Hydrology, erosion and nutrient cycling in a forest ecosystem in south Cameroon I II HYDROLOGY, EROSION AND NUTRIENT CYCLING IN A FOREST ECOSYSTEM IN SOUTH CAMEROON J.C. Ntonga, M.J. Waterloo and A.B. Ayangma, Tropenbos-Cameroon Documents 10 The Tropenbos-Cameroon Programme, Kribi, Cameroon 2002 III ABSTRACT J.C. Ntonga, M.J. Waterloo and A.B. Ayangma, 2002. Hydrology, erosion and nutrient cycling in a forest ecosystem in South Cameroon. The Tropenbos-Cameroon Programme, Kribi. Tropenbos-Cameroon Documents 10, XVIII + 60 pp.; 32 fig.; 9 tab.; 68 ref.; 0 Annexes Rainfall, water and sediment yields were measured and the evaporation quantified in three catchments (2.7-7.7 km2) covered with undisturbed rain forest, selectively logged forest and forest - shifting cultivation. The nutrient balance in the hydrological cycle was also quantified in an undisturbed forest area. Annual evaporation rates were similar in spite of the differences between land uses in the areas and in the regional variation in annual rainfall. On the other hand, the sediment yield depended strongly on the land uses practices. The results of the study of the nutrient budget in four compartments in the hydrological cycle showed that the throughfall and litter percolate constitute the main store of nutrients in the water cycle. In spite of the low nutrient input by rainfall, the losses by the outflow are low; this pattern has been described as an auto-sustained rainforest ecosystem. Keywords: catchment hydrology, undisturbed forest, selective logging, shifting cultivation, water yield, sediment yield, nutrient cycling, nutrient balance, Cameroon. EUROPEAN COMMISSION Directorate General for Development Development Policy Sustainable Development and Natural Resources L’Institut de Recherches Géologiques et Minières The Tropenbos-Cameroon Programme is a research programme executed under the joint responsibility of the Ministry of Environment and Forests of the Republic of Cameroon and Tropenbos International. This study was achieved with the financial contribution of the European Union Actions in Favour of Tropical Forests in Developing Countries Budget Line (b7-6201). The author is solely responsible for all opinions expressed in this document, and does not necessarily reflect that of the European Union and other donors. © 2002 Tropenbos International P.O. Box 232, NL-6700 AE Wageningen (the Netherlands). Phone: +31 317 495500; fax +31 317 495520; e-mail: [email protected] ISSN 1566-2152 No part of this publication may be reproduced or published in any form or by any means, or stored in a database or retrieval system, without the written permission of Tropenbos International. Tropenbos International assumes no liability for any losses resulting from the use of this document. IV TABLE OF CONTENTS PREFACE ...................................................................................................................................IX SUMMARY ................................................................................................................................XI RESUME.................................................................................................................................. XIII 1. INTRODUCTION..................................................................................................................... 1 1.1. Background and justification ............................................................................................. 1 1.2. Objectives of the study ...................................................................................................... 2 1.3. Outline ............................................................................................................................... 2 2. PHYSICAL ENVIRONMENT................................................................................................. 4 2.1. Location, topography and drainage system........................................................................ 4 2.2. Climate and hydrology....................................................................................................... 4 2.3. Geology and soils............................................................................................................... 8 2.4. Vegetation and land use..................................................................................................... 9 2.5. Study catchments ............................................................................................................. 10 2.5.1. The Songkwé catchment........................................................................................... 10 2.5.2. The Biboo - Minwo catchment................................................................................. 11 2.5.3. The Nyangong catchment......................................................................................... 11 3. METHODS ............................................................................................................................. 13 3.1. Hydrology and erosion..................................................................................................... 13 3.1.1. Field data acquisition................................................................................................ 13 3.1.2. Laboratory procedures and methods......................................................................... 14 3.1.3. Modelling procedure ................................................................................................ 15 3.2. Nutrient cycle study ......................................................................................................... 15 3.2.1. Field data acquisition................................................................................................ 15 3.2.2. Laboratory procedures and methods......................................................................... 16 4. HYDROLOGY ....................................................................................................................... 17 4.1. regional rainfall distribution ............................................................................................ 17 4.2. Rainfall interception ........................................................................................................ 19 4.3. Electrical conductivity and pH of stream water............................................................... 19 4.4. Soil hydrological characteristics...................................................................................... 22 4.4.1. Infiltration rate.......................................................................................................... 22 4.4.2. Soil cohesion and compaction .................................................................................. 22 4.4.3. Bulk density.............................................................................................................. 23 4.4.4. Aggregate stability.................................................................................................... 23 4.4.5. Soil roughness .......................................................................................................... 23 4.4.6. Moisture content of litter layer and biomass ............................................................ 24 4.5. Quantification of the water balance components............................................................. 24 4.6. Catchment sediment yield................................................................................................ 31 4.7. Hydrological modelling ................................................................................................... 34 4.8. Meteorology..................................................................................................................... 40 5. NUTRIENT CYCLING IN THE HYDROLOGICAL CYCLE ............................................. 43 5.1. Introduction...................................................................................................................... 43 5.2. Sampling and analysis ..................................................................................................... 43 5.2.1. Methodology on water sampling and chemical analyses.......................................... 43 5.2.2. Results of the analyses.............................................................................................. 44 5.3. Nutrient balance in the hydrological cycle ...................................................................... 50 5.3.1. Introduction .............................................................................................................. 50 5.3.2. Nutrient input by rainfall .......................................................................................... 52 5.3.3. Nutrient output in river flow..................................................................................... 52 5.3.4. Nutrient fluxes in throughfall and in litter percolates............................................... 52 5.3.5. Export or accumulation of major nutriments............................................................ 54 6. DISCUSSION AND CONCLUSIONS................................................................................... 55 V LIST OF TABLES Table 3.1: Geographical locations and elevations of the rain gauges in the TCP area and of those in Kribi and Ebolowa.......................................................................................................... 13 Table 4.1: Quantification of