Formulation, implementation and examination of vertical coordinate choices in the Global Navy Coastal Ocean Model (NCOM)

A 1/8° global version of the Navy Coastal Ocean Model (NCOM) is described with details of its formulation, implementation, and configuration of the vertical coordinate. NCOM is a baroclinic, hydrostatic, Boussinesq, free-surface ocean model that allows its vertical coordinate to consist of σ coordinates for the upper layers and z-levels below a user-specified depth. This flexibility allows implementation of a hybrid σ–z coordinate system that is expected to mitigate some of the weaknesses that can be associated with either pure coordinate option. For the global NCOM application, the σ–z coordinate is used to allow terrain-following σ coordinates in the upper ocean, providing better resolution and topographic fidelity in shelf regions where flow is most sensitive to its representation. Including z coordinates for deeper regions efficiently maintains high near-surface vertical resolution in the open ocean. Investigation into the impact of the selected coordinate system focuses on differences between atmospherically-forced free-running (no assimilation) global solutions using σ–z and pure z coordinates. Comparisons with independent temperature observations indicate that global NCOM using the σ–z coordinate has improved skill relative to its z coordinate implementation. Among other metrics, we show that in comparison with time series of surface temperature from National Oceanic Data Center (NODC) buoys, mostly located in coastal regions, root mean squared differences (RMSD) improved for 63% and correlation improved for 71% of the stations when σ–z coordinates were used instead of pure z. For the exclusively open-ocean Tropical Atmosphere-Ocean (TAO) buoys, differences between the simulations were small, with the σ–z showing smaller RMSD for 45% of the stations and higher correlation for 65% of the stations. Additional comparisons using temperature profile observations further confirm a tendency for improved performance using the hybrid σ–z coordinates.     

A warm,equable Cretaceous: The nature of the problem

The nature of the problem of warm equable paleoclimates is defined by investigating the mid-Cretaceous period. The problem consists of three components: (1) defining precisely the climatic state during any specified interval of geologic time; (2) specifying the external climatic forcing factors which may have been important; and (3) understanding the climatic response to any specific modifying influence. These components are characterized by limitations of critical importance in understanding paleoclimates. The nature of the problem of a warm Cretaceous is defined both qualitatively and quantitatively within these limits.     

The use of coexisting calcite-ankerite solid solutions as a geothermometer

Limited solid solution of Mg and Fe²⁺ occurs in calcite coexisting with dolomiteankerite. This substitution is strongly temperature-dependent. Experimentally determined calcite compositions co-existing with a dolomite phase are available in the binary system CaCO₃-MgCO₃ between 500° C and 900° C (Harker and Tuttle, 1955). This information is extrapolated to lower temperatures and is combined with three synthetic calcite-ankerite pairs determined at 400° C, 450° C and 500° C (Rosenberg, 1967).The compositions of six naturally occurring calcites coexisting with ankerites from Sofala, N.S.W. are not accurately known, but X-ray determined compositional limits assuming firstly Mg substitution then Fe substitution yield maximum and minimum values for both possibilities. These limits are plotted on the ternary CaCO₃-MgCO₃-FeCO₃ together with the experimentally derived solvus isotherms. Assuming equilibration at constant temperature, actual compositions of these natural calcites plot along the 415° C isotherm.     

Modelling the combination of birefringence retardations from strain envelopes around multiple inclusions in diamond

A paleo-alluvial 0.21 ct yellow diamond (L058) from Bingara (NSW) has three inclusions of coesite (two subequant crystals and one thin plate), each under more than 3.1 GPa internal pressure as measured by Raman spectroscopy. These inclusions cause overlapping birefringent retardation stress/strain haloes in the host diamond, visible under cross-polarised light. The complicated retardation pattern is quantified by mapping targeted retardation contours (170 nm, 270 nm and 380 nm) onto a photo of the diamond. A mathematical model of retardation is developed for each inclusion, and then the combined light retardations (CLR) are calculated using radial and tangential components with spherical and elliptical geometries. The CLR model reproduces most features of the measured data, but remaining differences may be due to local release of stress/strain by two short fractures radiating from one inclusion.     

Continental drift and the global pattern of sedimentation

The breakup of Pangaea through rifting and separation of the continents has special implications for the global pattern of sedimentation. The important initial conditions of Pangaea are area, elevation, the nature of the drainage and climate. The development of interior uplifts associated with rifting caused significant reorganization of drainage systems. Rifting and continental breakup result in unique sediment sequences on passive margins.The initial rift valleys were probably occupied by stratified fresh water lakes due to the equable Mesozoic climate, and during this phase might have accumulated about 7.5×10²¹ g of organic carbon; this would be 14% of the earth's total organic carbon concentrated on only 0.3% of the earth's surface. The sediments rich in organic carbon are expected to be typically overlain by evaporites.Sedimentation on the continental shelf is a complex interplay of thermal subsidence, sea level changes, sediment supply and isostatic adjustment.Beyond the shelf break, sedimentation rates in the world ocean appear to change significantly with time; during the Aptian-Albian, Campanian-Maastrichtian, Middle Eocene and Late Miocene-Quaternary overall sedimentation rates were about an order of magnitude higher than during the intervening periods. This variation is likely to be related to changing sediment supply responding to changes in sea level.

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Zusammenfassung Das Aufbrechen Pangaeas durch Spaltung und Separation der Kontinente hat spezielle Auswirkungen auf die globale Sedimentationsverteilung. Die bedeutenden Anfangsbedingungen Pangaeas sind Größe, Höhe, Abfluß- und Klimasituation. Die Entwicklung innerer Aufwölbungen, zusammen mit Spaltenbildung, hat eine entscheidende Reorganisation der Abflußsysteme bewirkt. Spaltung und Aufbrechen des Kontinents resultieren in einer einzigartigen Sedimentationsfolge an den passiven Rändern.Die anfänglichen Spaltungstäler waren wahrscheinlich von geschichteten Süßwasserseen eingenommen, hervorgerufen vom ausgeglichenen mesozoischen Klima. Während dieser Zeit können sich dort etwa 7,5 · 10²¹ g organischer Kohlenstoffe angesammelt haben, d. h. 14 % der weltweiten organischen Kohlenstoffe auf nur 0,3 % der Erdoberfläche. Die kohlenstoffreichen Sedimente sind typischerweise von Evaporiten überlagert.Sedimentation auf dem kontinentalen Schelf ist ein komplexes Zusammenspiel von Temperatursenkung, Wasserspiegelschwankungen, Sedimentsangebot und isostatischem Gleichgewicht.Jenseits des Schelfabhanges scheinen die Sedimentationsraten im Weltmeer signifikant mit der Zeit zu schwanken. Während des Aptiums-Albs, des Campaniums-Maastrichts, des Mittleren Eozäns und des Späten Miozän-Quartärs lagen die gesamten Sedimentationsraten um eine Größenordnung höher als in den dazwischenliegenden Zeiträumen. Diese Variation hängt wahrscheinlich mit einem geänderten Sedimentsangebot zusammen, das von Wasserspiegelschwankungen hervorgerufen wird.

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Résumé La dislocation de la Pangée, par fracturation (rifting) et séparation des continents, entraîne des implications particulières en ce qui concerne les modèles de sédimentation à l'échelle du globe. Les conditions initiales importantes de la Pangée sont la surface, l'altitude, la nature du drainage et le climat. Le développement de soulèvements intérieurs en association avec les processus de fracturation (rifting) fut la cause d'une importante réorganisation du système de drainage. Sur les marges continentales passives, des séquences sédimentaires uniqes résultent de ces processus de caussure (rifting), et de dislocation.Les vallées médianes (rifts) initiales ont été probablement occupées par des lacs à eaux douces stratifiées, dues au climat égal du Mésozoique, et ont pu durant cette période accumuler environ 7.5×20²¹ g. de carbone organique; cette quantité serait l'équivalent de 14% du carbone organique total du globe, concentrés sur 0.3% de sa surface. Les sédiments riches en carbone organique sont supposés être typiquement recouverts par des dépôts évaporitiques.La sédimentation sur le plateau continental se trouve en interaction complexe avec la subsidence thermique, les fluctuations du niveau des océans, les apports en sédiments et les ajustements isostatiques.Au delà de la limite externe du plateau continental, les taux de sédimentation dans les océans semblent varier de manière importante dans le temps; durant l'Aptien-Albien, le Campanien-Maastrichtien, L'Eocène moyen et le Miocène supérieur-Quaternaire, les taux d'ensemble de sédimentation ont été environ d'un ordre de grandeur plus élevé que durant les périodes intermédiaires. Cette variation est vraisemblablement liée aux changements des apports en sédiments, en réponse aux fluctuations du niveau océanique.

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Comparing Methods of Barometric Efficiency Characterization for Specific Storage Estimation

Groundwater responses to barometric pressure fluctuations are characterized using the concept of barometric efficiency (BE). For semiconfined and confined aquifers, BE values can be used to provide efficient, low-cost estimates of specific storage. This study compares, for the first time, eight existing methods of BE estimation. Comparisons were undertaken using data from the Peel region of Western Australia. Fourier analysis and regression deconvolution methods were used to estimate aquifer confinement status. The former approach was found to be robust and provided a quantitative basis for spatial comparisons of the degree of confinement. The latter approach was confounded by the presence of diurnal and/or semidiurnal signals. For wells at which semiconfined or confined responses were identified, frequency and time domain methods were used to estimate BE values. Most BE estimation methods were similarly confounded by diurnal and/or semidiurnal signals, with the exception of the Acworth et al. (2016) method. Specific storage values calculated from BE values were order-of-magnitude consistent with the results of four historical pumping tests. The methods implemented in this research provide efficient, low-cost alternatives to hydraulic testing for estimating aquifer confinement, as well as the BE and specific storage of semiconfined and confined aquifers. The frequency and duration of observations required by these methods are minimal; for example, typically requiring a minimum of four observations per day over a four month period. In some locations they may allow additional insights to be derived from existing groundwater hydrograph data.     

Establishing irrigation potential of a hillside aquifer in the African highlands

Feeding 9 billion people in 2050 will require sustainable development of all water resources, both surface and subsurface. Yet, little is known about the irrigation potential of hillside shallow aquifers in many highland settings in sub-Saharan Africa that are being considered for providing irrigation water during the dry monsoon phase for smallholder farmers. Information on the shallow groundwater being available in space and time on sloping lands might aid in increasing food production in the dry monsoon phase. Therefore, the research objective of this work is to estimate potential groundwater storage as a potential source of irrigation water for hillside aquifers where lateral subsurface flow is dominant. The research was carried out in the Robit Bata experimental watershed in the Lake Tana basin which is typical of many undulating watersheds in the Ethiopian highlands. Farmers have excavated more than 300 hand dug wells for irrigation. We used 42 of these wells to monitor water table fluctuation from April 16, 2014 to December 2015. Precipitation and runoff data were recorded for the same period. The temporal groundwater storage was estimated using two methods: one based on the water balance with rainfall as input and baseflow and evaporative losses leaving the watershed as outputs; the second based on the observed rise and fall of water levels in wells. We found that maximum groundwater storage was at the end of the rain phase in September after which it decreased linearly until the middle of December due to short groundwater retention times. In the remaining part of the dry season period, only wells located close to faults contained water. Thus, without additional water sources, sloping lands can only be used for significant irrigation inputs during the first 3 months out of the 8 months long dry season.     

Up‐scaling potential impacts on water flows from agricultural water interventions: opportunities and trade‐offs in the Osman Sagar catchment,Musi sub‐basin,India

Agricultural water management (AWM) has been shown to improve and secure yields in the tropics and has been suggested as an important way to combat poverty in the region. In this paper, we describe potential impacts on upstream and downstream flows of extensive AWM interventions, using the watershed development programme of the Osman Sagar catchment of Musi sub‐basin, Andhra Pradesh semi‐arid India, as an example. Various AWM interventions are compared with a non‐intervention state and the current state of the study area, using 31 years of data by application of the calibrated and validated ARCSWAT 2005 (Version 2.1.4a) modelling tool. Different AWM interventions contribute to improved livelihoods of upstream smallholder farmers by increasing soil moisture availability and groundwater recharge, which can subsequently be used for irrigation. The result is higher crop production and hence larger incomes. Moreover, lower flow intensities and sediment losses reduced by 30–50%, reducing the risk of flooding and sediment accumulation in the Osman Sagar drinking water reservoir. On the other hand, AWM interventions are predicted to result in reduced total water inflows to the Osman Sagar reservoir from 11% of the total annual rainfall (754 mm) recorded at present, to 8% if AWM interventions were implemented at large scale throughout the catchment. A cost–benefit analysis of AWM interventions showed that the highest net economic returns were achieved at intermediate intervention levels (only in‐situ AWM). Copyright © 2012 John Wiley & Sons, Ltd.