The impact of using area-averaged land surface properties —topography, vegetation condition, soil wetness—in calculations of intermediate scale (approximately 10 km) surface-atmosphere heat and moisture fluxes

It is commonly assumed that biophysically based soil-vegetation-atmosphere transfer (SVAT) models are scale-invariant with respect to the initial boundary conditions of topography, vegetation condition and soil moisture. In practice, SVAT models that have been developed and tested at the local scale (a few meters or a few tens of meters) are applied almost unmodified within general circulation models (GCMs) of the atmosphere, which have grid areas of 50–500 km². This study, which draws much of its substantive material from the papers of Sellers et al. (1992c, J. Geophys. Res., 97(D17): 19033–19060) and Sellers et al. (1995, J. Geophys. Res., 100(D12): 25607–25629), explores the validity of doing this. The work makes use of the FIFE-89 data set which was collected over a 2 km × 15 km grassland area in Kansas. The site was characterized by high variability in soil moisture and vegetation condition during the late growing season of 1989. The area also has moderate topography.

The 2 km × 15 km ‘testbed’ area was divided into 68 × 501 pixels of 30 m × 30 m spatial resolution, each of which could be assigned topographic, vegetation condition and soil moisture parameters from satellite and in situ observations gathered in FIFE-89. One or more of these surface fields was area-averaged in a series of simulation runs to determine the impact of using large-area means of these initial or boundary conditions on the area-integrated (aggregated) surface fluxes. The results of the study can be summarized as follows:

1. 1. analyses and some of the simulations indicated that the relationships describing the effects of moderate topography on the surface radiation budget are near-linear and thus largely scale-invariant. The relationships linking the simple ratio vegetation index (SR), the canopy conductance parameter (_F) and the canopy transpiration flux are also near-linear and similarly scale-invariant to first order. Because of this, it appears that simple area-averaging operations can be applied to these fields with relatively little impact on the calculated surface heat flux.

2. 2. The relationships linking surface and root-zone soil wetness to the soil surface and canopy transpiration rates are non-linear. However, simulation results and observations indicate that soil moisture variability decreases significantly as an area dries out, which partially cancels out the effects of these non-linear functions.In conclusion, it appears that simple averages of topographic slope and vegetation parameters can be used to calculate surface energy and heat fluxes over a wide range of spatial scales, from a few meters up to many kilometers at least for grassland sites and areas with moderate topography. Although the relationships between soil moisture and evapotranspiration are non-linear for intermediate soil wetnesses, the dynamics of soil drying act to progressively reduce soil moisture variability and thus the impacts of these non-linearities on the area-averaged surface fluxes. These findings indicate that we may be able to use mean values of topography, vegetation condition and soil moisture to calculate the surface-atmosphere fluxes of energy, heat and moisture at larger length scales, to within an acceptable accuracy for climate modeling work. However, further tests over areas with different vegetation types, soils and more extreme topography are required to improve our confidence in this approach.

     

The scaling characteristics of remotely-sensed variables for sparsely-vegetated heterogeneous landscapes

With increasing interest in airborne and satellite-based sensors for mapping regional and global energy balance, there is a need to determine the uncertainty involved in aggregating remotely-sensed variables [surface temperature (T_k) and reflectance (π)] and surface energy fluxes [sensible (H) and latent (λE) heat flux] over large areas. This uncertainty is directly related to two factors: (1) the nonlinearity of the relation between the sensor signal and T_k, π, H orλE; and (2) the heterogeneity of the site. In this study, we compiled several remotely-sensed data sets acquired at different locations within a semi-arid rangeland in Arizona, at a variety of spatial and temporal resolutions. These data sets provided the range of data heterogeneities necessary for an extensive analysis of data aggregation. The general technique to evaluate uncertainty was to compare remotely-sensed variables and energy balance components calculated in two ways: first, calculated at the pixel resolution and averaged to the coarser resolution; and second, calculated directly at the coarse resolution by aggregating the fine-resolution data to the coarse scale. Results showed that the error in the aggregation of T_k and π was negligible for a wide range of conditions. However, the error in aggregation of H and λE was highly influenced by the heterogeneity of the site. Errors in H larger than 50% were possible under certain conditions. The conditions associated with the largest aggregation errors in H were:

• sites which are composed of a mix of stable and unstable conditions;

• sites which have considerable variations in aerodynamic roughness, especially for highly unstable conditions where the difference between surface and air temperature is large; and

• sites which are characterized by patch vegetation, where the pixel resolution is less than or nearly-equal to the diameter of the vegetation ‘element’ (in most cases, the diameter of the dominant vegetation type or vegetation patch).

Thus, knowledge of the surface heterogeneity is essential for minimizing error in aggregation of H and λE. Two schemes are presented for quantifying surface heterogeneity as a first step in data aggregation. These results emphasized the need for caution in aggregation of energy balance components over heterogeneous landscapes with sparse or mixed vegetation types.     

Micro- and macro-dispersive fluxes in canopy flows

Resolving every detail of the three-dimensional canopy morphology and its underlying topography remains untenable when modeling high Reynolds number geophysical flows. How to represent the effects of such a complex morphological variability and any concomittant topographic variability into one-dimensional bulk flow representation remains a fundamental challenge to be confronted in canopy turbulence research. Theoretically, planar averaging to the scale of interest should be applied to the time-averaged mean momentum balance; however, such averaging gives rise to covariance or dispersive terms produced by spatial correlations of time-averaged quantities that remain ‘unclosed’ or require parameterization. When the averaging scale is commensurate with few canopy heights, these covariances can be labeled as ‘micro-dispersive’ stresses. When averaging is intended to eliminate low-wavenumber topographic variations, we refer to these covariances as ‘macro-dispersive’ terms. Two flume experiments were used to explore the magnitude and sign of both micro- and macro-dispersive fluxes relative to their conventional Reynolds stresses counterparts: a rod-canopy with variable roughness density and a dense rod canopy situated on gentle hilly terrain. When compared to the conventional momentum flux, the micro-dispersive fluxes in the lowest layers of sparse canopies can be significant (∼50%). For dense canopies, the dispersive terms remain negligible when compared to the conventional momentum fluxes throughout. For the macro-dispersive fluxes, model calculations suggest that these terms can be neglected relative to the Reynolds stresses for a deep canopy situated on a narrow hill. For the region in which topographic variations can interact with the pressure, both model calculations and flume experiments suggest that the macro-dispersive fluxes cannot be neglected, and their value can be 20% of the typical Reynolds stresses.     

近地表密度估计的重力贝叶斯分析方法及在云南地区的应用

基于布格重力异常相对于地形起伏光滑分布的约束条件,从一维自由空气重力异常数据出发,采用贝叶斯方法估算近地表岩石密度,同时采用三次B样条函数拟合布格重力异常,获取光滑分布的布格重力异常.数据拟合和光滑约束之间的权重采用Akaike贝叶斯准则(ABIC准则)自动确定.均匀剖分模型和不均匀剖分模型数据试验都验证了该方法的有效性.相关参数评价表明,足够多的样条系数可以提高估计结果的准确性,样条系数的个数接近测点数时可获得较稳定的估计结果.增大异常的噪声水平时,ABIC准则可有效地自动增大先验光滑约束的权重.云南地区两条重力剖面应用结果表明,剖面沿线的近地表密度值起伏变化明显(达2.45~2.8g·cm^-3),前寒武纪和古生代地层密度相对较高(主要为2.53~2.75g·cm^-3),而中生代密度较低(2.45~2.73g·cm^-3);本文估计的近地表密度结果与区域物性资料及地表地质特征较吻合;估计的剖面布格重力异常具有光滑性;红河断裂两侧近地表密度差异较大,可达0.4g·cm^-3.本文获得的两条剖面近地表密度结构和布格重力异常为该区深部结构与构造研究提供更可靠的重力基础数据.     

Measurement of soluble reactive phosphorus concentration profiles and fluxes in river-bed sediments using DET gel probes

DET (diffusive equilibrium in thin films) gel probes were used for sampling river-bed sediment porewaters, to characterise in situ soluble reactive phosphorus (SRP) concentration profiles and fluxes. DET probes were deployed in three contrasting rural streams: (1) a headwater ‘pristine’ stream, with minimal P inputs from low intensity grassland and no point sources, (2) an intensively cultivated arable catchment, and (3) a stream subject to high P loadings from sewage effluent and intensive arable farming. The DET results showed highly enriched porewater SRP concentrations of between ca. 400 and 5000 μg-P l⁻¹ in the sewage-impacted stream. In contrast, the arable and pristine streams had porewater SRP concentrations <70 μg-P l⁻¹ and <20 μg-P l⁻¹, respectively. Porewater SRP concentration profiles in both the sewage-impacted and arable-impacted streams showed well-defined vertical structure, indicating internal sources and sinks of SRP within the sediment. However, there was little variability in porewater SRP concentrations in the pristine stream. The DET porewater profiles indicated net diffusion of SRP (a) from the overlying river water into the surface sediment and (b) from subsurface sediment upwards towards the sediment–water interface. A mass balance for the sewage-impacted site showed that the influx of SRP into the surface sediments from the overlying river water was small (ca. 1% of the daily river SRP load). The DET results indicated that, in the arable and sewage-impacted streams, the surface ‘cap’ of fine sediment may play an important role in inhibiting upward movement of SRP from subsurface porewaters into the overlying river water, under steady-state, low-flow conditions.     

The scaling characteristics of soil parameters: From plot scale heterogeneity to subgrid parameterization

The variation in soil texture, surface moisture or vertical soil moisture gradient in larger scale atmospheric models may lead to significant variations in simulated surface fluxes of water and heat. The parameterization of soil moisture fluxes at spatial scales compatible with the grid size of distributed hydrological models and mesoscale atmospheric models ( 100 km²) faces principal problems which relate to the underlying microscopic or field scale heterogeneity in soil characteristics.

The most widely used parameterization in soil hydrology, the Darcy-Richards (DR) equation, is gaining increasing importance in mesoscale and climate modelling. This is mainly due to the need to introduce plant-interactive soil water depletion and stomatal conductance parameterizations and to improve the calculation of deep percolation and runoff. Covering a grid of several hundreds of square kilometres, the DR parameterization in soil-vegetation-atmosphere-transfer schemes (SVATs) is assumed to be scale-invariant. The parameters describing the non-linear, area-average soil hydraulic functions in this scale-invariant DR-equation should be treated as calibration-parameters, which do not necessarily have a physical meaning. The saturated hydraulic conductivity is one of the soil parameters to which the models show very high sensitivity. It is shown that saturated hydraulic conductivity can be scaled in both vertical and horizontal directions for large flow domains.

In this paper, a distinction is made between effective and aggregated soil parameters. Effective parameters are defined as area-average values or distributions over a domain with a single, distinct textural soil type. They can be obtained by scaling or inverse modelling. Aggregated soil parameters represent grid-domains with several textural soil types. In soil science dimensional methods have been developed to scale up soil hydraulic characteristics. With some specific assumptions, these techniques can be extrapolated from classical field-scale problems in soil heterogeneity to larger domains, compatible with the grid-size of large scale models. Particularly promising is the estimation of effective soil hydraulic parameters from area averaging measurements through inverse modelling of the unsaturated flow.

Techniques to scale and aggregate the soil characteristics presented in this paper qualify for direct or indirect use in large scale meteorological models. One of the interesting results is the effective behaviour of the reference curve, which can be obtained from similar media scaling. If the conclusions of this paper survive further studies, a relatively simple method will become available to parameterize soil variability at large scales. The inverse technique is found to provide effective soil parameters which perform well in predicting both the area-average evaporation and the area-average soil moisture fluxes, such as subsurface runoff. This is not the case for aggregated soil parameters. Obtained from regression relationships between soil textural composition and hydraulic characteristics, these aggregated parameters predict evaporation fluxes well, but fail to predict water balance terms such as percolation and runoff. This is a serious drawback which could eventually hamper the improvement of the representation of the hydrological cycle in mesoscale atmospheric models and in GCMs.     

On the effects of triangulated terrain resolution on distributed hydrologic model response

Distributed hydrologic models based on triangulated irregular networks (TIN) provide a means for computational efficiency in small to large‐scale watershed modelling through an adaptive, multiple resolution representation of complex basin topography. Despite previous research with TIN‐based hydrology models, the effect of triangulated terrain resolution on basin hydrologic response has received surprisingly little attention. Evaluating the impact of adaptive gridding on hydrologic response is important for determining the level of detail required in a terrain model. In this study, we address the spatial sensitivity of the TIN‐based Real‐time Integrated Basin Simulator (tRIBS) in order to assess the variability in the basin‐averaged and distributed hydrologic response (water balance, runoff mechanisms, surface saturation, groundwater dynamics) with respect to changes in topographic resolution. Prior to hydrologic simulations, we describe the generation of TIN models that effectively capture topographic and hydrographic variability from grid digital elevation models. In addition, we discuss the sampling methods and performance metrics utilized in the spatial aggregation of triangulated terrain models. For a 64 km² catchment in northeastern Oklahoma, we conduct a multiple resolution validation experiment by utilizing the tRIBS model over a wide range of spatial aggregation levels. Hydrologic performance is assessed as a function of the terrain resolution, with the variability in basin response attributed to variations in the coupled surface–subsurface dynamics. In particular, resolving the near‐stream, variable source area is found to be a key determinant of model behaviour as it controls the dynamic saturation pattern and its effect on rainfall partitioning. A relationship between the hydrologic sensitivity to resolution and the spatial aggregation of terrain attributes is presented as an effective means for selecting the model resolution. Finally, the study highlights the important effects of terrain resolution on distributed hydrologic model response and provides insight into the multiple resolution calibration and validation of TIN‐based hydrology models. Copyright © 2005 John Wiley & Sons, Ltd.     

Distributed parameterization of complex terrain

This paper addresses the incorporation of high resolution topography, soils and vegetation information into the simulation of land surface processes in atmospheric circulation models (ACM). Recent work has concentrated on detailed representation of one-dimensional exchange processes, implicitly assuming surface homogeneity over the atmospheric grid cell. Two approaches that could be taken to incorporate heterogeneity are the integration of a surface model over distributed, discrete portions of the landscape, or over a distribution function of the model parameters. However, the computational burden and parameter intensive nature of current land surface models in ACM limits the number of independent model runs and parameterizations that are feasible to accomplish for operational purposes. Therefore, simplications in the representation of the vertical exchange processes may be necessary to incorporate the effects of landscape variability and horizontal divergence of energy and water. The strategy is then to trade off the detail and rigor of point exchange calculations for the ability to repeat those calculations over extensive, complex terrain. It is clear the parameterization process for this approach must be automated such that large spatial databases collected from remotely sensed images, digital terrain models and digital maps can be efficiently summarized and transformed into the appropriate parameter sets. Ideally, the landscape should be partitioned into surface units that maximize between unit variance while minimizing within unit variance, although it is recognized that some level of surface heterogeneity will be retained at all scales. Therefore, the geographic data processing necessary to automate the distributed parameterization should be able to estimate or predict parameter distributional information within each surface unit.     

Diurnal cycle of surface energy fluxes in high mountain terrain: High-resolution fully coupled atmosphere-hydrology modelling and impact of lateral flow

Water and energy fluxes are inextricably interlinked within the interface of the land surface and the atmosphere. In the regional earth system models, the lower boundary parameterization of land surface neglects lateral hydrological processes, which may inadequately depict the surface water and energy fluxes variations, thus affecting the simulated atmospheric system through land-atmosphere feedbacks. Therefore, the main objective of this study is to evaluate the hydrologically enhanced regional climate modelling in order to represent the diurnal cycle of surface energy fluxes in high spatial and temporal resolution. In this study, the Weather Research and Forecasting model (WRF) and coupled WRF Hydrological modelling system (WRF-Hydro) are applied in a high alpine catchment in Northeastern Tibetan Plateau, the headwater area of the Heihe River. By evaluating and intercomparing model results by both models, the role of lateral flow processes on the surface energy fluxes dynamics is investigated. The model evaluations suggest that both WRF and coupled WRF-Hydro reasonably represent the diurnal variations of the near-surface meteorological fields, surface energy fluxes and hourly partitioning of available energy. By incorporating additional lateral flow processes, the coupled WRF-Hydro simulates higher surface soil moisture over the mountainous area, resulting in increased latent heat flux and decreased sensible heat flux of around 20–50 W/m² in their diurnal peak values during summertime, although the net radiation and ground heat fluxes remain almost unchanged. The simulation results show that the diurnal cycle of surface energy fluxes follows the local terrain and vegetation features. This highlights the importance of consideration of lateral flow processes over areas with heterogeneous terrain and land surfaces.     

Monitoring volcanic hazard using eddy covariance at Solfatara volcano, Naples, Italy

An eddy covariance (EC) station was deployed at Solfatara crater, Italy, June 8–25, 2001 to assess if EC could reliably monitor CO₂ fluxes continuously at this site. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes calculated in 30-min averages varied between 950 and 4460 g CO₂ m⁻² d⁻¹; the highest measurements were made downwind of degassing pools. Comparing turbulent fluxes with chamber measurements of surface fluxes using footprint models in diffuse degassing regions yielded an average difference of 0% (±4%), indicating that EC measurements are representative of surface fluxes at this volcanic site. Similar comparisons made downwind of degassing pools yielded emission rates from 12 to 27 t CO₂ d⁻¹ for these features. Reliable EC measurements (i.e. measurements with sufficient and stationary turbulence) were obtained primarily during daytime hours (08:00 and 20:00 local time) when the wind speed exceeded 2 m s⁻¹. Daily average EC fluxes varied by ±50% and variations were likely correlated to changes in atmospheric pressure. Variations in CO₂ emissions due to volcanic processes at depth would have to be on the same order of magnitude as the measured diurnal variability in order to be useful in predicting volcanic hazard. First-order models of magma emplacement suggest that emissions could exceed this rate for reasonable assumptions of magma movement. EC therefore provides a useful method of monitoring volcanic hazard at Solfatara. Further, EC can monitor significantly larger areas than can be monitored by previous methods.     

Tracing suspended sediment and particulate phosphorus sources in catchments

Information on suspended sediment and particulate P (PP) sources is an important requirement in many catchment-based diffuse source pollution studies, in order to assist with model validation and to provide information to support the development of effective sediment and phosphorus control strategies. Such information is, however, frequently unavailable or difficult to assemble. In the study reported, source fingerprinting procedures were successfully used to assemble this information for seven sub-catchments in the Hampshire Avon catchment and five sub-catchments in the Middle Herefordshire Wye catchment. The results provide important new information on the relative importance of the contributions from surface and channel/subsurface sources to the suspended sediment and PP fluxes from the catchments. In the Wye sub-catchments channel/subsurface sources contributed 40–55% of the overall suspended sediment flux and 21–43% of the PP flux from the catchments. Equivalent values for the Avon were 1–41% and 1–54%, respectively. Combination of the information on the relative importance of surface and channel/subsurface sources with measured suspended sediment fluxes has provided the first estimates of the specific fluxes of sediment and PP attributable to channel/subsurface sources for UK catchments. The former are as high as 15–20 t km⁻² year⁻¹ in some of the Wye sub-catchments, whereas the latter exceeded 0.1 kgP ha⁻¹ year⁻¹ in the same sub-catchments. The results emphasize the need to take account of potential contributions from channel/subsurface sources when using measured suspended sediment and PP flux data to validate predictions derived from models incorporating only surface contributions.     

陇东黄土高原下垫面不均匀性指标的建立及其对大孔径闪烁仪(LAS)观测感热通量的影响

目前定量研究下垫面不均匀性对大孔径闪烁仪(LAS)、涡动相关仪(ECS)观测感热通量差异的影响还比较少.本文利用黄土高原庆阳观测站2012年6、7月典型晴天两主风向范围E-SE和SW-W的陆面过程数据,在建立了下垫面不均匀性定量化指标的基础上,分析了下垫面不均匀性与地表温度变率的关系及其对LAS、ECS观测感热通量差异的影响.结果表明:庆阳站下垫面不均匀性大小η和地表温度变率г具有很好的相关性,相关系数达到0.566以上,证明了新建立指标的合理性.下垫面不均匀性大小η和LAS、ECS观测的感热通量差异HLAS-HECS具有很好的一致性,相关系数达到0.634.下垫面不均匀性对LAS和ECS的感热通量差异影响显著,下垫面越不均匀LAS和ECS测量的感热通量差异越大.对两主风向分别进行分析,在E-SE风向范围η和HLAS-HECS的相关系数为0.430,HLAS和HEC拟合的线性趋势系数为1.279,在SW-W风向范围η和HLAS-HECS的相关系数为0.680,HLAS和HEC拟合的线性趋势系数为1.297.下垫面不均匀性的影响程度越大,LAS和ECS观测的感热通量差异越大.     

Defining area-average parameters in meteorological models for land surfaces with mesoscale heterogeneity

After a brief review of existing methods developed to describe the subgrid variability of soil and vegetation at large scale, the paper summarizes aggregation results obtained in the framework of the Hapex-Mobilhy and EFEDA field experiments. The parameter aggregation method is based on the estimation of effective parameters for the soil and for the vegetation, representative of the large area. In conditions of moderate land-surface variability, the effective surface fluxes computed using a single column model forced by effective surface parameters are close to the areally-averaged turbulent fluxes estimated by a mesoscale model. A simple method for accounting for the subgrid variability of intercepted rain is tested.     

Methane in the hyporheic zone of a small lowland stream (Sitka, Czech Republic)

Methane concentrations and selected chemical parameters in interstitial water were examined along subsurface flowpath in two subsystems (hyporheic and parafluvial sediments) in the Sitka stream, Czech Republic. Interstitial methane concentrations exhibited a distinct spatial pattern. In the hyporheic downwelling zone where the sediments are relatively well oxygenated due to high hydrologic exchange with the surface water, low interstitial methane concentrations, averaging 9.3 μg CH₄/l, were found. In contrast, upwelling sediments and parafluvial sediments (active channel sediments lateral to the wetted channel) had significantly higher methane concentrations (p < 0.05, and p < 0.01, respectively), averaging 43.2 μg CH₄/l and 160.5 μg CH₄/l, respectively. Dissolved oxygen was the highest where surface water entered hyporheic/parafluvial sediments and decreased with water residence time in the sediments (p < 0.01). Nitrate concentrations decreased along the flowpath and were significantly lower at downstream end of the riffle (p < 0.001). Sulfate concentrations also show a slight decline with the water residence time, but differences were not significant. Effect of both nitrate and sulfate on methanogenesis is also discussed. The interstitial methane concentration significantly increased with surface water temperature (p < 0.001) and was negatively correlated with redox potential (p < 0.01) and dissolved oxygen (p < 0.05).     

Soil moisture variation in relation to topography and land use in a hillslope catchment of the Loess Plateau, China

The profile characteristics and the temporal dynamics of soil moisture variation were studied at 26 locations in Da Nangou catchment (3.5 km²) in the loess area of China. Soil moisture measurements were performed biweekly at five depths in the soil profile (0–5, 10–15, 20–25, 40–45 and 70–75 cm) from May to October 1998 using Delta-T theta probe. Soil moisture profile type and temporal variation type and their relationship to topography and land use were identified by detrended canonical correspondence analysis (DCCA) and correlation analysis. The profile distribution of time-averaged soil moisture content can be classified into three types i.e. decreasing-type, waving-type and increasing-type. The profile features of soil moisture (e.g. profile gradient and profile variability) are influenced by different environmental factors. The profile type of soil moisture is only attributed to land use while profile gradient and profile variability of soil moisture is mainly related to land use and topography (e.g. landform type and slope). The temporal dynamics of layer-averaged soil moisture content is grouped into three types including three-peak type, synchro-four-peak type and lagged-four-peak type. These types are controlled by topography rather than by land use. The temporal dynamic type of soil moisture shows significant correlation with relative elevation, slope, aspect, while temporal variance displays significant relation with slope shape. The mean soil moisture is related to both the profile and dynamics features of soil moisture and is controlled by both land use and topography (e.g. aspect, position, slope and relative elevation). The spatial variability of soil moisture across landscape varies with both soil depths and temporal evolution.     

Oxygen isotope evidence for the origin of enriched mantle beneath the mid-Atlantic ridge

Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ¹⁸O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ¹⁸O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ¹⁸O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.     

THE IMPORTANCE OF TOPOGRAPHIC CORRECTIONS ON MAGNETOTELLURIC RESPONSE DATA FROM RUGGED REGIONS OF ANATOLIA

Topographic irregularities cause some distortions of magnetotelluric (MT) fields. In the vicinity of a topographic feature, the TM-mode distortion increases with the height and inclination of the slope. It is well-known that TM-mode ( E _⊥) topographic effects are much greater than TE-mode ( E _ı) distortions.   We have made a study of MT anomalies in TM-mode due to two-dimensional topography. In order to reduce these effects, the distortion tensor stripping technique was used. After corrections, the resulting data can be interpreted as if they were obtained over a flat surface and depend only on the subsurface structure. However, this technique sometimes causes some geometrical distortions of the real subsurface structure.   One of our aims is to overcome this failure. We have modified the correction coefficients by considering the actual one-dimensional geology. Model studies showed that our approach is especially useful in removing the terrain effects on complex 2D subsurface structures.   The other purpose of this study is to emphasize the importance of a proper terrain correction for data from sites having mountainous topography over complex geology, e.g. strike-slip faults, suture zones and rift valleys. Some examples of MT data sets collected from the North Anatolian Fault Zone and from the thrust regions of the Western Taurides will be presented.     

Patch scale aggregation of heterogeneous land surface cover for mesoscale meteorological models

Theoretical studies dealing with aggregation of surface parameters at small scale are reviewed. Finding effective parameters for surface resistance is possible for most cases by taking simple geometric or arithmetic averages of the component resistances. The use of more sophisticated techniques such as the blending height improves the calculations. Resistances for heat and water vapour behave differently in heterogeneous terrain. A simple surface energy balance model is adapted to show the behaviour of the roughness length of heat and water vapour in heterogeneous terrain. It is suggested that this simple parameterization can adequately take into account the effect of variation in surface cover on the fluxes of heat and water vapour.     

Os isotope systematics in ocean island basalts

New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial ¹⁸⁶Os/¹⁸⁷Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial ¹⁸⁷Os/¹⁸⁶Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high ¹⁸⁶Os/¹⁸⁷Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.     

RESTORATION OF THE ORIGINAL TOPOGRAPHY OF THE XIAONANHAI LANDSLIDE IN CHONGQING AND CALCULATION OF ITS VOLUME

As documented in history, an M6¼ earthquake occurred between Qianjiang, Chongqing and Xianfeng, Hubei(also named the Daluba event)in 1856. This earthquake caused serious geological hazards, including a lot of landslides at Xiaonanhai, Wangdahai, Zhangshangjie and other places. Among them, the Xiaonanhai landslide is a gigantic one, which buried a village and blocked the river, creating a quake lake that has been preserved to this day. As the Xiaonanhai landslide is a historical earthquake-induced landslide, it is impossible to obtain the remote sensing image and DEM data before the earthquake, which brings certain difficulties to the estimation of landslide volume and the establishment of numerical simulation model. In this paper, the original topography before the earthquake is inferred by the methods of geomorphic analogy in adjacent areas and numerical simulation, and the volume of the Xiaonanhai landslide body is calculated. Firstly, the principle and application of UAV aerial photography are introduced. We employed an unmanned airplane to take pictures of the Xiaonanhai landslide and adjacent areas, yielding high-precision DOM images(digital orthophoto graph)and DEM data which permit generating terrain contours with a 25m interval. We also used the method of intensive manual depth measurement in waters to obtain the DEM data of bottom topography of Xiaonanhai quake lake. Based on field investigations, and combining terrain contours and DOM images, we described the sizes and forms of each slump mass in detail. Secondly, considering that the internal and external dynamic geological processes of shaping landforms in the same place are basically the same, the landforms such as ridges and valleys are also basically similar. Therefore, combining with the surrounding topography and landform of the Xiaonanhai area, we used MATLAB software to reconstruct two possible original landform models before the landslide. The original topography presented by model A is a relatively gentle slope, with a slope of 40°~50°, and the original topography presented by model B is a very high and steep slope, with a slope of 70°~80°. Thirdly, Geostudio software is used to conduct numerical simulation analysis on the slope stability. The safety factor of slope stability and the scale of landslide are analyzed under the conditions of static stability, seismic dynamic response and seismic dynamic response considering topographic amplification effect. The results show that large landslide is more likely to occur in model B, which is more consistent with the reality. In order to verify the credibility of recovered DEM data of valley bottom topography, we visited the government of Qianjiang District, collected the drilling data of 11 boreholes in two survey lines of Xiaonanhai weir dam. It is verified that the recovered valley bottom elevation is basically consistent with that revealed by the borehole data. Finally, according to the two kinds of topographic data before and after the landslide, the volume of the landslide is calculated by using the filling and excavation analysis function of ArcGIS software. There is a gap between the calculation results of filling and excavation, the filling data is 3×10⁶m³ larger than the excavation data. The reasons are mainly as follows: 1)Due to the disorderly accumulation of collapse blocks, the porosity of the accumulation body became larger, causing the volume of the fill to expand; 2)It has been more than 150a since the Xiaonanhai earthquake, and the landslide accumulation has been seriously reconstructed, therefore, there are some errors in the filling data; 3)The accumulation body in Xiaonanhai quake lake might be subject to erosion and siltation, this may affect the accuracy of the filling data. In conclusion, it is considered that the calculated results of the excavation are relatively reliable, with a volume of 4.3×10⁷m³.