Gambia River Basin Research Articles

Assessment of current and future trends in water resources in the Gambia River Basin in a context of climate change

Accurate assessment of water resources at the watershed level is crucial for effective integrated watershed management. While semi-distributed/distributed models require complex structures and large amounts of input data, conceptual models have gained attention as an alternative to watershed modeling. In this paper, the performance of the GR4J conceptual model for runoff simulation in the Gambia watershed at Simenti station is analyzed over the calibration (1981–1990) and validation period (1991–2000 and 2001–2010). The main inputs to conceptual models like GR4J are daily precipitation data and potential evapotranspiration (PET) measured from the same catchment or a nearby location. Calibration of these models is typically performed using the Nash–Sutcliffe daily efficiency with a bias penalty as the objective function. In this case, the GR4J model is calibrated using four optimization parameters. To evaluate the effectiveness of the model's runoff predictions, various statistical measures such as Nash–Sutcliffe efficiency, coefficient of determination, bias, and linear correlation coefficient are calculated. The results obtained in the Gambia watershed at Simenti station indicate satisfactory performance of the GR4J model in terms of forecast accuracy and computational efficiency. The Nash–Sutcliffe (Q) values are 0.623 and 0.711 during the calibration period (1981–1990) and the validation period (1991–2000), respectively. The average annual flow observed during the calibration period is 0.385 mm while it increases with a value of 0.603 mm during the validation period. As for the average flow simulated by the model, it is 0.142 mm during the calibration period (i.e., a delay of 0.142 mm compared to the observed flow), 0.626 mm in the validation period (i.e., an excess of 0.023 mm compared to the observed flow). However, this study is significant because it shows significant changes in all metrics in the watershed sample under different scenarios, especially the SSP245 and SSP585 scenarios over the period 2021–2100. These changes suggest a downward trend in flows, which would pose significant challenges for water management. Therefore, it is clear that sustainable water management would require substantial adaptation measures to cope with these changes.

Testing sensitivity of BILAN and GR2M models to climate conditions in the Gambia River Basin

Abstract This study investigates the performance of two lumped hydrological models, BILAN and GR2M, in simulating runoff across six catchments in the Gambia River Basin (Senegal) over a 30-year period employing a 7-year sliding window under different climatic conditions. The results revealed differences in overall performance and variable sensitivity of the models to hydrological conditions and calibration period lengths, stemming from their different structure and complexity. In particular, the BILAN model, which is based on a more complex set of parameters, showed better overall results in simulating dry conditions, while the GR2M model had superior performance in wet conditions. The study emphasized the importance of the length of the calibration period on model performance and on the reduction of uncertainty in the results. Extended calibration periods for both models narrowed the range of the Kling-Gupta Efficiency (KGE) values and reduced the loss of performance during the parameter transfer from calibration to validation. For the BILAN model, a longer calibration period also significantly reduced the variability of performance metric values. Conversely, for the GR2M model, the variability rate did not decrease with the length of the calibration periods. Testing both models under variable conditions underscored the crucial role of comprehending model structure, hydrological sensitivity, and calibration strategy effects on simulation accuracy and uncertainty for reliable results.

Detection of subsurface cavities in the structurally bounded Labe prefecture area, Republic of Guinea using two-dimensional electrical resistivity tomography (ERT)

ABSTRACT This study is conducted within the Gambia River Basin Development Organization Energy Project (OMVG) framework. The construction of a high voltage interconnection network fed by a hydropower station to supply electricity to OMVG Member States was done on stable ground. During the installation pits for the pylons in Labé Prefecture, Republic of Guinea, cavities were discovered in the geological formations during the initial planned route. Structurally, the study area is bounded by two major faults to the east and west of northeast direction. The geomorphology is made up of a central plateau, intermountain depressions, piedmont plains, talwegs, and glacis. Geological investigations were obtained by a geophysical Two-dimensional Electrical Resistivity Tomography (ERT) method. The results show that the synthetic lithological profile from top to base was sandy-clayey soil, hard cuirass, and a layer of gravel followed by a mottled zone comprising an indurated part and an imbibed part just before the source rock. The cavities were observed probably because of the dissolution of certain minerals constituting the weathering crust due to the infiltration, underground waters, and the seasonal variation in the level of the surface watercourses. Geophysical results showed a very good agreement with the visual observations.

Effects of Hydroclimatic Variability on the Spatial Dynamics of Ponds (Simenti, Kountadala and Oudassi) of the Gambia River Basin in the Niokolo Koba National Park (Senegal)

With climate change, West African watercourses have undergone profound changes with large runoff deficits, faster dryings and very severe low flows. These modifications have led to a sharp decrease in water resources and a drying up of some water bodies. This article evaluated the effects of hydroclimatic variability on the spatial dynamics of a few ponds (Simenti, Kountadala and Oudassi) in the Gambia Basin in the Niokolo Koba National Park, NKNP, (Senegal). Data from five sets of images are used for pond trend mapping (Landsat, Google Earth and Sentinel). The hydroclimatic data used in this study include precipitation, temperature, evaporation, and discharge data from the Gambia Basin subjected to trend (Mann-Kendall) and rupture (Pettitt) tests. The results show that the trend is positive and significant for temperatures and evaporation and negative and significant for precipitation at a 95% confidence level. The surface area of the ponds, from 40.3 ha in 1975, increased sharply to 79.3 ha in 1988, then rose slightly in 1999 with 80.2 ha and in 2010 with 83.4 ha before to shrink sharply from 67.8 ha in 2019 due to rainfall deficit and the expansion of aquatic vegetation. Recommendations were made on how to improve the problem. Faced with the scarcity and drying up of water in these ponds of Niokolo Koba Park, water transfer operations such as those noted with the Simenti pond to save animals, remain fundamental according to some conservatives.

Future Climate Change Impacts on Streamflows of Two Main West Africa River Basins: Senegal and Gambia

This research investigated the effect of climate change on the two main river basins of Senegal in West Africa: the Senegal and Gambia River Basins. We used downscaled projected future rainfall and potential evapotranspiration based on projected temperature from six General Circulation Models (CanESM2, CNRM, CSIRO, HadGEM2-CC, HadGEM2-ES, and MIROC5) and two scenarios (RCP4.5 and RCP8.5) to force the GR4J model. The GR4J model was calibrated and validated using observed daily rainfall, potential evapotranspiration from observed daily temperature, and streamflow data. For the cross-validation, two periods for each river basin were considered: 1961–1982 and 1983–2004 for the Senegal River Basin at Bafing Makana, and 1969–1985 and 1986–2000 for the Gambia River Basin at Mako. Model efficiency is evaluated using a multi-criteria function (Fagg) which aggregates Nash and Sutcliffe criteria, cumulative volume error, and mean volume error. Alternating periods of simulation for calibration and validation were used. This process allows us to choose the parameters that best reflect the rainfall-runoff relationship. Once the model was calibrated and validated, we simulated streamflow at Bafing Makana and Mako stations in the near future at a daily scale. The characteristic flow rates were calculated to evaluate their possible evolution under the projected climate scenarios at the 2050 horizon. For the near future (2050 horizon), compared to the 1971–2000 reference period, results showed that for both river basins, multi-model ensemble predicted a decrease of annual streamflow from 8% (Senegal River Basin) to 22% (Gambia River Basin) under the RCP4.5 scenario. Under the RCP8.5 scenario, the decrease is more pronounced: 16% (Senegal River Basin) and 26% (Gambia River Basin). The Gambia River Basin will be more affected by the climate change.

Precipitation trends in the Gambia River Basin (Senegal) for the period 1971-2010

Since the 1970s, West African streams experienced profound modifications connected to pluviometric deficit, which have influenced on a strong decrease of water resources. The objective of this study is to analyze trends of precipitation at the six selected stations in the Senegalese part of Gambia River basin, using graphical and statistical approach. Methodology is consisted of an analysis of monthly, seasonal and annual variability of precipitation for the 1971-2010 period, through Non-parametric tests. Study revealed a strong variability and often a contrasting evolution between various stations (K?dougou, Simenti, Tambacounda, Kounghel, Nioro and Goudiry). Between the periods 1971-1990 and 1991-2010, the value U of Mann-Whitney, had changed from 60 to 40 at Tambacounda, 64 to 59 at K?dougou and 63 to 51 at Simenti station, which showed non-significant trends in annual total precipitation for the studied period. Sen?s estimator revealed significant descending trends on K?dougou station (9.812 mm/year for the period 1971-1990 and 0.309 mm/year for the period 1991-2010), and Simenti station too. On the other hand (in the whole article), on the stations of Tambacounda, Goudiry and Koungheul, the reduction is noticed for the period 1971-1990 (3.658 mm/year on Tambacounda), and an increase is noticed in the period 1991-2010 (13.21 mm/year on Tambacounda). For the station Nioro, on other hand, increase is noted in both periods (3.801 mm/year and 8.841 mm/year). The assumption is that these significant trends of precipitation during the last four decades could be attributed to the climate changes.

Gender, Spirituality, and Economic Change in Rural Gambia: Agricultural Production in the Lower Gambia Region, c. 1830s–1940s

Across the Gambia River basin, farmers grew varieties of grains for local consumption and for sale. Maize, millet, and rice formed an important part of a complex social organization. In the mid-nineteenth century, however, the shift to cash crops changed important aspects of this social organization. The cash crop economy, which was highly restrictive, also encouraged social separation and alterations in the gender configurations of the region. The change from a household economy relying on growing grains for consumption to one of producing a legume to sell, which was eased along by the transformation of local spiritual ideas, resulted in alterations in settlement patterns impacting the lives of many female farmers. One particularly interesting area that scholars have noted but is yet to be fully developed is the ways in which religion or spirituality influenced African economic life. Therefore, the purpose of this article is to examine the ways spirituality or religion influenced the growth and the development of what became arguably the most important economic activity in West Africa during the second half of the nineteenth and the early twentieth centuries as well as gender dynamics in the affected societies.

Annulotrema (Monogenea: Dactylogyridae) from Hydrocynus brevis (Characiformes: Alestidae) in Senegal, with descriptions of two new species and remarks on Annulotrema pikei.

Two new species of Annulotrema Paperna & Thurston, 1969 were collected from the gills of the African tiger fish, Hydrocynus brevis, from the Gambia River basin in the Niokolo-Koba National Park, Senegal. Annulotrema besalis n. sp. is characterized by having a male copulatory organ (MCO) composed of an arcuate copulatory tube articulated to an eight-shaped accessory piece with terminal claw. The new species resembles Annulotrema pikei (Price, Peebles & Bamford, 1969) in having morphologically similar types of haptoral sclerites and MCO. As a result of the differential diagnosis made for A. besalis n. sp., new information on taxonomically important features of A. pikei is provided based on illustrations of the sclerotized parts of the holotype from Hydrocynus vittatus. The report of A. pikei on the gills of Hydrocynus forskahlii by Paperna in 1979 is shown to be erroneous. Annulotrema uncata n. sp. is similar to Annulotrema alestesimberi Paperna, 1973 in its possession of a coiled copulatory tube with about two and a half rings. Features distinguishing the new species include the sharply curved shaft of the ventral anchor, the base of the copulatory tube extending to a sock-like structure and a leech-shaped vagina. The necessity of emending the generic diagnosis of Annulotrema is briefly discussed.

Trends and Shifts in Time Series of Rainfall and Runoff in the Gambia River Watershed

For several decades, climate change and climate variability issues and their impacts on the hydrological regime of rivers have constituted a major topic for hydroclimatological sciences research and water resources planning policies. Understanding of these issues needs enough long time series of rainfall and runoff data covering a large period, and a comprehensive diagnosis of the existing trends and shifts in these time series of data. This can be done by applying robust statistical tests to relevant rainfall and runoff time annual series. The aim of this paper is to highlight the effect of climate change in the Gambia River Basin and its impacts on the availability of the water resources of this basin. To reach this objective, we have selected runoff time series of the Gambia River Basin at Mako, Kedougou Diaguéri streamgauges and rainfall time series at Koulountou’s rain gauge. Statistical tests for shift detection presented in the Khronostat software, such as Pettit, Hubert and Buishand ellipse tests are first used, Mann Kendall test for annual trend are then applied to check whether trends exist or not in these times series. When the null hypothesis of no trend is rejected, the non parametric Sen’s test is then applied to validate the Mann Kendall trend test and to estimate the magnitude of the trend and its direction. Tests for homogeneity show an increasing shift for rainfall time series of Koulountou raingauge and for runoff time series of Mako and Diaguéri and a decreasing shift for Kedougou streamgauge. According to the Mann Kendall trend test, there is an upward trend for Koulountou rainfall time series, and Mako and Diaguéri runoff time series, and a downward trend for Kedougou annual runoff time series. The Buishand ellipse and the Hubert test indicate generally the same year of the beginning of the shift. Interesting perspectives for decision makers in evaluation and precise management of water resources and water projects in the Gambia River basin are offered as well.

Two new species of Cichlidogyrus (Monogenea: Dactylogyridae) parasitizing the gills of African cichlid fishes (Perciformes) from Senegal: morphometric and molecular characterization

Two new species of Cichlidogyrus are described from the gills of three species of African cichlids collected from the Gambia River basin in the Niokolo-Koba National Park, Senegal: Cichlidogyrus dracolemma n. sp. from Hemichromis letourneuxi and Cichlidogyrus nageus n. sp. from Sarotherodon galilaeus and Tilapia guineensis. The new species are recognized on the basis of morphological differences in the male copulatory organs (MCOs) and apparent differences in the SSU rDNA and ITS1 sequence data. C. dracolemma n. sp. resembles specimens of Cichlidogyrus from Hemichromis bimaculatus, probably misidentified as Cichlidogyrus bychowskii by Paperna in 1965, in having morphologically similar types of haptoral sclerites and MCO. The new species is characterized by possessing an MCO with a kite-shaped termination of the accessory piece and a vagina distally surrounded by a circular superficial sclerotization of the body surface. Molecular phylogenetic analyses showed a strongly supported clade including C. dracolemma n. sp. and two species collected from Hemichromis fasciatus, namely Cichlidogyrus longicirrus Paperna, 1965 and Cichlidogyrus falcifer Dossou and Birgi, 1984. Based on both morphology and molecular phylogeny, C. nageus n. sp. appears to be closely related to Cichlidogyrus acerbus Dossou, 1982 obtained from S. galilaeus. Morphologically, C. nageus n. sp. is readily separated from all known congeners by the characteristic pestle shaped termination of the accessory piece of the MCO. Molecular phylogenetic reconstruction showed that C. dracolemma n. sp. and C. nageus n. sp. cluster with the Cichlidogyrus species possessing the same haptoral configuration and host preferences (on subfamily and tribe level, respectively).

Fighting Over Rice Swamps: Conflict and Community across the Gambia River Basin, Nineteenth and Early Twentieth Centuries

Fighting Over Rice Swamps: Conflict and Community across the Gambia River Basin, Nineteenth and Early Twentieth Centuries

“Re-claiming” Land in The Gambia: Gendered Property Rights and Environmental Intervention

By definition, land reclamation programs render marginally productive land resources more valuable to a broader set of users. The question of who gets access to rejuvenated lands is often highly political, however. Environmental managers “reclaim” land resources by rehabilitating them, but they simultaneously reanimate struggles over property rights in the process, allowing specific groups of resource users to literally and figuratively “re-claim” the land. Relying on data gathered during fourteen months of field work between 1989 and 1995, this paper analyzes the openings created by environmental policy reforms introduced over the past two decades along The Gambia River Basin, and the tactics and strategies rural Gambians have developed to manipulate these policies for personal gain. Specifically, I demonstrate how women market gardeners pressed “secondary” usufruct rights to great advantage to ease the economic impact of persistent drought conditions for the better part of a decade, only to have male lineage heads and community leaders “re-claim” the resources in question through donor-generated agroforestry and soil and water management projects. This is thus a study of the responses different community groups have made to a shifting international development agenda centered on environmental goals. It is simultaneously an analysis of those environmental policies and practices and their impact on gendered patterns of resource access and control within a set of critical rural livelihood systems.

The Mangrove Swamps of Keneba, Lower Gambia River Basin. III. Seasonal Variations in the Chloride and Water Content of Swamp Soils, with Observations on the Water Levels and Chloride Concentration of Free Soil Water Under a Barren Mud Flat During the Dry Season

The Mangrove Swamps of Keneba, Lower Gambia River Basin. III. Seasonal Variations in the Chloride and Water Content of Swamp Soils, with Observations on the Water Levels and Chloride Concentration of Free Soil Water Under a Barren Mud Flat During the Dry Season

The Mangrove Swamps of Keneba, Lower Gambia River Basin. II. Sulphur and pH in the Profiles of Swamp Soils

The Mangrove Swamps of Keneba, Lower Gambia River Basin. II. Sulphur and pH in the Profiles of Swamp Soils

The Mangrove Swamps of Keneba, Lower Gambia River Basin. I. Descriptive Notes on the Climate, the Mangrove Swamps and the Physical Composition of Their Soils

The Mangrove Swamps of Keneba, Lower Gambia River Basin. I. Descriptive Notes on the Climate, the Mangrove Swamps and the Physical Composition of Their Soils