Simulation of groundwater flow in a crystalline rock aquifer system in Southern Ghana – An evaluation of the effects of increased groundwater abstraction on the aquifers using a transient groundwater flow model

Monitored groundwater level data, well logs, and aquifer data as well as the relevant surface hydrological data were used to conceptualise the hydrogeological system of the Densu Basin in Southern Ghana. The objective was to numerically derive the hydraulic conductivity field for better characterization of the aquifer system and for simulating the effects of increasing groundwater abstraction on the aquifer system in the basin. The hydraulic conductivity field has been generated in this study through model calibration. This study finds that hydraulic conductivity ranges between a low of 2 m/d in the middle sections of the basin and about 40 m/d in the south. Clear differences in the underlying geology have been indicated in the distribution of aquifer hydraulic conductivities. This is in consonance with the general assertion that the hydrogeological properties of the aquifers in the crystalline basement terrains are controlled by the degree of fracturing and/or weathering of the country rock. The transient model suggest aquifer specific storage values to range between 6.0 × 10^?5 m^?1 and 2.1 × 10^?4 m^?1 which are within acceptable range of values normally quoted for similar lithologies in the literature. There is an apparent subtle decrease in groundwater recharge from about 13% of the annual precipitation in 2005 to about 10.3% of the precipitation in 2008. The transient model was used to simulate responses of the system to annual increment of groundwater abstraction by 20% at the 2008 recharge rates for the period 2009 – 2024. The results suggest that the system will not be able to sustain this level of abstraction as it would lead to a basin wide drawdown in the hydraulic head by 4 m by the end of the prediction period. It further suggests a safe annual increment in groundwater abstraction by 5% under business as usual recharge conditions. Identification and protection of groundwater recharge areas in the basin are recommended in order to safeguard the integrity of the resource under the scenario of increased abstraction for commercial activities in the basin. Copyright © 2012 John Wiley & Sons, Ltd.     

On the Principal Asteroseismic Diagnostic Signatures

Many stellar model calibrations against seismic data will utilize justthe so-called large and small frequency separations. In this report Iexplain in physical terms, based on resonance conditions forconstructive interference between simple waves,why those frequency separations depend on the variation of sound speedthrough the star in the manner in which they do. By so doing I hope toincrease appreciation of the nature of the stellar calibrations,and thereby provide a means for assessing the credibility of the results.     

On the excitation mechanism in roAp stars

We investigate a model for the excitation of high-order oscillations in roAp stars. In this model we assume that the strong concentration of magnetic field about the magnetic poles is enough to suppress convection. Thus the model considered is composed of two polar regions, in which convection is presumed to be suppressed totally, and an equatorial region, where the convection is unaffected. This model is generated by building pairs of locally spherically symmetrical equilibria to represent the polar and equatorial regions of the star, which are patched together below the base of the convection zone. Gravitational settling of heavy elements is taken into account by choosing appropriate chemical composition profiles for both the polar and equatorial regions. Our results indicate that the composite model is unstable against axisymmetric non-radial high-order modes of pulsation that are aligned with the magnetic poles. The oscillations are excited by the κ mechanism acting principally in the hydrogen ionization zones of the polar regions. The effect of the lateral inhomogeneity on the second frequency differences is also investigated; we find that the perturbation to them by the inhomogeneity is of the same order as the second differences themselves, thereby hindering potential attempts to use such differences to identify the degrees of the modes in a straightforward way.     

An asteroseismic signature of helium ionization

We investigate the influence of the ionization of helium on the low-degree acoustic oscillation frequencies in model solar-type stars. The signature in the oscillation frequencies characterizing the ionization-induced depression of the first adiabatic exponent γ is a superposition of two decaying periodic functions of frequency ν, with 'frequencies' that are approximately twice the acoustic depths of the centres of the He  i and He  ii ionization regions. That variation is probably best exhibited in the second frequency difference  Δ₂ν _{n ,  l} ≡ν _{n −1,  l} − 2ν _{n ,  l} +ν _{n +1,  l}   . We show how an analytic approximation to the variation of γ leads to a simple representation of this oscillatory contribution to Δ₂ν which can be used to characterize the γ variation, our intention being to use it as a seismic diagnostic of the helium abundance of the star. We emphasize that the objective is to characterize γ, not merely to find a formula for Δ₂ν that reproduces the data.     

Isopycnal mixing and convective adjustment in an ocean general circulation model

Abstract

Convective adjustment is examined in an ocean general circulation model which uses an isopycnal mixing parametrization. It is found that the use of an explicit convective adjustment scheme is not needed in a variety of equilibria and climate change scenario simulations. A numerical mechanism is proposed to explain this as well as the localized appearance of ‘negative’ diffusion.     

Monitoring precipitable water vapor in real-time using global navigation satellite systems

This paper addresses real-time monitoring of the precipitable water vapor (PWV) from GNSS measurements and presents some results obtained from 6-month long GNSS PWV experiments using international and domestic GNSS networks. In the real-time GNSS PWV monitoring system a server/client structure is employed to facilitate formation of PWV networks and single-differenced GNSS measurements are utilized to mitigate errors in GNSS satellites’ orbits and clocks. An issue relating to baseline length between the server and clients is discussed in detail and as a result the PWV monitor is configured to perform in two modes depending on the baseline length. The server estimates sequentially the zenith wet delay of the individual stations, which is then converted into the PWV of the stations. We evaluate system performance by comparing the real-time PWV solution with reference solutions including meteorological measurements obtained with radiosondes and deferred-time precision GNSS PWV solutions. Results showed that the standard deviation of difference between the real-time PWV and the reference solutions ranged from 2.1 to 3.4 mm in PWV for a 6-month long comparison, which was improved to 1.4 to 2.9 mm by reducing comparison period to 20 days in winter.     

“传统”农业景观旅游保护困局的反思（英文）

全球范围对"文化景观"重新燃起兴趣需要从多维方式来解释。传统栖息地,人民和他们在特殊环境里的生活方式的价值改变过程是和与遗产地有关的"文化、社会和经济建设"的引入密不可分的。自上世纪90年代开始,文化景观作为一种新型遗产列入联合国教科文组织世界遗产名录,它强调人类与环境相互作用的重要性,需要了解景观在时空上的动态性。观念的变化产生了对标志性景观和传统社区的"动态保护"。从交叉学科的视角来理解这一交互过程对规划和保护可持续遗产景观至关重要。遍及全世界的各试点项目和案例研究都引起了关于遗产景观的可持续性和旅游的至高无上作用的批判性反思。全球重要农业文化遗产(GIAHS)能否受益于旅游产生的经济资源,需要基于研究的方法来分析机会和预期,评估其战略政策和自上而下的政策。