B33D-1571 [WITHDRAWN]
Leaf Gas Exchange in Relict Spruce-Fir Cloud Forests of the Southern Appalachian Mountains, USA.
The relict spruce-fir (Picea rubens Sarg. - Abies fraseri (Pursh) Poir.) forests of the southern Appalachian mountains are found only on high altitude mountain tops that receive copious precipitation (>2000 mm annually) and experience frequent cloud immersion (~65% of the total growth season days). Cloud deposition accounts for up to 50% of the annual water budget for these high-elevation forests. Two sites in North Carolina were established to investigate the influences of cloudiness and cloud immersion on leaf gas exchange and water relations of Fraser fir: Mt. Mitchell (2028 m elevation) and Roan Mtn., NC (1890 m elevation). It was hypothesized that the cool, moist, and cloudy conditions at these sites would exert a strong influence on leaf carbon and water fluxes. Water status was high throughout all hours on measurement days, with xylem water potential always >-1.75 MPa and soil water content always >0.1 m3 m-3. Leaves were wet frequently (>60% of all hours) due to cloud immersion and nightly dewfall, which did not appear to limit photosynthesis, but may influence stomatal response and transpiration. Maximum photosynthesis (Amax) was about 15 umol CO2 m-2 s-1, and saturated at sunlight levels between 400-500 umol m-2 s-1. Maximum leaf conductance (gmax) and transpiration (Emax) were 0.31 mol m-2 s-1 and 3.9 mmol m-2 s-1, respectively, and were strongly associated with LAVD. At both sites, conductance and transpiration decreased exponentially as LAVD increased, with 50-75% reduction between 0-0.5 kPa. Mean instantaneous water use efficiency on clear days was 3.5 umol CO2 m-2 s-1/mmol H2O m-2 s-1 across all transpiration fluxes, but increased on cloudy and cloud-immersed days (range of 2.3 – 6.0 umol CO2 m-2 s-1/mmol H2O m-2 s-1) as transpiration increased. Leaf gas exchange appeared tightly coupled to the response of conductance to LAVD which maintained high water status, even at the relatively low LAVD of these cloud forests. Thus, the cloudy, humid environment of these refugial forests appears to exert a strong influence on leaf gas exchange and water relations of these montane species. Because global climate change is predicted to increase regional cloud ceiling levels, more research on cloud impacts on carbon gain and water relations is needed to predict the future survival of these relic forests.
B33D-1572
Will stomatal behavior lead to a positive or negative feedback to anticipated climatic change?
Some coupled climate-vegetation models use the Ball-Berry empirical relationship to simulate the effect of environmental conditions on stomatal conductance. This can lead to a positive feedback in the model, wherein the simulated vegetation closes its stomates due to higher pCO2 concentrations or water stress, resulting in less moisture cycling, enhancing stomatal closure, and eventually contributing to a non-vegetated state. Previous studies at the leaf, canopy, and regional scales have suggested that stomata function optimally to maximize carbon gain. Optimal stomatal functioning would suggest that stomata will react with a negative feedback to climatic change. Here we compare three transient climate-vegetation simulations forced with prescribed pCO2 concentrations over the 1850 -2100 time period. In the control simulation, we maximize the gross primary productivity with respect to the stomatal conductance parameter for pre-industrial conditions. This parameter is then held constant for the remainder of the simulation. In another simulation, we optimize the stomatal conductance parameter at regular intervals throughout the simulation period. In a third simulation, the stomatal conductance is calculated according to the Ball-Berry relationship. We then evaluate the simulations with regards to the modeled productivity and associated surface and boundary-layer characteristics.
B33D-1573
Vegetation and Climatic Controls on the Interannual Variability of Land Surface Energy Balances and Evapotranspiration Over an Oak-Savanna Ecosystem and an Annual Grassland
We report on a 6-year record of land surface energy balances and evapotranspiration (ET) measurements using the eddy covariance technique over an oak-savanna ecosystem (OS) and an annual grassland (AG) in California. Although two sites are just 2 km apart and received similar solar radiation, they revealed pronounced differences in net radiation, sensible heat flux and ET on an annual scale due to different phenological traits (i.e. live trees in summer at OS whereas dead grasses in summer at AG). Normalized ET (ETN, ET/ETeq) scaled with bulk surface conductance at both sites and ETN was higher for AG than OS on a monthly time scale. In contrast, annual ET was higher for OS than AG. The primary climatic driver for interannual variability of ET was precipitation and cloudiness in May and June. Annual ET at both sites decreased with annual solar radiation amount, indicating that these ecosystems are water-limited.
B33D-1574
Effects of Large Scale Poplar Plantations on the Hydrology of Semiarid Areas in Inner Mongolia
Tree plantation is widely practiced to counteract desertification. It often involves planting forests in semiarid and arid areas, which are naturally dominated by grass- or shrub steppe. Severe land degradation has been observed in the vast grasslands of Inner Mongolia, China. Under drying climates vegetation may naturally change from grass- to open shrub land. Current programs aim at a large number of poplar plantations to compose a super-scale shelterbelt to curb soil erosion, dust storms, and further loss of vegetation cover. However, the water consumption of poplar plantations can be expected to exceed the water use of grass- or shrub steppe. Hence, large-scale poplar plantation may significantly alter the water budget in the semiarid region. We compared Eddy- Covariance-derived evapotranspiration (ET) of a young poplar plantation and an adjacent shrub land south of the Yellow River in Inner Mongolia. In addition, ET from the semiarid site was compared with ET from an older poplar plantation growing under semi-humid conditions south of Beijing. In spite of 33% lower precipitation, ET was 6% higher from the young poplar plantation (236.52 mm) than from the natural shrub land (223.02 mm) based on the five-month period May- September 2006. The difference was mainly because of higher ET from the poplar plantation during the drier periods of the growing season. Further comparison with the older plantation outlined future potential of the poplars to exceed ET of the shrub land by 100-200%. To highlight potential hydrological consequences of large scale poplar plantations, ET values were set in relation to the total size of plantations projected for the area and the stream flow of the nearby Yellow River. Additional groundwater discharge by mature poplar plantations may equal 6.5-15% of the Yellow River mean stream flow. Thus, the water expenditure of poplar plantations renders them a questionable tool in sustainable arid-land management, particularly as climate predictions suggest higher temperatures and lower water availability in Inner Mongolia in the future.
B33D-1575
Eddy Covariance Method: Overview of General Guidelines and Conventional Workflow
Atmospheric flux measurements are widely used to estimate water, heat, carbon dioxide and trace gas exchange between the ecosystem and the atmosphere. The Eddy Covariance method is one of the most direct, defensible ways to measure and calculate turbulent fluxes within the atmospheric boundary layer. However, the method is mathematically complex, and requires significant care to set up and process data. These reasons may be why the method is currently used predominantly by micrometeorologists. Modern instruments and software can potentially expand the use of this method beyond micrometeorology and prove valuable for plant physiology, hydrology, biology, ecology, entomology, and other non-micrometeorological areas of research. The main challenge of the method for a non-expert is the complexity of system design, implementation, and processing of the large volume of data. In the past several years, efforts of the flux networks (e.g., FluxNet, Ameriflux, CarboEurope, Fluxnet-Canada, Asiaflux, etc.) have led to noticeable progress in unification of the terminology and general standardization of processing steps. The methodology itself, however, is difficult to unify, because various experimental sites and different purposes of studies dictate different treatments, and site-, measurement- and purpose-specific approaches. Here we present an overview of theory and typical workflow of the Eddy Covariance method in a format specifically designed to (i) familiarize a non-expert with general principles, requirements, applications, and processing steps of the conventional Eddy Covariance technique, (ii) to assist in further understanding the method through more advanced references such as textbooks, network guidelines and journal papers, (iii) to help technicians, students and new researchers in the field deployment of the Eddy Covariance method, and (iv) to assist in its use beyond micrometeorology. The overview is based, to a large degree, on the frequently asked questions received from new users of the Eddy Covariance method and relevant instrumentation, and employs non-technical language to be of practical use to those new to this field. Information is provided on theory of the method (including state of methodology, basic derivations, practical formulations, major assumptions and sources of errors, error treatment, and use in non- traditional terrains), practical workflow (e.g., experimental design, implementation, data processing, and quality control), alternative methods and applications, and the most frequently overlooked details of the measurements. References and access to an extended 141-page Eddy Covariance Guideline in three electronic formats are also provided. http://www.licor.com/eddyPresentation
B33D-1576
Application of Wireless Sensor Networks to measure the plant and soil characteristics within the transpiration process
Transpiration plays an important role in the land-atmospheric interactions. The performance of a land-surface model in the land-atmospheric system can be enhanced by the improvement of the representation of the internal phenomenon of the plant and the soil behavior during transpiration. This study aims to measure plant's internal phenomenon and soil behavior during the transpiration process by using a wireless sensor network. Sensor motes, as a part of the new wireless sensor network technology, have the ability to connect the observation instruments to collect the data. The sensor mote was designed with low power consumption and it is able to deliver the observation data to the base station through the wireless network. The programmable sensor mote also has the ability to work under different time intervals or trigger after certain events. This study tries to connect the measure instruments to the wireless sensor motes to measure the phenomenon inside the tree and within the soil. Initial results of this investigation will be presented and discussed.
B33D-1577
Inter-annual Variation in Growing Season Length of a Tropical Seasonal Forest in Northern Thailand
Growing season length is an important factor affecting energy balance and water and carbon cycling at deciduous forests. The impact of its inter-annual variation on annual energy and carbon exchange is likely to be critical especially in tropical region because of high radiant energy throughout the year; however, few studies investigated inter-annual variation in growing season length of tropical deciduous forests. This study revealed year-to-year variations in the canopy duration and transpiration period as measures of growing season length using time series data of radiative transmittance and heat pulse velocities of canopy trees in a teak plantation in northern Thailand from 2001 to 2006. This study also examined whether year-to-year variation in growing season revealed by field measurements could be detected by satellite NDVI data, as a first step to investigate the inter- annual variation in growing season length of deciduous forests over tropical monsoon region. Leaf-out and transpiration commenced earlier in 2001 than other years following exceptionally heavy rainfall in the late dry season, suggesting that enhanced soil moisture advanced leaf unfolding and start of transpiration. Leaf-fall and decline in transpiration at the beginning of 2003 were late in coming in correlation with a prolonged rainy season. Declines in transpiration were directly controlled by soil moisture at the beginning of the dry season. These results revealed that soil moisture is a major cause of large inter-annual variation in the growing season at this site. Seasonal variation in NDVI corresponded to that of LAI, while transpiration declined earlier than LAI and NDVI in the dry season. Year-to-year variation in canopy duration could be also detected by NDVI. The variation in canopy duration and transpiration period of this site from 2001-2006 spanned about 60 days. This was much larger than the inter-annual variations previously reported in temperate deciduous forests, implying a profound potential impact on energy, water, and carbon exchange on an annual time scale.
B33D-1578
How do forest ecosystems respond to a warm and dry weather event during the early growing season in eastern North America?
Many studies are being conducted under global Fluxnet to understand how forest ecosystems may respond to short-term weather events such as warm and dry periods. We measured energy, water vapour and carbon dioxide (CO2) fluxes in an age-sequence (66, 33, 17 and 5 year old) of temperate conifer forests (eastern white pine, -Pinus strobus L.) on the northwest shore of Lake Erie in southern Ontario, Canada, from 2003 to 2006 using the Eddy Covariance (EC) technique. Net ecosystem productivity (NEP) and water use efficiency (WUE) of the 17-yr-old stand was highest, followed by the 33-yr-old, 66-yr-old, and 5-yr-old forests. In 2005, eastern parts of North America experienced a warm and dry spring and early summer drought that significantly reduced NEP of these forest ecosystems. Compared to previous years, the warm and dry summer of 2005 resulted in a decrease of about 100-200 g C m -2 y -1 at all four sites, mostly because of a decrease in gross ecosystem productivity due to water stress and an increase in ecosystem respiration due to warm soil temperatures. The sensitivity of drought stress increased with increasing stand age. An EC flux component study utilizing sapflow and soil water content measurements in the 66-year stand indicated the occurrence of hydraulic redistribution during drought periods. The nightly increase in soil water (up to 0.50 mm) provided by hydraulic redistribution may have reduced drought intensity in the root zone by maintaining soil water contents at levels above the minimum observed soil moisture threshold (~ 0.07 mm) in these sandy soils. Similar reductions in measured NEP in 2005 were also observed in other forest ecosystems in the region. The results of this study indicate how eastern North American forest ecosystems might be affected by environmental stresses and drivers if this region experiences more extreme climatic conditions in future, including frequent warm and dry periods. http://www.science.mcmaster.ca/geo/faculty/arain/
B33D-1579
Vegetation Cover Decreases Evaporative Water Loss in a Wetland Ecosystem
Analysis of oxygen and hydrogen isotope ratios of water is a useful tool for quantitative measurements of water evaporation. Water molecules with the lighter isotopes, H216O, evaporate faster than H218O and DH16O, leaving the residual water enriched in D and 18O. Therefore, the greater the evaporation, the higher the δ18O and δD values in the remaining water body. Here we used stable isotope analyses to study evaporative processes in the a wetland water conservation area (WCA-1, South Florida Water Management District) where the primary purpose is to conserve regional water resources. Evaporation is one of the major paths of water loss in WCA-1. We collected water from 50 sampling stations located in the 145,920 acres of WCA-1 area for the months of August, September, and November 2006 and January 2007. Water samples were analyzed for oxygen and hydrogen isotope ratios. The results confirm that the water in this area is enriched by evaporation since a plot of water δD versus δ18O lies off the meteoric water line. However, the enrichment of 18O and D within WCA-1 is not homogeneous, with differences in δ18O values between stations of up to 2‰. We GIS mapped the δ18O values of water for the entire area and found the isotopic enrichment pattern is consistent through time. This result suggests that water at different locations in WCA-1 has different evaporation rates. Possible factors that contribute to this evaporation pattern are: distance to the peripheral canal discharge station, water depth, and vegetation coverage. To find out which is (are) the determining factor(s) affecting water evaporation of the area, we mapped δ18O values of water with elevation and vegetation type of WCA-1 and calculated average elevation and percentage of vegetation coverage of a 100m2 area around each sampling station. A multiple linear regression between δ18O values of water and average distance from the discharge gates, elevation, and percentage coverage indicate that the observed evaporation pattern is not caused by water depth. Distance from the discharge gates and percentage vegetation coverage are both significantly correlated with δ18O values of water. The effect of distance is related to the water turnover rate, i.e. the further the location is to a discharge station the greater the time the water at that location has been exposed to evaporation. In contrast, the higher the vegetation coverage the lower the loss of water through evaporation. In the future, we will determine if the effect of vegetation coverage in diminishing water loss by evaporation is annulled by the loss through transpiration.
B33D-1580
Revisiting the Boundary Layer Leaf Water Isotopic Model
The boundary layer (BL) model for oxygen or hydrogen isotopic composition of leaf water has been widely used in the past four decades, and has been incorporated into models that require information about leaf water isotopic variations. However, since its introduction, model predictions of the bulk leaf water have often exceeded observed isotopic enrichments. There are also cases in which the model yielded lower than observed isotopic enrichments of bulk leaf water. In general, underpredictions occur under relatively high humidity. In order to explain why the BL model overpredicts the isotopic composition, several modifications of the model have been proposed. However, no explanation exists for why the BL model underestimates observed isotopic enrichments. We recently developed a 2D model that successfully simulates the observed along-leaf 18O enrichment of pine needles, and can explain why the BL model could have over- or under-predicted the bulk leaf water δ18O values. In the BL model, bulk leaf water is isotopically equivalent to water at the evaporation site, fed directly by stem water. In a real leaf, however, stem water enters the base of the leaf and becomes progressively enriched in 18O towards the tip due to fractionation by transpiration, consistent with both our observations and behavior of the 2D model. Therefore, at least part of the leaf water, that near the base, would have isotopic values lower than water at transpiration sites predicted by the BL model, which might thus overestimate bulk isotope values. On the other hand, as water moves through a leaf, it becomes increasingly enriched in 18O, and the leaf water near the tip may have δ18O values well above the BL model prediction. Therefore, it is also possible for the BL model to underestimate the bulk leaf water δ18O. The actual isotopic composition of the bulk leaf water is a combination of these two effects. It is clear then that the BL model may not accurately predict the δ18O value of the bulk leaf water, because to do so would require the volumetric average of the δ18O in the depleted and enriched parts of the leaf to exactly equal the BL model prediction. Furthermore, if leaves are assumed to transpire fast under low humidity, our 2D model can also reproduce the humidity dependence of the discrepancy between observation and the BL model prediction. This suggests an interaction between environmental conditions and the physiological behavior of plants. If the simplicity of the BL model justifies its continued use, then it is important to investigate its accuracy further under different environmental conditions and for leaves with different morphologies and water transport pathways.
B33D-1581
The isotopic composition of water vapor in an irrigated urban area
The isotopic composition of water vapor in urban areas has rarely been measured, but has the potential to provide information about urban water cycle processes. We measured the isotope values of water vapor at two heights above an urban wetland, 20 m above an urban campus, and 30 m from the ocean. Diurnal isotope values at the beach and marsh showed a 5.8 per mil difference in d2H and 1 per mil difference in d18O, indicating the influence of local processes at each location. Isotope profiles show differences between the top and bottom of the marsh, supporting the idea of a local marsh isotopic signature. The Keeling plot approach and the mass- balance approach were used to partition evapotranspiration from the marsh. Both methods suggest that marsh water loss is dominated by transpiration. The data suggest that stable isotopes of water vapor can be a powerful tool to partition sources of evaporation and transpiration.
B33D-1582
A Study of Water Uptake by a Mature Amazonian Rainforest.
Approximately half of the Amazon evergreen forests is subjected to seasonal droughts of at least 3 months duration. Nonetheless, several plot-scale studies have shown that rainforests do not seem to exhibit a significant water stress during dry seasons. Moreover, recent analysis of remotely sensed spectral indices indicates an increased green leaf area and photosynthetic activity associated with these periods. These studies suggest both a specific phenology pattern and physiological water uptake adaptations to prolonged episodes of dry conditions. Several mechanisms have been proposed to support observations, including deep root function and hydraulic redistribution. In this study, we further investigate possible explanatory mechanisms. A vegetation-hydrology model that parameterizes the essential water-energy processes using a simplified three-leaf representation of canopy vertical structure is constructed. Deep 35 m profile is used to explicitly resolve the propagation of wet and dry cycles into the soil column. We focus on a site in Tapajós National Forest near km 67 of the Santarém- Cuiabá highway (BR-163), which was a part of the Brazilian-led Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-ECO). Meteorological data for the period of 2002-2005 are used as forcing. Land- surface fluxes, phenology, soil water, soil texture and water retention properties as well as observed profiles of root biomass are used to constrain the model. In a set of numerical experiments, we explore the implications of phenology pattern, soil texture effects in moisture redistribution, and possible niche separation in root water uptake among plant types.
B33D-1583
Modeling the Dynamic Root Water Uptake and its Hydrological Impact at the Reserva Jaru Site in Amazonia
An empirical approach is proposed to represent the impact of dynamic root water uptake by plant roots in two land surface models (CLM3 and IBIS2). It is compared with a more physically based approach that simulates the hydraulic redistribution, using the Reserva Jaru site in Amazonia as an example. For each model and each approach, two different root profiles are experimented on, an exponential profile and an observed, deeper profile. In both CLM3 and IBIS2, including dynamic root uptake significantly improves the model simulation of latent heat fluxes, regardless of what root profile is used. In both models, the impact of hydraulic redistribution (as accounted for by the physically based approach) is comparable to that of a low degree of dynamic root water uptake (as accounted for by the empirical approach). The latent heat flux simulation in IBIS2 is closer to observation than in CLM3, with or without the impact of dynamic root water uptake. Despite the model improvement due to including dynamic root water uptake, significant biases in soil moisture simulations remain. Assimilating soil moisture observations into the land surface models produces a remarkably improved latent heat flux simulation. Parameterization in surface and subsurface runoff (which influences soil moisture) is suggested as a likely cause for the severe biases in the default model simulations, which highlights the critical importance of correctly simulating the fundamental hydrological processes in land surface models.
B33D-1584
Soil Salinity Controls on Water and Carbon Cycling by Sunflower Plants
Agricultural effects on water cycling are of great importance for regional water resources management. These effects vary based on local soil and climate conditions, and are particularly modulated by high soil salinity levels, which stress plant growth and change their water use efficiency. Increasing salinization is predicted under hotter, drier conditions resulting from global climate change and from increased societal pressure on agricultural lands. This increased ionic presence creates a higher soil osmotic pressure that increases the resistance to water flow through the plant. This change also impacts the assimilation of carbon dioxide through the stomatal opening, and so affects rates of both photosynthesis and transpiration. Current agricultural and land-surface models that account for salinity do so in an overly empirical manner that cannot account for changes at different time scales in meteorological conditions. They tend to be ill equipped to examine how changing carbon dioxide levels may modify a plant's response to soil salinity. As a result, we present a new model of soil-vegetation- atmosphere water transfer that explicitly incorporates the role of soil salinity in changing this system's behavior. This model will allow for much greater flexibility in examining how vegetation may change the local water cycle under the joint impacts of both salinity and climate change. This model is supported by field research on the effects of salinity on sunflower plants in a large irrigation district in Inner Mongolia, China. Results presented include the role of salinity in changing stomatal regulation of water use efficiency, sub-canopy changes in leaf pressure, and changes in root activity. Modeling at sub-hourly time scales allows for a more precise understanding of how soil salinity changes the diurnal cycle of plant water use.
B33D-1585
Can an Alder Disease Influence the Controls of Ecosystem Water Flux?
In Alaska, nitrogen-fixing shrubs (Alnus spp.) comprise a large component of the vegetation and are important for water and energy flux in the Alaskan tundra, the Interior boreal forest, and south-central Alaska. Recently there has been high incidence and severity of a canker disease on Alnus which alters small scale controls (e.g., leaves, branches) of water and carbon flux and may have ecosystem scale effects in severely diseased areas. The fungal pathogen, Valsa melanodiscus (anamorph =Cytospora umbrina), kills active xylem and phloem, causes long, girdling cankers on stems, and is frequently associated with stem mortality in the thin leaf alder (Alnus tenuifolia) and green alder (Alnus crispa). We are just beginning to understand how this canker disease affects the physiology of its host and the implications for water loss and carbon fixation. An inoculation experiment at the University of Alaska is currently being conducted on well-watered and water-limited green alders to: 1) monitor the response of the water transport system to infection and canker development; 2) assess the ability of colonized alders to adjust water use efficiency; and 3) measure the effect of canker development on photosynthesis. Only 4 weeks after inoculation, stems (3-5 mm diam.) with incipient cankers had an overall reduction in total daily water loss and up to 50 percent decrease in daily peak water loss. Stems with incipient cankers were also unable to attain the maximum photosynthetic rate of healthy stems, and had lower light saturation points and quantum efficiencies. Further quantification of these relationships is underway. This study, in combination with recent field measurements of sapflow, will allow us to estimate the impact of varying degrees of disease severity on stand-level water flux. Alders are a keystone species and are currently responding to climate change by expanding into the Alaska tundra, increasing abundance in northern Alaska, and migrating into river drainages in the Interior. Thus, this canker disease poses the potential for wide ecophysiological consequences across the state of Alaska.
B33D-1586
Coupled Water and Carbon Exchange Processes in a Sagebrush-Steppe Ecosystem
The sagebrush-steppe ecosystem occupies nearly 43 million ha of semi arid, sage brush dominated land in the Great Basin region of United States and retains one of the largest carbon pools in USA. Eddy covariance data collected over a two month period (June- July, 2005) in the sagebrush-steppe dominated ecosystem in south- central Wyoming have shown soil moisture to be a dominant driver of carbon and water fluxes. As soil moisture decreased in July, non-shrub species senesced, leading to a decrease in photosynthesis and respiration. An integrated hydrology and photosynthesis model with a feedback loop to the soil respiration was developed to simulate the water and carbon fluxes. The working hypothesis was that the microbial and photosynthetic activities associated with sagebrush would be linked to two different soil moisture layers, thus helping to elucidate the dominant drivers of carbon and water fluxes. The model incorporates a two-layer soil moisture logic linking soil surface evaporation and microbial processes to the moisture content at a shallow soil depth and transpiration and photosynthesis to be a function of deeper root zone soil moisture. As the landscape was dominated by sagebrush, non sagebrush and bare soil, simulation was conducted separately for all the three components to obtain the equivalent carbon and water fluxes. While the model has been successful in simulating ET (r2 = 0.82), photosynthesis and respiration at high soil moisture content (theta > 0.15), further analysis into mechanisms behind the relatively poor model performance during periods of low soil moisture will be presented.
B33D-1587
Modeling the Hydrological Impact of Land-use Change in West Africa
Numerical simulations of idealized deforestation and overgrazing are performed for the Niger and Lake Chad basins of West Africa with a terrestrial ecosystem model IBIS (Integrated Biosphere Simulator) and an aquatic transport model THMB (Terrestrial Hydrology Model with Biogeochemistry). The study reveals how land use changes affect hydrological regimes at the watershed scale. Tropical forests, due to being situated in the regions of highest rainfall and exerting strong influence on evapotranspiration, have a disproportionately large impact on the water balance of the entire basin. Total deforestation of tropical forests increases the simulated runoff ratio from 0.15 to 0.44, and the annual streamflow by 35% to 65%, depending on location in the basin, although forests occupy only a small portion (<5%) of the total basin area. Complete removal of grassland and savanna, which occupy much greater areas of the basins, result in an increase in simulated annual streamflow by 33% to 91%. The hydrological response to progressive land cover change is non-linear and exhibits a threshold effect. There is no significant impact on the water yield and river discharge when the deforestation (thinning) percentage is below 50% or the overgrazing percentage below 70% for savanna and 80% for grassland areas; however, the water yield is increased dramatically when land cover change exceeds these thresholds. This threshold effect is a combined result of the non-linearity of the separate response of transpiration and soil and canopy evaporation to the imposed land cover changes.
B33D-1588
Seasonal patterns of photosynthetic gas-exchange and leaf reflectance characteristics in male and female riparian cottonwoods of southern Alberta
Riparian, or streamside, cottonwood trees ( Populus spp.) are dioecious phreatophytes of hydrological and ecological importance in arid and semi-arid ecosystems throughout the northern hemisphere. In southern Alberta, groundwater and soil moisture levels typically decline during the May to September growth season. To understand how narrowleaf cottonwoods ( Populus angustifolia James) respond to this seasonal decrease in moisture availability, we repeatedly measured photosynthetic gas exchange, leaf reflectance, chlorophyll fluorescence and stable carbon isotope composition (δ13C) in four male and four female trees of the Oldman River valley, throughout the 2006 growth season. Maximum light-saturated net photosynthesis rates (Amax), near 16 μmol m-2 s-1, occurred on day of year (DOY) 205, one month after peak soil moisture, but coincident with the maximum quantum efficiency of Photosystem II (Fv/Fm), chlorophyll index (CI) and scaled photochemical reflectance index (sPRI). CI data suggest that the early-season rise in Amax and Fv/Fm was partly due to growth in the chlorophyll pool. Thereafter, Amax fell to near 10 μmol m-2 s-1, largely due to its positive logarithmic relationship with stomatal conductance (gs; r2=0.89), which decreased from 559 to 246 mmol m-2 s-1 from DOY 205 to 237. The normalized difference vegetation index (NDVI), CI, sPRI and quantum yield of electron transfer at Photosystem II (ΦPSII) also declined in response to lower volumetric soil moisture content (θv) and increasing groundwater depth (Zgw). Little change in transpiration rate (E) was observed in response to changing environmental conditions, except on DOY 237, when a combination of unseasonably low vapour pressure deficit (D) and low θv above the deepening capillary fringe caused E to decrease. No significant difference was observed between the mean WUE (Amax/E) of males (2.1 ± 0.2 mmol mol-1) and females (2.5 ± 0.2 mmol mol-1; repeated measures ANOVA, df=6, F=2.39, p=0.16). Leaves of females showed a tendency for lower NDVI than those of males (repeated measures ANOVA, F=4.82, p=0.07). Otherwise, no significant differences were observed between males and females in any gas-exchange, leaf reflectance or fluorescence characteristic (repeated measures ANOVA, α=0.05) and δ13C remained in the -28.8 to -29.3 ‰ range throughout the season, in both sexes.
B33D-1589
Estimation of Net Radiation and Evapotranspiration in California Using MODIS Satellite Observations
Soil moisture links surface energy, water and biogeochemical cycles by several different pathways, including by influencing the partitioning of energy into latent and sensible heat and by regulating NPP and heterotrophic respiration fluxes. Evapotranspiration (ET) is a major pathway for water loss and its seasonal variation affects the seasonality of soil moisture and subsequently net ecosystem exchange. We developed an empirical ET algorithm using Ameriflux data and MODIS leaf area index to improve the estimation of soil moisture in CASA biogeochemical model at a regional scale. We estimated net radiation (Rn) using MODIS BRDF/albedo and skin temperature/emissivity products. A good agreement was found between satellite-based estimates and field- measured Rn from SURFRAD, Ameriflux, and 6 recently installed flux towers in southern California, with an absolute difference below 30 W m-2. The ground heat flux component of available energy was estimated using the fraction of vegetation derived from MODIS NDVI. We parameterized the Priestly-Taylor coefficient with leaf area index and soil moisture at an 8-day time scale using multi-year data from the Ameriflux sites. We validated this algorithm using ET measurements from May 2006 to April 2007 in southern California tower sites. The spatial distribution of annual mean Rn over California showed an increasing trend from desert to grassland ecosystems, and from grasslands to forests, reflecting decreasing albedo and surface temperature with increasing vegetation cover fraction. The Priestly-Taylor coefficients followed the phenology and the seasonality of soil moisture reasonably well, which leads to higher ET in spring rather than in summer- when Rn peaks. The seasonal cycle of net ecosystem exchange predicted by CASA with these improvements agreed reasonably well with those derived from California's eddy covariance measurements due to the improved seasonality of soil moisture.
B33D-1590
Water limitation of net primary productivity and the link to Water Use Efficiency
Monitoring, understanding and modeling carbon emission and fixation fluxes in combination with water demands are key variables to guide climate change stakeholders in the application of mitigation strategies since a strong coupling between the carbon and the hydrological cycle is observed. Water Use Efficiency (WUE) is the ratio of the net amount of carbon uptake by vegetation and the amount of water lost by evapotranspiration. WUE is the biogeochemical link between the carbon and hydrological cycles. Hence, the use of WUE as a proxy for the carbon and water cycle linkage is a promising tool to foster a better understanding and estimation of the impact of climate on terrestrial ecosystem. Field-experiments and point-simulations for a specific ecosystem allow to accurately estimate plant WUE. They however face scientists with the difficulty of scaling up WUE from the local to the regional scale. Remote sensing represents a technique providing spatial and temporal explicit datasets at regional to global scales and hence it is a promising approach to assess regional to global WUE by the joint estimation of net primary productivity (NPP) of plants and evapotranspiration at various spatial and temporal scales. In this study we present results on water limitation impact on ecosystem carbon uptake and release using an enhanced C-Fix model compared with validation data collected at EUROFLUX sites. It is demonstrated that if the water household of a terrestrial ecosystem is taken into account the balance between NPP and anthropogenic carbon emissions (ACE) for the European continent is changed quite significantly. Using water limited NPP at the country scale, we can reveal that many nations such as Czech/Slovakia, Hungary, Ireland, Italy and Switzerland change their NPP-ACE balance from positive to negative. These results were obtained with data acquired for Europe in 1997.
B33D-1591
Active Radar Soil Moisture Products for Water use Efficiency Estimation
The protection of water resources becomes more important with the increasing number of climate hazards. In order to protect water resources we have to fully understand the relationships between carbon and hydrological cycles and how these alter with increasing frequency of climate hazards. Numerous methods have been developed in order to monitor absorbed carbon, evapotranspiration, and soil moisture. Here, methods and current missions for soil moisture assessment at regional and global level will be introduced and their role in estimation of water use efficiency will be highlighted. Additionally, upscaling of soil moisture from local to regional scale will be discussed. http://www.ipf.tuwien.ac.at/radar/
B33D-1592
Mapping Water Use Efficiency for the Xinjiang Province in Northwestern China
The Tarim river basin, one of the basins integrated in the UNESCO – HELP Programme, is located in the Xinjiang Autonomous Region in Northwestern China. It is one of the world's largest closed hydrological drainage systems, and has ideal soils for agriculture,… if enough water is available. Farmers in the province produce one- sixth of China's total cotton production. Since the 1950's however, excessive land reclamation, over-grazing and increased utilization of water resources in the upper reaches of the basin intensified environmental degradation. This impacted on cattle stock reduction (by water shortage), withered poplars and vegetation, along the lower reaches of the basin. Riparian forest degraded with a reduction in its area of 200 000 ha in total. Higher water use combined with a gradual mean yearly temperature increase due to climate change caused serious hydrological problems in the basin. Since the 1970's a strong drying out of the lower region is elicited. This phenomenon degraded the basin's downstream ecosystems, with an expansion of desertification as a consequence. Quite conspicuously, the Tarim river has shortened by 320 km, onward the period mentioned above. In the lower region of the basin, the situation is very serious. It can in fact be described as an ecological disaster, with dying trees and vegetation, the drying out of rivers and as consequence a regression of the river fish population as well as increased salt deposits. In the ARCHIMOD bilateral project between Belgium and China, important objectives are the description of the phenomena cited above using hydraulic modelling for some sub-basins of the Tarim as well as the application of remote sensing to estimate evapotranspiration (ETR), soil moisture content (SMC) and water use efficiency (WUE) at the scale of the Xinjiang province. Te estimate WUE at this scale the carbon exchange model C-Fix was applied. Our paper focusses on the application of remote sensing to map WUE for the Xinjiang province, with the objective to estimate the surface areas eliciting a high water use by agricultural crops.
B33D-1593
Response of terrestrial vegetation to water and energy: the contribution of earth observation to estimate and understand Water Use Efficiency at multiple scales
Climate variability implies variable water availability and drought hazards in many parts of the world. Vegetation species, especially taking into account significant biodiversity, react very differently to water scarcity. The amount of biomass produced per unit volume of water transpired changes significantly across and within species, according to genetic characteristics. The latter represent the most significant resource towards adaptation of agriculture to climate change. On the other hand it remains difficult both to assess Water Use Efficiency (WUE) at larger spatial scales and to model WUE in a way sufficiently simple to allow inclusion in the large area and global climate models used to assess impacts and evaluate adaptation options. WUE is a ratio of extensive quantities and provides directly an upscaling constraint for all variables involved in related parameterizations. The first challenge is to observe WUE at any spatial scale larger than a single plant. At this scale biomass can be determined by direct sampling and transpiration can be measured to a satisfactory level of accuracy with sap- flow devices. At larger spatial scales no direct experimental method is available and heterogeneity makes attribution of estimated total water flux to specific vegetation types within the area observed rather difficult. Use of radiometric data collected from aircrafts and satellites is a practical approach to estimate and map both water flux and biomass, thus leading to WUE. When using satellite data this approach makes frequent observations and monitoring in time possible. This review presentation summarizes current approaches and trends in the estimation of vegetation-atmosphere water exchange and of biomass. Analysis of time series of indicators of vegetation response to water availability in terms of both ET and biomass will be presented on the basis of case- studies in Africa, South America, Europe and China.
B33D-1594
Water use Efficiency in a Blue oak ( Quercus douglasii) Savanna - a Combined Analysis of Stable Isotopes and Eddy Covariance Measurements
Understanding the relationship between carbon assimilation and water consumption by natural vegetation is needed to assess how changes in climate will affect plant carbon and water exchange as well as the energy fluxes of ecosystems. While climate change is expected to cause significant warming, most models also suggest changes in the timing and amount of precipitation received; thus implications of this type of change are particularly acute in Mediterranean regions of the world. Blue oak savannas are already exposed to broad variation in water availability and to severe droughts during the summer months. Our objective was to evaluate the trade-off between carbon gain and water loss (Water Use Efficiency) in this ecosystem at both the leaf and at the ecosystem scales. We monitored the ratio of the partial pressures of CO2 inside the leaf (Ci) and in the outside air (Ca) or Ci/Ca, during the summer months of three subsequent years. This ratio is determined by the balance between photosynthetic capacity and stomatal conductance to water loss. Leaf-level estimates for individual trees were based on the carbon isotope composition (δ13C) of bulk leaf tissue and of recently fixed carbohydrates (leaf soluble sugars). These leaf and individual tree based estimates were then compared with canopy-level estimates derived from continuous eddy covariance measurements of fluxes of CO2, water vapor and meteorological variables from two eddy covariance systems, one above (23m) and one below (2m) the tree canopy. We found that savanna Blue oak trees cope with severe drought through coordinated down-regulation of carbon and water fluxes, i.e. the ratio Ci/Ca remained stable over four summer months, despite decreasing soil water content and leaf water potentials. Stable C isotope composition of leaf soluble sugars is the most robust measure of Ci/Ca because it reflects the initial discrimination of photosynthetic products, without the confounding effects ascribed to storage, tissue chemical composition and time of tissue formation. Our findings at the leaf-level were confirmed at the ecosystem-level by using a two tower (above and below canopy) eddy covariance method.
B33D-1595
The Effect of Increased CO2 Mixing Ratio on Water Use Efficiency, Evapo-transpiration, Soil Moisture Content and Stem Flow in two Long-term Field Experiments
Stomatal opening in plant leaves control carbon and water exchange between vegetation and the atmosphere. Closure of these water-gates in response to increased atmospheric CO2 mixing ratio's, reduces transpiration under most laboratory and short term experimental conditions. Does this imply however, as atmospheric CO2 rises, and plant canopies expand, that evapo-transpiration (ETR), soil moisture content (SMC), and ecosystem water use efficiency (WUE) will increase? To test this question, field experiments have been and still are conducted using open top chambers. We have exposed native species in Florida Scrub to a carbon dioxide mixing ratio of nearly 700 ppmv CO2 for the past ten years and in Chesapeake Bay wetlands for 21 years. As a result of this treatment, in both ecosystems there was an increase in net ecosystem CO2 exchange and leaf area but a reduction of stomatal conductance, stem flow, transpiration, and ETR. For Florida scrub oak, these changes were also accompanied by an increase in soil moisture content as well.
B33D-1596
Estimating evapotranspiration over Yosemite National Park using a regional ecosystem model driven by satellite-based climate data.
Estimation of evapotranspiration over complex terrains is difficult because of the sparse distribution of ground- based observations and the large errors associated with extrapolating these data. Satellites, however, provide spatially continuous and consistent observations of key climate and vegetation variables, which may help us to overcome this difficulty. Here, we report our effort to estimate evapotranspiration over Yosemite National Park using a regional ecosystem model driven by satellite data (MODIS, GOES, and TM). The Terrestrial Observation and Prediction System (TOPS) was used in this study to simulate water and carbon fluxes. We also used the Variable Infiltration Capacity (VIC) routing model for validation of simulated water budgets using the discharge data at two locations in the park. The estimated evapotranspiration was reasonable both in terms of its spatial variability and in relation to the discharge data. Compared to other climate interpolation methods, this method is computationally inexpensive and can capture more detail for patchy regions like fire scares and logged areas.
B33D-1597
Global Remote Sensing of Water Use Efficiency: Initial Test and Application of a Synergistic Approach
Water use efficiency (WUE) provides a critical link between carbon and water cycles in terrestrial ecosystems. At leaf and canopy scales, WUE relates to the ratio of carbon fixed via photosynthesis to water lost through transpiration. WUE is mechanistically related to Ci/Ca, or the ratio of leaf internal to atmospheric CO2 concentrations. In turn, Ci/Ca ratios are related to discrimination against stable carbon isotopes during photosynthesis, an important constraint on the global carbon cycle. Methods to assess canopy WUE at canopy to global scales are therefore relevant to assessing plant Ci/Ca ratios and carbon isotope discrimination. We describe and test a synergistic method for remote sensing of water use efficiency that builds on rapidly developing methods for remote sensing of photosynthesis and transpiration. In this approach, photosynthesis (A) and transpiration (T) are assessed independently then combined to estimate WUE=A/T. We first describe methods for remote sensing of A and T based on visible and near-infrared reflectance, solar radiation, temperature and humidity, and methods of calculating Ci/Ca from WUE. We then validate these methods using eddy covariance and stable isotope measurements in a Mediterranean oak-grass savanna that exhibits dynamic seasonal changes in WUE and Ci/Ca. Finally, we apply the method at the global scale using satellite observations from the Advanced Very High Resolution Radiometer (AVHRR). In addition to highlighting spatial patterns and temporal trends in WUE, we demonstrate that knowledge of C3 or C4 photosynthetic pathway is not necessary for estimating transpiration when leaf area index (LAI) and net radiation (Rn) are used as inputs, but it is required for estimating photosynthesis. Knowledge of C4 plant distributions is therefore critical to remote sensing of regional to global scale WUE.