TC Cameroon: Scenario development and modelling of soil and water resources considering climate change

Report of the project:

• Cameroon - Scenario development and modeling of soil and water resources considering climate change in Cameroon

Background:
Cameroon in West-Central Africa is a country of unique diversity in landscape, climate and culture – Africa in miniature. It stretches from the tropical rainforest at the Gulf of Guinea to the southern shores of Lake Chad in the Sahel. Especially in northern Cameroon, impacts of climate change are already noticeable. The mean temperature increased from 1.0 to 2.5°C between 1971 and 2009 and changes in the distribution, quantity and intensity of precipitation have been observed.

Extreme events are detrimental to soil and water resources. Heavy rainfall events are responsible for an increased erosion, which negatively affects soil fertility. During such events, much of the water runs off, which contributes to uneven aquifer recharge. Longer and more frequent drought periods cause soils to dry out and less water is available for crops leading to low crop yields and low agricultural production. Dried-out soils are also less prone to water infiltration into the subsurface reducing aquifer recharge.

Project overview:
Building on the results from the previous project, the "Projet Sol-Eau-Climat (ProSEC)" aims to support technical staff from partner institutions consider the impacts of climate change on soil and water resources in the Adamawa and North Regions, and improve their advisory services to rural communities to become more resilient to climate change. The two partner institutions are IRGM (Institute of Geological and Mining Research) and IRAD (Agricultural Research Institute for Development).

ProSEC centres on two main axes:

• Participatory production of soil and water scenarios that consider, among others, climate change and population growth;
• Development of a numerical model that reproduces past and present soil-water interactions. The model will help to decide on adaptive water use measures once the scenarios are implemented.

The project has a strong capacity-building component in the development and use of numerical models.

Localisation and map of the pilot zones Source: BGR

Outcomes from the previous project:
The previous project "Projet Sol-Eau-Plante (ProSEP)" focussed on two pilot zones located in two different agro-ecological zones of Cameroon: the Bidou catchment (61 km²) in the Adamawa region (High Guinea savannah) and the Douka Longo catchment (678 km²) in the North region (Sudano-Sahel). The climate in both regions is characterised by a rainy season (from March/April to October/November) and a dry season. The length of the rainy season and the precipitation amount decreases towards the north, while the mean temperature increases.

Water resources monitoring in the pilot zones
To assess the catchment water balance, monitoring instruments were installed in each pilot zone:

• Rain collectors (2 per catchment): the rainfall amount has been manually recorded after each rain event since March 2021. Rain samples were also collected for isotope analyses and occasionally for standard chemistry, if the amount of water was sufficient.
• River gauges (2 per catchment): the river levels have been manually recorded twice daily since March 2020. Dataloggers installed in March 2021 supplement the manual recordings (initially every hour, then every 15 minutes since August 2022). All dataloggers measure the water temperature and some of them water conductivity.
• Dataloggers were also installed in boreholes (3 per catchment) and have been monitoring groundwater levels, temperature and, for some, conductivity on an hourly basis since November 2019 in the Bidou catchment and since March 2021 in the Douka Longo catchment.

Rain collector, river water level equipment at Gada Bidou station (Bidou catchment, Adamawa) and datalogger installed in a borehole at Tchabal (Bidou catchment, Adamawa) Source: BGR

Field activities
In addition to continuous monitoring, the project team carried out field activities to better understand the dynamics of each catchment. These include:

River discharge measurement at Gada Bidou station (Adamawa) Source: BGR

• River discharge measurements to produce rating curves (water level – discharge curve);
• Groundwater level surveys;
• Infiltration tests to assess the soil infiltration capacity and evaluate which areas are more prone to recharge or, on the contrary, to runoff;
• Pumping tests to estimate the hydraulic properties of the underlying aquifers;
• In-situ parameter measurements (temperature, pH, and electrical conductivity) followed by surface and groundwater sampling, and lab analyses for major and minor ions, trace elements, and isotopes;
• Soil sampling and lab analyses for major and minor ions, and trace elements;
• Rock sampling and lab analyses for major oxides, minor and trace elements.

Main findings
Rainfall data highlight spatial variability over short distances: the upstream part is less rainy than the downstream part in both catchments.

Owing to the position of the dataloggers in the Douka Longo River, only the manual readings can record the low water levels. In the Bidou catchment, manual readings can be used in case of datalogger failure. The higher temporal resolution of the dataloggers provide a better characterisation of the flood events.

Groundwater level in a borehole in Ngaoumokone (Adamawa) Source: BGR

Available groundwater time series are still short in the Douka Longo catchment. For the Bidou catchment, their preliminary interpretation indicates (at least partial) recovery of the aquifer during the rainy season. The piezometric maps also suggest an increase in water levels during the rainy season in some areas of the aquifer.

Saturated vertical hydraulic conductivities are similar in the two catchments (ca. 10^-6 – 10^-5 m/s) and characteristic of soils with a limited infiltration capacity.

The pumping tests suggest that the aquifer underlying the Bidou catchment is poorly permeable (ca. 10^-5 m/s) whereas the aquifer underlying the Douka Longo catchment has a higher productivity, although variable in space (10^-5 – 10^-3 m/s).

Stable isotopes in surface water and groundwater Source: BGR

Water resources are characterised by a low mineralisation. Due to its geology, the waters of the Douka Longo catchment are five times more mineralised. The majority of the samples in the two catchments are of Ca-HCO₃ type. Some samples exceed the World Health Organisation (WHO) guidelines for manganese and cobalt in the Bidou catchment (Nsata, 2020). About one-third of the groundwater samples exceed the WHO guideline for nitrates (NO₃^- ≤ 50 mg/L) in the Douka Longo catchment. Other exceedances are also observed in this catchment e. g. manganese, arsenic, barium, beryllium, cobalt, lead, and uranium (Chounna Yemele, 2020).

The precipitation isotope signatures in both catchments are generally not affected by significant evaporation except at the beginning and end of the rainy season. The groundwater isotope composition does not indicate any significant evaporation during recharge processes. Some surface water samples show isotope signature affected by evaporation.

Perspectives for ProSEC:
A good understanding of the catchments is a prerequisite for the development of conceptual and then numerical models. The data collected and their preliminary interpretation give insights into the dynamics of the two catchments. However, this preliminary interpretation reveals some data gaps. Only one preliminary rating curve (at the outlet of the Bidou catchment) is available. For the three other river gauges, data collection issues or a dry river during field visits prevent the production of the rating curves. Groundwater monitoring could be expanded by adding more dataloggers in each catchment to improve spatial coverage.

Flowchart showing the main stages of ProSEC Source: BGR

ProSEC seeks to improve data availability, both model input data (e.g. rainfall) and observation data (e.g. groundwater levels, river discharge) to validate model outputs. Identified data gaps do not preclude model development. Models will be revised and improved periodically as new data become available.

The development of future scenarios is independent of data collection in the two catchments. Some scenarios (climate, population) can be derived from existing global scenarios, but adapted to local conditions. The scenarios are developed in consultation with the project partners and local stakeholders.

Literature:

Master theses

• CHOUNNA YEMELE, G. (2020): Hydrogeochemical generation, content, mobility and flux of elements in watersheds draining sedimentary and igneous rocks; contribution to inorganic nutrients. - M. Eng., École Nationale Supérieure des Travaux Publics, Yaoundé, Cameroun. 124 pp.
• NSATA, S.A. (2020): Suitability of water resources for domestic and agricultural uses in Bidou and Douka Longo watersheds, Northern Cameroon. - M. Eng., École Nationale Supérieure des Travaux Publics, Yaoundé, Cameroun. 150 pp.

Technical reports

• NSANGOU, D., MONTCOUDIOL, N., SEEHAUSEN, L., NGAH, M., BASSOGOG, Z. & FANTONG, W.Y. (2023): Rapport de mission de terrain - Mission du 30 juin au 15 juillet 2023 dans les régions Adamaoua et Nord. Rapport technique du Projet ProSEC, 49 p. (PDF, 3 MB)
• MONTCOUDIOL, N., JOKAM NENKAM, T.L.L. & VASSOLO, S. (2021): Interprétation préliminaire des données collectées (2019–2021) pour la réalisation de bilans hydrologiques dans les bassins versants du Bidou et de Douka Longo, Cameroun. - Rapport technique N°7 de Projet ProSEP, 132 p. (PDF, 10 MB)
• JOKAM NENKAM, T.L.L., NGOLLO, T., FOZAO ATSAMO, D., NJIAYOUOM, M.N., DANDJIO, W.F., W., HAMAN, D.J.B., FODOUE, Y., AMINOU, A., MBODJI, B.J., TAKOUNDJOU, A.F. & FANTONG, W.Y. (2021): Rapport sur la mission de juillet à septembre 2021 dans les régions Adamaoua et Nord, Cameroun - Rapport technique N°6 de Projet ProSEP, 45 p. (PDF, 4 MB)
• JOKAM NENKAM, T.L.L., NGOLLO, T., TAKOUNDJOU, A.F., & FANTONG, W.Y. (2021): Rapport sur la mission de mars 2021 dans les régions Adamaoua et Nord. - Rapport technique N°3 de Projet ProSEP, 32 p. (PDF, 3 MB)
• JOKAM NENKAM, T.L.L., NGOLLO, T., NSATA, S.A. & FANTONG, W.Y. (2021): Rapport sur la mission de septembre 2020 dans les régions Adamaoua et Nord. - Rapport technique N°2 de Projet ProSEP, 33 p. (PDF, 2 MB)
• JOKAM NENKAM, T.L.L., TAKOUNDJOU, A.F., FANTONG, W.Y., NGOLLO, T., BASSAGOG, Z., FODOUE, Y., NJIAYOUOM, N., FOANO, W. & CHOUNNA YEMELE, G. (2020): Rapport sur la mission de terrain de février-mars 2020 dans les régions Adamaoua et Nord. - Rapport technique N°1 de Projet ProSEP, 45 p. (PDF, 2 MB)
• JOKAM NENKAM, T.L.L. (2019): Rapport de la mission de terrain (16-30 novembre 2019). - Rapport Projet ProSEP, 28 p. (PDF, 6 MB)

Peer-reviewed papers

• FANTONG, W.Y., CHOUNNA G., JOKAM NENKAM, T.L.L., FOUEPE, A.T., CHI FRU, E., VASSOLO, S., MONTCOUDIOL, N., FODOUE, Y., HAMAN, J.B.D., CARLIER, C., NBENDAH, P. & NKENG, G.E. (2023): Hydrogeochemistry of low agricultural soil yield in Sahelian and sub-tropical watersheds, Northern Cameroon. - Journal of African Earth Sciences, 19, 104823. DOI: 10.1016/j.jafrearsci.2022.104823
• TAKOUNJOU, A.F., DANDJIO, W.F., FODOUE, Y. FANTONG, W., NGAH, M., JOKAM NENKAM, T., CARLIER, C., VASSOLO, S., MONTCOUDIOL, N., ENOW TARKANG, C., CHOUNNA, G. & KRINGEL, R. (2022): Assessment of water availability for agricultural activities in the savannah Plateau of Adamawa-Cameroon. - Environmental Monitoring Assessment, 194, 813. DOI: 10.1007/s10661-022-10476-z