Meteorological, hydrological and hydrochemical parameters monitored on the Nyong network

The South-Cameroonian river network, which is studied in the frame of this CZO-BVET program, includes the 6 following stations: spring and outlet of the Mengong River near Nsimi village, Messam on Awout brook, Pont So’o on the So’o River, Mbalmayo and Olama on the Nyong River. Most of the meteorological and hydrological parameters are daily measured (or even continuously monitored), whereas the hydrochemical measurements and sampling are performed in the course of bimonthly field trips.

Pluviometry on the experimental Nsimi catchment is obtained from the daily recording of rainfall height by 14 pluviometers (noted RFm, mm/day); a pluviograph is also installed in this watershed. Monthly rainfall heights at Yaoundé, Abong-Mbang and Sangmelima stations are given by the Cameroonian Meteorological Office.

A batch of field instruments, which has been installed in a clearing of the Nsimi catchment, provide since the beginning of April 2004 the daily values of the following meteorological parameters: minimal (Tmin, °C), maximal (Tmax, °C) and mean (Tmean, °C) temperatures; mean air humidity (Hummean, %); instant (GRinstant, kW/m2) and cumulated (GRcumul, kJ/m2 ) global radiation; mean wind velocity (Vwind, m/s) and direction (Dirwind, in ° from North); pluviometry (RFday, mm/day), potential evapotranspiration (PETday, mm/day).

Discharge (m3 /s, except for the source and the outlet of the Nsimi watershed expressed in L/s) is determined from the daily-measured water stages by local observers on limnimetric scales, using calibration curves drawn from a gauging dataset in each station. At Nsimi outlet, a limnigraph allows us to record flood events.
The variations of the watertable level are followed up using piezometers which are daily monitored.

Water temperature (°C), electric conductivity (µS/cm) and pH are measured on the field at each trip.

Suspended Particulate Matter (SPM, mg/L) is determined in the Hydrological Research Center in Yaoundé, after filtration of 1liter of water on a cellulose acetate membrane with a 0.20 µm porosity. These measurements were not undertaken for the Nsimi spring.

Water samples for geochemical analysis are filtrated, stored into (generally HDPE) bottles, and kept refrigerated until they are sent to the LMTG in Toulouse (France).
Major cations (Na+ , K+ , Mg2+ , Ca2+ , in µmol/L) are analyzed from acidified (HNO3 ) water samples, by flame atomic absorption spectrometry (Perkin Elmer ZL 5100).
Major anions (Cl , NO3 , SO42- , in µmol/L) are obtained by high pressure liquid chromatography (Dionex ICS 2000) with eluent generator and AS18 column. For anions, as for cations, the detection limits are 1 µmol/L with a reproducibility of 2 µmol/L.
The concentration of hydrogenocarbonate (HCO3 , in µmol/L) is deduced from alkalinity, which is determined using an automatic titrator (716 DMS Titrino, Metrohm). Due to very low alkalinity values (<100 µeq/L), a very diluted HCl is needed and the reproducibility is rather mediocre (10 %).
Dissolved silica (expressed in mg/L SiO2 ) is determined on a colorimetric continuous-flow analyzer (Autoanalyzer III, Ax Flow) using ammonium heptamolybdate; the detection limit is lower than 0.1 mg/L.
Dissolved organic carbon (DOC, in mg/L) is measured through catalytic combustion in a Shimazu TOC 5000; the detection limit is 0.1 mg/l with a 5% reproducibility. In this case, the water samples were filtered on glassfiber membrane (type GF/F Whatman, 0.7 µm porosity) then acidified and stored in glass bottles. The Nsimi spring waters, which exhibit very low COD concentrations (<1mg/L) are no more analyzed since the beginning of 2005.
The comparison between the sum of cations and the sum of anions (Scat, San, in µeq/L) allows us to estimate the charge balance in waters; except at the spring, a strong anionic deficit is observed.
The total mineral dissolved charge (TMD, in mg/L) can be compared to the SPM and the DOC concentrations, which are expressed in the same unit.