Volta Basin Research Articles

Targeting investments in small-scale groundwater irrigation using Bayesian networks for a data-scarce river basin in Sub-Saharan Africa

Irrigation for smallholder farming systems is an important approach for sustainable intensification and increased productivity in Sub-Saharan Africa, provided investments in irrigation are properly targeted and accompanied by complementary improvements. Many GIS-based tools have been developed to identify suitable areas for investments in different types of small scale irrigation (SSI), but they do not explicitly address uncertainty on the data input and on the determination of factors that affect success of an investment in a given context. This paper addresses this problem by presenting an application of a decision-support targeting tool based on Bayesian networks (BNs) that can be used by non-expert policy-makers and investors to assess the potential success of specific technologies used for groundwater-based SSI. A case study application for the White Volta Basin in West Africa is presented to illustrate the BN approach.

On the potentials of multiple climate variables in assessing the spatio-temporal characteristics of hydrological droughts over the Volta Basin

Multiple drought episodes over the Volta basin in recent reports may lead to food insecurity and loss of revenue. However, drought studies over the Volta basin are rather generalised and largely undocumented due to sparse ground observations and unsuitable framework to determine their space-time occurrence. In this study, we examined the utility of standardised indicators (standardised precipitation index (SPI), standardised runoff index (SRI), standardised soil moisture index (SSI), and multivariate standardised drought index (MSDI)) and Gravity Recovery and Climate Experiment (GRACE) derived terrestrial water storage to assess hydrological drought characteristics over the basin. In order to determine the space-time patterns of hydrological drought in the basin, Independent Component Analysis (ICA), a higher order statistical technique was employed. The results show that SPI and SRI exhibit inconsistent behaviour in observed wet years presupposing a non-linear relationship that reflects the slow response of river discharge to precipitation especially after a previous extreme dry period. While the SPI and SSI show a linear relationship with a correlation of 0.63, the correlation between the MSDIs derived from combining precipitation/river discharge and precipitation/soil moisture indicates a significant value of 0.70 and shows an improved skill in hydrological drought monitoring over the Volta basin during the study period. The ICA-derived spatio-temporal hydrological drought patterns show Burkina Faso and the Lake Volta areas as predominantly drought zones. Further, the statistically significant negative correlations of pacific decadal oscillations (0.39 and 0.25) with temporal evolutions of drought in Burkina Faso and Ghana suggest the possible influence of low frequency large scale oscillations in the observed wet and dry regimes over the basin. Finally, our approach in drought assessment over the Volta basin contributes to a broad framework for hydrological drought monitoring that will complement existing methods while looking forward to a longer record of GRACE observations.

Evaluating groundwater recharge processes using stable isotope signatures—the Nabogo catchment of the White Volta, Ghana

This study evaluates the source and evolution of groundwater in parts of the Nabogo catchment of the White Volta Basin in Ghana. Porewater δ18O and δ2H signatures of vadose zone material were used together with the isotope signatures of rainwater, groundwater, and surface water in the area to suggest an evolutionary model and the mode of fluid fluxes through the unsaturated zone material in the area. This study finds that the local precipitation in the study area presents a relatively isotopically heavier signature compared to the average signature of global meteoric water. This is because the local meteoric water line (LMWL) has shallower slope, intercept, and average deuterium excess values compared to the global meteoric water line (GMWL). On the other hand, the local groundwater in the area presents relatively enriched isotopic signatures. Stable isotope profiles suggest piston flow as the main mechanism of vertical water movement in the vadose zone, indicating that direct groundwater recharge from local precipitation is principally based on this mechanism of transport. There is a progressive decline in the deuterium excess data of porewater vertically down the soil profile. This observation is consistent with an evolutionary pattern caused by fractionation processes attending evaporation of infiltrating water. Mineral-weathering processes have not been noted to have an effect on the isotopic signature of groundwater in the area. However, there appears to be a significant impact of evaporation on the total dissolved solid content of surface water in the area. In the case of groundwater, field-measured electrical conductivity (EC), total dissolved solids (TDSs), and pH are, respectively, in the ranges of 30–2407 μS/cm, 15–1196 mg/L, and 5.35–8.36. The EC, TDS, and pH are, respectively, in the ranges of 30–139 μS/cm, 15–70 mg/L, and 6.52–8.66 in surface water in the area.

Tightening-up NE Brazil and NW Africa connections: New U–Pb/Lu–Hf zircon data of a complete plate tectonic cycle in the Dahomey belt of the West Gondwana Orogen in Togo and Benin

The Dahomey belt in Togo and Benin is an important segment of the larger West Gondwana Orogen. Here, we review the geodynamic evolution of the Dahomey belt and discuss new U–Pb and Lu–Hf zircon data in light of similar data previously acquired on the geologically related Northern Borborema Province, in NE Brazil. Eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation. Passive margin deposits of the Atacora Structural Unit and lower units of the Volta Basin have detrital zircon signatures compatible with the flanking West Africa Craton. The arc-related magmatism resulted from the east-dipping subduction of the Goiás-Pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the Benino-Nigerian Shield between 670 and 610Ma. These granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input. Detrital zircon ages from syn-orogenic deposits in Benino-Nigerian Shield suggest that arc development could have started as early as 780Ma. The main period of melting in the internal part of the belt, the Benino-Nigerian Shield, is related to crustal thickening and occurs only ca. 30m.y. after initiation of the continental collision, marked by the ca. 610Ma ultra-high pressure (UHP) metamorphism recorded at Lato Hills. Foreland development represented by the upper units of the Volta basin developed soon after continental collision and persisted with the development of the west-verging thrust front synchronously with the main period of crustal melting due to collision at ca. 580Ma. The subvertical Transbrasiliano Lineament in South America, that corresponds to the Kandi Lineament in Africa, provides a present-day fit between NW Africa and NE Brazil. Restoration of the movement of the Transbrasiliano-Kandi Lineament (strike-slip plate boundary) places the Dahomey belt and Borborema Province (NE Brazil) along the same section of the West Gondwana Orogen. This configuration would explain some of the misfits previously discussed in the literature and aligns the UHP eclogites in Togo and NE Brazil.

Estimation of evapotranspiration losses in the vadose zone using stable isotopes and chloride mass balance method

This study used stable isotope and chloride data of porewater to evaluate water fluxes through the vadose zone and thereby estimate evapotranspiration water losses in parts of the Nabogo catchment of the White Volta Basin in Ghana. The overall objective was to develop a framework so that the recharge regime can be properly conceptualized for numerical hydrological modeling. Unsaturated zone material was sampled at regular intervals of 50 cm to a maximum depth of 300 cm in four different locations in the study area. Rainwater, groundwater and surface water were simultaneously sampled and analyzed for their δ18O and δ2H characteristics. Porewater, extracted from the unsaturated zone material was analyzed for the δ18O and δ2H and chloride content and profiles were created to gauge the isotopic evolution of precipitation and estimate evaporative losses at each interval of the sampling. The chloride mass balance technique was used to estimate the fraction of infiltrating water remaining at each interval of the sampling. Transpiration losses through the entire profile were then estimated. This study finds that the vertical infiltration of water through the vadose zone is dominated by piston flow and a mixture of piston and preferential flows. In the shallow subsurface (0.0–3.0 m), evaporative losses estimated from stable isotope data fall in the range of 29.3–52.4 % (322.3–576.4 mm/year) of the annual precipitation, with an average of 40 % (or 440 mm/year). Estimated vadose zone recharge at the maximum depth of sampling ranges between 11.1 and 185 mm/year with an average of 32.9 mm/year, representing 1.1, 18.5, and 3.29 % of the annual precipitation, respectively. Estimated transpiration losses within this interval range between 29.1 % (290 mm/year) and 69.5 % (695 mm/year), with an average of 54.7 % (547 mm/year) of the annual precipitation. Transpiration losses appear to increase down the profile and apparently account for a significant percentage of water losses in the vadose zone. A significant proportion of the original precipitation is lost within the upper 300 cm (3 m) of the vadose zone. The Water Table Fluctuations method was independently used to estimate saturated zone groundwater recharge and indicates that recharge rates range between 64.65 and 151.2 mm/year with an average of 102.5 mm/year which, respectively represent 5.9, 13.7, and 9.3 % of the average annual precipitation in the area. The apparently higher estimates from the water table fluctuations method may arise from uncertainties in the specific yield values used for the vadose zone material.

Hydrogeochemical Processes Influencing Groundwater Quality in the Black Volta Basin of Ghana

Hydrogeochemical processes influencing groundwater quality in the Black Volta Basin of Ghana have been studied. Groundwater samples were collected from 93 water points for physico-chemical analysis using standard methods. Major cation concentration was, generally, in the order of Ca²⁺> Na⁺>Mg²⁺>K⁺ while major anion concentration followed the order HCO₃->Cl->SO₄^2-. Mineral weathering (rock dissolution) and ion exchange were the controlling factors of groundwater composition. Stability diagrams show groundwater to be largely in equilibrium with Montmorillonite and Kaolinite and are quite reminiscent of climatic conditions pertaining to the geographical region of the basin. Ca-Mg-HCO₃ is the main groundwater type in the basin. Minor water types are Na-HCO₃ and Ca-Mg-SO₄. Principal Component Analysis (PCA), indicates that, HCO₃^- Ca²⁺, Mg²⁺, Na⁺ and F-contribute mainly to TDS and originate from the same-rock weathering.

Setting up agricultural water management interventions – learning from successful case studies in the Volta and Limpopo river basins

Abstract Long-term investments in agricultural water management (AWM) interventions in the Volta and Limpopo river basins have aimed at improving water availability and quality for smallholder farming systems. However, sustained and wider uptake of AWM technologies and approaches has not been as successful. We need to learn from successful AWM interventions, those interventions that have led to a sustained or increased uptake of AWM technologies or approaches, and which have led to improved well-being of farmers and livestock keepers in the rural development context of sub-Sahara Africa. This paper explores AWM interventions, specifically, the impacts these interventions have had and the factors contributing to the success of these interventions. In four countries within the Volta and Limpopo river basins, consultations were carried in 33 case studies of successful AWM interventions with implementing organisations and beneficiaries using a participatory GIS methodology. A systematic text analysis of 55 case study reports showed that these 33 interventions have had a positive impact on the well-being of beneficiaries and there was a sustained and wider uptake of the AWM technologies or approaches introduced. A clear demand for the technology, appropriate design of the technology, input support, training and capacity building, and a sense of ownership of the community helped to sustain the uptake of AWM technologies and approaches. We conclude that implementing organisations would benefit from investing in the soft components of an AWM intervention, as this will increase the likelihood of successful adoption and adaptation of the AWM technologies and approaches in the long-term.

Hydrological predictions for small ungauged watersheds in the Sudanian zone of the Volta basin in West Africa

Abstract Study region Hydrological observation networks in the West African region are not dense and reliable. Furthermore, the few available discharge data often present significant gaps. The Volta basin, the second largest transboundary basin in the region, is a typical example of a basin with inadequate hydrological networks. Study focus In this study, a prediction approach to determine monthly discharge in ungauged watersheds is developed. The approach is based on the calibration of two conceptual models for gauged watersheds and an estimation of models’ parameters from the physical and climatic characteristics of the watersheds. The models’ parameters were determined for each ungauged watershed through two different methods: the multiple linear regressions and the kriging method. The two methods were first validated on five gauged watersheds and then applied to the three ungauged watersheds. New hydrological insights for the region The application of the two hydrological models on the eight watersheds helped to produce relevant monthly runoff and to establish the annual hydrological balances from 1970 to 2000 for both gauged and ungauged watersheds. The developed method in this study could therefore help estimate runoff time series, which are of crucial importance when it comes to design hydraulic structures such as small reservoirs.

Toward a seasonal precipitation prediction system for West Africa: Performance of CFSv2 and high‐resolution dynamical downscaling

AbstractSeasonal precipitation forecasts are important sources of information for early drought and famine warnings in West Africa. This study presents an assessment of the monthly precipitation forecast of the Climate Forecast System version 2 (CFSv2) for three agroecological zones (Sudan‐Sahel, Sudan, and Guinean zone) of the Volta Basin. The CFSv2 performance is evaluated for the Sahel drought 1983 and for all August months of the reforecast period (1982–2009) with lead times up to 8 months using a quantile‐quantile transformation for bias correction. In addition, an operational experiment is performed for the rainy season 2013 to analyze the performance of a dynamical downscaling approach for this region. Twenty‐two CFSv2 ensemble members initialized in February 2013 are transferred to a resolution of 10 km × 10 km using the Weather and Research Forecasting (WRF) model. Since the uncorrected CFSv2 precipitation forecasts are characterized by a high uncertainty (up to 175% of the observed variability), the quantile‐quantile transformation can clearly reduce this overestimation with the potential to provide skillful and valuable early warnings of precipitation deficits and excess up to 6 months in ahead, particularly for the Sudan‐Sahel zone. The operational experiment illustrates that CFSv2‐WRF can reduce the CFSv2 uncertainty (up to 69%) for monthly precipitation and the onset of the rainy season but has still strong deficits regarding the northward progression of the rain belt. Further studies are necessary for a more robust assessment of the techniques applied in this study to confirm these promising outcomes.

Performance of small reservoir irrigated schemes in the Upper Volta basin: Case studies in Burkina Faso and Ghana

A major direct use of water from West African small reservoirs is irrigation. Analyzing the performances of irrigated agriculture is therefore a useful way to measure the impact of small reservoirs on food security and livelihoods of local communities. The aim of this study was to assess the performance of two such irrigated schemes in the Upper Volta basin (one in Burkina Faso and the other in Ghana) through participatory methods, to jointly identify major constraints, and to discuss possible solutions with local communities. The agronomic and economic performance of irrigated agriculture at both sites were far from satisfactory, due to the lack of maintenance of the small reservoirs and irrigation schemes, sub-optimal crop management, and poor product marketing. These issues were analyzed with the farmers across differing sub-schemes and cropping systems and some solutions were envisaged. Our assessment showed that farmers often had difficulty obtaining quality agricultural inputs and marketing their products. The poor performance of irrigated crops, due to poor condition of hydraulic infrastructures, poor agronomic management, and organizational failure provided only limited incomes for local households. Nevertheless, the existence and the many uses of small reservoirs improved food security and created indirect activities that also enhanced livelihoods. The local authorities generally considered preserving water to be a priority in small reservoir management, but the degradation of irrigation schemes could happen quickly and result in scheme failures, thus reducing indirect economic activities and causing under utilization or even abandonment of the small reservoir, unless appropriate measures are taken.

Modeling Impact of Climate Change on Water Resources and Agriculture Demand in the Volta Basin and other Basin Systems in Ghana

An assessment of the impacts of projected climate change on water availability and crop production in the Volta Basin and the southwestern and coastal basin systems of Ghana has been undertaken as a component of the impacts and adaptation study for Ghana by UNU-WIDER and the University of Ghana. Four climate change scenarios were considered in addition to a reference (no change) scenario—two dry and two wet scenarios. To conduct the analysis, a portion of a special framework using three water models was used; the framework is called the Strategic Analysis of Climate resilient Development (SACReD). First, the CliRun water balance model was used to simulate catchment runoffs using projected rainfall and temperature under the scenarios. Second, climate impacts on yields of the economically important Ghana crops were modeled using the AquaCrop software. Third, the Water Evaluation and Planning (WEAP) software was used for the water allocation modeling. The results show that all water demands (municipal, hydropower, and agriculture) cannot be simultaneously met currently, or under any of the scenarios used, including the wet scenarios. This calls for an evaluation of groundwater as an additional source of water supply and an integrated water resources management plan in the catchments to balance demand with supply and ensure sustainable socio-economic development. In addition, the AquaCrop model forecasts negative impacts for the crop yields studied, with some crops and regions seeing larger impacts than others.

Geophysical evidence for a major palaeochannel within the Obosum Group of the Volta Basin, Northern Region, Ghana

We have identified a 230km long palaeochannel cutting through the sediments of the Obosum Group within the Volta Basin, Ghana. The channel is visible in regional airborne magnetic data as a pair of parallel magnetic anomalies which we interpret to be lateral terrace placer deposits of magnetite or maghemite and show both meandering and braided geometries. The palaeochannel is also visible in the airborne radiometric data for part of its length, as well as in airborne electromagnetic data, where it is marked by a zone of low conductivity. The meanders have a wavelength of around 20km and together with the 2km spacing between magnetic anomalies, suggest a major palaeo-river system, of as yet unknown age, although based on published detrital zircon ages for the host sediments and geomorphological arguments it is probably between 509±68and 45Ma, and is comparable in size to the modern-day Niger River. Given the known occurrences of alluvial gold and diamonds in Ghana, and the general prospectivity of meandering and braided rivers systems, we believe the region has exploration potential for a range of economically valuable minerals, and could also potentially host a shallow aquifer.

Groundwater recharge processes in the Nasia sub-catchment of the White Volta Basin: Analysis of porewater characteristics in the unsaturated zone

Vertical infiltration of precipitation has been examined in this study for the purpose of evaluating groundwater recharge processes in parts of the Nasia sub-catchment of the White Volta Basin. As recharge is an essential component in the detailed assessment of groundwater resources potential in a basin, evaluating its processes is vital in determining the spatial and temporal variability of the resource. Stable isotope data of precipitation, groundwater, surface water and porewater in the area suggest that the local precipitation is largely enriched compared to global meteoric water. This is consistent with the prevailing local conditions in the region and ties in with observations in other parts of the sub-region. The groundwater and porewater data indicate that prior to, and in the process of infiltration and final percolation into the saturated zone, rainwater undergoes evaporative enrichment such that the finally recharged water plots along an evaporation line with a much shallower gradient and intercept compared to the global meteoric water line and the local meteoric water line. The isotope data further suggest that through the shallow unsaturated zone, a significant fraction of the initial precipitation would have been evaporated by a depth of 3.0m. Evaporation rates in the range of 38–49% have been estimated for the depth range of 0–3.0m based on the porewater stable isotope data. Details of the procedures and implications of high evaporation rates within such shallower depths are presented and discussed. Groundwater recharge rates estimated from the chloride mass balance technique report values in the range of 73.26mm/yr (390Mm3/yr)–109.89mm/yr (585.27Mm3/yr), with an average of 94mm/yr (500.6Mm3/yr). These translate into 6.6–10.9% of annual precipitation. Based on the current population trends and per capita water demand of 50L per capita per day, this study finds that the estimated recharge rates exceed the demand 59 times. This suggests significant promise for developing groundwater resources for climate-proof livelihood support projects in the terrain. However, much more detailed hydrogeological assessments are required to constrain the effects of lateral and vertical lithological variations on the availability of the recharged groundwater for immediate abstraction and use. The study also observes that since evaporation is one of the main dynamic processes influencing the fraction of the initial precipitation that finally reaches the saturated zone, projected increases in temperature during the next decade may increase evaporation rates of infiltrating rainwater leading to reduced groundwater rates.

Estimating water storage changes and sink terms in Volta Basin from satellite missions

The insufficiency of distributed in situ hydrological measurements is a major challenge for hydrological studies in many regions of the world. Satellite missions such as the Gravity Recovery and Climate Experiment (GRACE) and the Tropical Rainfall Measurement Mission (TRMM) can be used to improve our understanding of water resources beyond surface water in poorly gauged basins. In this study we combined GRACE and TRMM to investigate monthly estimates of evaporation plus runoff (sink terms) using the water balance equation for the period from January 2005 to December 2010 within the Volta Basin. These estimates have been validated by comparison with time series of sink terms (evaporation plus surface and subsurface runoff) from the Global Land Data Assimilation System (GLDAS). The results, for the period under consideration, show strong agreement between both time series, with a root mean square error (RMSE) of 20.2 mm/month (0.67 mm/d) and a correlation coefficient of 0.85. This illustrates the ability of GRACE to predict hydrological quantities, e.g. evaporation, in the Volta Basin. The water storage change data from GRACE and precipitation data from TRMM all show qualitative agreement, with evidence of basin saturation at approximately 73 mm in the equivalent water column at the annual and semi-annual time scales.

Peer review report 2 On “Hydrological predictions for small ungauged watersheds in the Sudanian zone of the Volta basin in West Africa”

Peer review report 2 On “Hydrological predictions for small ungauged watersheds in the Sudanian zone of the Volta basin in West Africa”

Peer review report 1 On “Hydrological predictions for small ungauged watersheds in the Sudanian zone of the Volta basin in West Africa”

Peer review report 1 On “Hydrological predictions for small ungauged watersheds in the Sudanian zone of the Volta basin in West Africa”

Detrital Zircon Geochronology of Mesozoic Sediments in the Rif and Middle Atlas Belts of Morocco: Provenance Constraints and Refinement of the West African Signature

Despite the presence of a deep history of continental evolution from Archean to Recent and a wealth of outcrop, existing studies of detrital zircon geochronology in Morocco are limited, focused mostly in the Cambrian and Neoproterozoic Pan-African exposures in the Anti-Atlas belt. We broaden the use of this tool by surveying the detrital zircon geochronology of three stratigraphic units from three distinct Mesozoic terranes: the Tisiren unit of the Maghrebian flysch domain, the Ketama unit of the Intrarif, and the Bou Rached sandstones of the Middle Atlas Mountains. Samples were analyzed for zircon U-Pb ages using laser-ablation inductively coupled plasma mass spectrometry, and zircon fission-track (ZFT) ages were analyzed using the emerging technique of scanning electron microscope high-density fission-track analysis. In total, 259 U-Pb ages are presented, along with 60 ZFT ages. U-Pb ages reveal dominant Pan-African (Cryogenian-Ediacaran) and West African cratonic (Paleoproterozoic) signatures that are typical of northwest Africa. All samples also include significant Mesoproterozoic U-Pb populations that are hitherto unknown in Morocco and scarce in North Africa. Proposed scenarios for the provenance of these zircons include delivery to Morocco from the Amazonian craton or the African Volta basin during Ordovician glaciation and/or delivery from Avalonian terranes during the Variscan orogeny. The Bou Rached sandstones contain a significant Ordovician-Devonian U-Pb population not found in the other samples and are of an age without a viable source currently dated in Morocco. An Avalonian source is likewise suggested for these Ordovician-Devonian ages. The Ketama and Tisiren units have comparable U-Pb ages, but their preliminary ZFT cooling age distributions differ, lending support to the idea that different source regions are required for the two domains.

Water balance of small reservoirs in the Volta basin: A case study of Boura reservoir in Burkina Faso

Effective water resources development and management is crucial for sustainable economic growth and poverty reduction in many developing countries. In West Africa, large numbers of small reservoirs with volumes from 104 to 107m3 play a considerable role in maintaining and developing activities in remote areas and in providing irrigation water for agriculture and supply water for both livestock and people. The lack of baseline data on reservoir operation hinders their optimal management. The objective of this study was to enhance the knowledge of water resources of the small reservoirs in order to improve their management in the context of multiple uses. This study was carried out on a small reservoir located in Southern Burkina Faso which was monitored for 2 years (from April 2012 to April 2014). A simple approach based on the mass conservation equation was developed for estimating reservoir fluxes. For a short hydrological monitoring period, the rainfall, evaporation and reservoir filling patterns revealed a different hydrological balance of the reservoir between these 2 years. A decrease of 32% in the annual rainfall leads to a 50% reduction in the annual runoff coefficient. The results showed that about 60% of water was lost by evaporation, whereas less than 20% of water caught in the reservoir was withdrawn for various uses. The available water resources in the studied system are largely sufficient to satisfy the current demands. There are still possibilities for developing uses of water storage and for enhancing the irrigation potential of the small reservoir. This analysis indicates that small reservoirs are under-performing. The results highlighted that estimating water fluxes in a reservoir is a central task to support water management authorities and stakeholders in operational strategies for water supply and irrigated agriculture.

Estimating soil erosion in sub-Saharan Africa based on landscape similarity mapping and using the revised universal soil loss equation (RUSLE)

Soil erosion is one of the major forms of land degradation in sub-Saharan Africa (SSA) with serious impact on agricultural productivity. Due to the absence of reliable data at appropriate resolution and differences in the methods used, there are discrepancies in soil erosion estimates at both continental and basin levels. This study attempts to contribute to the existing regional soil erosion estimates based on a two-stage approach. First, we partitioned SSA into environmental units, so-called similar environmental constraint envelops (SECEs), using broad scale data as proxies of erosion drivers. The SECEs are intended to provide spatial frame for scaling out modeled erosion results. Second, soil erosion estimate is made at two selected basins of the White Volta and the Nile using spatially distributed revised universal soil loss (RUSLE) model. The delineation of SECEs across SSA provided spatially differentiated clusters governed by the existence of similar environmental conditions and soil erosion risk levels. The RUSLE-based estimates show that soil erosion ranges between 0 to 120 t ha−1 yr−1 (overall mean of 35 t ha−1 yr−1) in the White Volta basin, and 0–650 t ha−1 yr−1 (overall mean of 75 t ha−1 yr−1) in the Nile basin. The soil loss estimates show an overall agreement with other studies conducted in the two basins. Our approach provides guidance on where empirically estimated soil erosion for a given SECE can be extrapolated to similar SECE’s with acceptable confidence and where finer SECE’s sub-units should be defined to further collapse the spatial variability of drivers of erosion.

Modelling West African Total Precipitation Depth: A Statistical Approach

Even though several reports over the past few decades indicate an increasing aridity over West Africa, attempts to establish the controlling factor(s) have not been successful. The traditional belief of the position of the Inter-tropical Convergence Zone (ITCZ) as the predominant factor over the region has been refuted by recent findings. Changes in major atmospheric circulations such as African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ) are being cited as major precipitation driving forces over the region. Thus, any attempt to predict long term precipitation events over the region using Global Circulation or Local Circulation Models could be flawed as the controlling factors are not fully elucidated yet. Successful prediction effort may require models which depend on past events as their inputs as in the case of time series models such as Autoregressive Integrated Moving Average (ARIMA) model. In this study, historical precipitation data was imported as time series data structure into an R programming language and was used to build appropriate Seasonal Multiplicative Autoregressive Integrated Moving Average model, ARIMA (p, d, q)*(P, D, Q). The model was then used to predict long term precipitation events over the Ghanaian segment of the Volta Basin which could be used in planning and implementation of development policies.