Caractérisation Hydrodynamique et Qualité des eaux de l’aquifère à nappe libre du bassin versant du Nkié (Yaoundé–Cameroun)

Entre septembre 2017 et août 2018, une étude visant à faire une caractérisation physique, à comprendre le comportement hydrodynamique et à évaluer la qualité des eaux de l’aquifère à nappe libre du bassin versant du Nkié a été menée. Lestravaux de terrain et de laboratoire menés conjointement ont consisté au suivi bimensuel des ouvrages hydrauliques préalablement identifiées, aux mesures in situ de la perméabilité, aux échantillonnages des sols et des eaux pour analyses de la granulométrie et des paramètres physico chimiques et bactériologiques. A l’issue de ces travaux, l’analyse granulométrique des sols du BV révèle une diminution de la taille des particules de la base vers le sommet du profil. Le report pondéral des différents niveaux de sols, permet de distinguer les sables graveleux, les sables, les graviers sableux et les argiles sableuses. Ces formations lithologiques ont des perméabilités qui varient entre 1,8.10-6 m.s -1 et 5,92.10-5 m.s -1 , illustrant ainsi qu’ellessont peu à assez perméables. Les débits journaliers dessources varient entre 0,69 l.s -1 (2,5 m3 .h-1 ) et 0,06 l.s -1 (0,22 m3 .h-1 ) en saison sèche (SS) et entre 0,86 l.s -1 (3,1 m3 .h-1 ) et 0,08 l.s -1 (0,3 m3 .h-1 ) en saison pluvieuse (SP). Les sources de plus grands débits seraient situées sur les principaux axes de drainage correspondant aux axes principaux de fracturation (N10-30, N40-50, N80-100, N110-120). L’analyse des cartes piézométriques révèle une convergence des lignes de courant d’eau vers le cours d’eau Nkié qui en est le principal axe de drainage de l’aquifère à nappe libre. Le bilan hydrique établi, montre que le réservoir d’altérites se recharge d’avril à novembre sous l’action de l’excédent pluviométrique et se vide de décembre à mars lors de l’étiage. Ainsi, le comportement hydrodynamique de la nappe est étroitement lié aux fluctuations pluviométriques. Les eaux de sources sont acides et peu minéralisées. Ce sont les eaux de type chloruré sodique et potassique. La présence remarquée des nitrates, des coliformes fécaux et des streptocoques fécaux dont les concentrations sont supérieures aux limites de toxicité définies par l’OMS (2011) induisent une pollution bactériologique et chimique de ces eaux. Elles nécessitent donc à cet effet un traitement préalable avant toute consommation. Between September 2017 and August 2018, a study aimed at carrying out a physical characterization, understanding the hydrodynamic behavior and evaluating the quality of the waters of the free water aquifer of the Nkié watershed was carried out. The field and laboratory work carried out jointly consisted of the fortnightly monitoring of the hydraulic structures previously identified, in situ measurements of permeability, soil and water sampling for analysis of particle size and physico-chemical and bacteriological parameters. At the end of this work, the particle size analysis of the BV soils reveals a reduction in the particle size from the base to the top of the profile. The weight carryover of the different soil levels makes it possible to distinguish gravelly sands, sands, sandy gravels and sandy clays. These lithological formations have permeabilities which vary between 1.8.10-6 m.s-1 and 5.92.10-5 m.s-1, thus illustrating that they are little to fairly permeable. The daily flow rates of the sources vary between 0.69 ls-1 (2.5 m3 .h-1 ) and 0.06 ls-1 (0.22 m3 .h-1 ) in the dry season (SS) and between 0.86 ls-1 (3.1 m3 .h-1 ) and 0.08 ls-1 (0.3 m3 .h1 ) in the rainy season (SP). The sources of larger flows would be located on the main drainage axes corresponding to the main fracturing axes (N10-30, N40-50, N80-100, N110-120). Analysis of the piezometric maps reveals a convergence of the streamlines towards the Nkié stream, which is the main drainage axis of the free aquifer. The established water balance shows that the alterite reservoir is recharged from April to November under the action of excess rainfall and is emptied from December to March during low water. Thus, the hydrodynamic behavior of the aquifer is closely linked to rainfall fluctuations. The spring waters are acidic and not very mineralized. These are waters of the sodium and potassium chloride type. The noted presence of nitrates, fecal coliforms and fecal streptococci whose concentrations are above the toxicity limits defined by the WHO (2011) induce bacteriological and chemical pollution of these waters. They therefore require prior treatment for this purpose before any consumption

Stable Carbon Isotopes δ13C as a Proxy for Characterizing Carbon Sources and Processes in a Small Tropical Headwater Catchment: Nsimi, Cameroon

Stream carbon fluxes are one of the major components in the global C cycle, yet the discrimination of the various sources of stream carbon remains to a large extent unclear and less is known about the biogeochemical transformations that accompany the transfer of C from soils to streams. Here, we used patterns in stream water and groundwater δ13C values in a small forested tropical headwater catchment to investigate the source and contribution from the soil carbon pools to stream organic and inorganic carbon behavior over seasonal scales. Stream organic carbon (DOC and POC) comes mainly from the upper rich soil organic carbon horizons and derived from total organic carbon (TOC) of biogenic source. The isotopic compositions δ13CTOC, δ13CDOC and δ13CPOC of these carbon species were very close (− 30‰ to − 26‰) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO2 and δ13CDIC indicated that besides the considerable CO2 evasion that occurs as DIC is transported from soils to streams, there were also other processes affecting the stream DIC pool. In-stream mineralization of DOC and mixing of atmospheric carbon had a significant influence on the δ13CDIC values. These processes which varied seasonally with hydrological changes represent the main control on DOC and DIC cycling in the wet tropical milieu. The rapid turnover of carbon on hillside soils, the transformation of TOC to DOC in wetland soils and further mineralization of stream DOC to DIC favor the evasion of C, making the zone a source of carbon to the atmosphere

Le cycle du carbone en domaine tropical humide (exemple du bassin versant forestier du Nyong au sud Cameroun)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Denudation rates on cratonic landscapes: comparison between suspended and dissolved fluxes, and Be-10 analysis in the Nyong and Sanaga River basins, south Cameroon

South Cameroon is located in a tropical and tectonically quiescent region, with landscapes characterized by thick highly weathered regolith, indicative of the long-term predominance of chemical weathering over erosion. Currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources. We analysed two of the main south Cameroon rivers: the Nyong River and Sanaga River. The Sanaga catchment undergoes a contrasted tropical climate from sub-humid mountainous and humid climate and is impacted by deforestation, agriculture, damming, mining and urbanization, especially in the Mbam sub-basin, draining the highly populated volcanic highlands. By contrast, the Nyong catchment, only under humid tropical climate, is preserved from anthropogenic disturbance with low population except in the region of Yaounde (Mefou sub-basin). Moreover the Nyong basin is dam-free and less impacted by agriculture and logging. We explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchment-averaged erosion and denudation datasets. The first one consists of an 11-year long gauging dataset, while the second one comes from cosmogenic radionuclides CRNs, here beryllium-10 (Be-10)] from sand sampled in the river mainstreams (timescale of tens to hundreds of thousands of years). Modern fluxes estimated from gauging data range from 5 to 100m/Ma (10 to 200t/km(2)/yr); our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60% and 40%, respectively, of the total denudation. Beryllium-10 denudation rates and sediment fluxes range from 4.8 to 40.3m/Ma or 13 to 109t/km(2)/yr, respectively, after correction for quartz enrichment. These fluxes are slightly less than the modern fluxes observed in Cameroon and other stable tropical areas. The highest Be-10-derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact. Copyright (c) 2016 John Wiley & Sons, Ltd

Polar pesticide contamination of an urban and peri-urban tropical watershed affected by agricultural activities (Yaoundé, Center Region, Cameroon)

(IF 2.91; Q1)International audienceUrban agriculture is crucial to local populations, but the risk of it contaminating water has rarely been documented. The aim of this study was to assess pesticide contamination of surface waters from the Méfou watershed (Yaoundé, Cameroon) by 32 selected herbicides, fungicides, and insecticides (mainly polar) according to their local application, using both grab sampling and polar organic compounds integrative samplers (POCIS). Three sampling campaigns were conducted in the March/April and October/November 2015 and June/July 2016 rainy seasons in urban and peri-urban areas. The majority of the targeted compounds were detected. The quantification frequencies of eight pesticides were more than 20% with both POCIS and grab sampling, and that of diuron and atrazine reached 100%. Spatial differences in contamination were evidenced with higher contamination in urban than peri-urban rivers. In particular, diuron was identified as an urban contaminant of concern because its concentrations frequently exceeded the European water quality guideline of 0.200 μg/L in freshwater and may thus represent an ecological risk due to a risk quotient > 1 for algae observed in 94% of grab samples. This study raises concerns about the impacts of urban agriculture on the quality of water resources and to a larger extent on the health of the inhabitants of cities in developing countries

Polar pesticide contamination of an urban and peri-urban tropical watershed affected by agricultural activities (Yaoundé, Center Region, Cameroon)

(IF 2.91; Q1)International audienceUrban agriculture is crucial to local populations, but the risk of it contaminating water has rarely been documented. The aim of this study was to assess pesticide contamination of surface waters from the Méfou watershed (Yaoundé, Cameroon) by 32 selected herbicides, fungicides, and insecticides (mainly polar) according to their local application, using both grab sampling and polar organic compounds integrative samplers (POCIS). Three sampling campaigns were conducted in the March/April and October/November 2015 and June/July 2016 rainy seasons in urban and peri-urban areas. The majority of the targeted compounds were detected. The quantification frequencies of eight pesticides were more than 20% with both POCIS and grab sampling, and that of diuron and atrazine reached 100%. Spatial differences in contamination were evidenced with higher contamination in urban than peri-urban rivers. In particular, diuron was identified as an urban contaminant of concern because its concentrations frequently exceeded the European water quality guideline of 0.200 μg/L in freshwater and may thus represent an ecological risk due to a risk quotient > 1 for algae observed in 94% of grab samples. This study raises concerns about the impacts of urban agriculture on the quality of water resources and to a larger extent on the health of the inhabitants of cities in developing countries

Urban flood susceptibility modelling using AHP and GIS approach: case of the Mfoundi watershed at Yaoundé in the South-Cameroon plateau

International audienceFloods are considered as the natural hazards that affect the world's major metropolises the most. Thus, the present study aimed at evaluating the sensitivity to flood risks of the Mfoundi watershed (96.5 km2) located in the heart of the Cameroonian political capital in a tropical humid forest zone, more precisely in the South Cameroon plateau. The methodological approach adopted was to identify the factors that most favor the risk of flooding in the area from intense literature review and field investigations; the analysis of these factors and the calculation of the Flood Harzard Index (FHI) using the Analytical Hierarchy Process (AHP) approach coupled with the Geographical Information System (GIS) environment. The results reveal that among the ten parameters of the natural environment (elevation, drainage density, rainfall, slope, distance from the river, topographic humidity, hydraulic conductivity, groundwater level, geology and land cover) selected, the land cover, elevation and the geology are the factors that most influences the flooding phenomenon in the area. The value of Flood Hazard Index (FHI) varied from 4.16 to 9.16, the higher the value, the more sensitive the area is to the risk of flooding. Five main classes of flood susceptibility are highlighted: very low, low, moderate, high and very high, representing 9.50, 26, 23, 22 and 19.5%, respectively of the study area. To validate the efficiency of the obtained flood susceptibility map, the adopted Area Under the Curve (AUC) method shows a very good accuracy (0.84 or 84%). The results of this study constitute a basic tool for decision-making for environmental management by public authorities and decentralised territorial authorities with territorial jurisdiction

Impact of Land Use Change on the Hydrological Processes of a Peri-Urban Tropical Catchment: The Méfou and Mfoundi basins, Yaoundé, Cameroon

The city of Yaoundé in Cameroon is exposed to a continuous population increase from 60,000 inhabitants in 1960to more than 2,500,000 nowadays accompanied by a notable urbanization which tripled over the last 30 years.The case of Yaoundé is representative of many African cities. The aim of this work is to analyze and modelthe hydrological behavior of the peri-urban catchments of the Méfou (425 km2) englobing the city of Yaoundé,taking into account the evolution of land use during the last four decades. The methodology has two steps: i)analysis of the hydro-meteorological and cartographic data in order to identify the main hydrological processes;ii) the implementation of a spatially distributed modeling approach and a parameterization strategy adapted to thespecificities of the site with scarce data and taking into account the temporal evolution of land use. Scarce dailyrainfall/runoff data are available on some periods between 1968 and 2014. Land use maps were also available onthe study period. The basin altitude ranges between 700 and 1000 m. Annual rainfall ranges between 1550 and 2060mm. The analysis of rainfall data didn’t show any modification of the rainfall regime during the study period. Themean annual runoff coefficient on the Méfou is 36.7% (with a specific discharge of 18.5 m3/s/km2) and remainspractically unchanged during the study period. The Mfoundi tributary (38 km2) located in the urban zone, has seenthe maximum urbanization extension from 8% in 1951 to nearly 28% in 2005, while the forest zone has decreasedfrom 78% to 6%. The direct consequence was an increase of 100 % of annual runoff coefficient (from 36.7% in1960 to 78% today corresponding to an increase of the specific annual discharge from 18.5 m3/s/km2in 1968 to42.4 m3/s/km2today); moreover we observe an increase of 200% of runoff coefficient during the rainy season(September to November). Then, the HEC-HMS spatially distributed hydrological model was used at a daily timestep to simulate the impact of land use change evolution on the hydrograph at both the Mfoundi and the Méfoubasins. Calibration/validation strategies were undertaken on different periods, 1965-1970, 1971-1974, 1975-1977,1984-1986, 2005-2006, 2010-2011 and 2012-2013 in order to identify the evolution of model parameters with theevolution of land use change. Results show that the parameter characterizing the soil surface infiltrability is themost sensitive. Moreover, we observe a correlation between the value of the optimized parameter and the areaof the urbanized zone. However, the remaining model parameters seem constant during the whole study period.Finally the model was applied to simulate and compare various scenarios of urbanization extension and land usechang

A non-stationary model for reconstruction of historical annual runoff on tropical catchments under increasing urbanization (Yaoundé, Cameroon)

International audienceInter-tropical regions are nowadays faced to major land-use changes in data-sparse context leading to difficulties to assess hydrological signatures and their evolution. This work is part of the theme Panta Rhei of the IAHS, and aims to develop a combined approach of data acquisition and a new semi-distributed model taking into account land-use changes to reconstruct and predict annual runoff on an urban catchment. Applications were conducted on the Mefou catchment at Nsimalen (421 km2; Yaoundé, Cameroon) under rapid increase in urbanization since 1960. The data acquisition step combines an historical data processing and a short-term spatially-dense dedicated instrumentation (2017–2018), leading to 12 donor catchments, 6 from historical studies and 6 from the instrumentation presenting various topographic, soil and land-use characteristics. We developed an annual rainfall-runoff model based on mathematical relationships similar to the SCS model. The model needs the definition of a hydrological index I which is time variable and enables to take into account land-use changes and non-stationary relationships between rainfall and runoff. The index I is an empirical indicator defined as a combination of several components such as topography, soil, and land-use. The rules for the construction of I are obtained from data analysis on donor catchments. Then, the model was calibrated on donor catchments. Finally, two applications were conducted on eight target catchments composing the Mefou in order: (i) to study the spatial hydrological functioning and calculate the water balance during the short instrumentation period; (ii) to reconstruct the hydrograph at the Mefou and to simulate the impact of future scenarios of land-use and urbanization. Results show that that the Mfoundi catchment, integrating the three more urbanized sub-catchments, contributes near to 40 % of the Mefou despite covering only 23 % of the basin. The most urbanized sub-catchments present annual runoff coefficient about 0.86 against 0.24 for the most natural sub-catchments. The second result is the reconstruction of historical annual runoff from 1930–2017 with r2 = 0.68, RMSE = 99 mm and a mean absolute normalized error Ē = 14.5 % over the 29 observed years. The reconstruction of the annual runoff at Nsimalen confirms the moderate impact of urbanization on annual runoff before 1980. However, a decrease of about 50 % of the forest cover and an increase from 10 % to 35 % of the urban area between 1980 and 2017 are associated with an increase of 53 % of annual runoff coefficient for the Mefou at Nsimalen (0.44 against 0.29). Application for a fictive plausible scenario of urbanization in 2030 leads to an increase of more than 85 % of the annual runoff in comparison of the values observed in 1980. The coupled experimental-modelling approach proposed herein opens promising perspectives regarding the evaluation of the annual runoff in catchments under changes