Akpofure Taigbenu | University of the Witwatersrand (original) (raw)

Papers by Akpofure Taigbenu

HAL (Le Centre pour la Communication Scientifique Directe), 2006

Sustainable catchment management requires the integration of the hydrologic, environmental and so... more Sustainable catchment management requires the integration of the hydrologic, environmental and socio-economic components that occur within the catchment. Most existing models deal individually with each component. Modeling of the components alone may not simulate the complete system effectively as physical/environmental and socio-economic systems have emergent properties which relate to the whole, and not merely to the sum of the parts. A realistic framework that integrates a range of models (hydrologic and socio-economic) and datasets will therefore provide the necessary platform for assessing the impacts of proposed policy and management strategies on livelihoods. This paper introduces a framework for the coupling of hydrologic and socio-economic models. The model framework will account for the interactions between water availability, farmer behavior, agricultural productivity, and will estimate the socio-economic gains from improvement in the allocation and efficiency of water use. The paper further suggests ways in which such models can be developed and used. The integration model will test and evaluate a given policy on the farmer income and the quality of livelihood outcomes. The approach aims to explore, rather than predict, the future and is not oriented towards optimization. By exploring alternative scenarios the user exercises choice of the best bet solutions of proposed policy and management practices. The model framework will be applied in the Olifants River Basin, South Africa at quaternary level (lowest water management area) as a case study for an in-depth investigation

International Journal of Agricultural Sustainability, Sep 3, 2018

Multiple agricultural water management (AWM) technologies are being promoted worldwide in rainfed... more Multiple agricultural water management (AWM) technologies are being promoted worldwide in rainfed agro-ecological production systems, such as the Limpopo River Basin, to close the yield gap, enhance food security and reduce poverty, but evidences on yield gains and environmental impacts are varied. This paper conducts a review of the performance of AWM technologies against conventional farmer practices to produce adequate evidence on cereal yield and field runoff changes. With the interrogation of literature from 1980 to 2013 using seven AWM groupings, enough evidence was found that AWM technologies can deliver substantial benefits of increased crop yield and water productivity with reduced environmental impacts. Using random effects model, the standardized mean difference (SMD) of yield between AWM and control was 0.27, while SMD of water productivity was 0.46, indicating the effectiveness of the technologies (SMD > 0). Subgroup analyses showed greatest yield responses on silty-clay-loam, clay-loam and sandy soils compared to clay and loam-sandy soils, and higher yield increase under low rainfall regime (200-500 mm) than under high rainfall regime (500-800 mm). Large yield change variations for different AWM technologies present a huge opportunity for meeting the existing yield gaps and enhancing coping capacity in dry years and under climate change.

WIT transactions on modelling and simulation, Aug 23, 1999

Flows in variably saturated media are not only of profound interest to numerical analysts, engine... more Flows in variably saturated media are not only of profound interest to numerical analysts, engineers and scientists because of the challenge they pose as a result of their highly nonlinear constitutive relations, but also because of their importance in many fields of engineering such as drainage, irrigation, environmental, soil, and petroleum engineering. In this paper, the Picard and Newton-Raphson (N-R) algorithms are incorporated into the Green element method (GEM) to simulate these flows. The Green element method offers a viable means of implementing the singular boundary integral theory so that the theory is of more general application, and computational efficiency is enhanced [1]. Here the GEM discretizes the integro-differential equation in space with rectangular elements and with a generalized 2-level time discretization scheme in time, while the system of nonlinear discretized element equations are linearized by the Picard and Newton-Raphson (N-R) algorithms. Calculations carried on three numerical examples of infiltration in unsaturated soils in 2-D. spatial dimensions indicate better convergence of the N-R algorithm at comparable computational cost as the Picard algorithm.

European Journal of Computational Mechanics, May 11, 2018

When pollution spills occur, they impact on the quality of water in underlying aquifers. Such spi... more When pollution spills occur, they impact on the quality of water in underlying aquifers. Such spills can be modelled as instantaneous pollution sources, and estimating their strengths from the concentration plumes they produce is an inverse problem which is addressed in this paper by the Green element method (GEM). Estimating the strengths of such spills, making use of the concentration data at various locations and times, is an inverse problem whose solution is often associated with non-uniqueness, non-existence and instability. Here the GEM is used to predict the strengths of pollution spills from measured concentration data at internal observation points. The performance of the methodology is illustrated using two numerical examples in which the contaminant plumes are from multiple point and distributed pollution sources. Single and multiple episodes of pollution injections are accommodated in both examples. It is observed that GEM is more accurate in predicting the strengths of distributed instantaneous pollution sources than point sources because of the discontinuities of the latter in both the spatial and temporal dimensions.

Advances in Engineering Software, Mar 1, 1998

Our earlier paper demonstrated that the singular boundary element (BE) theory, which hitherto had... more Our earlier paper demonstrated that the singular boundary element (BE) theory, which hitherto had been considered unsuitable for the highly nonlinear variably saturated transient Row problem, could be applied to the same problem when implemented along the lines of the Green element method (GEM) (Taigbenu, A. E. and Onyejekwe, 0. O., Green element simulations of transient nonlinear unsaturated flow equation, Applied Mathematical Modelling, 1995, 19, 675-684). Here, the transient onedimensional unsaturated flow problem is revisited with a Green element model which incorporates the cubic Hermitian interpolation basis functions to approximate the distribution of the primary variable and the soil constitutive relations. Because the soil parameters vary appreciably by several orders of magnitude over small intervals of soil moisture, approximating those parameters by linear interpolation functions, as earlier done, could be inadequate, and this fact is demonstrated here using two numerical examples of infiltration into vertical soil columns. With the first example, there is good agreement between the solutions of Hermitian GEM and Hermitian finite element method (FBM) which confirms that the discrepancy of less amount of soil moisture in the linear GEM solution, earlier observed, is due to an error from interpolation (Taigbenu, A. E. and Onyejekwe, 0. O., Green element simulations of transient nonlinear unsaturated flow equation, Applied Mathematical Modelling, 1995, 19,675-684). The second example serves to show that comparable accuracy between Hermitian and linear GE models can be achieved using a fewer number of elements in the former model.

Physics and Chemistry of the Earth, Parts A/B/C, 2016

Abstract The mappings of poverty and food insecurity were carried out for the rural districts of ... more Abstract The mappings of poverty and food insecurity were carried out for the rural districts of the four riparian countries (Botswana, Mozambique, South Africa and Zimbabwe) of the Limpopo river basin using the results of national surveys that were conducted between 2003 and 2013. The analysis shows lower range of food insecure persons (0–40%) than poverty stricken persons (0–95%) that is attributable to enhanced government and non-government food safety networks in the basin countries, the dynamic and transitory nature of food insecurity which depends on the timings of the surveys in relation to harvests, markets and food prices, and the limited dimension of food insecurity in relation to poverty which tends to be a more structural and pervasive socio-economic condition. The usefulness of this study in influencing policies and strategies targeted at alleviating poverty and improving rural livelihoods lies with using food insecurity mappings to address short-term socio-economic conditions and poverty mappings to address more structural and long-term deprivations. Using the poverty line of $1.25/day per person (2008–2013) in the basin, Zimbabwe had the highest percentage of 68.7% of its rural population classified as poor, followed by Mozambique with 68.2%, South Africa with 56.1% and Botswana with 20%. While average poverty reduction of 6.4% was observed between 2003 and 2009 in Botswana, its population growth of 20.1% indicated no real poverty reduction. Similar observations are made about Mozambique and Zimbabwe where population growth outstripped poverty reductions. In contrast, both average poverty levels and population increased by 4.3% and 11%, respectively, in South Africa from 2007 to 2010. While areas of high food insecurity and poverty consistently coincide with low water availability, it does not indicate a simple cause–effect relationship between water, poverty and food insecurity. With limited water resources, rural folks in the basin require stronger institutions, increased investments and support to enable them generate sufficient income from their rain-fed farming livelihood to break out of the poverty cycle.

WIT Transactions on State of the Art in Science and Engineering, 2007

This chapter presents some solutions to the diffusion-convection equation that are based on the b... more This chapter presents some solutions to the diffusion-convection equation that are based on the boundary element theory. Four formulations are discussed, but solutions from only three of them are presented. The formulations represent different replications of the differential equation along the lines of the singular integral theory. Their fundamental solutions come from different linear parts of the differential operator. The elliptic diffusion (ED) formulation that is based on the ED operator shows the most promise. Attempts at avoiding domain integrations through the dual reciprocity method are discussed, while full domain discretization through the Green element method for ease of evaluation of domain integrations and in solving heterogeneous and nonlinear transport is presented. The solutions from the three formulations to the nonlinear Burgers' equation are also presented, with the ED formulation exhibiting superior performance.

The Green Element Method, 1999

Water SA, 2005

Judicious management of a groundwater system requires an understanding of its hydrogeology and re... more Judicious management of a groundwater system requires an understanding of its hydrogeology and response to various recharge and pumping stresses. However, in developing countries, groundwater resource evaluations are hampered by a lack of adequate data that will allow for its complete characterisation. Under such circumstances it is not uncommon for ad hoc groundwater management measures to be embarked upon, especially during drought conditions. These were the conditions that existed during the 1991/92 drought when the CSIR Stellenbosch evaluated the groundwater resource of an urban aquifer in Bulawayo, Zimbabwe. Their recommendations revealed that about 3.5×10 6 m 3 /a could be safely abstracted from the aquifer. In this work, a more comprehensive hydrogeological investigation was carried out which included pumping tests, estimation of abstraction rates and recharge, and numerical modelling of the aquifer. The investigations indicate that the aquifer is unconfined with hydraulic conductivity and specific yield ranging from 0.1 m/d to 2.09 m/d and 0.02 to 0.11, respectively. Recharge estimates indicate an annual recharge of 105.5 mm with 38.4%, 52.1% and 9.5% accounting respectively for direct recharge, water mains and sewer leakages. Furthermore, a long-term sustainable annual abstraction of 6.1×10 6 m 3 or 15% of current city water demand can be obtained from the aquifer.

Advances in Engineering Software, 1998

WIT transactions on modelling and simulation, Jun 11, 2013

Inverse contaminant transport problems, unlike direct problems, may result in non-unique and unst... more Inverse contaminant transport problems, unlike direct problems, may result in non-unique and unstable solutions because of the ill-conditioned nature of the coefficient matrix. In this work the Green element method (GEM) is used to solve steady inverse contaminant transport problems. The ill-conditioned, overdetermined system of equations that arises from the Green element discretization is solved by the least square method with the singular value decomposition technique and Tikhonov regularization. Two examples of steady inverse contaminant transport problems with constant and variable velocity are simulated by GEM with good prediction obtained for the concentration and fluxes.

Pollution of groundwater often occurs because of unknown disposal of toxic wastes, especially fro... more Pollution of groundwater often occurs because of unknown disposal of toxic wastes, especially from industrial sites, or due to undetected leakage from pipes, waste storage containers, or underground tanks. The determination of pollution sources by using only concentration measurement data in the aquifer is analogous to reconstructing the history of events that has occurred in the aquifer over a time horizon. Identification of unknown groundwater pollution sources becomes more difficult when the hydrogeologic flow and transport parameters are also unknown. A trained artificial neural network (ANN) can be utilized to simultaneously solve the problems of estimating unknown groundwater pollution sources and estimating unknown hydrogeologic parameters (hydraulic conductivity, porosity, and dispersivities). In this article, the universal function approximation property of a multilayer, feed-forward ANN was utilized to estimate temporally and spatially varying unknown pollution sources, as well as to provide a reliable estimation of unknown flow and transport parameters. ANN was trained on patterns of simulated data using a back-propagation algorithm. A set of source fluxes and temporally varying simulated concentration measurements constituted the pattern for training. This article also describes a potential applicability of this methodology by using an illustrative example. Additionally, the methodology performance is evaluated under varying concentration measurement errors. The limited performance evaluations show that the proposed methodology performs reasonably well even with large measurement errors.

European Journal of Computational Mechanics, 2018

International Journal of Computer Mathematics, 1999