Modelling Of Water Fluxes Research Articles

DYNAMICS OF LEAF AREA INDEX AND CANOPY OPENNESS FOR THREE FOREST COMMUNITIES IN THE WARM TEMPERATE ZONE OF CHINA

Aims Leaf area index (LAI) and canopy openness are two important biophysical parameters of canopy structure related to water and carbon fluxes and light interception. They are not only key variables in ecological and climate models, but also important indicators of forest ecosystem productivity. Accurate estimates of LAI and canopy openness are required in studies of ecophysiology, atmosphere-ecosystem interactions, and global change. Many researches have been done in Dongling Mountain in the warm temperate zone; however, fewer studies have focused on LAI and canopy openness. The objective of this study is to measure and analyze the dynamics of LAI and canopy openness of three typical forest communities in this area. Results will be useful in testing and validating LAI estimations based on remote sensing, providing reference for study of canopy heterogeneity and its effects and modeling carbon, water and energy fluxes from stand to landscape levels. Methods We used hemispherical photographs taken from the ground through a fisheye lens for indirectly assessing LAI and canopy openness. Photos were taken in May, August, September and November 2005 in deciduous broad-leaved forest (DBF), Larix principis-rupprechtii forest (LF) and Pinus tabulaeformis forest (PF). Important findings The LAI of DBF and LF increased gradually with plant growth and development to August. The lowest value of canopy openness appeared in November. DBF maintained a higher LAI in August and was more open in November compared to LF. For PF, we observed few changes of LAI and canopy openness, which we attribute to leaf retention of this evergreen species. In all forests, canopy openness varied inversely with LAI. The correlation was exponential and significant; therefore, canopy openness is a good indicator of LAI in these forests.

Slab dehydration in the Earth's mantle transition zone

Because of the high water solubility in wadsleyite and ringwoodite, the mantle transition zone is possibly a large water reservoir. The potentially high water content of the Earth's mantle transition zone is a key element of several global mantle dynamic models. Nevertheless, to keep the transition zone relatively wet, the tendency for convection to distribute water over the entire mantle has to be offset by other mechanisms. One such mechanism could be linked to the dehydration of the slab. Studies of slab dehydration mainly focus on hydrous minerals phase diagrams that provide the depth of water exsolution. In this study, we investigate the water diffusion process that occurs prior to such exsolution. The diffusion process is controlled by both chemical gradients and thermal gradients via the temperature dependence of water solubility. We have addressed how this diffusion phenomenon influences slab water transport into the transition zone by developing a theoretical model of coupled thermal and chemical diffusion in a flat lying slab stalled in the transition zone. Model solutions demonstrate that even if intrinsic water solubility is decreasing with increasing temperature (which normally would act to impede a chemical diffusion from the slab), the water concentration profile adjusts to reduce the adverse effects of temperature-dependent solubility. The self-adjustment entails the concentration of water reaching a local maximum below the top edge of the stagnant slab; under specific conditions, this concentration maximum can reach the solubility limit and thus exsolve fluid (i.e. create of hydrous melt or aqueous silicate fluid). Consequently, model water flux computations are relatively independent of solubility variation and indicate that much of the water flux introduced by relatively well hydrated slabs is taken up into the transition zone by diffusion.

Effect of aggregating spatial parameters on modelling forest carbon and water fluxes

Estimating spatial variability of carbon and water fluxes is an essential task in ecological modelling. In this article, the sensitivity of carbon and water fluxes to the spatial variability of biochemical and structural properties of canopies is assessed in beech forests using a process-based model (CASTANEA). Firstly, a sensitivity analysis was carried out by varying simultaneously a combination of six key parameters within a realistic range: the above ground wood biomass ( B), the soil water reserve ( S WR), the canopy clumping factor ( C F), the leaf area index ( L), the leaf mass per area of sunlit leaves ( M sun) and the leaf nitrogen content ( N). Secondly, three spatial scales of variability were considered using three study sites whose areas ranged from 0.8 to 1000 ha. The first area studied was a heterogeneous stand located in old-growth forest in Fontainebleau (south of Paris, France). The spatial variability of the biophysical and biochemical ecosystem characteristics in 80 m 2 out of 100 m 2 was determined. For the two other case studies, we selected a sample of nine plots in which the key input parameters were measured. Sensitivity analysis indicated that photosynthesis and ecosystem respiration show a moderate non-linear response to L, S WR and B. In spite of these non-linear responses, the three case studies revealed that using parameters averaged over the whole area, induces only a slight bias in the estimation of carbon fluxes and almost no bias in the estimation of water fluxes. The implication of the low sensitivity of carbon and water fluxes to parameter aggregation is discussed in relation to the general problem of the scaling up fluxes from ecosystems to large forest regions.

Cross-shore velocity shear, eddies and heterogeneity in water column properties over fringing coral reefs: West Maui, Hawaii

A multi-day hydrographic survey cruise was conducted to acquire spatially extensive, but temporally limited, high-resolution, three-dimensional measurements of currents, temperature, salinity and turbidity off West Maui in the summer of 2003 to better understand coastal dynamics along a complex island shoreline with coral reefs. These data complement long-term, high-resolution tide, wave, current, temperature, salinity and turbidity measurements made at a number of fixed locations in the study area starting in 2001. Analyses of these hydrographic data, in conjunction with numerous field observations, evoke the following conceptual model of water and turbidity flux along West Maui. Wave- and wind-driven flows appear to be the primary control on flow over shallower portions of the reefs while tidal and subtidal currents dominate flow over the outer portions of the reefs and insular shelf. When the direction of these flows counter one another, which is quite common, they cause a zone of cross-shore horizontal shear and often form a front, with turbid, lower-salinity water inshore of the front and clear, higher-salinity water offshore of the front. It is not clear whether these zones of high shear and fronts are the cause or the result of the location of the fore reef, but they appear to be correlated alongshore over relatively large horizontal distances (orders of kilometers). When two flows converge or when a single flow is bathymetrically steered, eddies can be generated that, in the absence of large ocean surface waves, tend to accumulate material. Areas of higher turbidity and lower salinity tend to correlate with regions of poor coral health or the absence of well-developed reefs, suggesting that the oceanographic processes that concentrate and/or transport nutrients, contaminants, low-salinity water or suspended sediment might strongly influence coral reef ecosystem health and sustainability.

The use of dynamic modeling in assessing tritium phytoremediation

To minimize movement of tritium into surface waters at the Mixed Waste Management Facility at the Savannah River Site, tritium-contaminated groundwater released to the surface along seeps in the hillside is being retained in a constructed pond and used to irrigate forest acreage that lies over the contaminated groundwater. Management of the application of tritium-contaminated irrigation water needs to be evaluated in the context of the large amount of rainfall relative to evapotranspiration, the strong seasonality in evapotranspiration, and intra-annual and interannual variability in precipitation in this region. A dynamic simulation model of water and tritium fluxes in the soil-plant-atmosphere continuum was developed to assess the efficiency (tritium transpired/tritium applied) of several irrigation management strategies. The model was parameterized using soil-water content data measured at 18 sites for the first year of the project and evaluated using tritium activity measurements made at the same 18 sites over 2.5 yr. The model was then used to evaluate several irrigation strategies. The 25-yr efficiencies (tritium transpired/tritium applied) of the irrigation strategies were related to the quantity of irrigation water applied. There was a strong (r2 = 0.99) negative linear relationship between irrigation water applied and efficiency. When a quasi-steady state has been reached in the system, the annual efficiencies of all the irrigation strategies were negatively correlated with annual rainfall. Quantification of these relationships allows irrigation managers to choose irrigation strategies based on desired long-term system efficiency, which differ with climate and irrigation strategy.

ArcEGMO-URBAN — Hydrological model for point sources in river basins

The new model ArcEGMO-URBAN aims at deterministic and spatiotemporal modelling of water, nitrogen and phosphorus fluxes from all urbanised areas of a river basin considering all potential sources. Pollution loads are calculated for discrete urban patches and balanced on the level of hydrological subbasins. Modelling results can be defined by the user of any level of spatial and/or temporal aggregation, e.g. matter balances for river basins or river sections and years or months, respectively. To process spatial data, a Geographic Information System is linked to the model. Information on urban land use and general characteristics of river basins is based on digital coverages, partly generated from remote-sensing data. Moreover, statistical data, e.g. on population, sewer systems, wastewater treatment plants etc. are included. Stormwater runoff from impervious surfaces is calculated as one input to the sewer network. Wastewater is considered with its main sewer system, pumping stations and treatment plants. Finally, the discharge is balanced for discrete river sections. Modelling results attest ArcEGMO-URBAN its ability to realistically quantify matter fluxes and major pollution sources as well as their seasonal variation. This makes the model an applicable tool for the analysis of scenarios with e.g. varying population distribution or climatic and technological conditions.

Particle Density of Aspen, Spruce, and Pine Forest Floors in Alberta, Canada

Soil particle density (ρs), the ratio of the mass of soil solids to the volume of solids, is used to derive such properties as soil porosity and heat capacity, which are critical to understanding and modeling water, energy, and nutrient fluxes through forested landscapes. Values of forest floor ρs and organic matter particle density (ρo) vary widely in the literature, so it is difficult to know which values are appropriate under different circumstances. We measured ρs, ρo, bulk density (ρb), loss‐on‐ignition (LOI), and total C for a range of forest types in northern Alberta, Canada. Although samples were obtained from a diverse range of forest types, our measured values of forest floor ρs (1.52–1.60 Mg m−3) and calculated values of ρo (1.41–1.44 Mg m−3) showed no statistically significant differences among stand types. The measured values of ρs and ρo were greater than many values in the literature, potentially due to differences in measurement methods. Measurements of ρs should be performed across a range of forest floor and organic soil types to refine our understanding of the variation in this fundamental soil property.

Temporal persistence of spatial patterns in throughfall

Spatial and temporal variability of throughfall beneath forests are potentially important controls on soil processes, watershed hydrology, and biogeochemistry. We used a set of 94 rain gauges to measure variability of throughfall beneath three forest stands in the Pacific Northwest, USA. The length scale over which throughfall amounts were correlated (spatial correlation lengths) was between one-half- and one crown diameter in mid-age and old stands of conifers. In a deciduous stand, the spatial correlation length was about one crown diameter when in leaf condition and throughfall was not correlated spatially in leaf-off condition. Spatial patterns of storm-total throughfall were temporally stable in two ways: semivariograms, which provide a measure of the continuity of a spatial phenomenon, were similar among storms, and throughfall amounts of an individual gauge could be predicted relative to the plot average. Time stability plots of throughfall amounts, normalized with respect to mean and variance, were useful for comparing temporal persistence of spatial throughfall variability among stands. Together, semivariograms and time stability plots appear to be suitable descriptors of throughfall variability for modeling water flux at the soil surface.

Impacts of Climate Variation and Catchment Area on Water Balance and Lake Hydrologic Type in Groundwater-Dominated Systems: A Generic Lake Model

Lakes are a major geologic feature in humid regions, and multiple lake hydrologic types exist with varying physical and chemical characteristics, connections among lakes, and relationships to the landscape. The authors developed a model of water fluxes through major components of groundwater-dominated lake catchments in a region containing thousands of lakes, the Northern Highland Lake District (NHLD) of northern Wisconsin and the Upper Peninsula of Michigan. The model was calibrated with data from widely differing lakes using the same set of simple equations to represent the hydrologic type, water residence time, and amount and timing of stream and groundwater flows of representative lakes in today's climate. The authors investigated the sensitivity of the water balance of a set of three connected representative lakes and their catchments to systematic increases and decreases in the precipitation regime, and contrasted results using lake-specific morphometry to those for a lake having size and shape parameters typical of the region. Results indicate that a common set of equations can successfully represent major water balance characteristics of the three basic lake hydrologic types (hydraulically mounded, groundwater flowthrough, and drainage) in the NHLD. Sensitivity of modeled lakes varied by lake type, with drainage lakes more strongly buffered against substantial hydrologic changes in extreme climate scenarios. Catchment-scale water budgets differed substantially among lakes of different types, yet can be understood along a continuum of relative catchment size. These results suggest that a simple model of lake and catchment water balance can be extended to entire lake districts, where the detailed morphometry of most lakes is not well known.

Assessment of the effects of spatial resolutions on daily water flux simulations

Impacts of spatially distributed precipitation and soil heterogeneity on modeling water fluxes at different spatial resolutions are investigated using the Three-layer Variable Infiltration Capacity (VIC-3L) land surface model at the Blue River watershed in Oklahoma. In this study, hourly grid-based NEXRAD (Next Generation Radar) Stage III radar precipitation data approximately at 4×4 km 2 resolution are used to compute daily precipitation at spatial resolutions of 1/32, 1/16, 1/8, 1/4, 1/2 and 1 degree based on an area weighted average method. Soil parameters at the corresponding six spatial resolutions are derived from the State Soil Geographic (STATSGO) soil data. The forcing data of daily maximum and minimum temperature, wind speed, and vegetation parameters are disaggregated/aggregated directly to finer/coarser spatial resolutions based on the University of Washington (UW) data, which are gridded at 1/8 degree spatial resolution. Our study suggests that a critical spatial resolution for the VIC-3L model may exist for the study watershed. For spatial resolutions finer than the critical resolution, one does not necessarily obtain better model performance in terms of runoff, evapotranspiration, and total zone soil moisture with increasing spatial resolution if the VIC-3L model parameters are calibrated at each spatial resolution. Also, model parameters calibrated at a coarse resolution can be applied to finer resolutions to obtain generally comparable results. However, model parameters calibrated at finer resolutions cannot result in comparable results when applied to resolutions coarser than the identified critical resolution. In addition, while soil moisture of the total zone is more sensitive to the spatial distributions of soil properties, runoff and evaporation are more sensitive to the spatial distribution of daily precipitation at the watershed being studied.

A Reply to R. Keim’s Comment on “Measurement and modelling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada”

A Reply to R. Keim’s Comment on “Measurement and modelling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada”

Comment on “Measurement and modeling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada”

Comment on “Measurement and modeling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada”

A simple tropical ecosystem model of carbon, water and energy fluxes

A simple tropical ecosystem model (SITE) was developed to study the response of tropical ecosystems to environmental conditions. SITE fills the niche of an ecosystem model of intermediate complexity, sophisticated enough to be used to study the fast dynamics of tropical ecosystems, while simple enough to be used to introduce the ecosystem modelling concepts to students and inexperienced modellers. SITE is a dynamic model that incorporates several processes: canopy infrared radiation balance, solar radiation balance, aerodynamic processes, canopy physiology and transpiration, balance of water intercepted by the canopy, transport of mass and energy in the atmosphere, soil heat flux, soil water flux and carbon balance. It is structured with one canopy layer and two soil layers, and is forced by hourly data of temperature, radiation balance, precipitation, humidity and wind, and simulates the fluxes of CO 2, water and energy, as well as the dynamics of carbon in the ecosystem. For calibration and validation, we used fluxes of CO 2, water vapour and sensible heat, measured at a primary evergreen forest site in Eastern Amazonia, Brazil. Even though SITE is considerably less complex than other models of similar goals, it reproduces well the hourly variability of the fluxes of CO 2 and water vapour, and simulates the seasonal scale balance of those elements properly. SITE is available as a 1200-line FORTRAN code, and as a computer spreadsheet. We believe the model will be useful to help train the next generation of tropical ecologists in the use of ecosystem models.

The use of the language interface of R: two examples for modelling water flux and solute transport

Beyond its statistical usage, R is a noteworthy programming environment for C and Fortran code. We describe features of the programming interface and illustrate it by a simple example and a more complex example that is of interest on its own. The latter makes available SWMS_2D within the R environment, a free software for two-dimensional modelling of water fluxes and solute transport by Simunek, Vogel and van Genuchten (Technical Report 132, US Salinity Laboratory, Riverside, CA, 197pp.).

Measurement and modelling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada

During the growing-season of 1995 throughfall, stemflow, and canopy interception loss fluxes from a mature temperate deciduous stand in southern Ontario, Canada were observed to be 77.5±1.0, 3.7±1.3, and 18.8±3.8% of the 259.3 mm incident precipitation input, respectively. Simple empirical equations were derived for estimating the magnitude of each canopy water flux, while equations were also derived for estimating the spatial variability associated with throughfall and stemflow within this stand. The number of throughfall collectors required to sample this flux to within 5 and 10% of the mean at the 95% confidence interval was found to average 37 (range=17–81) and 10 (range=5–21), respectively for incident precipitation events >4 mm. For events <2 mm it was found that an unmanageable number of collectors would be required, on average, to adequately sample this flux. The Helvey and Patric [Water Resources Res. 1 (1965) 193] and the revised Gash [J. Hydrol. 190 (1997) 141] models were evaluated for further application within this forest-type. The Helvey and Patric model was found to systematically underestimate canopy interception loss for events >7 mm, while the revised Gash model was found to systematically overestimate this flux for events with corresponding rainfall intensities≥7 mm h −1. When optimised canopy storage capacities were applied to the revised Gash model the overestimate was reduced from 6.1 to 1.1 mm, or from 12.5 to 2.3% of the observed flux. The finding that the optimised canopy storage capacity value for storms with rainfall intensities≥7 mm h −1 is smaller than that derived for events with rainfall intensities <7 mm h −1 supports the Calder [J. Hydrol. 185 (1996) 363] hypothesis that the storage capacity of a stand is reduced when rain-drops and thus rainfall intensities are large. The need for increased research on the influence stand and meteorological factors have on this and other Gash model parameters is discussed.

Web-based modelling of energy, water and matter fluxes to support decision making in mesoscale catchments––the integrative perspective of GLOWA-Danube

The GLOWA-initiative (Global Change of the water cycle), funded by the German Ministry of Research and Education (BMBF), has been established to address the manifold consequences of Global Change on regional water resources in a variety of catchment areas with different natural and cultural characteristics. Within this framework, the GLOWA-Danube project is dealing with the Upper Danube watershed as a representative mesoscale test site (∼75.000 km 2) for mountain-foreland regions in the temperate mid-latitudes. The principle objective is to identify, examine and develop new techniques of coupled distributed modelling for the integration of natural and socio-economic sciences. The transdisciplinary research in GLOWA-Danube develops an integrated decision support system, called DANUBIA, to investigate the sustainability of future water use. GLOWA-Danube, which is scheduled for a total run-time of eight years to operationally implement and establish DANUBIA, comprises a university-based network of experts with water-related competence in the fields of engineering, natural and social sciences. Co-operation with a network of stakeholders in water resources management of the Upper Danube catchment ensures that practical issues and future problems in the water sector of the region can be addressed. In order to synthesize a common understanding between the project partners, a standardized notation of parameters and functions and a platform-independent structure of computational methods and interfaces has been established, by making use of the unified modelling language, an industry standard for the structuring and co-ordination of large projects in software development [Booch et al., The Unified Modelling Language User Guide, Addison-Wesley, Reading, 1999]. DANUBIA is object-oriented, spatially distributed and raster-based at its core. It applies the concept of “proxels” (process pixels) as its basic objects, which have different dimensions depending on the viewing scale and connect to their environment through fluxes. The presented paper excerpts the hydrological view point of GLOWA-Danube, its approach of model coupling and network-based communication, and object-oriented techniques to simulate physical processes and interactions at the land surface. The mechanisms and technologies applied to communicate data and model parameters across the typical discipline borders are demonstrated from the perspective of the Landsurface object. It comprises the capabilities of interdependent expert models for energy exchange at various surface types, snowmelt, soil water movement, runoff formation and plant growth in a distributed Java-based modelling environment using the remote method invocation [Pitt et al., Java.rmi: The Remote Method Invocation Guide, Addison Wesley Professional, Reading, 2001, p. 320]. The presented text summarizes the GLOWA-Danube concept and shows the state of an implemented DANUBIA prototype after completion of the first project-year (2001).

Modeling water transportation and storage in sapwood — model development and validation

I have described a single tree water flux model that combines the stomatal conductance model based on plant physiological knowledge with the concept of fluid mechanics used to describe water transportation and storage in sapwood. The model was parameterized and validated using field data of the water budget of Cryptomeria japonica [J. Jpn. For. Soc. 78 (1996) 66] determined by weighing a suspended cut tree. In their experiment, water transportation and storage in the sapwood and leaves and the sapwood potential were examined under ordinary conditions and extreme conditions where water uptake was stopped by sealing the cut stem surface. Numerical experiments show the information to reproduce the observed results: low intensity of stomatal response to decreasing leaf water potential and lower hydraulic conductance of the crown sapwood than the conductance of the trunk sapwood. The model reproduced the observed results under both ordinary and extreme conditions using only information of observed physical size and hydraulic properties, except for the information on low values of hydraulic conductance of the crown sapwood.

Gas exchange and hydraulic properties in the crowns of two tree species in a Panamanian moist forest

Hydraulic properties and gas exchange were measured in branches of two tropical tree species (Simarouba amara Aubl. and Tapirira guianensis Aubl.) in a moist lowland forest in Panama. Branch-level sapflow, leaf-level stomatal conductance, and water potential measurements, along with measurements of specific hydraulic conductivity of stems in crown tops, were used to relate hydraulic parameters to leaf conductance in two individuals of each species. Branches of the taller trees for each species (28 m, 31 m) showed much higher leaf-specific hydraulic conductance and leaf vapor-phase conductance than those of the smaller trees (18m, 23m). This was probably related to the leaf-to-sapwood area ratio in branches of taller trees, which was less than half that in branches of smaller trees. Dye staining showed evidence of massive cavitation in all trees, indicating that stomata do not control leaf water potential to prevent xylem cavitation in these species. Stomatal conductance of intact leaves also appeared to be insensitive to leaf area removal treatment of nearby foliage. Nevertheless, a simple mass-balance model of water flux combining hydraulic and vapor transport was in close agreement with observed maximal vapor-phase conductance in the four trees (r2=0.98, P=0.006). Our results suggest that the major organismal control over water flux in these species is by structural (leaf area) rather than physiological (stomatal) means.

Modelling water fluxes for the analysis of diffuse pollution at the river basin scale

Diffuse pollution is a significant and sometimes even major component of surface water pollution. Diffuse inputs of pollutants to the surface water are related to runoff of precipitation. This means that the analysis of diffuse pollutant fluxes from the land surface to the surface water requires an analysis of water fluxes. In this paper we have modelled the average long-term total runoff, groundwater recharge index and groundwater residence times for two large European river basins (Rhine and Elbe). We applied and compared two independently developed and recently published methods. We found that with the available large-scale databases and methods we could simulate successfully the regional patterns of the average long-term total runoff. The reported groundwater recharge indices and groundwater residence times should be interpreted as estimates based on available knowledge and databases. They do not represent absolute values, but illustrate the possible travel times and spatial patterns of the different runoff components that have to be taken into account for the analysis of diffuse pollution at large regional and temporal scales. Copyright © 2000 John Wiley & Sons, Ltd.

Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy

A technique used for hydroponics was adapted to measure instantaneous root water uptake from the soil for a leaf succulent CAM species, Agave deserti. Comparisons were made to previously modelled water fluxes for A. deserti and to Encelia farinosa, a non-succulent C3species. Net CO2uptake and transpiration forA. deserti under well-watered conditions occurred primarily at night whereas root water uptake was relatively constant over 24 h. Leaf thickness decreased when transpiration commenced and then increased when recharge from the stem and soil occurred, consistent with previous models. A drought of 90 d eliminated net CO2uptake and transpiration and reduced the water content of leaves by 62%. Rewetting the entire root system for 7 d led to a full recovery of leaf water storage but only 56% of maximal net CO2uptake. Root water uptake was maximal immediately after rewetting, which replenished root water content, and decreased to a steady rate by 14 d. When only the distal 50% of the root system was rewetted, the time for net CO2uptake and leaf water storage to recover increased, but by 30 d gas exchange and leaf water storage were similar to 100% rewetting. Rewetting 10 or 20% of the root system resulted in much less water uptake; these plants did not recover leaf water storage or gas exchange by 30 d after rewetting. A redundancy in the root system of A. deserti apparently exists for daily water uptake requirements under wet conditions but the entire root system is required for rapid recovery from drought.