02 May 2019

02 May 2019

Review status: this preprint is currently under review for the journal HESS.

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

Camille Jourdan1,2, Valérie Borrell-Estupina3, David Sebag4, Jean-Jacques Braun5,6, Jean-Pierre Bedimo Bedimo6,†, François Colin2, Armand Crabit2, Alain Fezeu7, Cécile Llovel8, Jules Rémy Ndam Ngoupayou9, Benjamin Ngounou Ngatcha10, Sandra Van-Exter11, Eric Servat1, and Roger Moussa2 Camille Jourdan et al.
  • 1OSU OREME, Univ Montpellier, Montpellier, France
  • 2LISAH, Univ Montpellier, INRA, IRD, SupAgro, Montpellier, France
  • 3HSM, Univ Montpellier, CNRS, IRD, Montpellier, France
  • 4Normandie Univ, UNIROUEN, UNICAEN, CNRS, M2C, Rouen, France
  • 5GET, CNRS, IRD, University of Toulouse, Toulouse, France
  • 6Institut de Recherches Géologiques et Minières, Centre de Recherches Hydrologiques, Yaoundé, Cameroon
  • 7French National Research Institute for Development (IRD), Yaoundé, Cameroon
  • 8WSP France, Toulouse, France
  • 9Laboratoire de Géologie de l'ingénieur et d'Altérologie, Département des Sciences dela Terre et de l'Univers, Faculté des Sciences, Université de Yaoundé I, BP 812, Yaoundé, Cameroun
  • 10Department of Earth Sciences, Faculty of Sciences, University of Ngaoundéré, Ngaoundere, Cameroon
  • 11GM, Univ Montpellier, CNRS, Université des Antilles, Montpellier, France
  • deceased

Abstract. Inter-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.

Camille Jourdan et al.

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Camille Jourdan et al.

Camille Jourdan et al.


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Short summary
In the theme Panta Rhei, this paper aims to develop a combined approach of data acquisition and a new semi-distributed non-stationary model taking into account land-use changes to reconstruct and predict annual runoff on an urban catchment in a data-sparse context. We use historical data and deploy a complementary short-term spatially-dense dedicated instrumentation. Applications were conducted on the tropical Mefou catchment (Yaoundé, Cameroon) to assess contributions of sub-catchments.