

Water, carbon, and nitrogen cycles are all critical for humans and ecosystems and have strong links to climate. These cycles have been perturbed by human activity throughout human history. However, these influences have accelerated in the past five decades or so, with correspondingly significant regional and global changes. Water, C, and N cycles show notable relationships and feedbacks. These feedbacks must be understood within the context of the proposed initiative to (1) address the causes of water cycle variability and predictability, and (2) anticipate or avoid adverse impacts of climate variation on water resources and aquatic and terrestrial ecosystems.

Human activities have profoundly affected hydrologic processes (e.g., Potter 1991) and nitrogen cycling (e.g., Galloway et al. 1995) in terrestrial and freshwater aquatic ecosystems. Wetlands have been drained for flood control and agricultural development. Estimates of U.S. wetland areas altered alone are very large -- the loss of over 50% of the Everglades in the last century being a prominent example. Agricultural practices have markedly altered the availability of nitrogen for transport to aquatic ecosystems. For example, annual applications of nitrogen fertilizers on U.S. farmlands increased approximately five-fold between 1960 and 1980 (Alexander and Smith, 1990), and more than 233 million U.S. acres were treated with nitrogen fertilizer in 1997 (USDA, 1997).

The construction of dams on major rivers worldwide has affected flow regimes, and with them C and N fluxes and ecosystem dynamics. Changes in erosion and sedimentation alter channel and floodplain morphology, with important feedbacks to water, carbon, and nitrogen cycles. Further, sediment from erosion can have long-lasting influence on river hydrology (e.g., Meade 1982). Land use changes affect hydrological processes and these interact with carbon and nutrients in many significant ways.

The rate at which water moves through terrestrial and aquatic ecosystems often has a major impacts, such as rates at which dissolved and particulate materials are leached from the land to water courses. Land use and land management affect the hydrological response of a system and thus nutrient fluxes. Conversely, changes in nutrient regimes (e.g., through fertilization) change plant growth rates and sometimes community structure.

These effects change evapotranspiration, surficial soil characteristics, and hydrological response of watersheds to rainfall. Better understanding the interactions and feedback mechanisms among water, C, and N cycles, and on their linkages with ecosystems is essential to simulate variability and change in these linked processes. Important interactions occur at time scales ranging from weather and similar short-term events to seasonal, decadal, and still longer duration events.

Increasing nitrogen concentrations in groundwaters, rivers, lakes and reservoirs, and coastal oceans can have adverse effects on both human health and ecosystem functioning. High concentrations of nitrate are associated with methemoglobinemia (blue-baby syndrome). Nitrate concentrations in groundwater in some rural areas exceed the EPA drinking water standard, diminishing the value of this water supply. Concerns about nitrogen contamination in drinking water are even more pervasive in other parts of the world.

Phytoplankton and aquatic plants in coastal oceans and estuaries are commonly nitrogen-limited, so that increasing nitrogen inflow into rivers leads to eutrophication. Algal blooms are a problem in many estuaries in the United States as they are worldwide. In addition, hypoxia caused by excess phytoplankton growth has led to major ecosystem disruptions in the Mississippi delta region, the Chesapeake Bay, and elsewhere (Box 4.1).
 

Box 4-1 Recent Notable Oxygen Losses from Important Coastal Waters

Understanding and quantifying carbon cycle effects on climate and climate change is a central goal of the Carbon Cycle Science Plan (1999). Similarly, understanding and quantifying climate effects of the water cycle are central to this initiative. Nitrogen cycling also plays a role in a diverse array of global changes, many of which link to an even broader array of effects through interactions with carbon and water. The mechanisms through which ecosystems regulate uptake and emission of nitrogen are not fully understood, but it is known that agricultural practices and land use can have a large impact on N fluxes.

In summary, changes in carbon (C) and nitrogen (N) fluxes are connected to changes in water cycle, land use patterns, agricultural practices, urban development, and vegetation. Changes in C-N cycles in turn induce further changes in the water cycle itself, which can have adverse impacts on terrestrial and aquatic resources. Carbon dioxide levels in the atmosphere affect plants' water use efficiency through stomatal response and total leaf area, thereby altering local and regional water cycles. Atmospheric nitrogen deposition can have a fertilizer effect, inducing changes in both water and carbon cycles. Emissions of nitrous oxide, a strong greenhouse gas, have greatly increased because of human-induced alterations of the nitrogen cycle. Increased worldwide use of nitrogen fertilizer has also directly affected surface water and groundwater chemistry.

Major river systems deliver nitrogen from the continents, leading to eutrophication of coastal waters. Hydrologic extremes, whether floods or droughts, significantly impact water quality (in terms of sediment and salinity as well as carbon and nitrogen). Anticipating or avoiding these changes and impacts requires a fundamental understanding of the links among the C, N, and water cycles in terrestrial and inland aquatic systems. It is also important to understand factors driving the human activities that impact vegetation distribution and water quality, and to evaluate the sensitivity of these activities to a range of policy alternatives. Quantitative models that capture these linkages and feedbacks are essential to improve our ability to evaluate potential ecosystem changes, predict climate variations, and manage water resources effectively.

The program elements of the proposed water cycle initiative that are discussed below will address the most pressing issues on the impacts from changes and variability in the coupled water-C-N cycles. The first of this area's two main research thrusts concerns land-atmosphere transfers. In this realm, water, C, and N cycles are linked directly by the tight coupling of cycles through terrestrial vegetation. In these terrestrial systems, feedbacks among cycles occur at all time scales. Short-time-scale fluxes from, to, and within plant canopies are linked to each otherin part because stomata act to regulate transfers to and from leaves, but also respond to environmental conditions (e.g., Jarvis and McNaughton, 1986). Precipitation is affected by vegetation feedbacks on time scales of individual storms, but also across growing seasons (e.g., Pielke et al., 1999).

Finally, climate changes can lead to shifts in large-scale vegetation patterns (e.g., Shugart, 1998), which in turn can accentuate or mitigate climate changes through feedbacks among the water, nitrogen, carbon, and energy cycles. There is great uncertainty about the fate of nitrogen added to the landscape by atmospheric deposition or fertilizer application -- only a fraction of nitrogen applied runs off with drainage water. One aim of the proposed work is to account for components and coupled exchanges of the water-C-N cycles fully and simultaneously (Aber, 1999) at all important time scales through modeling, observational, and experimental projects.

The second research thrust in this area concerns aquatic systems. Aquatic systems are very strongly affected by changes in water and nutrient cycling, but they have less direct coupled interactions with the global water cycle than do terrestrial systems. (There are important feedbacks between aquatic systems and climate through the nitrogen cycle. Human activities have doubled emissions of N₂O from coastal areas and these are a significant fraction of total emissions to the atmosphere from all sources (Kroeze and Seitzinger, 1998)). Fluxes to aquatic ecosystems, as well as fluxes from such systems also are important in global cycles. Fluxes of nitrogen and carbon, along with those of sediments, other nutrients, and contaminants, from upland source areas to freshwater and coastal ecosystems have profound effects on aquatic ecosystems and on water resources (e.g., Naiman et al., 1995). A critical scientific need exists to quantify these fluxes and to better understand how human activities affect them.

Why? Quantifying the critical, dominant couplings of biological and hydrological processes requires measuring mass fluxes. Current measurement programs are not up to this task. At most terrestrial sites, programs measure concentrations of various constituents in air, but pay little attention to measuring corresponding hydrological variables at the same time scales. Such joint measurements are required to estimate mass fluxes and to establish important linkages among processes.

Current monitoring approaches for aquatic systems were designed for such purposes as local water quality evaluation, rather than for quantifying C and N flux coupling to the hydrological cycle. There is currently a major mismatch in temporal and spatial sampling scales for the processes that need to be quantified. Without appropriate monitoring information during major hydrologic events, no full accounting is possible of nutrients and pesticides transported by streams, and a full understanding of the effects of these contaminants on the health and living resources of receiving waters, such as the Chesapeake Bay, is therefore limited (Fuhrer et al., 1999).

How? These goals will be accomplished through a well-designed program of observation using new technologies.

Goal 2: Develop a quantitative predictive framework for water, C, and N fluxes coupled to ecosystem responses.

Why? Understanding both aquatic and terrestrial processes requires that ongoing observations be linked to the continued development and testing of models. Developing terrestrial C-N-water models with state-of-the-art representations for the component processes on multiple temporal and spatial scales requires new understanding of the processes to be described, whether for freshwater or coastal aquatic systems.

How? Improved process models, linked to observational and experimental data, will be developed and tested.

Why? Human activities affect water and nutrient cycles both intensively and extensively. To avoid or mitigate unwanted human effects on water resources requires distinguishing human-induced changes from natural variability. Future management of water resources will require a strong scientific base, including work that integrates social, economic, and ecological elements (NRC, 1999b).

How? Observations will be made on a variety of natural and disturbed systems. Paleoclimate records will be examined to deduce preindustrial natural variability. Models will be improved to incorporate information about human and natural disturbances. In particular, information will be developed on economic, biological, and physical factors driving human disturbances, such as nitrogen loadings from agricultural operations and shifts in vegetation cover due to changes in land use. Analysis and modeling efforts will be conducted in sufficient detail to develop predictive capabilities, allowing researchers to examine the potential consequences of policies designed to mitigate adverse impacts. A fully integrated knowledge transfer program will be established, which will coordinate with national and state water quality management efforts and with watershed-scale land use planning activities.

New methods must be developed to acquire and manage data to elucidate links among water, C, and N cycles for better understanding of ecosystem processes. These new methods must quantify critical cycle components. Better data are also needed on water use and on institutions.

Fluxes of Water, Carbon, and Nitrogen at Vegetated Surfaces. Considerable effort has been dedicated to measuring long-term CO₂ fluxes between vegetation systems and the atmosphere, and some notable successes have been realized (e.g., Wofsy et al., 1993). Currently, 36 U.S. sites are operational in the U.S. to monitor Net Ecosystem Exchange (NEE) of carbon as part of the AmeriFlux initiative. For Europe, 15 sites have already published long-term NEE as part of the EuroFlux initiative (Valentini et al., 2000), while 27 other sites distributed in Japan, India, Australia, Brazil, and South Africa have been initiated. These monitoring sites are now part of the international FluxNet (Kaiser, 1998), a long-term measurement network of carbon dioxide exchange integrated into a consistent, quality assured, documented dataset. This initiative requires further expansion to understand coupled water-C-N cycles.

The network itself lacks (1) systematic measurements of N and detailed water cycle components, (2) a clear methodology to integrate its findings at regional and global scales, and (3) measurements of vegetation attributes and their dynamic physiological properties. This program element thus proposes a coordinated expansion of FluxNet to identify and monitor components of the coupled water-C-N cycle. The program element takes advantage of ongoing efforts toward understanding the global carbon cycle. We propose expansion in two stages: the first requires U.S. flux monitoring sites to be expanded in both number of sites and scope of measurements (e.g., to include water and N monitoring), while the second strives for a full expansion of the international FluxNet.

New instrumentation for measuring concentrations of gaseous and liquid N species is required, along with expanded soil water measurements. Recent advances in laser diode technology permit gaseous N species concentration to be measured at 20 Hz, a sampling rate ample for conducting eddy-covariance flux measurements between land and atmosphere. At present, the cost of such instrumentation is prohibitive for routine flux monitoring of N species. Similar cost barriers exist for liquid N deposition and transport. Most AmeriFlux sites measure and resolve temporal variation in water vapor fluxes between the land and atmosphere via eddy-covariance methods. A critical augmentation is new measurements of the dynamics of the water table, soil moisture, soil water tension, soil hydraulic properties, interception and throughfall, and root properties.

These measurements, along with subsurface N-species measurements, will support analysis of nitrogen transformations (e.g., nitrification-denitrification in root soil) induced by atmospheric N deposition. These N transformations produce a wide range of N-species in liquid and gaseous phases (e.g., soluble NH₃, NO₂, NO₃), within the soil-vegetation-atmosphere continuum (plant NO₂, NO₃, NH₃, organic N, gaseous N). Few of these species can be routinely measured at the temporal resolution needed to capture their transport and transformation at the desired time scales. These new data will provide the basis for exploring the coupled impacts of C and N on water cycling in changing terrestrial ecosystems in the face of changing atmospheric forcing (e.g., Farquahar et al., 1980; Collatz et al., 1991).

The integration of measured network fluxes is needed at regional and global scales. Satellite remote-sensing data are potentially useful to integrate network results beyond landscape scales. EOS-MODIS data are being employed in support of carbon studies (Running et al., 1999). New sensor platforms, such as NASA's Terra and Aqua, offer opportunities for spatial integration of water and N cycle component models using remotely sensed terrestrial and atmospheric data to generalize (or scale) network results to regional and global scales. The addition of new flux monitoring sites can also be designed to balance the need to sample diverse biomes with the need to sample at a hierarchy of spatial scales to identify and test techniques for integrating from landscape to regional and global scales.

One major complication in implementing these scaled results in a global modeling framework is subgrid heterogeneity, which affects the description of land surface fluxes (Avissar, 1993). Vegetation classification using new, nonlinear, hierarchical tree-like scaling (Hansen et al., 1996) offers promise for describing subgrid variability. Finally, the development of dielectric (Crow et al., 2000) and laser vegetation imaging sensors (Weishampel et al., 2000) offers unique opportunities to measure and resolve the spatial variation and dynamics of three-dimensional leaf area distribution and near-surface soil moisture fields.

Fluxes of Water, Carbon, Nitrogen, and Sediment in Rivers. Despite considerable efforts over the past several decades, mass fluxes of dissolved and suspended constituents in river systems are poorly known. In large part, this deficiency has arisen because water quality sampling has been intended to assess concentrations, not to understand processes or to estimate fluxes. Thus, most data on stream chemical composition are not linked to measurements of discharge. Even when discharge measurements are available, water quality samples are typically sparse in time relative to discharge.

Given that concentrations can change significantly with river discharge, flux estimates are highly suspect under these conditions. But discharge values are critical to calculate global biogeochemical budgets (e.g., Seitzinger and Kroeze, 1998) and also "as one of the best integrated measures of the success of clean water efforts, free from biases caused by changes in total discharge from year to year" (NRC 1999b, p. 129).

We have only a general conceptual understanding of how in-stream processes modify nutrient fluxes in rivers and how hydrologic variability influences those processes. Recent studies of individual streams (e.g., Mulholland and Hill, 1997; Burns 1998) and regional analyses of river basins (Alexander et al., 2000) have indicated the existence of important in-stream sinks for nutrients, particularly in low-order streams. In-stream sinks can be important controls on riverine fluxes of nutrients across the landscape.

To accumulate the necessary data, coordination of water flux and other measurements will be critical. For example, NSF is now designing a new program, NEON (National Ecological Observation Network). A strong hydrological component in NEON could address the critical question of linked variations in water and nutrient cycles. The enhancement of ongoing monitoring programs to include data across disciplines, and on appropriate geographic scales, will provide some of the major improvements necessary for better understanding of global biogeochemistry and better management of water resources.

For example, USDA reports data on U.S. fertilizer use for major crops, but the data are not organized on the basis of hydrologic units. The LUCC (Land Use and Cover Change) program of the IGBP is observing land use dynamics and conducting process studies, but these activities have not encompassed hydrologic effects. Addressing such linkages will be vital for understanding how land management decisions and land use changes will affect water quality.

As urged by a recent NRC committee, stream-gauging and monitoring network design should emphasize adequate temporal resolution, sampling of storm events, and measurement of appropriate ancillary hydrological and biogeochemical data; these activities should use the highest possible quality of sampling and analysis (NRC, 1999b). Nevertheless, even in such major programs as NEON, EPA Environmental Monitoring and Assessment Program (EMAP), USGS National Water Quality Assessment (NAWQA), and others, flux estimates will necessarily be limited because resource constraints limit sampling frequency.

Data from standard sampling programs must be augmented through instruments capable of near-continuous measurement and through judicious use of remote sensing. Program design should also include the many volunteer and cooperative water quality monitoring networks in the United States and worldwide.

In Situ Sensors. In situ measurement using new micro- and nanotechnologies has the potential to overcome constraints in monitoring C and N variations associated with water cycle variations. Conductivity and temperature are already monitored at some stream-gauging stations. Other sensors are being developed to monitor dissolved oxygen and carbon dioxide reliably; these sensors should allow continuous estimates of respiration and photosynthesis, processes relevant to carbon storage and transport. In principle, similar instruments should allow in situ measurement of many other constituents, including C and N species.

Unfortunately, use of in situ monitoring technology in water chemical analysis has been limited, because of the large investment required for development. This program element would support the necessary development and testing to make such in situ instruments available at a reasonable cost. Development and testing could involve a partnership of federal agencies and the private sector.

The types of detectors best suited for these instruments are photometers, spectrophotometers, and fluorometers. Photometers can directly measure turbidity, an indicator of particulate matter, which generally contains significant amounts of C and N. Measurement of dissolved N species requires the addition of one or more reagents. Thus, instruments would be designed to store reagents and to release them in prescribed amounts. Reactions with reagents will generate colored species that can then be detected with a spectrophotometer. Fluorometers can detect both chlorophyll in particulate matter and dissolved organic material. To test and implement monitoring using these detectors would require a period of overlap with traditional sample collection at sites that are part of established monitoring networks.

Remote Sensing. Existing and improved remote-sensing tools provide significant opportunities to enhance the temporal and spatial resolution of available data. Some remotely operated devices can be installed permanently at sites, while others can be deployed periodically using aircraft, and still others make use of satellite systems. In this program element, we envision a balance of ground-based and remote data collection.

Spectral radiometers mounted at sites would be useful to detect changes in periphyton on the streambed. Periphyton concentrations could be used to estimate N retention as well as fluxes of organic N. Radiometers would also be used to monitor riparian wetlands.

We can now detect algal blooms in the open ocean using aircraft and satellite instruments such as MODIS and SeaWIFS. These tools have not yet been applied to monitoring similar processes in large rivers, lakes, and reservoirs. Remote sensing can detect transparency/turbidity, pigments (e.g., chlorophyll), and color or fluorescence (to determine dissolved organic carbon [DOC]), which are three critical measurements associated with C and N flux.

Important issues in applying satellite remote sensing are spatial resolution, georeferencing, image analysis, and frequency of cloud-free images as they relate to hydrological and ecological events such as snowmelt and algal blooms. Satellite and aircraft measurements will be calibrated with ground-based observations at particular locations. In this manner, hydrologic events that we expect to be critical to C and N fluxes can be examined as natural experiments, with the appropriate level of resolution to evaluate quantitative models.

Integrated Database Development. To best use hydrologic and water quality data from ongoing and past monitoring programs, the data must be archived in sustainable, accessible data and information systems. As noted by the NRC Committee on Hydrologic Science, the most comprehensive national system for water quality data, the EPA-maintained STORET, has many limitations as an archive (NRC, 1999a). Moreover, this system is limited to U.S. measurements and does not include the hydrologic data needed to examine linkages between water quality parameters and water fluxes. Data and information systems that can address linked global cycles of water, C, and N must incorporate both hydrologic and water quality data from throughout the world and store the data in readily accessible format for comparison, synthesis, and testing of conceptual models.

Substantial advances have been made in the past decade in the design of relational databases. These databases can integrate data collected at a wide range of spatial and temporal scales. They are organized through a spatial framework and have been developed to handle very large quantities of data. The databases can be linked to a geographic information system (GIS) by defining the location of the data collection site. Although software for both databases and GIS will continually evolve, the existing systems are sufficiently mature that future upgrades to commercial software should be able to carry forward data organized in a relational manner. Thus, the usefulness of the data incorporated in the initial database can be safeguarded for the future.

Development of the proposed database will require a large up-front investment in software, hardware, and personnel. But the rewards will benefit many scientists in the community as the data are probed to test new predictions about the coupling of hydrologic, ecological, and biogeochemical processes. Types of continuous or discrete monitoring data that could be integrated in this database include the following:

• Precipitation
• Air and water temperature
• Light, wind speed, and humidity
• Soil moisture
• Leaf area index
• Snowpack
• River flow
• Lake level
• Water conductivity and salinity
• pH
• Suspended sediment
• Chlorophyll a
• Color
• DIC, DOC, POC,
• PON, inorganic N species and possibly other nutrients
• Agricultural practices data (e.g., cropped acres, animal units, nitrogen applications)
• Land use data (e.g., percent impervious surface).

Human Influences on the Water Cycle and Their Links to Nutrient Cycles. Although some water uses are metered, much water use that is governed by riparian rights or involves extraction from privately owned wells is unmetered and unmeasured. This gap in observation makers it difficult to estimate net fluxes between surface and ground waters and to model processes affecting the chemical composition of freshwater (e.g., salinization). Thus, the gap also makes it hard to estimate the effects of human activities on water supply and quality. Other important observations are also missing. For example, it is important to classify and catalogue the institutions (organizations and sets of rules) governing water use in vulnerable regions.

In addition, spatially disaggregated observations on land uses and agricultural practices that affect water quality will be important for managing non-point-source contamination of water bodies. For example, watershed-scale monitoring of the quantity and timing of fertilizer applications, tillage practices, and land use changes are needed to understand how variations in these factors affect water quality and runoff characteristics in response to hydrologic events.

Program Element 2 (Goals 1, 2, and 3):

Field Studies

Experimental studies on the coupling of water, C, and N cycles will be used to test quantitative formulations for controlling processes, and to explore the behavior of key forcings outside the range of current ambient conditions. Experiments can also reveal important responses of C and N cycles to forcing from simultaneous changes in multiple hydrologic and ecological factors, such as climate and land use. In watersheds, the processes controlling water and nutrient cycles operate over a range of spatial and temporal scales, from responses of stream communities to rainfall events to changes in vegetation in response to climatic shifts or disturbance. Experimental studies can rely on natural extremes in seasonal events to reveal couplings or can use purposeful manipulations.

Experimental studies should be performed at carefully selected sites that provide a range of climate regimes (e.g., humid/warm temperate, humid/cool temperate, arid) and land use types (e.g., forest, grassland, urban, agricultural). The experiments must be designed for effective interfaces with observational studies and models. Well-designed experiments can be powerful tools to make research findings accessible to a broad range of stakeholders. Additionally, paleo-records can reveal past changes in vegetation distribution and provide data on past changes in climate.

Natural Experiments. Long-term experimental nested basins provide essential data on water balance, flow pathways in terrestrial ecosystems, and aquatic ecosystem and biogeochemical processes that control reactive transport and storage of C and N. This information is invaluable in developing models and methods to scale hydrological variables and C and N species, characterize basin-scale variability, and understand the limits of predictability. The goal of this program element is to operate three to five nested basins in the United States during a decadal program. These basins will represent several bioclimatic zones, geological settings, and land use characteristics, as well as scales, with the largest being of actual continental scale.

The basins will represent water-limited, energy-limited, and nutrient-limited systems. Closed basins may prove to be valuable in research because they can be used to study long-term budgets (accumulation) of nutrients, carbon, salinity, and so forth, in addition to water itself. Such closed basin networks are common in the central United States and Canada, in particular, the glaciated prairie wetlands found from Iowa to Saskatchewan, with forested closed wetlands continuing into northern Canada. Sites must be selected to take full advantage of existing programs with historical records and records of continuing research.

Natural experiments play a key role in the continuum between long-term observations and purposeful manipulations. One significant limitation of the existing network of experimental sites is the relatively small scale of most basins studied, which limits the opportunity to observe natural experiments, as well as restricting the research scale. Thus, larger scale study areas need to be designated, areas that include relatively undisturbed along with agricultural and developed lands.

Manipulative Experiments. Long-term studies of nested small- to large-scale basins provide a framework for designing and conducting small-scale manipulative experiments. These experiments will achieve the process understanding required for the model development component of the science plan. For example, experiments involving whole-catchment additions of reactive tracers are needed to study controls on hydrologic fluxes from land to water, particularly the ways that hydrologic variability and nutrient -- water cycle interactions affect the transfer of nutrients from terrestrial to aquatic systems.

The Carbon Cycle Science Plan (CCSP; USGCRP 1999) describes the need to improve process models regarding the terrestrial component of the carbon cycle and associated ecosystem changes. This need extends to the coupling of water and nitrogen cycles as well, as noted in the CCSP. Thus, the need for improved process models, linked to observational and experimental data, is an important element of the water cycle plan as well.

Some existing models have been relatively successful in reproducing the range boundaries of major trees and shrubs as a function of current climate conditions. These models have been used to estimate shifts in these ranges under potential climate change scenarios. However, the existing models were not designed to account for the rates at which ecotones, the boundaries between different ecosystems such as grassland and forest, may shift in response to changes in temperature and precipitation. Shifts in ecotones, such as the conversion of wetlands to drier upland vegetation, can alter not only fluxes of carbon and nitrogen, but also rates of infiltration and evaporation. These modifications to hydrologic processes, in turn, create feedbacks to the water cycle.

Such feedbacks to the water cycle act in conjunction with feedbacks from changes in C and N cycles. The results of these feedbacks are presently uncertain; together they could either stabilize ecotones or induce further changes in vegetation communities. Past-recorded changes in vegetation may have been interlinked with climate changes. For example, Claussen et al. (1999) hypothesize that the desertification of North Africa some 5,000 years ago resulted from a complex feedback interaction between solar insolation, vegetation, and sea ice (Box 4.2).

Box 4-2 Links Between Vegetation and Climate

Background photograph (taken by Philipp Hoelzmann) shows rock paintings of mid-Holocene fauna near Zolat el Hammad, North Sudan.  Today this region, like the largest part of the Sahara, is a hyperarid desert.  The desert formed some 5,000 to 6,000 years ago.  Ensemble simulations using a coupled atmosphere-ocean-vegetation model reveal a rather abrupt change in Saharan vegetation (superimposed blue lines indicate fractional vegetation cover), which was triggered by subtle changes in Northern hemisphere summer insolation (Claussen et al.,1999).  Copyright American Geophysical Union, reprinted with permission. Climate variability during the present interglacial, the Holocene, has been rather smooth compared to the last glacial period.  Nevertheless, the Holocene has seen some abrupt climate changes.  One of these changes, the desertification of the Saharan and Arabian region some 5,000 to 6,000 years ago, was presumably quite important for human society.  It could have been the stimulus for the foundation of civilizations along the Nile, Euphrates, and Tigris rivers.  Saharan and Arabian desertification may well have been triggered by subtle variations in the Earth's orbit, strongly amplified by atmosphere-vegetation feedbacks in the subtropics.  The timing of this transition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.

Understanding of terrestrial processes requires that ongoing observations be linked to continued model development and testing. Often, model limitations serve as signposts in formulating and testing new hypotheses. Current model frontiers include simulations of the effects of CO₂ on a full suite of plant processes, of dynamic interactions between carbon and nitrogen budgets, of hydrologic changes (such as drying or thawing of boreal peat), and of vegetation dynamics, such as successional changes over long time scales. Models must also be improved to incorporate information about human and natural disturbances of the land surface. Current models emphasize physiological and biogeochemical processes and largely neglect the carbon storage dynamics induced by cultivation, forest harvest, fire, and fire suppression.

Knowing the rate of carbon sequestration in lakes, reservoirs and peatlands, as well as in oceans, along with controls on these rates, is essential to understanding the global carbon cycle. Improved modeling ability is needed to anticipate changes in carbon delivery to freshwater aquatic ecosystems and carbon export from them, as affected by changes in the hydrologic cycle, specifically the supply of water to these systems and changes in residence times of water within them. Both storage of carbon in peat and carbon release by methane emissions are greatest when water tables are high. This is another mechanism by which variability in the water cycle is linked directly with variability in the carbon cycle.

There are also important feedbacks among the nitrogen cycle, ecosystem functioning, and global and regional water cycles. The next level of improvement in estimating regional evaporation from plants is likely to result from a combination of new observations (e.g., remote sensing) and models of vegetation that include photosynthesis and the assimilation of nutrients. Knowledge of coupling between the cycling of nitrogen -- a limiting factor for growth in diverse aquatic and terrestrial ecosystems -- and water and carbon cycling is needed for models of land surface -- atmosphere interactions to improve estimates of regional water fluxes in terrestrial ecosystems.

Nitrogen export from major river basins to coastal ecosystems is the cause of significant eutrophication, with resulting extensive zones of low oxygen concentration in coastal waters worldwide. We recognize that elements such as phosphorus and iron often limit production in aquatic systems; but we emphasize nitrogen in this science plan. The nitrogen problem has important direct impacts on human well-being, including impacts on fisheries caused by toxic algal blooms and diminished recreational resources. To inform possible management actions to mitigate these effects or control nitrogen export, models must be developed and refined to link hydrological transport of nitrogen from forested, agricultural, urban, and suburban lands with in-river process and routing.

Although nutrient export from basins can be a significant problem, nutrients are of course critical to the sustainability of riparian ecosystems, which often harbor a large percentage of a region's biological diversity. Riparian, like terrestrial, ecosystems exist due to the fortuitous intertwining of biogeochemical and hydrologic cycles. Available water and nutrients determine and sustain these ecosystems. Disruptions in either cycle can cause major ecosystem perturbations and can shift species distributions. Although the interconnectivity of the cycles is not well understood, hydrologic inputs have been tied to major nutrient inflows and cycling rates. Many semiarid systems are potentially nitrogen-limited and have the ability to take up nitrogen throughout the year. However, if there are interruptions to hydrologic inputs (e.g., through intercepted groundwater or major flooding) and concomitant decreases in vegetation, the ability of the riparian system to retain nitrogen declines, resulting in a net export of nitrogen from the system.

The modeling of river-basin N export is not well advanced. Lacking a general theoretical framework has led to the simple expedient of relying on empirical approaches (e.g., Johnes et al., 1996; Johnes and Heathwaite 1997; Meeuwig, 1999; Valiela et al., 1997). Such approaches lack a significant hydrological component and are quite limited in their ability to predict impacts from future changes in land use. Process-based models have been developed, but are limited by gaps in scientific knowledge (e.g., Whitehead et al., 1998; Choi and Blood, 1999; Thomann, 1998). Also, ecologists have advanced a conceptual model of aquatic functioning called the "river continuum model."

But this conceptual model has not been formulated in mathematical terms, in large part because no comprehensive set of field observations on a major river system is available. A program should be established to foster the development and testing of integrated river-basin models for nitrogen export. Because of the close linkages discussed above, these models will necessarily include carbon (and therefore sediment) transport, and changes in ecosystem processes brought about during transport through rivers, reservoirs, and their associated ecosystems.

Three key societal needs are related directly to the interactions among water, carbon, and other constituent cycles. The first is understanding implications of land use decisions on carbon sequestration, and in particular, on the storage, mobilization, and movement of carbon in and among wetlands, lakes, and reservoirs. The second is understanding riverine transport of sediment and associated pollutants to estuarine and coastal systems. The third is prediction of organic carbon concentrations in drinking water supplies, which is increasingly the focus of disinfection requirements.

Notwithstanding important application needs related to predicting water movement at the land surface (see sections on applications in Chapters 2 and 3), predictive capability for carbon cycling and other constituents is in its infancy. Beyond the need for better predictive tools linking water and carbon cycles (detailed elsewhere in this chapter), studies of human activities are needed that link to water and constituent cycles. Relevant human activities include, for instance, extraction of water from surface and underground sources, release of water into surface waters and the ground, alterations in the technology of water use, and changes in water management institutions.

Forces driving these human activities must also be understood to develop accurate forecasts of changes in hydrological systems and to estimate future vulnerabilities of societies and ecosystems to water cycle changes. Understanding the hydrological effects of human activities will help inform land use planning decisions. To benefit fully from this research, water use models must be coupled to models of water supply (e.g., aquifer recharge, streamflow, precipitation) and to models describing influences of land use changes on water and nutrient cycles, and sediment transport.

Applications research in coupling water and constituent cycles will reflect the increasing emphasis of the U.S. Global Change Research Program (USGCRP) on integrated assessment of the regional impacts of global change. There is a need to involve not only researchers in natural and social sciences, but also decision makers, resource users, educators, and others who need more and better information about climate and its impacts. Applications research must be initiated to develop better means to connect water cycle research and its applications in water resources management, that is, two-way communication and knowledge transfer between researchers and the management and applications community.

An integrated data and information system should be developed. Because relevant data are currently archived by different agencies, an interagency planning group would be required at the outset. (The National Water Quality Monitoring Council, as Powell (1995) suggested, might be an appropriate body.) This group should identify the location and formats of the relevant data, evaluate the current databases that may already be structured as relational databases, and prioritize steps for entraining old data and new data into the integrated database. The NASA Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (see http://www-eosdis.ornl.gov/) focuses on biogeochemical dynamics and may be able to provide critical capabilities for this effort. The effort should also include ongoing system testing by analysis of the integrated data. The approach should rely on commercially available software to ensure the ability to upgrade as more advanced versions of commercial relational databases become available. The effort should include data recovery and "data mining." It is critical that sufficient resources be devoted to this effort to ensure the availability of important data to the entire community.

Advanced technology should be imported into this field to enable order-of-magnitude improvements to in situ monitoring. The initiative could be set up to use a consortium of private, university, and government labs. The program should be designed with priorities for different chemical species, and so to attempt parallel development of several types of detector systems. These detectors would also be useful in a wide range of routine water quality studies and should include sediment monitoring. This investment would have beneficial carryover into other applied areas.

Observations

Establish a network of in situ monitoring stations near the mouths of major U.S. rivers, along with a program to encourage international partners to establish comparable networks globally. As in situ measurement technology is developed, sensors could be deployed at gauging stations near the mouths of major rivers, with an emphasis on rivers draining into estuaries that have exhibited hazardous algal blooms in the past decade. The data from these stations would be immediately useful to indicate modeling directions for coupled water cycle, C, and N models, and also to reveal the complexity of conditions involving nutrient loadings and current weather patterns that induce hazardous algal blooms. These data would provide an early indication of the level of observational and modeling complexity required to achieve useful predictive understanding.

The establishment of similar networks using in situ sensors with international partners should also be encouraged. This approach could yield comparable data sets for inflows to shared oceanic regions, such as the Gulf of Mexico, and to consistent information on nutrient loading for other regions. Also, any developments in using such data to anticipate hazardous algal blooms could then be rapidly shared. Finally, rapid transfer of these advances could allow less developed countries to "leapfrog" the current limited field sampling and laboratory-intensive technology, in a manner comparable to the recent quick spread of cellular telephones worldwide in areas without land-based telephone line networks.

Develop a suite of aircraft- and satellite-based sensors to monitor parameters (e.g., turbidity, color, pigments) related to carbon and nitrogen concentrations in freshwater ecosystems. With the lead of the ocean sciences research community, remote-sensing technology has advanced to a degree that now permits frequent, distributed observations of water quality in major rivers. High spatial resolution and moderate spectral resolution instruments will be needed. Use of commercial data and commercial partnerships should be considered, as an alternative to developing independent research missions.

Strategically expand and augment 200 to 300 existing streamflow and water quality monitoring stations to provide long-term, high-frequency records of C and N as well as water, to characterize fluxes, and to compare with and calibrate remotely sensed data. New measurement stations within river basins need to be developed, in parallel with those at the mouths of major rivers. The network expansion should be carefully planned to advance the science objectives of this plan as rapidly as possible, and to encourage the application of advanced technology to ongoing water quality monitoring studies. The first wave of deployment beyond the mouths of major rivers should support nested watershed studies and allow the extension of the results from these studies to larger spatial scales. This first-wave deployment should also include key stations upriver on the Mississippi River system, because of the significant relevant research base that already exists on this system, and the critical questions related to hypoxia at the river's outlet.

Build cooperative worldwide programs, including volunteer efforts, to monitor water quality parameters of interest. Compile the results of these programs to improve the existing database. While U.S. government -- supported measurement systems will form the backbone of the research efforts, global coverage can only occur through cooperative efforts with other professional and volunteer programs. A body should be established and supported to set standards, maintain quality assurance and quality control, and guide scientific cooperation.

Process Studies

Establish nested basin studies in three to five river systems with varying land cover and levels of human disturbance and regulation (examples of possible basins include the Mississippi, the Potomac, and at least one high-latitude river). These studies should employ in situ measurements and remote-sensing technologies to characterize and improve understanding of linked water, C, and N transport and transformation processes. Studies on both terrestrial and aquatic ecosystems should be part of the program. These studies should be coordinated with ongoing studies and with new studies relating to other program elements, so that costs of streamflow and other hydrologic monitoring will be borne by these other studies. Basins should be selected over a range of bio-hydro-climatic conditions (water-limited, energy-limited, and nutrient-limited systems), so that models will be adequately pushed and tested. Where practical, the research program should communicate with local watershed management organizations. Such organizations could assist the research by developing and maintaining detailed local data on land use and management practices, including fertilizer applications, water use, animal husbandry, and other relevant variables.

Modeling

Develop process models of coupled water, carbon, and nitrogen transport and transformation in aquatic ecosystems and other terrestrial components of the hydrologic cycle (e.g., soil and groundwater) that can be tested against data from integrated databases and results of field studies. Model development and testing will also require focused, small-scale experimental studies to elucidate processes. It is anticipated that these experimental studieswould be supported through competitive grants programs and agency research.

Augment, as appropriate, modeling being conducted as part of the Carbon Cycle Science Plan, to better identify linkages among carbon, water, and nitrogen cycles. Process studies at intensively instrumented sites should be conducted to define appropriate model structures for predicting the coupled cycling of C, N, and water across vegetated land surfaces. The existing network of tower sites (Ameriflux/Fluxnet) should be expanded with the dual objectives of covering representative biomes and providing an appropriate array for a multiscale grid that would support analysis of prediction efforts over regional and continental areas. Rigorous data analysis tools must be developed to identify the flow and transformation of information content through these biophysical systems, such as how rainfall anomalies affect soil moisture. Subjects of study should include nitrogen cycling, root function, foliar chemistry, stomatal function, and, ultimately, latent heat exchange to the atmosphere, with impacts to space-time fields of downwind precipitation.

Develop dynamic vegetation models to provide realistic boundary conditions in long-term integrations of atmospheric models. Successes have been achieved by including vegetation functional controls on surface water and energy balances over meteorological time scales. These successes must be followed with efforts to handle the vegetation's slower, structural responses to changes in climate and land use. Because these vegetation changes (e.g., in structure, density, and species distribution) affect water cycling, they must be dynamically included in water cycle models to accurately understand and predict the behavior of the water cycle over the longer time scales during which vegetation distributions shift and change. Through these efforts we can identify the attributes of changes in energy and water inputs that induce positive (and negative) feedbacks on energy and water balances.

This initiative would draw equally on data sets from the network of tower sites (including coverage of vegetation) and joint historical records of climate and vegetation changes. Disentangling human-induced changes from natural changes in past vegetation distribution is an important component of this effort. Analysis of the pathways through which a changing climate interacts with a dynamic biosphere (relative to carbon, water, and energy exchange) would support model development integrating atmospheric forcing, land surface mass and energy fluxes, and vegetation dynamics. The combined influences of carbon, nitrogen, and water must be harnessed to predict the trajectory of vegetation shifts and their feedbacks on spatial and temporal distribution of water through infiltration, retention, and transpiration.

Establish a knowledge transfer program. In the chapter's earlier discussion of Program Element 5, three areas of focus were identified for applications research on linkages between water and constituent transport cycles. This applications initiative is intended to foster advances in all three areas, by "fast tracking" connections among the research communities, operational agencies, and other decision makers. Of the three areas, the first -- implications of land use decisions on transport and fate of carbon and other constituents -- must have as its target a relationship with land use planners. This goal could be addressed initially through a series of workshops targeted at land use planners to promote information transfer about the effects of land use planning decisions on aquatic transport and storage of carbon and other constituents. Depending on needs identified at these workshops, an applications research agenda could be developed.

The second thrust area -- riverine transport of constituents to estuarine and coastal systems -- should first attempt to coordinate research in land-based runoff and transport processes, and coastal and estuarine research. A modest effort in this area is currently supported through NASA's Office of Global Programs, though much more is needed. Initially, a workshop should be held to address gaps in ongoing research, with subsequent outreach activities aimed at the coastal zone management community. In the third area -- predicting organic carbon concentrations in drinking water -- the target operational community is municipal water supply agencies, who need better information about the dynamics of organic carbon in water supply sources to identify treatment requirements. This need would have to be addressed through a combination of targeted research and cooperative projects with water supply agencies. The fellowship system suggested under the knowledge transfer initiative in Chapter 2, along with research solicitations targeted at applications, is one suggested mechanism.

Continue to study References. Return to The Global Water Cycle Study.
_____
* Courtesy of United States Climate Change Global Research Program.
Suite 250, 1717 Pennsylvania Ave, NW, Washington, DC 20006.
Telephone: 1-202-223-6262.
The U.S. Global Change Research Program was Codified by Congress in 1990 under P.L. 101-606.
See original at < http://www.usgcrp.gov/usgcrp/Library/watercycle/wcsgreport2001/wcsg2001chapter4.htm >.

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