POTENTIAL CROP PRODUCTION

Stromgaard, Peter: Potential Crop Production—illustrated by an example from West Africa, Geografisk Tidsskrift, 77: 6—12, København, June 1, 1978.

Different ways of describing and modelling the process of primary production are compared, and the concept of potential crop production is elaborated.

Two mathematical models are suggested, converted into FORTRAN IV, and used for an estimation of potential productivity in Ghana.     

Bayesian power-law regression with a location parameter,with applications for construction of discharge rating curves

This paper presents a Bayesian approach for fitting the standard power-law rating curve model to a set of stage-discharge measurements. Methods for eliciting both regional and at-site prior information, and issues concerning the determination of prior forms, are discussed. An efficient MCMC algorithm for the specific problem is derived. The appropriateness of the proposed method is demonstrated by applying the model to both simulated and real-life data. However, some problems came to light in the applications, and these are discussed.     

Bayesian methods for estimating multi-segment discharge rating curves

This study explores Bayesian methods for handling compound stage–discharge relationships, a problem which arises in many natural rivers. It is assumed: (1) the stage–discharge relationship in each rating curve segment is a power-law with a location parameter, or zero-plane displacement; (2) the segment transitions are abrupt and continuous; and (3) multiplicative measurement errors are of equal variance. The rating curve fitting procedure is then formulated as a piecewise regression problem where the number of segments and the associated changepoints are assumed unknown. Procedures are developed for describing both global and site-specific prior distributions for all rating curve parameters, including the changepoints. Estimation and uncertainty analysis is evaluated using Markov chain Monte Carlo simulation (MCMC) techniques. The first model explored accounts for parameter and model uncertainties in the interpolated area, i.e. within the range of available stage–discharge measurements. A second model is constructed in an attempt to include the uncertainty in extrapolation, which is necessary when the rating curve is used to estimate discharges beyond the highest or lowest measurement. This is done by assuming that the rate of changepoints both inside and outside the measured area follows a Poisson process. The theory is applied to actual data from Norwegian gauging stations. The MCMC solutions give results that appear sensible and useful for inferential purposes, though the latter model needs further efforts in order to obtain a more efficient simulation scheme.     

A New method for identifying possible causal relationships between CO2, total solar irradiance and global temperature change

The study compares two formulas for calculating the daily evapotranspiration ET₀ for a reference crop. The first formula was proposed by Allen et al. (AL), while the second one was proposed by Katerji and Perrier with the addition of the carbon dioxide (CO₂) effect on evapotranspiration (KP). The study analyses the impact of the calculation by the two formulas on the irrigation requirement (IR). Both formulas are based on the Penman-Monteith equation but adopt different approaches for parameterising the canopy resistance r _c . In the AL formula, r _c is assumed constant and not sensitive to climate change, whereas in the KP formula, r _c is first parameterised as a function of climatic variables, then ET₀ is corrected for the air CO₂ concentration. The two formulas were compared in two periods. The first period involves data from two sites in the Mediterranean region within a measured climate change period (1981–2006) when all the input climatic variables were measured. The second period (2070–2100) involves data from a future climate change period at one site when the input climatic variables were forecasted for two future climate scenarios (A2 and B2). The annual cumulated values of ET₀ calculated by the AL formula are systematically lower than those determined by the KP formula. The differences between the ET₀ estimation with the AL and KP formulas have a strong impact on the determination of the IR for the reference crop. In fact, for the two periods, the annual values of IR when ET₀ is calculated by the AL formula are systematically lower than those calculated by the KP formula. For the actual measured climate change period, this reduction varied from 26 to 28 %, while for the future climate change period, it varied based on the scenario from 16 % (A2) to 20 % (B2).     

Analysis of shallow gas and fluid migration within the Plio-Pleistocene sedimentary succession of the SW Barents Sea continental margin using 3D seismic data

Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the 2–3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration and advanced interpretation techniques, they provide surprisingly detailed images of inferred gas accumulations and the sedimentary environments in which they occur. The presence of gas is inferred from seismic reflection segments with anomalously high amplitude and reversed phase, compared with the seafloor reflection, so-called bright spots. Fluid migration is inferred from vertical zones of acoustic masking and acoustic pipes. The 3D seismic volume allows a spatial analysis of amplitude anomalies inferred to reflect the presence of gas and fluids. At several locations, seismic attribute maps reveal detailed images of flat spots, inferred to represent gas–water interfaces. The data indicate a focused fluid migration system, where sub-vertical faults and zones of highly fractured sediments are conduits for the migration of gas-bearing fluids in Plio-Pleistocene sediments. Gas is interpreted to appear in high-porosity fan-shaped sediment lobes, channel and delta deposits, glacigenic debris flows and sediment blocks, probably sealed by low-permeability, clayey till and/or (glacio)marine sediments. Gas and fluid flow are here attributed mainly to rapid Plio-Pleistocene sedimentation that loaded large amounts of sedimentary material over lower-density, fine-grained Eocene oozes. This probably caused pore-fluid dewatering of the high-fluid content oozes through a network of polygonal faults. The study area is suggested to have experienced cycles of fluid expulsion and hydrocarbon migration associated with glacial–interglacial cycles.     

BIBLIOGRAPHIES

Abstract

On a regional scale (from about 10 to 10⁵km²) estimations of actual evapotranspiration ET for water balance equations are generally missing. The various methods available at present are briefly described and discussed. The use of remote sensing, in particular infra-red thermometry by satellite, in conjunction with soil energy budget data, is the only method to provide satisfactory data on the same scale in space and lime for water balance models. This method is being studied at the INRA Bioclimatology Station at Avignon in the framework of the European ‘Tellus’ project for using HCMM satellite data. Systems for continuously recording the components of the energy budget and ET have been installed in the Crau plain, north of Marseilles, in both a dry area and a large area of irrigated pastures. By combining the recorded measurements with the satellite data and starting from values for elements of 25 ha (500 m × 500 m), the ET for the whole of the Crau plain can be estimated. First results are analysed and the possibilities and limitations of the method are discussed.     

Late-Quaternary biogeographic scenarios for the brown bear (Ursus arctos), a wild mammal model species

This review provides an up-to-date synthesis of the matrilineal phylogeography of a uniquely well-studied Holarctic mammal, the brown bear. We extend current knowledge by presenting a DNA sequence derived from one of the earliest known fossils of a polar bear (dated to 115 000 years before present), a species that shares a paraphyletic mitochondrial association with brown bears. A molecular clock analysis of 140 mitochondrial DNA sequences, including our new polar bear sequence, provides novel insights into the times of origin for different brown bear clades. We propose a number of regional biogeographic scenarios based on genetic data, divergence time estimates and paleontological records. The case of the brown bear provides an example for researchers working with less well-studied taxa: it shows clearly that phylogeographic models based on patterns of modern genetic variation alone can be substantially improved by including data on historical patterns of genetic diversity in the form of ancient DNA sequences derived from accurately dated samples and by using an approach to divergence-time estimation that suits the data under analysis. Using such approaches it has been possible to (i) establish that the processes shaping modern genetic diversity in brown bears acted recently, within the last three glacial cycles; (ii) distinguish among hypotheses concerning species’ responses to climatic oscillations in accordance with the lack of phylogeographic structure that existed in brown bears prior to the last glacial maximum (LGM); (iii) reassess theories linking monophyletic brown bear populations to particular LGM refuge areas; and (iv) identify vicariance events and track analogous patterns of migration by brown bears out of Eurasia to North America and Japan.     

Timing of the younger dryas glacial maximum in western Norway

This study precisely constrains the timing of the Younger Dryas (YD) glacial maximum in south‐western Norway by utilizing sediment records from lake basins. Two of the basins, located on the distal side of the mapped Herdla–Halsnøy Moraine, received meltwater directly from the ice sheet only when the ice margin reached its maximum extent during the YD. In the cores, the ice maximum is represented by well‐defined units with meltwater deposits, dominantly laminated silt. Plant macrofossils in the sediment sequences are common and we obtained 18 radiocarbon ages from one of the cores. By applying Bayesian age–depth modelling we obtained a precise date for this meltwater event and thereby also for the timing of the YD glacial maximum. We conclude that the ice‐sheet advance culminated at the Halsnøy Moraine at 11 760 ± 120 cal a BP, and that the ice margin stayed in this position for 170 ± 120 years. The subsequent retreat started at 11 590 ± 100 cal a BP, i.e. close to the YD/Holocene boundary. Withdrawal was probably triggered by abrupt climatic warming at this time. Copyright © 2011 John Wiley & Sons, Ltd.