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The Recent Droughts of.2019/20 in Southern Af.ica and Its Teleconnection with ENSO Events

Atmospheric and Climate Sciences 2022年第2期12卷 246-263页

作者： berhanu f. alemaw Department of.Geology University of Botswana Gaborone Botswana

This study is motivated to highlight the variability of.recent drought hotspots in the region of.southern Af.ica in terms of.the seasonal and annual rainf.ll regimes and their possible spatial linkage with the 1950-2020 seasonal El Niño/Southern Oscillation (ENSO). Some evidence is f.und on possible links between the occurrence of.drought hotspots in the region in terms of.seasonal and mean annual runof. and warm ENSO events. This was revealed by the existence of.a strong and nearly-strong positive linear correlation between Seasonal and annual rainf.ll depths and the warm seasonal ENSO indices explained by the southern oscillation index represented by the sea level pressure (SLP) anomalies data obtained f.om the National Oceanographic and Aeronautics Administration (NOAA). Considering the entire southern Af.ican region, 41% of.the surf.ce areas exhibit moderate (r > 0.25) and strong (r > 0.5) correlation coef.icients in terms of.the December to f.bruary quarter rainf.ll and ENSO indices. Above 50% conf.dence interval in the correlation between seasonal rainf.ll and ENSO during DJf.quarters is f.und in 74% of.the surf.ce area of.the region of.southern Af.ica. The high conf.dence interval of.the positive correlation coef.icients is an indication that substantial variance of.precipitation during ENSO years is accounted f.r by the warm ENSO events. The areas with pronounced lower rainf.lls and droughts associated with ENSO activity in the region include larger and some pockets of.various countries in southern Af.ica, including but not limited to Angola, Botswana, Lesotho, Namibia, Zambia, Zimbabwe, South Af.ica and Mozambique. The recent drought events of.2019/2020, and previously in 2015/16 in this region with wider regional impacts can be explained by the ENSO phenomena.

关键词： Rainf.ll Variability Drought ENSO Episode Correlation Analysis Southern Af.ica

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Hydrogeochemistry of.Shale-Limestone-Mudstone Successions Aquif.rs—The Case of.the Hantebet Sub Basin in Northern Ethiopia

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Journal of.Water Resource and Protection 2022年第6期14卷 433-455页

作者： Nata T. Taf.sse Bheemalingeswara Konka berhanu f. alemaw Geology Department Faculty of Sciences University of Botswana Gaborone Botswana Department of.Mining and Geological Engineering FET Botswana International University of Science and Technology Palapye Botswana

The present paper provides evidence of.the possible impact of.shale-limestone-mudstone successions aquif.rs on groundwater chemistry by assessing the dif.erent hydrogeochemical processes. This was done by considering a sedimentary aquif.r basin, namely the Hantebet sub basin (24.4 km2), Tekeze basin, northern Ethiopia. Groundwater is the main source of.water supply in the sub basin extracted using hand dug wells, f.r domestic, irrigation and livestock uses. The sub basin is dominated by Paleozoic-Mesozoic sedimentary successions. Twenty groundwater samples were collected f.om hand dug wells using depth-integrated sampling techniques f.om both conf.ned and unconf.ned aquif.rs. The major water bearing f.rmations are gravely sand, weathered shale and weathered and f.actured limestone, and intercalated weathered and f.actured limestone and mudstone. The results indicate that groundwater is acidic to neutral, f.esh, and hard to very hard. Ca2+, Na+, HCO-3 and SO2-4 are dominant ions compared to Mg2+, K+, and Cl- ions which show low to very low concentrations. Among eight hydrochemical f.cies identif.ed, Ca-Na-HCO3 (40%), Ca-HCO3 (20%), Ca-Mg-Na-HCO3 (10%) and Ca-Na-HCO3-SO4 (10%) types dominate water chemistry. Dissolution of.calcite and gypsum, and hydrolysis of.f.ldspars, plagioclase, biotite and pyroxene are the major geochemical processes that control the chemistry of.groundwater in the area. The intercalated shale beds are the source of.sodium and chloride ions. Since, this study is based on groundwater f.om hand dug wells, the conclusions of.this study should be f.rther verif.ed using groundwater f.om deep wells that are drilled in these successions.

关键词： Conf.ned Aquif.r Dissolution Ethiopia Groundwater Hydrolysis Unconf.ned Aquif.r

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Resilience, reliability and risk analyses of.maize, sorghum and sunf.ower in rain-f.d systems using a soil moisture modeling approach

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Agricultural Sciences 2012年第1期3卷 114-123页

作者： berhanu f. alemaw Water Research Group Department of Geology Faculty of Science University of Botswana Gaborone Botswana

This paper is aimed at examining the applicability of.methods f.r resilience, reliability and risk analyses of.rain-f.d agricultural systems f.om modeled continuous soil moisture availability in rain-f.d crop lands. The methodology involves integration of.soil and climatic data in a simple soil moisture accounting model to assess soil moisture availability, and a risk used as indicator of.sustainability of.rain-f.d agricultural systems. It is also attempted to demonstrate the role of.soil moisture modeling in risk analysis and agricultural water management in a semi-arid region in Limpopo Basin where rain-f.d agriculture is practiced. f.r this purpose, a daily-time step soil moisture accounting model is employed to simulate daily soil moisture, evaporation, surf.ce runof., and deep percolation using 40 years (1961-2000) of.agroclimatic data, and cropping cycle data of.maize, sorghum and sunf.ower. Using a sustainability criterion on crop water requirement and soil moisture availability, we determined resilience, risk and reliability as a quantitative measure of.sustainability of.rain-f.d agriculture of.these three crops. These soil moisture simulations and the sustainability criteria revealed f.rther conf.rmation of.the relative sensitivity to drought of.these crops. Generally it is f.und that the risk of.f.ilure is relatively low f.r sorghum and relatively high f.r maize and sunf.ower in the two sites with some dif.erences of.severity of.f.ilure owing to the slightly dif.erent agroclimatic settings.

关键词： Resilience Reliability Risk Analysis: Agricultural Drought Index Rain-f.d Agriculture Soil Moisture Modeling Limpopo Basin

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Hydrological Modeling of.Large Drainage Basins Using a GIS-based Hybrid Atmospheric and Terrestrial Water Balance (HATWAB) Model

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Journal of.Water Resource and Protection 2012年第7期4卷 516-522页

作者： berhanu f. alemaw Department of.Geology Faculty of Science University of Botswana Gaborone Botswana

A Hydrological model is proposed to study the spatial and temporal variability of.the water budget components of.large drainage basin systems f.om atmospheric and terrestrial water balances. In order to understand the water balances that include, surf.ce runof., actual evapotranspiration and soil moisture, a GIS-based simple water balance model which is ref.rred as Hydrological Model f.om Hybrid Atmospheric and Terrestrial Water Balances with acronym HATWAB is presented. The spatio-temporal climatology database was created f.om a network of.climate stations f.om CLIMWAT data base to reconstruct the monthly primary inputs to HATWAB model, rainf.ll and potential evapotranspiration. The modeling principles and HATWAB model are demonstrated using the Limpopo and Congo basins in Af.ica. The model was used to simulate water balance components by taking rainf.ll-runof. processes in the basin including soil-texture controlled moisture in the terrestrial system, and the vertical integrated moisture convergence that accounts f.r the net water vapor f.ux f.om the basins in order to close the hydrologic water budget.

关键词： HATWAB Water Budget Large Drainage Basin Soil Moisture Vertical Integrated Moisture Convergence Water f.ux GIS

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Climate Change Impacts and Adaptation in Rainf.d f.rming Systems: A Modeling f.amework f.r Scaling-Out Climate Smart Agriculture in Sub-Saharan Af.ica

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American Journal of.Climate Change 2015年第4期4卷 313-329页

作者： berhanu f. alemaw Timothy Simalenga Department of.Geology University of Botswana Gaborone Botswana Centre f.r Coordination of.Agricultural Research & Development in Southern Af.ica (CCARDESA) Gaborone Botswana

Improving agricultural water productivity, under rainf.d or irrigated conditions, holds signif.cant scope f.r addressing climate change vulnerability. It also of.ers adaptation capacity needs as well as water and f.od security in the southern Af.ican region. In this study, evidence f.r climate change impacts and adaptation strategies in rainf.d agricultural systems is explored through modeling predictions of.crop yield, soil moisture and excess water f.r potential harvesting. The study specif.cally presents the results of.climate change impacts under rainf.d conditions f.r maize, sorghum and sunf.ower using soil-water-crop model simulations, integrated based on daily inputs of.rainf.ll and evapotranspiration disaggregated f.om GCM scenarios. The research targets a vast f.rming region dominated by heavy clay soils where rainf.d agriculture is a dominant practice. The potential f.r improving soil water productivity and improved water harvesting have been explored as ways of.climate change mitigation and adaptation measures. This can be utilized to explore and design appropriate conservation agriculture and adaptation practices in similar agro-ecological environments, and create opportunities f.r outscaling f.r much wider areas. The results of.this study can suggest the need f.r possible policy ref.nements towards reducing vulnerability and adaptation to climate change in rainf.d f.rming systems.

关键词： Climate Change Adaptation Rainf.d f.rming Systems A Modeling Climate Smart Agriculture Southern Af.ica

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Regionalization of.Rainf.ll Intensity-Duration-f.equency (IDf. Curves in Botswana

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Journal of.Water Resource and Protection 2016年第12期8卷 1128-1144页

作者： berhanu f. alemaw Ron T. Chaoka Department of.Geology University of Botswana Gaborone Botswana

A regional analysis of.design storms, def.ned as the expected rainf.ll intensity f.r given storm duration and return period, is conducted to determine storm Rainf.ll Intensity-Duration-f.equency (IDf. relationships. The ultimate purpose was to determine IDf.curves f.r homogeneous regions identif.ed in Botswana. Three homogeneous regions were identif.ed based on topographic and rainf.ll characteristics which were constructed with the K-Means Clustering algorithm. Using the mean annual rainf.ll and the 24 hr annual maximum rainf.ll as an indicator of.rainf.ll intensity f.r each homogeneous region, IDf.curves and maps of.rainf.ll intensities of.1 to 24 hr and above durations were produced. The Gamma and Lognormal probability distribution f.nctions were able to provide estimates of.rainf.ll depths f.r low and medium return periods (up to 100 years) in any location in each homogeneous region of.Botswana.

关键词： IDf.Curves Design Storm Probability Distributions Clustering Regionalization

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Regional Design Storm and f.ood Modelling—Risk Implications in Ungauged Catchments

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Journal of.Water Resource and Protection 2016年第13期8卷 1211-1221页

作者： berhanu f. alemaw Ron T. Chaoka Department of.Geology University of Botswana Gaborone Botswana

Most planned developments in a catchment f.r control of.excess water using a culvert, bridge or dam spillway are located at a site in a stream where there are no discharge measurements. Even though, f.r gauged catchments a number of.established f.ood f.equency models and rainf.ll-runof. models do exist, f.r ungauged catchments mostly regional f.ood f.equency and event-based rainf.ll-runof. models are used, which depend on regional parameters. In this paper, a regional approach f.r design f.oods is presented and risk implication f.r design of.drainage structures assessed. A case study in light of.the above has been considered at f.ur ungauged sites in the Limpopo Drainage Basin in north-eastern Botswana.

关键词： Ungauged Catchments Drainage Structures Dams Design f.ood Risk Assessment

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Urban Stormwater and Sewerage Modelling: An Approach f.r Peak Runof. and Volume Assessment

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Journal of.Water Resource and Protection 2021年第11期13卷 855-880页

作者： berhanu f. alemaw Nata T. Taf.sse Department of.Geology University of Botswana Gaborone Botswana

In this study, an integrated approach f.r runof. estimation using the storm water management model (SWMM) was applied to undertake a stormwater and sewerage modelling in urban setting. The main objectives of.the research and this manuscript include overload detection of.sewer systems during extreme rainf.ll events with SWMM and to model and predict the relationship between precipitation parameters and overf.ooding of.sewer collection system that includes emergency ponds to relieve f.ow f.om pump stations. The study takes into account monitored concurrent extreme rainf.ll event data and peak wet weather f.ows observed at outf.ll of.collection system entering a wastewater treatment plant draining the urban centre. In the study SWMM was modif.ed and adapted f.r the Tati and Ntshe conf.uence rivers draining the urban centre of.f.ancistown in Northern Botswana. Landuse, soil, geological, drainage and sewerage network and imperviousness data sets were acquired and developed in GIS database. The runof. coef.icient is f.und to range between 0.12 and 0.24 in the studied catchments. The calibrated model was able to predict the observed outputs with reasonable accuracy f.r calibration datasets of.two peak f.ood events of.2016-Jan 12 and 2017-f.b 16 and verif.cation f.ood events of.2016-f.b 05 and 2017-f.b 26. f.r six watersheds that drain the study area considered with a seventh entire collection system catchment area, we have evaluated the model perf.rmance using dif.erent criteria. We have f.und that correlation coef.icients range f.om 0.539 to 0.813 and NSE ranges f.om 40.9% to 89.0%, and RSR ranges f.om 0.330 to 0.812 f.r the calibration datasets. Whereas, f.r the verif.cation dataset, the correlation coef.icients range f.om 0.539 to 0.813 and NSE values range f.om 40.9% to 89.0%, and RSR values range f.om 0.330 to 0.812. Using the criteria adopted, the SWMM-simulated runof. values are in acceptable agreement with the observed hydrographs.

关键词： SWMM Peak Runof. Runof. Volume Sewerage System Wastewater Treatment Plant Calibration Verif.cation

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Assessment of.Sedimentation Impacts on Small Dams—A Case of.Small Reservoirs in the Lotsane Catchment

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Journal of.Water Resource and Protection 2013年第12期5卷 1127-1131页

作者： berhanu f. alemaw Malebogo Majauale Timoty Simalenga Centre f.r Coordination of.Agricultural Research & Development in Southern Af.ica (CCARDESA) Gaborone Botswana University of.Botswana Department of Geology Gaborone Botswana

Sedimentation is a major problem f.r agricultural dams in Botswana, as it reduces the storage capacity and lif. span of.the reservoirs. The process of.sedimentation starts f.om day one of.the impounding of.water in any given reservoir. Even though a provision is made f.r every reservoir during planning f.r a certain storage capacity, specif.cally f.r sediment deposition, called dead storage, a major portion of.the sediment gets deposited f.r many years of.the reservoir’s lif. in areas other than the dead storage, and this trend cannot be reversed at easy cost. This study is aimed at the analysis of.prevailing sedimentation processes in the nearby dozens of.dams f.und in the Lotsane catchment located within the Limpopo Basin of.Botswana, and f.cuses on assessment of.annual sedimentation rate. A spatial analysis and modelling study was conducted based on the Revised Universal Soil Loss Equation and GIS to determine sediment yield and degree of.impact of.each reservoir f.r a given landscape, rainf.ll and catchment heterogeneity. f.eld observations and soil sampling were carried out in order to determine the f.ctors that lead to reservoir sedimentation. Spatial data on the dams in Lotsane catchment were also collected f.om Ministry of.Agriculture, which were used f.r ground-truthing, GIS-based calculations and model validation. The average sediment rate and sediment delivery ratio were f.und to be 1.74 t/ha/year and 81%, respectively. These are usef.l parameters to estimate service lif. of.the dams and plan remedial measures related to sedimentation problems.

关键词： Sedimentation Erosion Revised Universal Soil Loss Equation Sediment Yield Sediment Delivery Ratio Small Reservoirs Botswana

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Regional Drought Severity Assessment at a Basin Scale in the Limpopo Drainage System

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Journal of.Water Resource and Protection 2013年第11期5卷 1110-1116页

作者： berhanu f. alemaw J. M. Kileshye-Onema D. Love Department of.Geology University of Botswana Gaborone Botswana WaterNet University of Zimbabwe Harare Zimbabwe

A spatial analysis of.drought characteristics in the Limpopo basin is undertaken to evaluate its regional implications to water management challenges. In this study, drought duration, f.equency and severity are investigated. In addition drought Severity-Area-f.equency (SAf. curves were constructed. The entire Limpopo River Basin is subdivided into f.ur homogeneous regions based on topographic and climate variations in the basin, which was constructed with the K-Means Clustering algorithm. Using the medium range time series of.the Standardized Precipitation Index (SPI) as an indicator of.drought f.r each homogeneous region, monthly and annual SAf.curves and maps of.probability of.drought occurrence were produced. The results indicated localized severe droughts in higher f.equencies compared to moderate to severe low f.equency droughts spread over wider areas in the basin. The approach can be used to develop improved drought indicators to assess the relationship between drought hazard and vulnerability and to enhance the perf.rmance of.methods currently used f.r drought f.recasting. In general, this preliminary investigation reveals that the western part of.the basin will f.ce a higher risk of.drought when compared to other regions of.the Limpopo Basin in terms of.the medium-term drought patterns. The Limpopo Basin is water stressed and livelihood challenges remain high, thus impacts of.droughts and related resilience options should be taken into account in the f.rmulation of.regional sustainable water resources development strategies.

关键词： Drought Standardized Precipitation Index Drought Severity Drought Severity-Area-f.equency (SAf. Curves Limpopo Basin

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