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5
Water-Energy- Vegetation Interactions
OVERVIEW
Over the last decade, considerable progress has been made in under-
standing and modeling both the climate system in general and certain re-
lated components of the terrestrial biosphere (e.g., biophysical-atmospheric
exchanges such as radiation and water and heat fluxes, ecosystem dynam-
ics, and Face gas exchange and biogeochem~cal cycles). As yet, however,
there have been few successful efforts, either in modeling or in data acqui-
siiion, to link the activities of the physical climate and the terrestrial bio-
sphere so as to further understand and improve capabilities to predict global
change.
This chapter focuses on the interactions between the vegetated land sur-
face and the atmosphere, particularly on the exchanges of energy, water,
sensible heat, and carbon dioxide between the two. The aim of the research
strategy discussed here Is not merely to describe such exchanges but to fully
understand them so that predictive modeling can be used to explore possible
future states of the earth system. To do this, it will be necessary to make
comprehensive, biophysically based models of the atmosphere and land bio-
sphere with measurable state parameters as prognostic variables, e.g., tem-
perature and humidity for the atmosphere, and albedo, leaf area index, and
This chapter was prepared for the Committee on Global Change from the contri-
butions of Piers J. Sellers, University of Maryland, Chair; John Bredehoft, U.S.
Geological Survey; Christoper Field, Carnegie Institute of Washington; Inez Fung,
NASAJGoddard Institute for Space Studies; Alan Hope, San Diego State University;
Gordon McBean, University of British Columbia; and William Reiners, University
of Wyoming.
131
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132
RESEARCH STRATEGIES FOR THE USGCRP
photosynthetic capacity for the land. To calibrate, initialize, and validate
these models, it will be necessary to acquire a broad range of data covering
the space-time domain, from continuous global-scale monitoring to intense,
high-resolution field observations. Briefly stated, the goals of the research
strategy are as follows:
· To develop models that realistically describe the interaction between
the land biota and the atmosphere with particular reference to the exchanges
of energy, water, heat, and carbon dioxide. Ultimately, these models should
be adequate for exploring the consequences of global change in terms of
perturbations to the climate system and terrestrial ecosystems. The models
will have to cover a wide range of spatial scales (millimeters to global) and
time scales (seconds to millennia).
· To collect data that can be used to initialize, validate, and prescribe
boundary conditions for the models described above. Additionally, the data
are to be used for monitoring the global environment and testing new hypotheses
and for diagnostic or retrospective studies.
· To conduct manipulative experiments, field campaigns, and process
studies to improve our understanding of the processes controlling the trans-
fer of energy, water, heat, and carbon dioxide between the land surface and
the atmosphere at appropriate scales and to develop better methods for
quantifying the controls.
Figure 5.1 shows the relationship between the modeling, the process
studies (experiments and laboratory work), and the large-scale data acquisition
program proposed here, and the interdependence and coordination needed
among these activities. In general, the modeling studies are intended to
distill the results of process studies to provide a realistic predictive capability.
The model predictions can be compared with field experiment observations
on local scales and with data from the long-term monitoring effort and
global data sets on regional and global scales, respectively. The flow of
information should be two-way, as the results from the modeling activities
will determine which variables should be observed at which time and space
scales.
To achieve these goals, two things must be done. First, the existing
research activities must be adjusted and focused to promote the coordinated,
interdisciplinary studies necessary for the collection of specialized data sets
and the construction of a new generation of models. Second, efforts must
be undertaken to address the most crucial resource as of now, which is not
hardware or data, but trained scientists.
The remainder of the chapter addresses the particulars of data needs,
modeling, infrastructure, and resources, with particular emphasis on those
areas that are weakly supported at present (see Tables 5.1, 5.2, and 5.3 for
activities in monitoring, manipulative experiments, and field experiments,
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WATER -ENERGY-VEGETATION INTERACTIONS
1 1
Model Construction
and Testing
Manipulative ~Field ~Process 10
Expenment s Cn m paigns S tudies
. Fine spatial scaled
discontinuous J
_I Integer ated Expenmental Ef Sorts |
| Hydrology, Ecology, Biogeochemistry
Lon~term Monitoring Studies
Global Data Set Acquisition l
133
Monitonng
pLarge Spatial Scale,
l continuous J
FIClURE 5.1 Relationship between modeling, process studies, and over data acqui-
sinon activities. The numbers on the right-hand side of the figure denote die ap-
proximate number of field sites worldwide.
respectively). The sections on data needs and modeling follow the structure
outlined in Figure 5.1 by first reviewing global data set needs and then
reviewing data needs at progressively finer spatial and temporal scales. It
should be remembered throughout that the proposed strategy was conceived
as a whole and that coordination of the component activities represents a
significant challenge by itself.
DATA NEEDS AND EXPERIMENTS
A wide range of data needs to be gathered, processed, and integrated to
provide an information base for modeling and diagnostic studies of the
earth system. These include satellite, atmospheric, and in situ observations
operating on an extensive and more-or-less continuous basis to provide
"monitoring" information, and focused, coordinated observations the product
of field and in vitro experiments to provide the insight necessary for model
and algorithm development. These two kinds of data sets should be regarded
as complementary, as the monitoring data sets will partially determine the
list of items to be addressed by experiment and the experimental results
should lead to improvement of the data processing methodologies applied to
the monitoring data set.
It should be remembered that all types of land cover need to be ad
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134
TABLE 5.1 Monitoring Programs (Ongoing)
RESEARCH STRATEGIES FOR THE USGCRP
Activity Program Agency/Coun~y
Meteorological data Climate and global change NOAA, national
program meteorological
agencies
Satellite data products National Environmental NOAA, national
Satellite, Data, and meteorological
Information Service and agencies
weather service
Earth radiation budget
Cloud climatology
Earth Radiation Budget NASA
Experiment
International Satellite Cloud NASA
Climatology Project
Vegetation index (AVHRR) GIMMS; National NASA, NOAA
Environmental Satellite,
Data, and Information
Service; and other national
programs
Retrospective land cover IRAP/Intemational Satellite NASA
studies Land Surface Climatology
Project
Soil moisture, vegetation Goddard Space Flight Center NASA
cover
Snow and ice Goddard Space Flight Center NASA, NOAA
Runoff ? WCRP
Carbon dioxide Background Air Pollution NOAA
Monitoring Network
TABLE 5.2 Manipulation Experiments/Gradient Studies (Ongoing)
Activity
Program
Agency
Carbon dioxide enrichment,
small crops
Plastic ecosystem response,
remote sensing
Effect of land use changes
on hydrology (forests)
Effect of land use changes
on hydrology (forests)
and nutrient treatments
Florida Area Cumulus DOE/USDA
Experiment
Experimental lakes area
. · .
universities
Experimental lakes area
Valdai, RSFSR
Hubbard Brook, Coweeta
Canada
USSR
USA
Crop and pasture WorldwideAgricultural
fertilization agencies
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WA TER -EVER G Y- VEGETA TI ON INTERA C TI ONS
TABLE 5.3a Field Campaigns (Ongoing)
135
Lead
Agency/
Activity Date Program Location Country
Large-scale 1986 HAPEX France France
hydrometeorology
(100 km)2
Energy balance, 1987, 1989 FIFE Kansas, USA NASA
biophysics, 1988 on KUREX Kursk, USSR USSR
meteorology,
remote sensing
(15 km)2
TABLE 5.3b Field Campaigns (Planned)
Lead
Agency/
Activity Date Program Location County
Global Energy
Water Cycle
Experiment
Arid zone,
energy, water
cycle, remote
sensing (100 km)2
Arid zone,
energy, water
cycle, remote
sensing (15 km)2
Tropical forest,
energy, water
cycle, remote
l990s
Global Energy
and Water Cycle
Experiment
1992 HAPEX-II
1991 IFEDA
Global WMO
Niger
France
Spain European
Community
1990s ABRACOS Brazil U.K., Brazil
sensing
Boreal forest, 1994 BOREAS
energy, water
cycle, tropospheric
chemistry, remote
sensing
Canada NASA, Canada
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RESEARCH STRATEGIES FOR THE USGCRP
dressed by the data acquisition effort and experiments, not just "natural,
undisturbed" ecosystems. In particular, agricultural systems require a high
priority as targets for studies at all levels monitoring, process studies, and
modeling. Integration with national agricultural research programs will be
essential.
Particular needs for each kind of data set are discussed below.
Global Data Needs
Global data are needed for the specification of boundary conditions for
global models as well as for the framework for analyzing and detecting
global change. The data would come from extensive ground and aircraft
surveys as well as from satellites. It should be stated at the outset that for
satellite data to be useful for detecting and monitoring interannual and longer-
term changes (1) calibration of satellite data must be of the highest priority,
(2) rigorous correction for atmospheric effects and for viewing geometry
must be performed, and (3) the calibration and correction procedures must
be carried out by data centers and information systems to make the products
available to the scientific user community in a timely fashion.
In terms of looking for global change indicators, it should be remembered
that a large archive of satellite data (20 years' worth) already exists. More
resources should be made available to study these records to determine
whether changes or the effects of changes can be detected at a usable level
of accuracy.
The following data are required:
.
Satellite data. The most obvious need is for the integration of existing
techniques into information systems that can deliver calibrated, geometri-
cally corrected, atmospherically corrected, and registered data products to
the scientific user community in a timely fashion. Until this is done, remotely
sensed data will continue to be underused, if not unused, as research scien-
tists are forced to complete the whole task themselves from satellite sen-
sor counts all the way through to derived products and only then toward
their own scientific goals. Better means must be found.
It is proposed that more than one effort be initiated to address this task.
To be effective, individuals involved will have to form a wide pool of
talents and be drawn from a diverse population instrument engineers, atmo-
spheric physicists, scientific user groups, and information scientists and
will also have to work across agencies. They will have to develop a means
for selecting and implementing satellite data algorithms for operational processing
of the data stream. The development of these information systems is probably
the highest-priority task facing the community at present.
The research on satellite data sets involves the continuing improvement
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WATER -ENERGY- VEGETATION INTERACTIONS
137
of techniques for obtaining area-averaged parameters from coarse-resolu-
tion data. More research needs to be done to apply scaling methods so that
knowledge gained from infrequent high-resolution data (e.g., Landsat) can
be applied to the continuously acquired global coverage data sets (e.g.,
Advanced Very High Resolution Radar (AVHRR)~.
· Land transformation data. Any kind of large-scale land transformation
needs to be documented in a uniform way as part of the monitoring program.
If possible, this should be connected with the satellite data acquisition effort.
Topographic data. At present there are no reliable high-resolution
topographic data sets for the globe. The resolution of available global data
sets for the land surface is either 5 or 10 minutes, depending on the continent.
Present data are inadequate for many land surface studies. There is a need
(1) to compile, archive, and make available additional existing topographic
data; (2) to prepare new topographic maps from space-borne (e.g., Systeme
Probatoire d'Observation de la Terre (SPOT)) data; and (3) ultimately to fly
a dedicated mission (or missions) to acquire a coherent set of topographic
data that can be made available to earth scientists in readily usable forms.
Vegetation data. There are several digital data sets of vegetation for
the globe. The spatial resolution ranges from 0.5° x 0.5° to much coarser
for the globe. The resolution of vegetation information is much coarser,
and ranges from about 10 blames to more than 150 vegetation types. Some
of these vegetation data sets represent potential or climax vegetation, and
others include land use and modification. The accuracy of the data sets has
to be improved. The feasibility of land surface classification using satellite
data has now been demonstrated. Improvement of this new technique in
conjunction with ground surveys is critical, not just for mapping vegetation,
but also for developing the capability for detecting changes in the structure
and function of vegetation.
It should be recognized that the appropriate grouping of vegetation types
for evapotranspiration and water cycling may be different from the appropriate
grouping for, say, surface energy balance. Research into appropriate classification
schemes is needed.
Vegetation function data. The phonology of vegetation is important
for determining the timing and amount of water released through the land
surface. There is limited tabular information on leaf area indices for different
vegetation types. The normalized difference vegetation index (NDVI), available
from polar orbiting satellites, gives information on the seasonal march of
vegetation greenness globally and should be applied to the water studies. In
order that the NDVI be used for detecting interannual and longer-term changes
in phonology and water cycling, calibration of satellite instruments and
rigorous correction for atmospheric effects must be of the highest priority.
These studies should be carried out in conjunction with surface validation
efforts (see following sections).
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138
RESEARCH STRATEGIES FOR THE USGCRP
· Soil data. Most global soil data sets are digital versions of the Food
and Agriculture Organization (FAO) soil maps. Improvements to soil data
sets are on the research agenda of several international groups (e.g., Soil
and Terrain Data Base (SOTER) of the International Soil Science Society).
Soil units, texture, and other parameters in the FAG soil maps are important
for hydrologic studies, as are hydraulic potential and rooting depth of vegetation.
Some information about these parameters has been gleaned from the litera-
ture, but an extensive survey of different vegetation-soil complexes should
be carried out.
· Soil moisture data. The temporal and spatial variations of soil moisture
are not well known. In most models, the soil-water-holding capacity asso-
ciated with different vegetation types is specified, but there is no observed
climatology of the field capacity. Field capacity climatologies derived as
residuals from budget calculations suffer from inaccuracies in the other
terms in the water budget, such as rainfall and evapotranspiration. The
capability for measuring soil moisture from space, especially in the presence
of vegetation, must be developed as far as possible. Meanwhile, because it
is impossible to map soil moisture distributions globally, surface-based studies
should be carried out to understand the vegetation and soil characteristics
that determine soil moisture amount and its spatial variations (see the section
"Remote Sensing" below).
· Meteorological data. Near-surface atmospheric humidity, rainfall, near-
surface air temperature, and wind speed are the driving forces for atmosphere-
surface fluxes of water vapor. At present, the weather station network is
the primary source for such data. Such networks must be maintained at
mid-latitudes and expanded in polar and tropical regions. Technologies for
automating these weather stations exist and should be further explored and
developed, including such technologies as telemetric tipping rain gauges
and ceilometers. Commonly, these data are used by the National Meteoro-
logical Center and then discarded. Efforts should be made to integrate some
of these data into a research data base.
· Rainfall data. Rainfall varies on small scales. It also exhibits a
distinct diurnal cycle, which varies geographically. Quantifying and understanding
this variability are important to the scaling-up of local site studies to the
gross resolution of global models. Thermal infrared radiation at the top of
the atmosphere has been shown to be a useful index of convective precipitation,
which is a source of small-scale variability. Existing satellite data (e.g.,
from AVHRR) should be analyzed to investigate the spatial and temporal
variability of precipitation and to determine the appropriate precipitation
statistics to describe rainfall variability on a global scale.
Microwave measurements of precipitation from satellites offer a real hope
for providing a global rainfall climatology. More efforts should be committed
to research and validation in this area.
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WA TER -ENER G Y- VEGETA T10N INTERA C TI ONS
139
· Runoff data. River runoff determines the freshwater inputs to the
world oceans. Broecker (1989) has suggested that changes in runoff may
trigger changes in deepwater formation and consequently changes in cli-
mate. The USGS has an extensive gauge network that measures, among
other things, daily flow rates of the rivers and tributaries in the United
States. Similar gauge data have yielded a long-term record of the flow of
the Amazon River. Such networks must be maintained, and if possible,
expanded to other major rivers of the world.
For closed drainage basins (see the section "Integrated Monitoring and
Process Studies" below), river flow data must be analyzed in conjunction
with contemporaneous precipitation and other weather data to develop and
test hydrological models and to determine the frequency and~accuracy of
river flow data for global change.
· Surrogate and corroborative data.
As there are several sources of
water vapor to the atmosphere, the unique signatures of each water exchange
process will provide information to validate models and hypotheses of the
terrestrial hydrological cycle. These signatures include oxygen and hydro-
gen isotopes of water. Also, for C3 plants, the simultaneous exchange of
hydrogen oxide and carbon dioxide through stomata makes the temporal
variation of carbon dioxide and its stable isotopes carbon-13 and oxygen-18
critical cross-checks for evapotranspiration.
· Photometric data. Standardization, coordination, and augmentation of
sun photometry measurements are needed to allow the collection of atmospheric
optical thickness data. These data are required for the routine atmospheric
correction of satellite data.
Long-Term Monitoring
A global network of minimally instrumented sites is needed to provide
data for
diagnosing the effects of climatic change over long periods of time,
· testing basic models and hypotheses, and
· anchor stations to perform satellite algorithm inversion.
The first objective, diagnosing the effects of climatic change, will re-
quire different measurement strategies in different parts of the world. These
effects may manifest themselves as changes in characteristics such as vegetation
cover or composition, mass and heat fluxes, or the hydrological balance.
Sites need to be identified that can act primarily as "barometers" that will
reflect the impact of climatic shifts on different ecosystems while also pro-
viding data for the second objective, the long-term validation of models.
This large, diffuse network of stations should total several hundred sites
worldwide, and therefore optimal use will need to be made of existing
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RESEARCH STRATEGIES FOR THE USGCRP
resources and monitoring programs such as the Long-Term Ecological Re-
search (LTER) network or the proposed observatories of IGBP and WCRP.
The range of sites should encompass the world's major vegetation types and
biomes, including agricultural systems.
The long-term monitoring stations should be located at the centers and
edges of ecosystems. This arrangement would provide data that may indi-
cate the resilience of ecosystems to changes as well as the shifts on the
borders where initial changes could be expected. The full benefit of the
long-term data sets will be realized only if sufficient resources are set aside
for basic research that will analyze the data sets. Furthermore, it will be
essential to provide adequate funds for technical personnel to maintain good
quality data collection programs over an extended period.
Long-term monitoring of selected hydrological, climatological, and chemical
variables has been conducted by a variety of agencies in the past. A reexamination
of these monitoring efforts is required with a view to extending and coordinating
the monitoring activities. Some of the automated data collection devices
may be modified to record additional variables.
At minimum, these sites should be committed to collecting meteorologi-
cal data and conducting periodic surveys of the vegetation and soils in their
locale.
Integrated Monitoring and Process Studies
Out of the larger set of long-term monitoring sites, a few (fewer than one
hundred worldwide) special sites should be selected for integrated studies.
There should be a significant and integrated scientific commitment to these
sites in the areas of hydrology, biogeochemical cycling, ecology, and satel-
lite monitoring.
Facilities at these sites serve three purposes. First, they interface with
operations at the next higher level of intensity (field campaigns and integrated
ecosystem experiments) by providing starting points with infrastructure and
background information sufficient to ensure high returns. Second, for fa-
cilities at the next lower level of intensity (long-term monitoring sites),
studies at the higher-intensity sites will identify variables to monitor, define
appropriate sampling intervals, and validate integrated models tailored to
the specifics of each biome. Third, these facilities for high-intensity moni-
toring will be the primary barometers for changes in subtle parameters that
affect aspects of ecosystem function without immediate or profound effects
on structure.
It is anticipated that the energy, water, and carbon balance models to be
tested using data sets from these sites will utilize meteorological data (e.g.,
temperature, humidity, precipitation, wind speed, and radiation) to provide
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WATER-ENERGY-VEGETAT10N INTERACTIONS
141
the forcings and satellite data to provide the slowly changing surface boundary
conditions (e.g., photosynthetic capacity and soil moisture estimates).
An ultimate goal for this measurement-modeling exercise is to develop
the models that can be driven by the combined remote sensing and meteoro-
logical data sets to calculate fields of surface fluxes and associated forcings
on the important ecosystem processes over the whole globe.
Accordingly, the proposed criteria for site selection are as follows:
· located within important biome centers or on biome transition zones,
· existing long-term research archive, and if possible, a paleoecological
record (see chapter 3),
.
nearby research institute for site support,
"feasible" topography for flux measurement,
· presence of gauged or gaugeable watersheds of a reasonable size (5 to
20 km2),
and
away from excessive anthropogenic impacts, e.g., large-scale air pollution,
.
logistics: airfields nearby and road access.
To satisfy the modeling requirements, the stations would routinely acquire
the following kinds of data:
.
hydrological,
meteorological (including radiation balance),
ecosystem structure and productivity,
land use and soil information,
atmospheric optical depth,
large-area surface fluxes (tower),
trace gas concentrations, and occasionally fluxes, and
selected satellite data.
Most of the data should be taken within a concentrated area of roughly
20 x 20 km, which allows a reasonable area for the sampling of satellite
observations and airborne flux measurement. However, this core site should
be located within a larger similar zone (100 x 100 km), which could be used
for studies of the spatial and temporal variability of some of the parameters.
Hydrological data. Various components of the hydrological cycle at
local and regional scales are expected to respond to climatic shifts. There-
fore studies of snow pack dynamics should include analyses of snowline
retreat patterns over time and space. The timing of runoff originating from
snow packs may be a significant indicator of general atmospheric warming
at higher elevations.
Reservoir levels (minus consumptive use) can be taken as a long-term
integrated measure of large-scale (regional) water surplus or deficit. Reser
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WATER -ENERGY-VEGETATION INTERACTIONS
153
and exchanges of sensible and latent heat and carbon dioxide (biophysical
control of evapotranspiration and photosynthesis). Essentially, the LSPs
assume a static ecosystem structure and a prescribed phonology, which in
turn define the albedo and roughness length characteristics of a given area
and the evapotranspiration response as a function of soil moisture. Gener-
ally, the surface vegetation type, and hence albedo and roughness length, is
prescribed from data, and the soil moisture field is initialized from offline
climatological studies. As a result, these models in their current state have
a limited utility for the study of global change because they merely represent
an improvement over the abiotic "bucket" models described by Budyko
(1974) and Eagleson (1982~.
General circulation models of the atmosphere have improved considerably
over 30 years of development to the point where they are the preferred tools
for weather prediction and the study of climate. However, it is clear that
these models will be subject to certain limitations for the foreseeable future.
Most importantly, the models will be limited in terms of spatial resolution,
representation of small-scale (subgrid scale) processes, and duration of run.
It should also be remembered that each model possesses its own climatology,
which differs from reality, and that an adequate description of the model
climatology normally requires an extended series of runs. The normal GCM
time step is on the order of 10 to 30 minutes (times much longer than this
can lead to serious systematic error in the description of dynamical or physical
processes), and this effectively limits the number or the length of runs that
can be executed and analyzed by a research group.
Ecosystem dynamics models operate in an entirely different time and
space domain from the LSP-GCM combinations, generally working on small
spatial scales (meters to kilometers) and long time scales, integrating over
centuries or millennia with time steps of up to one month. (For the time
being, the discussion will exclude the "biogeographical" type of model,
which describes the continental or global distribution of vegetation forma-
tions on the basis of "mean" climatology. These descriptions are not dynamic,
as they operate as direct single-solution transforms of imposed climatic
fields.)
It is likely that global change in the real world will affect elements of all
three systems, as shown in Figure 5.4. The sequence of changes could be as
follows:
1. A change in the physical climate system would bring about a direct
biophysical response from the biota. For example, near-surface temperature
or humidity changes would have a direct impact on photosynthesis and
evapotranspiration rates (fluxes).
2. Changes in the surface biophysical response would directly affect the
near-surface climate. The resulting feedback could be positive, neutral, or
negative, depending on the circumstances.
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154
RESEARCH STRATEGIES FOR THE USGCRP
/~
_ ~
PHYSICAL CLIMATE
SYSTEM
~ Heat, Mass
| B | Fluxes
BIOPHYSICS
Albedo
Roughness
Hydrology
~ CO2, CH4 Release
IBI
BIOCHEMISTRY
~Nutrient,
\ / Carbon
\r) Pools
Tem rature,
Beater l l
Relations JO
~C~
ECOSYSTEM STATE
Community Composition,
Structure, Pedology
FIGURE 5.4 Important interactions between the vegetated land surface arid Me
atmosphere with respect to global change. (a) Influence of charges in the physical
climate system on the biophysical characteristics and ecology of the biome. (b)
Changes in nutrient cycling rates; release of carbon dioxide arid methane from soil
carbon pool back to We atmosphere. (c) Ecological change in species composition
results in changes in land surface characteristics of albedo, roughness, and soil
moisture availability with possible feedbacks on near-surface climatology.
3. Changes in the near-surface climate or the surface biophysical re-
sponse would have a direct impact on the forcing functions acting on ecosystem
dynamics. Obviously, changes in ecosystem processes would be the first
manifestation (e.g., rates of decomposition and mineralization), but these
could be followed by changes in the gross structure (e.g., species composi-
tion and standing biomass). The degree, if any, of such changes is again
highly variable and dependent on the locale and site history.
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WA TER -ENER G Y- VEGETA TI ON INTERA C TI ONS
155
4. Changes in ecosystem structure and function can be expected to feed
back onto the physical and possibly the chemical climate system.
As yet, the tools to investigate these phenomena have not been integrated
in a scientifically defensible way. Although there has been some progress
in coupling LSPs with GCMs, it is highly unlikely that either will be directly
coupled with EDMs in the near future because of the gross disparity in time
and space scales.
It can be convincingly argued that direct coupling is highly undesirable
in any case. To force EDMs, many of which incorporate stochastic descriptions
of ecological processes, good representations of the mean and variability of
a region's climate must be applied: thus there is a need to repetitively
apply many variations of a climatology to an EDM before a credible ensemble
of results can be collected. In addition, the spatial scale of most EDMs is
not consistent with that of GCMs: the answer, of course, is not to increase
GCM spatial resolution to finer and finer scales, as this would result in
problems similar to those discussed regarding temporal scales. For both of
the above reasons it is clear that direct links between LSP-GCMs and EDMs
are both impracticable and undesirable. However, before significant progress
can be made in the area of medium- to long-term atmosphere-biosphere
interactions, it will be necessary to construct more rigorous linkages be-
tween the LSP-GCMs and EDMs. It is proposed that this be done by
constructing "forcing modules," to convert GCM output into "forcing"
climatologies for EDMs, and "aggregation modules" to aggregate the effects
of ecosystem dynamics changes into representative LSP parameter sets.
In spite of the general state of modeling described above, every effort
should be made to support model development in every direction~CMs,
LSPs, and EDMs-as these represent the greatest opportunities for predict-
ing the mechanisms and effects of global change. Some specific needs that
merit special attention or that have not been addressed in previous reports
are listed below.
Intermodel Transfer Packages
As discussed above, two kinds of intermodal transfer packages (ITPs) are
needed the first to allow communication from LSP-GCMs to EDMs and
the second to allow communication in the other direction.
The first, the forcing module, would accept GCM output and generate
the requisite "climate" for EDM applications. The module should take into
account the following:
· Biases due to GCM climatology.
· Effects of GCM resolution and the parameterization of subgrid-scale
processes.
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RESEARCH STRATEGIES FOR THE USGCRP
· The likely range of microclimates produced by variations in topogra-
phy, soil moisture, and pedological effects.
· The frequency and type of "extreme" events associated with a clima-
tology in addition to the description of the mean condition.
.
Results obtained from process studies and monitoring data sets.
Forcing modules would necessarily have to incorporate some background
knowledge of the GCM's structure and performance. In this sense, they
would be far more than simple extensions of quasi-stochastic "weather generators."
Aggregation modules would be used to analyze the results of EDM runs
and would generate the requisite grid-scale parameters for LSP-GCMs. To
a degree, the aggregation module is an inversion of the forcing module in
that it attempts to generalize and integrate the specific and different results
of EDM runs. The modules should take account of the following points:
· Integration of physiological characteristics probably cannot be done in
a linear, arithmetic fashion. The "importance" of the contributions of dif-
ferent organisms to various fluxes, and so on, must be taken into account.
· The impact of spatially varying soil moisture should also be integrated
to take account of nonlinear effects on the surface fluxes.
.
Where appropriate, the effects of landscape pattern, e.g., repeating
topographic units, should be integrated using ensemble averaging techniques.
Phenological Descriptions for LSPs
As discussed above, it is impracticable to place full EDMs within GCMs.
However, some elements of vegetation phonology could be formalized and
placed within LSPs. In particular, the following physiological phenomena
should be described as functions of GCM prognostic variables (temperature,
humidity, soil moisture, radiation, and so on): time series of green leaf area
index; rooting depth; and roughness length, albedo, photosynthetic capacity,
and maximum canopy conductance if these are not functions of leaf area
index and rooting depth.
The models should be able to describe the seasonal course of vegetation
attributes and provide a crude response to large interannual variations in
precipitation.
Hydrological Models
Land hydrological modeling is currently split into two effectively
noncommunicating camps:
· "Wet" hydrology, an extension of the traditional hydrology, which
had its roots in engineering applications (e.g., channel routing and storm
flow response). Many recent research efforts have been focused on small
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WATER -ENERGY- VEGETATION INTERACTIONS
157
or regional-scale catchment models, with spatially distributed descriptions
of rainfall interception, overland flow, and infiltration. Usually, these mod-
els have fairly simple evapotranspiration descriptions.
· "Green" hydrology, mainly concerned with the study of the biophysics
of the evapotranspiration process using soil-plant-atmosphere models. The
same basic models have been applied to describe processes on small-scale
(agricultural) sites up to the scale of GCM grid areas.
Clearly some linkage between the different kinds of models is required.
In particular, greater efforts must be expended to make the biophysical
models into better descriptions of spatially heterogeneous surfaces. Also,
some distillation of the wet hydrology models must be introduced into the
LSPs: currently, all the LSPs use simple one-dimensional descriptions of
the infiltration process with very simplistic representations of overland flow
and spatial heterogeneity. The goal should be to provide good descriptions
of total runoff losses as integrated over a month or more, rather than the
extremely difficult objective of reproducing the correct timing of runoff
losses.
Surface/Planetary Boundary Layer Models
Many of the problems in describing realistic feedback mechanisms be-
tween the surfaces and troposphere are associated with the description of
mixing processes in the planetary boundary layer. Correct description of
these is also important for some modeling inversion techniques driven by
satellite remote sensing (e.g., determination of surface heat fluxes using
meteorological and satellite data). Modeling efforts to address both goals
should be encouraged.
Ecosystem Structure Models
The models discussed in previous sections can describe the effects of
changes in land surface biophysical parameters (e.g., albedo, roughness,
and moisture availability) and biogeochemical properties (e.g., nutrient cy-
cling rates) as they are defined by a given ecosystem status (e.g., species
composition, soil microecology, vegetation health, and leaf area index).
Modeling techniques for describing ecosystem structures must be developed
in parallel with the models discussed in previous sections. These models
should address the issues of alteration of ecosystem structure due to changes
in (1) the physical climate system, (2) atmospheric chemistry, and (3) land
use change.
Depending on the intensity and type of change imposed in a given re-
gion, the ecosystem structure may be altered slightly (e.g., by adjustments
in carbon dioxide exchange rates) or drastically (e.g., with changes in species
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158
RESEARCH STRATEGIES FOR THE USGCRP
composition or leaf area index). All types of changes may feed back onto
the physical or chemical climate system (see Figure 5.4).
Prediction of terrestrial ecosystem change can be approached by using
biogeographical methods as well as by using "full-up" ecosystem simula-
tion models. It is strongly recommended that research efforts be encouraged
on a broad front: the preliminary biogeographical models will provide us
with the means to explore the possible sensitivity of land-surface- atmosphere
interactions to changes in surface conditions. This approach is represented
by Loop I in Figure 5.5, where a climatic change scenario leads to a simple
definition of a new (steady state) distribution of biomes, which is then fed
back through the climate modeling process to test for second-order effects
due to the induced change in surface cover. Results from this kind of study
will indicate which regions and blames are important in terms of inducing
second-order effects and thus merit further studies and interaction using
kinetic ecosystem structure models (see Loops II and III in Figure 5.5~. In
this respect, none of the models should be regarded as an ultimate replace-
ment for the rest; they have different roles depending on the level of detail
required by either the climate modeling effort or the interests of biologists
working on the effects of global change. In all cases the intermodel transfer
packages discussed in the section above will have to be used as communication
models.
Radiative Transfer/Plant Physiology Models
A number of modeling efforts have been partially successful in retrieving
vegetation attributes from remotely sensed optical data. For the most part,
these have concentrated on obtaining values for biometric properties such
as leaf area index or biomass. More recently, efforts have been made to
calculate physiological properties from observed radiances, including canopy
(area-averaged) photosynthetic capacity and stomata! conductance. Research
continues in the use of radar and passive microwave instruments for interpreting
vegetation properties, but so far these efforts have focused on the retrieval
of biometric properties and the classification of landscapes into different
cover types.
The field of remote sensing of biospheric functioning is on the verge of
providing invaluable information for the study of global change. The key to
progress is clearly in the development of satellite data algorithms that can
calculate appropriate states and rates associated with the terrestrial vegetation
along with estimates of the uncertainty attached to each derived value. Such
algorithms will have to address the following problems: satellite sensor
calibration, sun-target-sensor geometry and the effect of atmospheric scat-
tering, radiation transfer within the vegetation canopy and soil, and the
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WA TER -ENER G Y- VEGETA TI ON INTERA C TI ONS
( GIS ~
o
._
CD
in
3
o
-
CtS
in
-
C\5
in
Cot
on
to
-
Ct
-
159
(a DESIGNATE MODEL REGION
(I) Define initial and
boundary conditions
Select
scenario
-
~Large scale ~
(correlative modelJ
(O Define new climate
or other changes on
environmental template
l
(O Create 1 st approximation
of new vegetation
(no kinetics)
Smallscale ~
simulation modelJ
Create 2nd
approximation of
corrected new vegetation
(1 st order kinetics)
~ 1
Application of
knowledge of animal soil
spersal landuse effected
-
111
Create 3rd approximation
(finer kinetics)
Output to ecosystem
function models
FIGURE 5.5 Proposed approach for modeling vegetation (ecosystem) charge sub-
ject to large changes in forcing functions.
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160
RESEARCH STRATEGIES FOR THE USGC8P
relationship between canopy scattering properties and relevant vegetation
properties.
Although parts of the above problem can best be addressed by special-
ized researchers working in a loose confederation, the final goal of an integrated
radiance-to-surface parameter algorithm must be kept firmly in mind. The
achievement of the goal will require coordination of the talents of scientists
working in several different disciplines: remote sensing technology, atmospheric
physics, radiative transfer, plant physiology, and modeling. Ultimately, the
effort could provide scientists with the means to calculate carbon, water,
and energy fluxes over the land surface from satellite data.
Soil Genesis Models
Many of the models discussed above require some basic information
about soil properties for their operation-soil physics properties in the case
of GCMs and soil optical and nutrient properties in the case of the canopy
radiative transfer/physiology models. Some of these properties can be derived
using soil genesis models, which require data on the parent material, climatological
regime, and vegetation cover as input. While these models cannot provide
definitive production in most cases, they could have potential in terms of
filling the gaps between reliable observations.
Sensitivity Analyses
All of the above sections have addressed the need to advance the realism
and sophistication of different modeling efforts, in essence forming a "broad
front" approach to the component parts of the issue of terrestrial biosphere-
atmosphere interactions. Another important task must be coordinated with
all of these a sensitivity study on the effect of errors or uncertainties in
the input data set of each model on the calculated product. Ultimately, it is
hoped that all the individual models will provide products that can be used
as input or validation for other models. For example, the products from the
remote sensing algorithms could be used to prescribe surface conditions for
GCM studies.
To make the best use of research resources, including money, time, equipment,
and personnel, it is important that the sensitivity of each class of models to
variations in their input parameter set be well understood. For example, if
analyses indicate that GCM-simulated climates for continental interiors are
sensitive to the successional stage of the vegetation cover there, it would be
highly desirable to increase the flow of resources for improving the ecosystem
dynamics models, which could then provide more realistic boundary condi-
tions for the GCM. For this and similar problems, a gradualist approach to
the sensitivity problem should be used. Simple schemes or parameter prescriptions
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WATER-ENERGY-VEG~ATION I=ERACTlONS
161
should be used to determine the sensitivity of the model in question to
variations in the input parameter set. The results of these basic tests should
determine whether or not to invest heavily in more sophisticated approaches.
Summary
The above analysis has called out the need to vigorously promote re-
search efforts in a few areas of obvious weakness. However, it should be
reemphasized Hat modeling efforts in all the relevant areas should be supported
to a much greater extent than they are now; these include the ecosystem
dynamics models, land surface parameterizations, and atmospheric general
circulation models. The descendants of current models will be the tools for
understanding and predicting global change.
INFRASTRUCTURE
The modeling and data gathering tasks discussed in previous sections
will not contribute to the overall goal of understanding and predicting global
change unless there is a continuing effort to coordinate the activities. De-
fining the form of a governing coordinating body is beyond the scope of
this document, but describing its purview is a necessity.
Operational Observations
There is no question that the array of operationally acquired meteorologi-
cal, oceanographic, space-based, and other data is invaluable for earth sys-
tem studies. However, most of the existing networks are not suitably configured
for this work (e.g., aviation forecasting dominates many meteorological
activities), and insufficient resources are dedicated to storing the data. All
operational systems need to be considered as possible contributors to the
earth system science effort, and a means of prioritizing and storing important
data types needs to be formalized. Assembling a self-consistent long-term
record of variables important for earth system science will require the
implementation of new measurement networks, new information systems,
and a high degree of collaboration among agencies.
Satellite Data Processing
The need to produce useful satellite data products for the scientific com-
munity has been emphasized above. A mechanism is needed to specify the
list of desired products, with associated accuracy and precision requirements,
and transmit this to the agencies so that resources, facilities, and personnel
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162
RESEARCH STRATEGIES FOR THE USGCRP
are dedicated to the task. This need is as urgent as the need for the devel-
opment and launch of a new generation of instruments.
Centers for Research and Monitoring
Centers for research and monitoring should be the sites where more in-
tensive, coordinated experiments and scientist training, as well as continu-
ous monitoring-type observations, take place. An international effort should
be made to establish such centers so that these research tasks are directly
addressed. The centers should be sited in areas that are representative of a
large and important vegetation formation, a "sensitive" area (e.g., a transition
zone), or a benchmark area where there is a long research history and
archive. To address the goals of ensuring a continuous, high-quality moni-
toring effort while suitable for intensive field experiments of the scale of
FIFE or larger, there should be permanent research staff attached to each
center. These staff, in cooperation with visiting scientists, should also carry
out an educative and training function.
Education
As noted in other chapters, there is currently a critical shortage of trained
researchers to carry out the task of earth system science research. A coordinated
approach is required to recruit good students into the field and to train them
to be able to participate in interdisciplinary research. This will take money,
effort, and organization.
Interagency and International Coordination
There is a need to integrate the planning and implementation of measure-
ment networks, modeling efforts, experiments, and education at the interagency
and ~nterna~aonal levels. This requires He interlocldng of experienced bureaucrats
· · · .
ant ~ practicing scientists.
Coordination among most large-scale experiments is usually fairly haphazard,
and thus over-redundancy and gaps continue to plague their operational
implementation. A central clearinghouse, or at least an information exchange,
would be useful. Such clearinghouses lead to better coordination in the use
of instruments and personnel.
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Representative terms from entire chapter:
climatic change
