630 US ISSN 0271-9916 December 1982 RESEARCH EXTENSION SERrES 024 Soil Erosion by Water in the Tropics S. A. EI-Swaify, E. W. Dangler, and C. L. Armstrong HITAHR • COLLEGE OF TROPICAL AGRICULTURE AND HUMAN RESOURCES • UNIVERSITY OF HAWAII BEST AVAILABLE COpy SOIL EROSION BY WATER IN THE TROPICS S. A. EI-Swaify, E. W. Dangler, and C. L. Armstrong Department ofAgronomy and Soil Science College ofTropical Agriculture and Human Resources University ofHawaii Honolulu, Hawaii BESTAVAILABLE COpy CONTENTS Illustrations vii Tables ix Acknowledgments xi Abbreviations xiii Synopsis and Recommendations xv 1. Introduction 1 Forms ofwater erosion 1 Tolerance limits 3 Special considerations for the tropics 6 2. Extent ofWater Erosion in the Tropics 9 Approaches, methods, and scales ofassessment 9 Rainfall erosion in the tropics-general trends 12 Inventory ofrainfall erosion in the tropics 13 Tropical Africa 14 Tropical Asia 31 Tropical Australia, Papua New Guinea, and Pacific Islands 44 Tropical South America 45 Central America 53 Caribbean Islands 55 Changes in the extent oferosion 58 3. Impact of Rainfall Erosion in the Tropics 60 Impact on soil productivity 60 Flood hazards 69 Sedimentation and usefulness ofreservoirs and waterways 72 Other environmental impacts .: 74 4. Predictability Parameters for Rainfall Erosion in the Tropics 75 Conditions favoring high rates ofsoil loss 75 Quantitative parameters for prediction 76 Prevailing land-use patterns and farming systems 108 5. Erosion Control Measures 119 Traditional systems 121 Developed systems 121 Vegetative control methods 124 Mechanical control methods 134 6. Priority Needs for Problem Solving 146 Information dissemination 146 Research needs ............................................................ 146 Extension, advisory, and information delivery services 149 Training needs 149 Literature Cited 150 Index 169 About the Authors 173 v ILLUSTRATIONS MAPS 1. Tropical climates ofthe earth 2 2. World soil classification 7 3. Fournier's map oferosion risk in Africa south ofthe Sahara 16 4. Soil erosion hazards in Zimbabwe (Rhodesia) 19 5. Relationship between erosion and stock routes in the Kisongo catchment, central Tanzania 26 6. Erosion studies in Ethiopia 28 7. Geology and structure ofeastern Nigeria 30 8. Soil erosion in the Navrongo-Bawku area, northern Ghana 31 9. Agroclimatic zones and major soil regions in Sri Lanka 33 10. The Mahaweli Ganga basin, Sri Lanka 34 11. Extent and types oferosion in India 36 12. Rainfall patterns ofthe Philippines 42 13. Estimated erosion status ofKauai, Hawaii 46 14. Estimated erosion status ofOahu, Hawaii 46 15. Estimated erosion status ofMolokai, Hawaii 47 16. Estimated erosion status ofLanai, Hawaii 47 17. Estimated erosion status ofMaui, Hawaii 48 18. Estimated erosion status ofthe island ofHawaii 48 19. Mean annual rainfall variability for the world 80 20. Mean monthly rainfall in Asia/Australia 81 21 . Mean monthly rainfall in the Americas 81 22. Mean monthly rainfall in the Pacific 82 23. Mean monthly rainfall in Africa· 82 24. Estimated average annual values ofthe rainfall erosion index in the major islands ofHawaii 84 25. Isoerodent map ofJava and Madura 86 26. Isoerodent map ofIndia 88 27. Isoerodent map ofwest central Africa ~ 89 28. Mean annual erosivity over Zimbabwe (Rhodesia) 90 29. Relative erodibility ofTaiwan slopeland soils 100 Vll FIGURES 1. Schematic illustrations ofmajor forms oferosion by water 4 2. Measurement oferosion around vegetation 9 3. Relation between sediment delivery ratio and catchment size 11 4. Relation ofmean annual sediment yield to drainage area in Tanzania compared to eastern Wyoming. 11 5. Changes in soil erosion trends with rainfall 13 6. Frequency ofgullies on three types ofland tenure in Zimbabwe (Rhodesia) 18 7. Sediment yields observed under various land-use patterns in the semiarid rangelands ofKenya 22 8. Extent ofland that will have soil cover removed completely over various periods in three study areas in Kenya 22 9. Annual rainfall pattern for Thailand's northern highlands, 1975-1977 39 10. Changes in soil productivity as influenced by the degradation-aggradation processes 61 11. Potential soil and nutrient loss from an unprotected tropical Oxisol in relation to steepness ofslope 64 12. Effects ofslope and crop rotation on total nutrient loss in eroded sediments during 1973 64 13. Effects ofdifferent mulch rates and slopes on total nutrient losses in runoffwater 64 14. Total loss oforganic carbon in eroded sediments, 1973 65 15. Yields ofmaize in relation to topsoil depth, United States 65 16. Effects of simulated erosion on yields ofmaize on a cobbly soil, Malaysia 67 17. Effects of simulated erosion on yields ofmaize on a deep soil, Malaysia ., 67 18. Effect ofsurface soil removal on grain yield on Alfisols 67 19. Relationship ofthree levels ofproduction ofmaize to soil loss and slope 68 20. Examples ofcropland burial by erosional sediments 69 21. Wischmeier's nomograph for soil erodibility estimation 95 22. Roth, Nelson, and Romkens' nomograph for soil erodibility estimation 96 23. Relationships between K, E, and soil erodibility F 102 24. Relationship between gradient and soil loss under simulated rainfall for residual soils on Oahu, Hawaii 103 25. Relationship between gradient and soil loss under simulated rainfall for volcanic ash soils on the island ofHawaii 103 26. Comparative soil erosion from cropped and fallow Vertisols 106 27. Relationship between catchment vegetation and soil erosion in selected rivers ofAfrica and Asia 106 28. Combined mulch and canopy effects at different fall distances 113 29. Effect ofplant residue mulch on soil loss 115 30. Flow chart of selected soil erosion practices for water-induced soil erosion 122 31. The brush matting technique 129 32. The wattling and staking technique 129 33. Generalized diagram ofstrip cropping 131 34. Major types ofterraces 136 35. Horizontal and vertical interval measurement for spacing terraces 137 36. Major kinds ofwaterways 138 37. Various kinds of spillways 140 38. Guide to structure selection 142 39. A rock-fill check dam 142 40. Temporary dam structures 143 41. Use ofworn tires for stabilizing slopes 145 Vill TABLES 1. Classes oferosion .............................................. .' ................ 10 2. Rates oferosion ofthe continents 13 3. Estimated annual soil erosion within drainage basins ofselected rivers ofthe tropics 15 4. Percentage ofland area ofvarious agricultural settlements affected by various degrees oferosion 17 5. Explanation ofmajor categories ofhazard shown in Map 4 20 6. Estimates ofaverage Cenozoic erosion rates in Kenya 23 7. Soil denudation rates in seven catchment basins in Tanzania compared with data from one set of soil erosion plots 24 8. Areal inventory oflandforms, land use, and soil erosion in the catchments ofMatumbulu, Msalatu, and Imagi reservoirs, Tanzania 25 9. Estimated extent ofsevere sheet erosion and gully volumes, Kisongo catchment, central Tanzania 25 10. Altitudinal zones ofrainfall, land use, and soil erosion in the Morogoro River catchment, Tanzania .. 27 11. Estimated extent and types oferosion in India .......................................... 35 12. Suspended sediment loads ofsome Southeast Asian rivers 38 13. Erosion extent in the Philippines 41 14. Degree and extent ofsoil erosion in areas licensed to timber companies within the Agno River basin, Philippines 43 15. Actively eroding areas and erosion rates for the Hawaiian Islands 49 16. Relationship between climate, vegetation, and soil erosion in Latin America 50 17. Estimate ofpotential erosion in the Andean region ofPeru 51 18. Some quantitative effects ofhuman activities on surface erosion 59 19. Analytical data from some eroded and noneroded Ferralsols (Oxisols) in Tanzania 62 20. Nutrients in runofffrom selected plots in conventionally tilled tobacco on three soils in Puerto Rico, 1967 63 21. Effect ofdepth oftopsoil on yields ofcorn and oats on Tama silt loam, Iowa, United States 65 22. Effect ofdepth oftopsoil on yields ofcorn, cotton, and oats on Cecil soil, Georgia, United States 65 23. Effect ofdepth oftopsoil on yield ofcorn 66 24. Relation ofcrop yields to depths oftopsoil 66 25. Effect ofdepth ofsoil removed on root development. 68 26. Data on siltation in selected reservoirs in India 73 27. Rainfall in selected tropical and subtropical locations 78 28. Interrelationships between raindrop size, storm kinetic energy, and intensity, as established by various workers ................................................................ 83 29. Summary ofalternative estimates for the rainfall erosion index and its components 89 30. Selected data for high rainfall intensity rates in the tropics 91 31. Selected data for rainfall erosivity according to the EI30 index 91 32. Erodibility ofAlfisols or Aridisols 92 33. Erodibility ofOxisols 92 34. Erodibility ofInceptisols 93 35. Erodibility ofMollisols or Vertisols 93 IX 36. Erodibility ofUltisols 93 37. Erodibility ofsoils with miscellaneous classifications 94 38. Nomograph-predicted erodibility values for some Brazil soils 97 39. Nomograph-predicted erodibility values for some Sri Lanka soils 97 40. Comparison ofrange ofproperties used t~ develop existing nomographs with those encountered in selected Hawaii soils 98 41. Comparison ofnomograph-predicted and experimental erodibility values for Hawaii soils 98 42. Regression equations for soil properties and erodibility ofHawaii soils 99 43. Soil erodibility values used in the SLEMSA Model 101 44. Table for calculation