Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models

The presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed‐shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed‐shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges. Copyright © 2014 John Wiley & Sons, Ltd.     

Amorphous silica mobilization by inter‐rill erosion: insights from rainfall experiments

Amorphous silica (ASi) carried in suspension by rivers is an important component in the global Si budget. Water erosion processes in cultivated catchments are likely to drive ASi delivery to the river system. However, no studies have investigated the controls on ASi mobilization by erosional processes in croplands. Rainfall experiments were performed on split fields (i.e. a part conventionally ploughed and a part under reduced tillage) to simulate ASi mobilization by inter‐rill erosion in croplands, and identify its dependency on soil, field and rainfall characteristics. The ASi content of the soil and the inter‐rill erosion rate were determined as the major controls on ASi mobilization. Variables such as tillage technique and crop type did not have a consistent direct or indirect effect. Inter‐rill erosion is clearly selective with respect to ASi, indicating association of ASi with the fine soil fraction and with soil organic carbon. Our experiments demonstrate that erosion increases due to human perturbation will increase the delivery of reactive Si to aquatic systems. We estimate that globally, c. 7% of all reactive Si that enters aquatic systems is derived from erosion of agricultural soils. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of Obstacles on the Aerodynamic Roughness of the Netherlands

The aim of this study was to analyse the influence of large- and small-scale obstacles (orography, tree lines, and dikes) on the effective aerodynamic roughness of the Netherlands, a relatively flat, small-scale landscape. The roughness averaging approach was based on drag coefficients. The effective roughness was locally dominated by small-scale obstacles such as tree lines and dikes. Even at a regional scale (40,000 km2), the small-scale obstacle drag was of the same order of magnitude as the shear stress due to landuse. The neglect of those obstacles on a regional scale would result in approximately 10% overestimated averaged windspeed at 10~m above the surface. It was concluded that small-scale obstacles need to be taken into account to calculate the aerodynamic roughness of flat landscapes. Orography was of minor importance in this lowland country.     

Low biomass of macrobenthic fauna at a tropical mudflat: An effect of latitude?

The macrobenthic animal biomass of the intertidal area of the Sembilang peninsula of South Sumatra, Indonesia, has been studied in 2004. Each month (March–August) 21 core samples were taken at each of six sampling stations. Macrobenthic fauna were identified at the lowest taxonomical level possible and counted. Biomass was measured as ash-free dry mass (afdm). The average biomass over all stations and months was 3.62 g afdm m⁻², the highest biomass (47.45 g afdm m⁻²) found at a station in one month was due to abundant occurrence of the bivalve Anadara granosa. Low biomass of macrobenthic fauna at Sembilang peninsula cannot easily be explained but is in line with low biomasses found elsewhere in the tropics. For that reason we analyzed a data set of 268 soft-bottom intertidal biomasses collected world-wide to look for a relationship with latitude. It was shown that average biomass of intertidal macrobenthic fauna in the tropics was significantly (p < 0.05) lower than that at non-tropical sites. A significant second-order relationship between biomass of macrobenthic fauna and latitude was established.     

The Energy Balance Experiment EBEX-2000. Part I: overview and energy balance

An overview of the Energy Balance Experiment (EBEX-2000) is given. This experiment studied the ability of state-of-the-art measurements to close the surface energy balance over a surface (a vegetative canopy with large evapotranspiration) where closure has been difficult to obtain. A flood-irrigated cotton field over uniform terrain was used, though aerial imagery and direct flux measurements showed that the surface still was inhomogeneous. All major terms of the surface energy balance were measured at nine sites to characterize the spatial variability across the field. Included in these observations was an estimate of heat storage in the plant canopy. The resultant imbalance still was 10%, which exceeds the estimated measurement error. We speculate that horizontal advection in the layer between the canopy top and our flux measurement height may cause this imbalance, though our estimates of this term using our measurements resulted in values less than what would be required to balance the budget. The National Center for Atmospheric Research is supported by the National Science Foundation     

Trace element partitioning between ilmenite, armalcolite and anhydrous silicate melt: Implications for the formation of lunar high-Ti mare basalts

We performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite (Ilm), armalcolite (Arm) and anhydrous lunar silicate melt, to constrain geochemical models of the formation of titanium-rich melts in the Moon. Experiments were performed in graphite-lined platinum capsules at pressures and temperatures ranging from 1.1 to 2.3 GPa and 1300-1400 °C using a synthetic Ti-enriched Apollo ‘black glass’ composition in the CaO-FeO-MgO-Al₂O₃-TiO₂-SiO₂ system. Ilmenite-melt and armalcolite-melt partition coefficients (D) show highly incompatible values for the rare earth elements (REE) with the light REE more incompatible compared to the heavy REE ( 0.0020 ± 0.0010 to 0.069 ± 0.010 for ilmenite; 0.0048 ± 0.0023 to 0.041 ± 0.008 for armalcolite). D values for the high field strength elements vary from highly incompatible for Th, U and to a lesser extent W (for ilmenite: 0.0013 ± 0.0008, 0.0035 ± 0.0015 and 0.039 ± 0.005, and for armalcolite 0.008 ± 0.003, 0.0048 ± 0.0022 and 0.062 ± 0.03), to mildly incompatible for Nb, Ta, Zr, and Hf (e.g. 0.28 ± 0.05 and : 0.76 ± 0.07). Both minerals fractionate the high field strength elements with D_Ta/D_Nb and D_Hf/D_Zr between 1.3 and 1.6 for ilmenite and 1.3 and 1.4 for armalcolite. Armalcolite is slightly more efficient at fractionating Hf from W during lunar magma ocean crystallisation, with D_Hf/D_W = 12-13 compared to 6.7-7.5 for ilmenite. The transition metals vary from mildly incompatible to compatible, with the highest compatibilities for Cr in ilmenite (D ∼ 7.5) and V in armalcolite (D ∼ 8.1). D values show no clear variation with pressure in the small range covered.Crystal lattice strain modelling of D values for di-, tri- and tetravalent trace elements shows that in ilmenite, divalent elements prefer to substitute for Fe while armalcolite data suggest REE replacing Mg. Tetravalent cations appear to preferentially substitute for Ti in both minerals, with the exception of Th and U that likely substitute for the larger Fe or Mg cations. Crystal lattice strain modelling is also used to identify and correct for very small (∼0.3 wt.%) melt contamination of trace element concentration determinations in crystals.Our results are used to model the Lu-Hf-Ti concentrations of lunar high-Ti mare basalts. The combination of their subchondritic Lu/Hf ratios and high TiO₂ contents requires preferential dissolution of ilmenite or armalcolite from late-stage, lunar magma ocean cumulates into low-Ti partial melts of deeper pyroxene-rich cumulates.     

Two-dimensional equilibrium of a neutral sheet in a gravitational field

We solve the two-dimensional MHD-equations to find the equilibrium structure of a neutral sheet having its axis parallel to the gravitational field in an atmosphere. The evolution of an initially plane sheet which is a self-consistent, non-equilibrium solution of the MHD-equations, is followed until static equilibrium of forces is obtained. The effect of field line anchoring in a cool dense layer at the bottom of the sheet is taken into account.Presently at The Auroral Observatory, Boks 953, 9001 Tromsø, Norway.     

Late Quaternary deglaciation and climate history of the Larsemann Hills (East Antarctica)

The Late Quaternary climate history of the Larsemann Hills has been reconstructed using siliceous microfossils (diatoms, chrysophytes and silicoflagellates) in sediment cores extracted from three isolation lakes. Results show that the western peninsula, Stornes, and offshore islands were ice‐covered between 30 000 yr BP and 13 500 cal. yr BP. From 13 500 cal. yr BP (shortly after the Antarctic Cold Reversal) the coastal lakes of the Larsemann Hills were deglaciated and biogenic sedimentation commenced. Between 13 500 and 11 500 cal. yr BP conditions were warmer and wetter than during the preceding glacial period, but still colder than today. From 11 500 to 9500 cal. yr BP there is evidence for wet and warm conditions, which probably is related to the early Holocene climate optimum, recorded in Antarctic ice cores. Between 9500 and 7400 cal. yr BP dry and cold conditions are inferred from high lake‐water salinities, and low water levels and an extended duration of nearshore sea‐ice. A second climate optimum occurred between 7400 and 5230 cal. yr BP when stratified, open water conditions during spring and summer characterised the marine coast of Prydz Bay. From 5230 until 2750 cal. yr BP sea‐ice duration in Prydz Bay increased, with conditions similar to the present day. A short return to stratified, open water conditions and a reduction in nearshore winter sea‐ice extent is evident between 2750 and 2200 cal. yr BP. Simultaneously, reconstructions of lake water depth and salinity suggests relatively humid and warm conditions on land between 3000 and 2000 cal. yr BP, which corresponds to a Holocene Hypsithermal reported elsewhere in Antarctica. Finally, dry conditions are recorded around 2000, between 760 and 690, and between 280 and 140 cal. yr BP. These data are consistent with ice‐core records from Antarctica and support the hypothesis that lacustrine and marine sediments on land can be used to evaluate the effect of long‐term climate change on the terrestrial environment. Copyright © 2004 John Wiley & Sons, Ltd.     

Average fluxes from heterogeneous vegetated regions

Using a surface-layer model, fluxes of heat and momentum have been calculated for flat regions with regularly spaced step changes in surface roughness and stomatal resistance. The distance between successive step changes is limited to 10 km in order to fill the gap between micro-meteorological measurements and meso-scale models. A single-layer big leaf model of the vegetation is compared with a multi-layer model to assess the performance of the former in the determination of surface fluxes in heterogeneous terrain.The sub-models of vegetation and atmosphere are mainly based on well-known theory. However, a modification of the mixing-length closure of atmospheric exchange is included to achieve a more realistic calculation of fluxes near step changes at the surface. Measurements, presented in the literature, are used to determine the mixing-length parameters and to validate the calculated fluxes downwind of a change in vegetation cover.The single-layer model, well validated for homogeneous surfaces, underestimates the effects of local advection upon the surface fluxes as this model neglects air flow across the edges of tall vegetation. Using the multi-layer model, local advection results in an increase of up to 50% in regional momentum flux and smaller changes in regional evaporation. Even widely spaced heterogeneities appear to influence regional fluxes.     

A new technique to estimate net groundwater use across large irrigated areas by combining remote sensing and water balance approaches, Rechna Doab, Pakistan

Over-exploitation of groundwater resources threatens the future of irrigated agriculture, especially in the arid and semi-arid regions of the world. In order to reverse this trend, and to ensure future food security, the achievement of sustainable groundwater use is ranking high on the agenda of water policy makers. Spatio-temporally distributed information on net groundwater use—i.e. the difference between tubewell withdrawals for irrigation and net recharge—is often unknown at the river basin scale. Conventionally, groundwater use is estimated from tubewell inventories or phreatic surface fluctuations. There are shortcomings related to the application of these approaches. An alternative methodology for computing the various water balance components of the unsaturated zone by using geo-information techniques is provided in this paper. With this approach, groundwater recharge will not be quantified explicitly, but is part of net groundwater use, and the spatial variation can be quantitatively described. Records of routine climatic data, canal discharges at major offtakes, phreatic surface depth fluctuations, and simplified information on soil textural properties are required as input data into this new Geographic Information System and Remote Sensing tool. The Rechna Doab region (approximately 2.97 million ha), located in the Indus basin irrigation system of Pakistan, has been used as a case study. On an annual basis, an areal average net groundwater use of 82 mm year^–1 was estimated. The current result deviates 65% from the specific yield method. The deviation from estimates using tubewell withdrawal related data is even higher.

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Resumen La sobre-explotación de recursos de agua subterránea amenaza el futuro de la agricultura de riego, especialmente en las regiones áridas y semi-áridas del mundo. Para revertir esta tendencia, y para garantizar seguridad alimentaria futura, la meta del uso sostenible del agua subterránea se encuentra alto en la agenda de los políticos. Información espacial y temporal en cuanto al uso neto de agua subterránea- i.e. la diferencia entre las extracciones de agua de pozos entubados para riego y recarga neta- se desconoce frecuentemente a la escala de cuenca hidrográfica. Generalmente, el uso de agua subterránea se estima a partir de inventarios de pozos o fluctuaciones de superficies freáticas. Existen deficiencias en relación con las aplicaciones de estos enfoques. En este artículo se aporta una metodología alternativa para calcular los diferentes componentes del balance hídrico de la zona no saturada utilizando técnicas geoinformativas. Aunque con este enfoque no se cuantifica de manera explícita la recarga de agua subterránea, la cual es parte del uso neto de agua subterránea, puede describirse cuantitativamente la variación espacial. Para esta nueva herramienta de Sistemas de Información Geográfica y Sensores Remotos se requieren datos de entrada como registros rutinarios de datos climáticos, descargas de canales en salidas principales, fluctuaciones de profundidades de superficies freáticas, e información simplificada de las propiedades texturales de los suelos. Se ha utilizado como estudio de caso la región Rechna Doab (aproximadamente 2.97 millones ha), localizada en el sistema de riego de la cuenca Indus de Pakistán. Se ha estimado un uso promedio areal anual de agua subterránea de 82 mm año^–1. El resultado obtenido difiere en un 65% del método de productividad específica. La diferencia en relación a estimados provenientes de extracciones en pozos entubados es aún mucho más alta.

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Résumé La surexploitation des ressources en eau souterraine menace le futur de lagriculture irrigué, spécialement dans les zones arides et semi-arides du monde. De manière à renverser la tendance, et dassurer la sécurité alimentaire, lutilisation durable des eaux souterraines est devenue une priorité dans lagenda des politiques de leau. La distribution spatio-temporelle de lusage net de leau souterraine (la différence entre l ‹eau pompée et la recharge nette) est rarement connue à léchelle dun bassin versant. Conventionnellement, lutilisation des eaux souterraines est estimée à partir des données de rabattement ou les données de fluctuation du niveau de la nappe phréatique. Il y a des défauts dans ces approches. Une méthodologie alternative pour calculer les différents composants de la balance hydrologique est présentée dans cet article. Avec cette approche, la recharge des eaux souterraines ne sera pas quantifiée de manière explicite, mais sera considérée comme une part de lutilisation nette en eau souterraine, et la variation spatiale peut être décrite quantitativement. Les chroniques des données climatiques, les débits du réseau hydrographique majeur, les fluctuations de la surface de la nappe phréatique, et des données basiques sur la texture du sol sont nécessaires et sont rentrées dans un nouveau Système dInformation Géographique et outil de télédétection. La région de Rechna Doab au Pakistan, environ 2.97 millions dhectare, localisée dans le bassin irrigué de lIndus, a été utilisé comme cas détude. Sur base annuelle, lutilisation nette de leau souterraine est estimée à 82 mm. en moyenne. Le résultat obtenu diffère de 65% du résultat de la méthode du débit spécifique. La différence avec le résultat obtenu en observant le rabattement des puits est encore plus élevée.