Summer evapotranspiration in western Siberia: a comparison between eddy covariance and Penman method formulations

Evapotranspiration is difficult to quantify because of the many factors and complex processes that influence it. Several empirical methods have been developed over the years to estimate potential evapotranspiration based on easily available parameters. Directly measured data of actual evapotranspiration have been rather sparse in the past and still need to be improved in particular regions like western Siberia. The transition zone between the warm temperate and cold temperate continental climates is very sensitive to climate change, and water stress is an increasingly important issue in these regions with a highly dynamic agricultural activity. So there is a growing need to estimate actual evapotranspiration. Widely usable approximations are needed. In this study, the values of potential evapotranspiration computed with the original version, and eight modifications of the Penman formulation were compared and related to the actual evapotranspiration measured by eddy covariance over a grassland area in western Siberia. The original 1948 and 1963 Penman formulations are best for estimating potential evapotranspiration in the transition zone between the forest steppes and the pre‐taiga. A nearly linear relationship between the potential and actual evapotranspiration was found. A simple modification of the Penman equation (i.e. the multiplication of the result by a factor of 0.47) is suggested for approximating the actual evapotranspiration based on standard meteorological data for the region. The original Penman formulation is most robust and will provide the widest applicability in the future under changing climate and environmental conditions. In this context, it is further recommended not to neglect the ventilation term of the Penman equation, which is often assumed to be negligibly small. A detailed correlation analysis showed that under dry soil conditions, the vegetation largely contributed to the actual evapotranspiration and, in contrast to widely held expectations, that the Penman equation is best adapted to vegetated surfaces. Copyright © 2015 John Wiley & Sons, Ltd.     

Studie über Höhenschwankungen der F2-Schicht: I

Zusammenfassung Die mittags gemessenen scheinbaren Höhen (hF2) der F2-Schicht unterliegen jährlichen Schwankungen, die örtlich verschieden sind. Die Jahresmittelwerte und die Amplitude der ersten Harmonischen (einfache jährliche Periode) zeigen eine deutliche Abhängigkeit von der geographischen Breite des Beobachtungsortes. Während die Phase der einfachen Periode Höhenmaxima zum höchsten Sonnenstand ergibt, ist die Phase der doppelten Periode um rund 180° verschoben, so dass in äquatorialen Gebieten die Höhenmaxima zu den Solstitien auftreten.

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The virtual height of the ionospheric F2-layer varies with the location of the ionospheric observatory and with the season. The annual mean of these heights and likewise the annual component of their variation varies with the geographical latitude. The greatest variation appears in high latitude, on the other hand the annual variation disappears on the equator. There remains only a half-yearly component. The maxima of F₂-heights appear in high latitude at summer-solstice and on the equator at both solstices.

     

Analysis of absorption effects on the dynamic response of dam reservoir systems by boundary element methods

Using reciprocal theorems for dynamic and static boundary value problems, boundary integral equations are presented for wave propagation in elastic, isotropic media and compressible, inviscid fluids in the time domain as well as in the frequency domain. For the analysis of fluid–soil and fluid–structure systems, suitable coupling conditions are prescribed along the interfaces. The numerical treatment of the boundary integral equations consists of a point collocation and of a discretization of the boundary, in which constant and linear approximation functions are assumed. Step-by-step integration is applied to the time-dependent equations, where again the states are taken to be linear and constant over each time interval. These boundary element procedures are used to analyse the response of dams due to horizontal and vertical ground motions considering dam–water interaction and absorption of hydrodynamic pressure waves at the reservoir bottom or at the far end into the soil medium. Both the frequency response and the impulse generated transient response are investigated.     

Determination Of The Surface Drag Coefficient

This study examines the dependence of the surface drag coefficienton stability, wind speed, mesoscale modulation of the turbulent flux and method of calculation of the drag coefficient. Data sets over grassland, sparse grass, heather and two forest sites are analyzed. For significantly unstable conditions, the drag coefficient does not depend systematically on z/L but decreases with wind speed for fixed intervals of z/L, where L is the Obukhov length. Even though the drag coefficient for weak wind conditions is sensitive to the exact method of calculation and choice of averaging time, the decrease of the drag coefficient with wind speed occurs for all of the calculation methods. A classification of flux calculation methods is constructed, which unifies the most common previous approaches.The roughness length corresponding to the usual Monin–Obukhovstability functions decreases with increasing wind speed. This dependence on wind speed cannot be eliminated by adjusting the stability functions. If physical, the decrease of the roughness length with increasing wind speed might be due to the decreasing role of viscous effectsand streamlining of the vegetation, although these effects cannot be isolated from existing atmospheric data.For weak winds, both the mean flow and the stress vector often meander significantly in response to mesoscale motions. The relationship between meandering of the stress and wind vectors is examined. For weak winds, the drag coefficient can be sensitive to the method of calculation, partly due to meandering of the stress vector.     

Die Bodenbedeckung des Englischen Kanals und die maximalen Gezeitenstromgeschwindigkeiten

Zusammenfassung Es wurde eine Bodenkarte des Englischen Kanals neu zusammengestellt und darin vier Bodenarten unterschieden: Schlick, Sand, Kies und grober Sand sowie Fels. Ihre Verbreitung wurde mit der Verbreitung der Geschwindigkeitsstufen der maximalen Gezeitenströme dadurch verglichen, da die entsprechenden Areale ausgemessen wurden. Es ergaben sich sowohl zahlenmäig für das Gesamtgebiet wie auch für zahlreiche Einzelfälle enge Beziehungen zwischen den maximalen Gezeitenströmen an der Oberfläche des Englischen Kanals und den Korngröen der Bodenablagerungen.

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The covering of the bottom of the English Channel and the maximum velocities of the tidal currents

Summary A new bottom chart of the English Channel has been designed in which four kinds of bottom covering are distinguished: Mud, sand, shingle and gravel, and rocks. Close correlations were found between the maximum tidal currents at the surface and the grain sizes of the sediments of the English Channel, both numerically for the whole area and for numerous particular cases.

     

10Be exposure age constraints on the Late Weichselian ice‐sheet geometry and dynamics in inter‐ice‐stream areas,western Svalbard

The Late Weichselian ice sheet of western Svalbard was characterized by ice streams and inter‐ice‐stream areas. To reconstruct its geometry and dynamics we investigated the glacial geology of two areas on the island of Prins Karls Forland and the Mitrahalvøya peninsula. Cosmogenic ¹⁰Be surface exposure dating of glacial erratics and bedrock was used to constrain past ice thickness, providing minimum estimates in both areas. Contrary to previous studies, we found that Prins Karls Forland experienced a westward ice flux from Spitsbergen. Ice thickness reached >470 m a.s.l., and warm‐based conditions occurred periodically. Local deglaciation took place between 16 and 13 ka. At Mitrahalvøya, glacier ice draining the Krossfjorden basin reached >300 m a.s.l., and local deglaciation occurred at c. 13 ka. We propose the following succession of events for the last deglaciation. After the maximum glacier extent, ice streams in the cross‐shelf troughs and fjords retreated, tributary ice streams formed in Forlandsundet and Krossfjorden, and, finally, local ice caps were isolated over both Prins Karls Forland and Mitrahalvøya and their adjacent shelves.     

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