H51H-0859
Controls on Nitrate Spatial Variability in Paine Run Catchment of Shenandoah National Park
This research examines the catchment-scale controls on in-stream nitrate concentrations by (1) quantifying nitrate spatial variability in a headwater catchment and (2) determining the biophysical processes underlying this variability. The Shenandoah Watershed Study (SWAS) established thirty-eight stream sampling sites in the Paine Run catchment to collect field data on stream chemistry, discharge and transient storage. An evaluation of SWAS data at these sites from the early 1990s to 2007 reveals spatial and temporal variability in nitrate concentrations following the gypsy moth defoliation. We observed high in-stream nitrate concentrations with elevation and an apparent dilution at lower elevations. Main topographic descriptors related to the spatial distribution of nitrate, elevation and contributing area, are associated with differing biophysical factors such as soil residence time, bacterial denitrification, vegetation and mineralization. Previous studies have demonstrated that the physical properties of hyporheic zones can strongly influence denitrification rates. We examined this in the Paine Run catchment with tracer tests to evaluate dilution effects and predict stream outflow and inflow from hyporheic zones responsible for denitrification. We then looked for biophysical processes responsible for higher nitrate levels at higher elevation by using the OTIS model for transient storage to evaluate hyporhiec zones in Paine Run. We also established a method to evaluate soil parameters for depth and permeability. By identifying the controls on nitrate inputs, transport and denitrification, we isolated a set of criteria applied to a quantitative model for nitrate spatial variability. This research has important implications for defining nutrient availability both within the stream network and at the outlet of forested headwater catchments.
H51H-0860
IMPACT OF STORM CHARACTERISTICS AND LAND USE ON NUTRIENT EXPORT IN TWO CENTRAL INDIANA WATERSHEDS, USA
This study investigates nitrate and dissolved organic carbon (DOC) export during three spring storm events in an agricultural watershed and a mixed agricultural/urban land use watershed in till landscape in Central Indiana, USA. The objectives of the study are (1) to determine how land use affects water, nitrate, and DOC delivery (timing, amount) to streams during spring storms, and (2) to determine nitrate and DOC flow pathways to streams during storms. High frequency stream sampling of nutrients and cations, coupled with hydrograph separations using δ18O, was used to identify water flow pathways and event and pre-event water contributions to the streams. Results indicate that nitrate and DOC concentrations display distinct temporal patterns during spring storm events. DOC concentration increased with stormflow, and peaked with discharge and the peak in event water regardless of land use or storm characteristics. Nitrate concentrations followed Ca2+, Mg2+, and Na+ trajectories and decreased with stormflow in both watersheds. In addition, the nitrate concentration peak was delayed relative to DOC in the mixed land use watershed. Data suggest that during storms, DOC is exported either via overland flow or via preferential flow through soil macropores. On the other hand, nitrate appears to be mainly delivered to streams in association with pre-event water via subsurface flow. This study contributes to a better understanding of nutrient export pathways during storms for a variety of land uses and to the development of better management strategies and nutrient loading models at the watershed scale.
H51H-0861
Changes in N and P Fluxes due to Agricultural Land Use in the Mississippi River System
Riverine flux is one of the most important terms balancing many global elemental cycles including nitrogen and phosphorus. Using US Geological Survey Water Data and a variety of GIS layers including yearly precipitation, we investigated the agricultural land use effects on both riverine discharge and fluxes of N and P from the Mississippi River system. Results demonstrate that there has been a large increase in water discharge due to agricultural land use and that the changes in discharge affected fluxes of N and P significantly. Dependency of the fluxes on regions and spatial variation will be discussed.
H51H-0862
Interannual Variability in the Carbon, Water and Energy Balances at the BERMS Flux Towers in Relation to Watershed-Scale Stream flow
The Boreal Ecosystem Research and Monitoring Sites (BERMS) network of eddy-covariance flux towers is located in central Saskatchewan, Canada, near the southern limit of the boreal forest. The 1999-2006 study period includes three extreme drought years (2001-2003) followed by three extreme wet years (2004-2006). We analyze interannual variability in the water balance at stand and watershed scales and explore the coupling of the carbon and water cycles at three mature forest sites and one wetland site.
H51H-0863
Comparison of Flow Distribution Algorithms for Estimating Spatio-temporal Soil Water Content With an Eco-hydrological Model
In an eco-hydrological model, RHESSys, soil water content (SWC) estimations are variable with flow distribution algorithms, by which some of model input data are calculated. The aim of this study was to compare three flow algorithms to calculate wetness index and flow table. Single Flow Direction (SFD), two types of Multi Flow Direction (MFD), and Demon algorithms were modified to produce wetness index of semi-distribution hydrological model and flow table of distribution hydrological model. The resultants were used to estimate SWC's, which were compared with observed values. Our results showed that effects of topography was better reflected and maps of soil water distribution were more natural in MFD and Demon algorithms than in SFD. Volumetric SWC of SFD was on average 0.30 v/v, and that of MFD 0.27 v/v, indicating that output of total water amount were also variable with flow distribution algorithms. We found that spatial distribution as well as total amount of SWC was different depending on flow distribution algorithms. Average RMSE's of SWC estimated with the modified SFD, MDF, and Demon algorithms were 0.13 (0.11~0.15), 0.11 (0.09~0.14), and 0.12 (0.09~0.15), respectively. Among the flow distribution algorithms, MFD was the most reliable.
H51H-0864
Moisture and Climatic Forcings on Sphagnum Productivity in a Cutover Peatland
Gross ecosystem production (GEP) was measured at a sub-boreal ombotrophic peatland (Cacouna Bog) 15 km NE of Riviere-du-Loup, Quebec, in the summers of 2005 and 2006. The Cacouna bog was extensively mined between 1940 and 1970 using the block cut method. The combination of exposed, high bulk density peat and low water table (c. 30 cm) has produced a succession vegetation community of ericaceous shrubs, invasive trees, and only c. 10% Sphagnum moss cover. Chamber-based measurements were made at three locations arranged longitudinally along a cut trench. Volumetric water content (VWC) and temperature probes at four depths, tensiometers, and a meteorological station provided high temporal resolution moisture and climatic data. June to August precipitation in 2005 was 167 mm below the 30-year mean; in 2006, the deviation was only 17 mm below normal. The cool and dry summer of 2005 depressed VWC by 10-20 %, soil water tension by 20-40 mb, and maximum GEP by 5-10 g C m-2 d-1 as compared to 2006. Instantaneous GEP did not decline during prolonged rain-free periods in August 2005 where soil tensions exceeded –100 mb, within the hypothetical range of hyaline cell drainage and reduced GEP. The response of these Sphagnum cushions points to a resilience to hydrologic stress at intermediate age (35 years since establishment) not otherwise observed in younger Sphagnum cushions less than 5 years since establishment. These findings have implications on the parameterization of sensitivity to moisture stress in bog growth models, and to management of restored and naturally regenerating peatlands.
H51H-0865
Geophysical Evidence for Abiotic Controls on Peatland Patterning at Multiple Scales
The autogenic and allogenic controls on the formation of distinctive and dramatic vegetation patterning found in northern peatlands remain unclear. Groundwater model studies and investigations using point measurements lack intensive data over multiple scales, primarily due to the intensive time required and difficult logistics required to work in these remote ecosystems. We provide geophysical evidence that lithological controls on vegetation patterning exist at multiple scales in ombrotrophic peatlands of northern Minnesota and Maine. Surveys using electrical imaging methods (including resistivity, induced polarization, and ground penetrating radar) at sites in the Red Lake Peatland Complex (160 km2), as well as Kanokolus Bog (1.65 km2) and the Caribou Bog Peatland Complex (22 km2) in Maine reveal sharp vegetation gradients coinciding with changes in the mineral soil lithology. In contrast, large-scale, continuous, patterned zones found in the Red lake Complex coincide with strikingly uniform mineral soil lithology as inferred from the geophysical images. Small-scale (0.3 km2) vegetation patterns observed in Caribou Bog also coincide with small scale lithologic changes in both the mineral and organic deposits. These results provide evidence that the subsurface hydrogeologic framework regulates vegetation patterning in peatlands across multiple scales, presumably by regulating (1) the supply of mineral solutes to the surface vegetation water, and (2) water levels within the organic soil.
H51H-0866
Effects of Paleoclimate and Time-Varying Canopy Structures on Paleo-Water Fluxes
We combined a long-term (18,000 years) climatological record and time-varying vegetation conditions to evaluate the role that climate change and vegetation might play in paleo-water fluxes in arid settings. The HYDRUS-1D model, which solves Richards Equation for variably saturated flow, the convection-dispersion equation for chloride transport and the heat flow equation, was used to simulate water flux and chloride (Cl) transport. Six distinct case studies were compared, for different boundary conditions and root distributions. A Mojave Desert- type canopy including evergreens, drought deciduous shrubs, annuals, grasses, and succulents was used as representative vegetation to transpire soil water, and was modeled using ground cover percentage and leaf area index (LAI) as the bases for partitioning evapotranspiration (ET). The results showed that, under water limited conditions, realistic root zone distributions and climate sequences (including extreme events) were both needed to simulate the accumulation of Cl in Mojave Desert soils. Results also showed that increasing precipitation intensity affected paleo-water fluxes. However, contrary to the results of other researchers, we found that simulated chloride bulges were located at depths of around 20-30 cm, rather than at the base of the root zone, if current and normal climate conditions were applied. Moreover, the climatic shift beginning in the late Pleistocene was not the major reason for the chloride accumulation.
H51H-0867
Recharge in Karst Shrublands of Central Texas: Monitoring Drip Rates in Shallow Caves
The exceedingly complex subsurface hydrology of karst landscapes presents formidable challenges to understanding recharge rates and the relationships between rainfall and recharge. In this study, we have established a network of drip collectors and monitoring stations in shallow caves in the Edwards Plateau to better understand the dynamics of recharge and eventually for determining the effect of woody plants on recharge rates. Understanding recharge rates has direct relevance for management of the Edwards Aquifer, which serves as the main source of fresh water for the city of San Antonio and surrounding communities, As population around San Antonio continues to grow so does the demand for water, in turn, a need to address the supply exists. We have instrumented two caves that lie within the Camp Bullis Training Facility north of San Antonio, Texas. Data collected at each site record precipitation on the surface and measure recharge inside the caves. Monitoring of natural rainfall events at these sites began in October 2004. To date, all monitoring and data collection has occurred with the juniper canopy in place. Results have shown that cave recharge is influenced by 1) rainfall intensity and duration, 2) antecedent soil moisture condition, 3) depth of soil, and 4) surface geology. We plan to remove the tree canopy in the summer of 2008 and continue monitoring cave recharge in response to natural and re-created rainfall events. Comparing data collected with and without juniper cover in place will allow us to determine if recharge may be increased by reducing tree cover.
H51H-0868
Water Source Utilization of Hammock and Pine Rockland Plant Communities in the Everglades, USA.
South Florida has a mosaic of plant communities resulting from topographical differences, spatially varying hydroperiods and fire. The only plant communities not flooded in the wet season are hardwood hammocks and often pine rocklands. Natural fires burn off litter accumulated in pine rocklands, with the exception of organic matter in sinkholes in the limestone bedrock. This relative lack of soil is thought to constrain pineland plants in the Everglades to depend upon groundwater that is typically low in nutrients. In contrast, adjoining hardwood hammocks have accumulated an organic soil layer that traps rainwater and nutrients. Plants in hammocks may be able to utilize this water and thereby access nutrients present in the litter. Hammocks are thus viewed as localized areas of high nutrients and instances of vegetation feedback upon the oligotrophic everglades landscape enabling establishment and survival of flood-intolerant tropical hardwood species. This study examines water source use and couples it to foliar nutrient concentrations of plants found in hammocks and pinelands. We examined the δ2H and δ18O of stem waters in plants in Everglades National Park and compared those with the δ2H and δ18O of potential water sources. In the wet season hammock plants accessed both groundwater and water in the surface organic soil layer while in the dry season they relied more on groundwater. A similar seasonal shift was observed in pineland plants; however groundwater constituted a much higher proportion of total water uptake throughout the year under observation. Concomitant with differential water utilization by hammock and pineland plant communities, we observed hammock plants having a significantly higher annual mean foliar N and P concentration than pineland plants. Most hammock species are intolerant of flooded soils and are thus constrained by the high water table in the wet season, yet access the lowered groundwater table in the dry season due to drying up of surface soilwater. This dependence on a relatively narrow seasonal range of water table depth has important implications for South Florida water resource management that can affect these ecologically important upland communities in the Everglades. Being the only emergent areas in the wet season, hammocks provide habitat for a wide range of flora and fauna.
H51H-0869
Vegetation Dynamics And Soil Moisture: Consequences For Hydrologic Modeling
Current global population growth and economical development accelerates land cover conversion in many parts of the world. Introducing non-native species and woody species encroachment, with different water demands, can affect the partitioning of hydrological fluxes. The impacts on the hydrologic cycle at local to regional scales are poorly understood. The present study investigates the hydrologic implications of land use conversion from native vegetation to rubber. We first compare the vegetation dynamics of rubber (Hevea brasiliensis), a non- native specie in Southeast Asia, to the other main vegetation types in the study area. The experimental catchment, Nam Ken (69km 2), is located in the Xishuangbanna Prefecture (21 °N, 100 °E), in the south of Yunnan province in South China. From 2005 to 2006, we collected continuous records of 2 m deep soil moisture profiles in four different land covers (tea, secondary forest, grassland and rubber), and measured surface radiation in tea and rubber canopies. Our observations show that root water uptake by rubber during the dry season is controlled by the change of day-length, whereas water demand of the native vegetation starts with the arrival of the first monsoon rainfall. The different root water uptake dynamics of rubber result in distinct depletion of deeper layer soil moisture. Traditional evapotranspiration and soil moisture models are unable to simulate this specific behavior, thus a different conceptual model is needed to predict hydrologic changes due to land use conversion in the area.
H51H-0870
Soil Moisture Patterns on Conifer and Aspen Hillslopes in an Alpine Catchment of Northern Utah
Conifer invasion has been considered a significant factor explaining the decline in the extent of aspen cover in Utah. A consequence of this change in vegetation type may be lower water yield from mountain headwater catchments in the region. In this study, soil moisture patterns and snow accumulation were compared between conifer and aspen dominated hillslopes to understand the differences in (1) the timing of surface water inputs from snow melt and (2) how the wetting front of these inputs travels through the soil profile vertically and laterally. Nested soil moisture probes were installed in the summer of 2006 on two hillslopes adjacent to a stream in a headwater catchment of the Ogden River basin. One transect of eight nests lies perpendicular to the stream in an aspen dominated stand and another in a conifer stand. Nests consist of three probes at 5, 20 and 100cm or as deep as we could install the probe (45 to 110cm). Peak snow water equivalent was measured along each of the transects at a 4m interval. Snow surveys revealed similar average snow water equivalents between aspen and conifer stands but a greater variability of snow pack in the conifer stand. Soil profiles under the two vegetation types were found to vary greatly with aspen having a deeper profile with a higher clay content and conifer having a shallow and extremely rocky profile. Hourly volumetric soil water contents showed that water inputs from fall precipitation did not penetrate to the deeper layers of the soil profiles (60 to 100cm) in either vegetation type. At 20cm depth, fall precipitation increased saturation to 30% in the conifer profiles and 60%, twice as much, in the aspen. These levels of saturation were sustained throughout the winter months after which peak soil moisture in the spring showed all depths reaching 90% or greater saturation in both vegetation types. Spring snow melt water saturated the entire profile and sensors at all depths reacted to melt water inputs. Peak soil moisture in the spring snow melt came earlier on the aspen hillslope, probably because of the high saturation level sustained over the winter. The study has shown that for most of the year water inputs remain in the soil profile and only during the spring snow melt is there a chance for percolation to deeper groundwater storage. Because the degree of saturation over the winter was greater in the aspen hillslope, the duration of peak soil moisture was greater in the aspen than in the conifer hillslope.
H51H-0871
Native Trees and Salt Cedar: Quantifying Transpiration at Intermittent and Perennial Streamflows on the San Pedro River
Native cottonwood-willow forests that historically dominated south-western riparian areas are being replaced by salt cedar (Tamarix ramosissima) on the majority of regulated western rivers. Some studies of salt cedar have indicated its water use is considerably greater than native trees and depletes alluvial aquifers of groundwater; however, other studies have shown low to moderate water use by salt cedar. Results have varied on temporal and spatial scales making it difficult to draw firm conclusions. We compared whole plant transpiration by native riparian cottonwood (Populus fremontii) trees and salt cedar in co-occurring communities at the upper and lower San Pedro River in Arizona during 2006 and 2007, respectively. Water use by both species was monitored and quantified using the heat balance sap flow technique at intermittent and perennial reaches during the pre- monsoon season, a period of high atmospheric water demand. Our 2006 measurements in a riparian transition zone at an intermittent reach of the San Pedro River appeared to differ with earlier studies that salt cedar has higher transpiration rates, as cottonwoods and salt cedar demonstrated similar, low transpiration rates. However transpiration results from a 2007 study on these same species at a perennial reach of the San Pedro River indicate significantly higher transpiration by salt cedar and moderate increases for cottonwoods compared to the intermittent site.
H51H-0872
Reconstructing Two Decades of Aquatic Invertebrate Diversity and Density along an Alluvial Plain River
Lotic invertebrate communities vary in time and space in response to flow variability. When stream flow data exist they can be used to predict community responses to flow conditions. It is rare, however, that long-term, high- frequency hydrologic data exist for more than one or a few points along a river. This data scarcity in space restricts our understanding of the spatial variability of biotic response in rivers whose hydrology varies greatly along their course. To explore space-time variation in flow-invertebrate relationships, we reconstructed two decades of daily river flows down the length of the Selwyn River mainstem as it crosses the Canterbury Plains of SE New Zealand. This 60 km-long mainstem is perennial at the upper and lower ends and ephemeral and intermittent in its middle reaches. Flow at 18 cross-sections along the mainstem was calculated as a function of the flow at two stage recorders located in the perennial reaches. Data from 38 months of intermittent gauging at the cross- sections was used to calibrate model parameters. Flow interpolation between cross-sections provided longitudinal continuity. Hydrologic metrics (e.g., frequency and duration of dry and wet phases) were derived from the reconstructed time series. These metrics were used to test relationships between invertebrate taxonomic richness and density and hydrological conditions at 15 sites along the river where invertebrate samples had been collected. Several hydrologic metrics were significant predictors of the invertebrate metrics. Multivariate linear models were used to hind-cast invertebrate richness and density in time and space along the entire study domain.
H51H-0873
Using Multiple Watershed-scale Dye Tracing Tests to Study Water and Solute Transport in Naturally Obstructed Stream Channels
Temporary storage of surface water at channel sides and pools significantly affects water and solute transport downstream in watersheds. Beavers, natural "stream channel engineers", build dams which obstruct stream flow and temporarily store water in small to large ponds within stream channels. These ponds substantially delay water movement and increase the water residence time in the system. To study how water and solutes move through these obstructed stream channels, we did multiple dye tracing tests at Cherry Creek, a main tributary to Red Canyon Creek (Wind River Range, Wyoming). First we surveyed beaver dam distributions in detail within the study reaches. We then introduced dyes four times from July 2nd to 6th, 2007 using a scale-up approach. The observation site was fixed at the mouth of Cherry Creek, and 1.5 grams of Rhodamine WT (RWT) dye was injected sequentially at upstream sites with increasing test reach length. The reach lengths scaled up from 500m to 2.5 km. A field fluorometer recorded RWT concentrations every 15 seconds. The results show non-linear decreases of the peak concentration of the dye tracing cloud as the reach scaled up. Also, the times to 1.) the arrivals of the leading edges (Tl), 2.) the peak concentrations (Tp) and 3.) the tailing edges (Tt) and 4) the durations of the tracer cloud (Td) behaved non-linearly as function of length scale. For example, plots of arrivals of leading edges and tailing edges with scale distance appear to define curves of the form; Tl=27.665e1.07× Distance (r2=0.99) and Tt=162.62e0.8551× Distance (r2=0.99), respectively. The greatest non-linearity occurred for the time of tailing and the least for the time of leading edge. These observations are consistent with what would be expected with greater density of dams and/or storage volumes as the reach length increased upgradient. To come to a first approximation, we are currently modeling the breakthrough curves with the solute transport code OTIS to address the relative differences in average travel velocity, longitudinal dispersion, and storage parameters from the mouth to the headwaters of the creek.
H51H-0874
Field experiments to assess the links between bed-material entrainment and invertebrate drift in gravel-bed rivers during floods
Entrainment of sediment from small (m2) patches of fine material in gravel-bed rivers is poorly understood. Although morphological features such as these patches play an important role in sediment transport, bedload studies have largely neglected the incipient motion of sediment from within them. Here we present the details of a newly developed device (a portable flume) that allows controlled flow manipulation in natural river channels, producing low bedload transport rates from patches of stream bed. We have already demonstrated (Gibbins et al., 2007) that such low rates produce marked invertebrate drift from patches and hence have ecological relevance. He we show how the device can be used to address questions related to incipient bedload transport and its hydraulic controls, questions which conventional approaches are unable to tackle adequately. The flume is placed in the river-bed, isolating the target patch of fine material. When the hinged doors fitted to the upstream end of the flume are opened, more water is funnelled into the flume, increasing hydraulic forces acting on the target patch. To produce further increases, a Perspex sheet can be slid vertically in an out of the flume, leaving a gap between its base and the stream bed. Water is forced under pressure between the bottom of the sheet and the stream bed, increasing velocity and shear stress over the patch. Field experiments using the flume were carried out in the Ribera Salada River (NE Iberian Peninsula), an upland unmodified stream in the Ebro basin. These indicated that the increment in near-bed velocity over the patches averages 120%, with velocity >2 m/s in some cases. The maximum bedload transport rate created by the manipulations was close to 7 g/ms, a value typically attained during the early stages of floods in the study section of the Ribera Salada. Simultaneous measurements of invertebrate drift during the experiments indicated that a marked increase in the loss of animals from the bed occurs at the point when sediment becomes unstable and bedload transport is initiated. Calculations suggest that at the highest shear stresses created by the experiments (i.e. 40 N/m2), the river-bed may be denuded of animals within a 30 minute period. The preliminary experiments in the Ribera Salada demonstrate the potential of the flume to study patch entrainment and sediment transport in patchy rivers, as well as to provide insights into the ecological consequences of these physical processes.
H51H-0875
Ecohydrological Controls on Intra-Basin Alpine Subarctic Water Balances
In the mountainous Canadian subarctic, elevation gradients control the disposition of vegetation, permafrost, and characteristics of the soil profile. How intra-basin ecosystems combine to control catchment-scale water and biogeochimcal cycling is uncertain. To this end, a multi-year ecohydrological investigation was undertaken in Granger Basin (GB), a 7.6 km2 sub-basin of the Wolf Creek Research Basin, Yukon Territory, Canada. GB was divided into four sub-basins based on the dominant vegetation and permafrost status, and the timing and magnitude of hydrological processes were compared using hydrometric and hydrochemical methods. Vegetation plays an important role in end-of-winter snow accumulation as snow redistribution by wind is controlled by roughness length. In sub-basins of GB with tall shrubs, snow accumulation is enhanced compared with areas of short shrubs and tundra vegetation. The timing of melt was staggered with elevation, although melt-rates were similar among the sub-basins. Runoff was enhanced at the expense of infiltration in tall shrub areas due to high snow water equivalent and antecedent soil moisture. In the high-elevation tundra sub-basin, thin soils with cold ground temperatures resulted in increased surface runoff. For the freshet period, the lower and upper sub-basins accounted for 81 % of runoff while accounting for 58 % of the total basin area. Two-component isotopic hydrograph separation revealed that during melt, pre-event water dominated in all sub-basins, yet those with greater permafrost disposition and taller shrubs had increased event-water. Dissolved organic carbon (DOC) spiked prior to peak freshet in each sub-basin except for the highest with thin soils, and was associated with flushing of surficial organic soils. For the post-melt period, all sub-basins have similar runoff contributions. Solute and stable isotope data indicate that in sub-basins dominated by permafrost, supra-permafrost runoff pathways predominate as flow pathways are confined above the permafrost aquitard. In contrast, lower elevation zones supply runoff via deeper subsurface flow pathways with increased levels of dissolved solutes. With regards to DOC, sub-basins dominated by permafrost supply the bulk of DOC to the stream because of near-surface pathways. Results highlight the importance of vegetation, the soil profile and frozen ground status in controlling hydrological and hydrochemical fluxes. Future changes in vegetation, which are occurring rapidly in the subarctic, are expected to have a large impact on the hydrology and biogeochemistry of these systems.
H51H-0876
Potential evapotranspiration viewed from the perspective of constructal theory
In this study we investigated the evapotranspiration phenomena from the point of view of the thermodynamic constructal theory. When applied to the vegetation cover at the surface of the ground, the constructal law states that plants are adapting to gain maximum access to available resources and at the same time to minimize the internal irreversibilities of the system, that is the entropy generation rate. The analysis of plants under heat constraints showed that the optimal state of plant given potential conditions can be achieved if the optimal plant temperature defining the maximum productivity state is equal to the average air temperature which in turn has to be equal to the average vegetation temperature. To test this hypothesis, we modeled the vegetation cover as a thermodynamic system exchanging heat and mass with the atmosphere. The input data for the model were provided by the FLUXNET network. For 32 sites around the globe, we investigated the variation of the vegetation state with respect to the stomatal resistance rs. We showed that plants can thermoregulate their temperature by means of the stomatal resistance and that there is a critical stomatal resistance, rsmin, corresponding to a minimum entropy generation rate. We showed that, for sites characterized with highly evolved plants, the optimal thermodynamic state defined by rsmin is indeed selected by plants when potential condtions are met. When potential conditions are not met, plants will adapt such that the average state of vegetation will be as close as possible (given external constraints) to the optimal state defined by rsmin.
H51H-0877
Green Alder Pattern in Relation to Slope-Area Scaling Regimes of a Headwater Basin in the Eastern Italian Alps
The landscape of headwater alpine basins is strongly influenced by erosion processes. The scaling relationship between the local slope of a given point on the landscape and its drainage area reveals information about the dominant erosion process over geomorphic time scales. There has been significant research literature which documents how vegetation distribution is coupled with local topography. Understanding the interrelationship between the vegetation, especially between some plant species than others with local topography will help us to better understand how the landscape change due to a specific geomorphic process is related to vegetation change. In this paper a steep Alpine debris flow-landslide dominated headwater catchment with uniform lithology substrata, and vegetation characterized by various grass, shrub, and forest species has been considered. The basin was chosen as study area because it is representative of the lithological and physiographical conditions frequently observed in the Carnia region (Eastern Italian Alps). The work has focused on the analysis of scaling regimes of local slope versus contributing area in relation to each vegetation types with particular attention to the spatial distribution of Alnus viridis, the main shrub species present in the area. Alnus viridis, also known as Green Alder, is an early successional shrub growing up to 4 m that invades screes, landslide scree, avalanche debris on talus slopes, avalanche slide paths and pastures in the subalpine zone of the Alps. It is widespread on moist, north-exposed medium-steep slopes on silicious bedrock at an altitude ranging from 1500 to 2000 m a.s.l.. LiDAR-derived DSM (Digital Surface Model) served as the basis to evaluate the distribution of vegetation canopies. The LiDAR bare-ground elevation points were used for the DTM (Digital Terrain Model) interpolation at the same resolution of the DSM. The results revealed that there is significant relationship between the local slope and drainage area depicting the natural location of Green Alder and other vegetation types. Profound landslide/debris flow topographic signature is detected in areas where Green Alder prevails. Its pioneering ability to colonize the landslide scars and headwater channel hollows, makes it as a key species in the analysis of geomorphic processes at the hillslope/valley transition. The work has broad implications in geomorphology, landscape ecology, landscape evolution with vegetation dynamics for studies in high-altitude extreme-climate Alpine regions.
H51H-0878
Long-term Ecohydrologic Pattern Optimization at the Hillslope Scale
The optimization of vegetation structure at the plot scale has been reviewed in various terms, including the minimization of water stress and maximization of productivity across the different systems. However, ecosystem patches exist as part of a drainage chain, or catena, that share some degree of dependency on productivity and resource uses with other patches along flowpaths. Especially at a mountainous forest where lateral water fluxes through shallow soil columns are dominant, it will increase the heterogeneity of spatial distribution and the dependence on the topographic gradients. Ecosystem patches linked along hydrologic flowpaths would optimize their vegetation density to local climate, soil and topographic conditions in the absence of significant human manipulation. Therefore, spatial pattern of current vegetation density at the hillslope scale is a good estimator for spatio-temporal dynamics of the root layer moisture. We estimated the spatial distribution of vegetation density from the relationship between point-measured leaf area index information and vegetation indices from the fine- scale remote sensed data. And then, eco-hydrologic model (RHESSys) was deliberately optimized not only with point measurements (e.g. streamflow data, TDR soil moisture data), but also with spatial information (e.g. spatial vegetation density) within the multi-criteria concept. Behavioral parameter spaces related to the spatial distribution of rooting depth were evaluated with rooting depth measurements from soil pits. We tried to examine which hillslope level properties the catenary sequence of patches optimize (e.g. maximizing productivity or water use efficiency, minimizing water stress) and their meaning in terms of susceptibility to the climate fluctuation (e.g. drought) with long-term ecohydrological model simulation.
H51H-0879
Scaling Heterogeneous Soil Hydraulic Properties Using Canopy/Interspace Distributions in a Mojave Desert Ecosystem
Desert piedmonts are a mosaic of interspersed vegetation and open soil or interspaces. The distribution of perennial plants in arid regions is ultimately tied to available soil moisture. Surface soils in deserts undergo different pedologic processes depending on the proximity to plant canopies. For example, bioturbation and the accumulation of aeolian material and organic matter around plant canopies result in a mound-like formation around perennial plant canopies, whereas interspace areas tend to be microtopographic low points with reduced organic matter. Differences in soil structure and texture in undercanopy and interspace microsites can be significant, thus affecting infiltration, plant available water and ET. In this study, we sought to answer the questions: do soil hydraulic properties vary predictably from the undercanopy to interspace at the plot scale, and if so, how does this heterogeneous parameter field affect large-scale hydrologic processes of a heterogeneous landscape in the Mojave Desert? To answer these questions, a total of four radial transects was run on each of six shrubs (three each of L. tridentata and L. paladin) at the Mojave Global Change Facility (MGCF), located at the Nevada Test Site, USA. The extent of heterogeneity in soil physical and hydraulic properties (texture, bulk density, hydraulic conductivity functions K(h)) was measured across microsites by soil sampling and analysis, and by using up to 7 mini-disk tension infiltrometers (MDTI) spaced at 25-cm increments in linear array across a distance of 150 cm. Significant gradients of soil physical and hydraulic properties were observed from canopy to interspace microsites at 1.2 times the mean mound diameter. Despite a decrease in bulk density and fines under shrub canopies, a consistent trend of increasing K(h) and decreasing Gardner's alpha with increasing radial distance from shrubs was measured. Using the results of observed gradients around canopies, hydraulic property variability, and the large scale vegetation cover and distribution, two different methods of upscaling ( p-norm and site-specific pedotransfer function [PTF]) were compared during simulations of the ecosystem water balance. Results showed that effective K(h) from the p-norm approach resulted in a similar total water balance compared to the PTF method, but partitioning of water to root uptake differed significantly.
H51H-0880
A New Technique for Up-scaling Sap Flow Transpiration Measurements to Stand or Landscape Scale Fluxes
Measurements of individual tree transpiration, obtained using the sap flow technique, are easier to collect and less expensive than other traditional measurements of ecosystem evapotranspiration, such as eddy-covariance and lysimetery. Up-scaling these point measurements to a stand or a landscape level, however, is a challenge, especially in water-controlled ecosystems. At these scales, sap flow cannot be treated solely as a function of diameter; available soil moisture strongly influences transpiration, and this can vary considerably across a landscape. In this study, geostatistical and partitional clustering methods were used to locate a network of sap flow and soil moisture sensors across a California Oak Savanna. Eight "representative trees" were monitored; each was systematically selected to represent a subgroup of the population within a 200 x 200 m plot. All trees in a subgroup had similar diameters and soil moisture status and were presumed to have correspondingly similar sap flow. The sensors collected half-hour data over the course of the 2007 growing season, during which unusually low rainfall occurred. The sap flow data for each tree were transformed into specific water flux, and a total stand level water flux was computed at hourly and daily time-steps. Large diameter trees in wet areas typically contributed to almost 40% of the total stand flux, while they accounted for less than 10% of the total population. To test the method, these fluxes were then compared to the measurements of stand level tree transpiration collected using the eddy- covariance towers on site. In the future, this technique could be used to measure transpiration of targeted trees over a broader area or in terrain or situations where eddy-covariance is not feasible.
H51H-0881
Effects of Soil Erosion on Ecohydrology of Constructed Slopes From Opencast Coal Mining in a Mediterranean-Continental Environment.
Numerous works have revealed strong links between hydrological processes, soil moisture, and the structure and function of biological communities. Nevertheless, the influence of soil erosion on soil-water-plant interactions has been poorly documented, particularly on constructed slopes, wherein soil erosion has a key role for restoration success. The main objective of this work is to investigate the eco-hydrological implications of soil erosion on constructed slopes from the opencast coal mining in a Mediterranean-Continental environment: the Teruel coalfield (Spain). Water deficit is the main limiting factor for revegetation success in Mediterranean-Continental environments. Soil moisture in these artificial systems is controlled by feedback mechanisms between soil erosion and vegetation. Our hypothesis states that a major effect of soil erosion on plant communities deals with the increase of the climatic water deficit. Thus, the mechanisms involved are as follows: the crust formation, the reduction of soil superficial roughness and the efficient overland flow evacuation by means of rill network. The final result is a net reduction of water availability for plants through the increase of run-off and the decrease of infiltration and re- infiltration processes at slope scale. Moreover, the lack of vegetation development leads to increase run-off and soil erosion. This work has been carried out in five constructed slopes which were reclaimed with similar treatments and substrata, but differ in vegetation cover and erosion (from 0 up to 120 t/ha/year). These differences are mainly triggered by different amounts of run-on coming from external sources as mining tracks and banks. During 2005- 06 hydrological year, we monitored run-off and sediment yield as well as soil moisture and vegetation traits (cover, biomass, water potential) on these slopes. Our results reinforce the general hypothesis. In this way, on the least eroded slopes, biological control of hydrological processes leads to maximize infiltration rates, increasing the net water availability for plant growth. On the other hand, the most eroded slopes, physically controlled by intensive run-off generation, show the lowest water availabilities, increasing the water stress borne by vegetation during the end of spring and summer.
H51H-0882
Climate Change Effects on Temperate Forest Ecosystems: Putting Groundwater Into the Equation
Many attempts have been made to predict the influence of changing climate on ecosystems. Models developed for this purpose most often concentrate on vegetation in connection with soil moisture but usually omit groundwater. However in temperate climate groundwater can have a profound effect on the reaction of vegetation to climate change, because it strongly influences the spatial temporal distribution of soil moisture and therefore water and oxygen stress of vegetation. Here we focus on the effect of climate change on the zonation of vegetation and groundwater dynamics along a hill slope. To study this we developed a fully coupled hydrological-vegetation model, for a temperate forest ecosystem. The vegetation model is based on the carbon assimilation model of Farquhar et al. (1980) and the extension of Daly et al. (2004), which includes transpiration for the vegetation and accounts for the response to low soil moisture content. Further extensions have been made to account for vegetation response to high soil moisture contents due to high groundwater levels, vegetation growth and light competition. To simulate the hydrology the saturated-unsaturated flow model STARWARS (van Beek 2002) has been used. This coupled model was compared to measured semi-hourly flux tower data of H2O and CO2, showing good results. Long simulation runs of 1000 years were performed to study the effect of climate change. The results show that changing rainfall regimes have a small but significant influence on transpiration and carbon assimilation of the vegetation and groundwater recharge, but it has a large influence on interception evaporation. Change in precipitation affects vegetation types the most in areas where groundwater is near the surface. Due to the buffering capacity of both the vegetation and the hydrological system the effect on growth rate of vegetation is small, resulting in relatively small changes in the zonation of vegetation between dry adapted and wet adapted tree species. This model shows the importance of using a coupled groundwater vegetation model in temperate lowland areas. The coupled hydrological-vegetation model allows for detailed studies of changes in spatial temporal patterns of vegetation under changing climate.