in soil erosion prediction: a case study of the sg

UNIVERSITI PUTRA MALAYSIA

APPLICATION OF GEOGRAPHIC INFORMATION SYSTEM (GIS) IN SOIL EROSION PREDICTION: A CASE STUDY OF THE SG. HIJAU

WATERSHED, FRASER'S HILL, PAHANG

ANASTASIA NG SUAN KIM

FH 2003 4

APPLICATION OF GEOGRAPHIC INFORMATION SYSTEM (GIS) IN SOIL EROSION PREDICTION: A CASE STUDY OF THE SG. HIJAU

WATERSHED, FRASER'S HILL, PAHANG

By


Thesis submitted in partial fulfillment of the requirement for the Degree of Master Science in Tropical Forest Resource Management in the

Faculty of Forestry, Universiti Putra Malaysia, Selangor.

May, 2003

APPLICATION OF GEOGRAPHIC INFORMATION SYSTEM (GIS) IN SOIL

EROSION PREDICTION: A CASE STUDY OF THE SG. HIJAU WATERSHED,

FRASER'S HILL, PAHANG.

By


2003

Chairman: Associate Prof. Dr. Lai Food See

Faculty: Forestry

A study was conducted to assess soil erosion using Geographical Information System

(GIS) and Universal Soil Loss Equations (USLE) for the Sg. Hijau watershed. The study

area, located in Fraser's Hill, Pahang is 1 .5 km2 in size and lies between 1 060 and 1 350

m a.s.l. A GIS software, ARC/INFO version 3 .4 was used to develop the database.

Analysis was carried out using the ArcView Spatial Analyst Version 1 . 1 and ArcView

3D Analyst. Results obtained show that about 48% of the Sg. Hijau catchment falls under

the slope gradient of 12°_25° while 26% of the area under the slope of more than 25°.

Four empirical methods based on the applications of erosivity were used for soil erosion

computation. They were from Roose ( 1 975), Balamurugan ( 1 990), Morgan (1 974) and

rainfall of more than 25 mmIhr adopted in this study. By comparison, Roose generated

the highest erosion rate with 1 .757 t/halyr followed by Balamurugan with 0.685 t/halyr,

25 mm/hr with 0.567 t/halyr and lastly, Morgan with 0.532 t/halyr. Erosion rates of less

than 1 t/halyr were computed for most of the area in the study watershed: soil erosion

rates ranged from 0.363 to 0.642 tlhalyr. The results obtained were also comparable to

measured soil loss from erosion plots in other studies under similar conditions. Most

studies have shown that erosion seldom exceeds 1 t/halyr under forest conditions. This

study showed that soil erosion rates could be calculated using USLE within the GIS

environment. Apart from employing GIS as an easy-to-use database and toolkit for

modeling, GIS techniques could be used to assess the uncertainty and validity of spatial

erosion models. The use in catchment hydrological and erosion modeling offers

considerable potential.

n

APPLIKASI SISTEM MAKLUMAT GEOGRAFIK (GIS) DALAM MERAMAL

HAKISAN TANAH: SATU KAHAN KES DI KA WASAN TADAHAN AIR SG

HIJAU, FRASER'S HILL, PAHANG

Oleh


2003

Pengerusi: Profesor Madya Dr. Lai Food See

Fakulti : Perhutanan

Satu kajian telah dijalankan untuk menentukan hakisan tanah dengan menggunakan Sistem Maklumat Geografi (GIS) di kawasan tadahan air Sg. Hijau. Kawasan tadahan air Sg. Hijau terletak di Fraser's Hill, Pahang, dengan saiz keluasan 1 .5 km2 dan terletak di antara 1 060 hingga 1 350 m pada paras laut. Perisian GIS, iaitu ARC/INFO versi 3 .4 digunakan untuk membina pangkalan data. Analisis data dijalankan dengan menggunakan perisian ArcView Spatial Analyst Versi I . l a dan ArcView 3D Analyst. Keputusan menunjukkan bahawa sebanyak 48% daripada Sg. Hijau berada dalam lingkungan cerun 12° - 25° manakala 26% daripada kawasan Sg. Hijau berada dalam cerun lebih dari 25°. Empat kaedah empirik berdasarkan kadar hujan telah digunakan dalam menentukan kadar hakisan tanah. Kaedah itu adalah dari Roose ( 1975), Balamurugan (1 990), Morgan (1 974) dan kadar hujan lebih dari 25 mmlj (R>25 mmlj) yang digunakan dalam kajian ini. Daripada keempat-empat kaedah empirik, kaedah Roose memberi kadar hakisam yang tinggi dengan 1 .757 t/halthn diikuti dengan Balamurugan iaitu 0.685 t/halthn, 25mm1j dengan 0.567 t/halthn dan akhimya, Morgan dengan 0.532 t/halthn. Kadar hakisan tanah kurang dari 1 t/halthn diperolehi di kebanyakkan kawasan Sg. Hijau dalam kajian ini, iaitu dalam kadar lingkungan 0.363 ke 0.642 t/halthn. Keputusan yang diperolehi juga adalah setanding dengan keputusan hakisan tanah yang diukur di petak hakisan dalam kajian lain dibawah keadaan yang sama. Kebanyakkan kajian menunjukkan bahawa kadar hakisan tanah jarang melebihi 1 t/halthn di dalam kawasan hutan. Kajian ini telah menunjukkan bahawa selain menggunakan perisian GIS sebagai alat yang mudah digunakan malah GIS mempunyai potensi untuk menaksir pangkalan data dalam hidrologi tadahan air dan model hakisan.

ill

Name of Candidate

Title of Thesis

Date: May, 2003

APPROVAL SHEET

Anastasia Ng Suan Kim

Application of Geographic Information System (GIS) in

Soil Erosion Prediction: A Case Study in The Sg. Hijau

Watershed, Fraser's Hill, Pahang.

Approved by:

Assoc. Prof. Dr. Lai Food See

(Supervisor)

kJ a. Lu·

Assoc. Prof. r. Ahmad Ainuddin bin Nuruddin

(Examiner)

Prof. Dr. Mohd. Hamami bin Sahri

(Dean of Faculty of Forestry)

IV

Specially c£edicated to

5l1y 6eCoved 5l1otlier, 5l1ary and my Cwing dad; PauC 5l1y sister, PeCicia

5l1y 6rotlier eiliis famiCy, Leonard; Prisciffa ei}lnnie,

}lnd to a{{ my friends

'11ianuou for your Cove, prayers, patience and encouragement,

'Without your support, I wi{{ not 6e wliere I am today,

I Cove you aCC

v

ACKNOWLEDGEMENTS

Firstly, I would like to thank the Almighty God for granting me the strength

and confidence that I need to complete this thesis. I especially would like to express

my deepest gratitude and appreciation to my supervisor, Assoc. Prof. Dr. Lai Food

See for his invaluable help, dedicated efforts, guidance and patience throughout this

study.

I am particularly grateful to Albert Tan for his kind assistance, knowledge

and advice. Many thanks also, to my friends for their moral support and most of all,

their invaluable friendship.

Finally, I would like to express my deepest love and gratitude to my family

members for their unending prayers, support and encouragement during the course

of this study.

vi

TABLE OF CONTENTS

Page

TITLE PAGE . .... ... .. .. . . . .. . . . .. . . .. . . .... .... . . .. . .. . . . . . . . . . . . . . . . . . . . .. . . . ... ... . . . . ABSTRACT ... , ... , ................. '" .... , ... , ................... " .. , .... '" .. . .. . .... 11 ABSTRAK. ... .. . . . . . . . . .. . . . . ........ . . . .. . . ... .. . . . . .. . . . . . . . .. . . . . . . . .. . .. . .... . .. . . . .. 111 APPROY AL SHEET.. ..... .. . . . . . . . . . . . ..... . . . . .. . . . . . . .. ....... ... . . . . . . .... ......... iv DEDICATION . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . ... . . .... . . . . . . . . . ... .... . . . . . . .. . v ACKNOWLEDGEMENTS . . . . . . . . . . . ... . . .. . . .. . . . . . . .. . . ......... ........ ... . . . . . . .. vi TABLE OF CONTENTS . . . . ... . ... . . . . . . .. . . . .. .. .. . .... . . ... ......... ... .... . . .. .. . . . vii LIST OF TABLES. . . . . . .. .. . . . . . .. .. . .. .. . . . . . .. . . . . ... . . .. ..... .. . . . . .. . .. . . .. ..... . . .. ix LIST OF FIGURES... . . .. . .. . . . . ..... . .... . . .. . . . .. . ... .... ..... . ... . . ... . ... .... . ... . . x LIST OF APPENDICES .. ..... ... . .. .. , . . . . .. . . . . .. . .. . . . . .. . .. . .. . .. . .. . .. . . . . .. . . . . . . xi

CHAPTER

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 General. . .. . .. . . . . .. . .. . .. . .. . .. . . . . . . . . . . . . . . . . .. . . . . .. . .. . . . . .. . .. . . . . .. 1 1.2 Problem Statement ............ , ........... , . .. . ... .. . .. . .. ... . ... . . ..... 2 1.3 Objectives of the Study.. . . . .. . . . .. . . .. . . . ....... ....... . .... . . ........ 3

2 LITERATURE REVIEW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Introduction.. ..... ...... ... . .... . . ....... ... . . . . . . . . . .. . . . .. . ........... 4 2.2 Soil Erosion.......... . ... . . ... . . . . . . . . . . . . . ........ . .... . . . . .... . . . .. . ... 5 2.3 Types of Erosion . . . . . . .. . . . .. . . . . . . . .. . ................................. 6

2.3.1 Wind Erosion.. . . . . . ... .... . . .... ........ . ........ . ...... . . .. 7 2.3.2 Water Erosion.... . ....... . . ........ . ... ................. . .... 8 2.3.3 Water and Wind Erosion.... . .. . . . . . . ... . ... .. ............. 10

2.4 Erosion Models . . . .. . . . . . . . . . .. . . . . . . . ... . ... . . . . . . .. . ..... ... . . ... . .. . . . 11 2.5 Universal Soil Loss Equation . .... .... , .................... , . .. . . . . .... 13

2.5.1 Limitations of the USLE ModeL.. . .. . . ... . . .. . .... ... . . . 21 2.6 Revised Universal Soil Loss Equation (USLE) . . .. . .... ..... ... . .. . 22

2.6.1 Limitation ofRUSLE............ . . . . . . .... . ... .... . . . . . .. . . 30 2.7 GIS and Soil Erosion...... ........ . .. . . ... ..................... . ... . .. . 31

2.7.1 GIS and Erosion Model Linkages. ............. .. .... ..... 33 2.7.2 Limitations of GIS.. ......... .......... ...................... 34

2.8 Mountains .... . . . ... .... . . ... . ... . . .. . . ........ ... . ........ ... . .... , . .. . .. 36 2.8.1 Mountains and GIS... . . . . ... . ... . . .................. . .... . . . 38

2.9 Conclusion. . ... . . ... .... ... . . ... .... . . . .. . ...... . . . .. ............ . ... .... 40

vii

3

4

5

MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "

3.2 Description of Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2.1 Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3.1 Base Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. .

3.4 Slope Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Universal Soil Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Soil Erosion Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Data Analysis . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.7.1 SPSS Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . .... . . .. . . . . . . . ... . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Basin Slope Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " . . , . ... . 4.3 Mean Soil Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Boxplot for Fraser's Hill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .

4.4.1 Comparison of Box plots of Other Studies . . . . . . . . . . . . . . . 4.5 Frequency Distribution of Erosion Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.5.1 Comparisons of Frequency Distribution of Erosion Rates With Other Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.6 Comparison of Soil Erosion Rates With Other Studies . . . . . . . . . . . . 4.6.1 Comparison Based on Selected Rainfall Values . . .. . . ..

4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . ... .

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Computation of Soil Erosion Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42 42 42 42 43 43 43 43 45 45 49 49 49 51

52 52 52 54 57 60 61

64 64 64 75

76 76 76 77

BIBLIOGRAPHy. . . . . . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

APPENDICES. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 94

viii

LIST OF TABLES

Table

2.1 Global extent affected by wind and water erosion . . . . . . . . . . . . . , . . . . . . ,

3.1 Primary layers of Sg. Hijau Watershed, Fraser's Hill . . . . . . . . . . . . . . . . . . .

3.2 Slope classification of Fraser's Hill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . ,

4.1 Slope distribution of watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . .

4.2 Mean erosion rates of the study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,

4.3 Statistics for boxplot at different rainfall . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . .

4.4 Mean soil erosion rates (t/ha/yr) of different catchment areas . . . . . . . .

4.5 Erosion rates based on specific rainfall of different catchment

Page

7

44

45

54

55

59

65

areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.6 Comparison of soil erosion rates in Malaysia and other countries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . 67

ix

LIST OF FIGURES

Figure Page

4.1 Slope classification of Fraser's Hill . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 53

4.2 Soil erosion rates based on hypothetical annual rainfall in Fraser Hill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.3 Boxplot of erosion based on various rainfall values . . . . . . . . . . . . . .. . 58

4.4 Frequency distributions of erosion rates of Fraser's Hill . . . . . . . . . 62

4.5 Comparisons of erosion rates in selected study catchments . . . . . . 70

4.6 Soil erosion for Fraser's Hill using 25 mmlhr . . . . . . . . . . . . . . . . . . . . . . . 71

4.7 Soil erosion for Fraser's Hill using Morgan method . . . . . . . . . . . . . . 72

4.8 Soil erosion for Fraser's Hill using Roose method . . . . . . . . . . . . . . . . . 73

4.9 Soil erosion for Fraser's Hill using Balamurugan method . . . . . . . . 74

x

LIST OF APPENDICES

Appendix

1 Hypothetical annual rainfall values from 1 700 mm to 3000mm

Page

for Fraser's Hill.. ................. ........ ..... .................... ..... 94

2 Outliers and extreme values for Fraser's Hill.... .... ... ............ 97

3 Boxplot of erosion based on various values for Sg. Pansun...... 1 00

4 Boxplot of erosion based on various values for Sg. Weng 5..... 1 01

5 Boxplot of erosion based on various values for Sg. Lawing... ... 1 02

6 Boxplot of erosion based on various values for Sg. Chongkak. . .. . .. ... . .. . .. . . . . .. . . . . .. . .. . .. . . . . ... .. . ... .. . .. . .. . .. .... . . 1 03

7 Boxplot of erosion based on various values for Sg. Weng 2. . ... 1 04

8 Boxplot of erosion based on various values for Sg. Weng 3. . ... 1 05

9 Boxplot of erosion based on various values for Sg. Weng 1 ..... 1 06

1 0 Boxplot of erosion based on various values for Sg. Batangsi.... 1 07

11 Statistics for boxplot at rainfall of 1800 mm. . .. . .... . . . . .. . .. . . . ... 1 08

12 Statistics for boxplot at rainfall of 2650 mm.............. .......... 1 09

1 3 Statistics for boxplot a t rainfall of 3000 mm.. .. . .. . .. . . . . .... . . . ... 1 1 0

14 Frequency distribution of erosion rates of Sg. Pansun..... ........ 1 1 1

15 Frequency distribution of erosion rates ofSg. Weng 5..... ..... ... 1 1 3

1 6 Frequency distribution of erosion rates of Sg. Lawing............ 1 1 5

1 7 Frequency distribution o f erosion rates of Sg. Chongkak... .... .. 1 1 7

1 8 Frequency distribution o f erosion rates of Sg. Weng 2. . . . . . .... . . 1 1 9

19 Frequency distribution of erosion rates of Sg. Weng 3. . . . . . .... . . 1 21

20 Frequency distribution of erosion rates ofSg. Weng 1 .. .......... 1 23

21 Frequency distribution of erosion rates ofSg. Batangsi.. ..... .... 1 25

xi

22 Soil erosion rates based on hypothetical annual rainfall for Sg. Pansun........................................................................ 127

23 Soil erosion rates based on hypothetical annual rainfall for Sg. Weng 5....................................................................... 128

24 Soil erosion rates based on hypothetical annual rainfall for Sg. Lawing ........................................ ............................... 129

25 Soil erosion rates based on hypothetical annual rainfall for Sg. Chongkak.................................................................... 130



28 Soil erosion rates based on hypothetical annual rainfall for Sg. Weng 1.................................. ........................ ............. 133

29 Soil erosion rates based on hypothetical annual rainfall for Sg. Batangsi. . . . . . . . .. . . . . . . . .. . .. . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . .. . . . . .. . . . . .. 134

xii

CHAPTER ONE

INTRODUCTION

1.1 General

Although soil is an essential natural resource it is being degraded at an

unprecedented scale, both in rate and geographical extent. Soil degradation is caused by

soil erosion through the process of detachment and transportation of soil material from one

place to another through the actions of wind, water or action of rain. While numerous

research have been done over the years throughout the world, the problem of soil erosion

still persists and recent information indicates the soil loss rates may indeed again be on the

rise (Carter, 1 977).

When a vegetation or forest area is removed, the natural equilibrium between soil

building and soil removal is disturbed. As a result, soil erosion will cause serious

deterioration of hydrological conditions of all watersheds. Soil erosion not only hinders

sustainable land management, but can also cause off site environmental problems such as

siltation of lakes and reservoirs.

These days, with the growing importance of Information Technology and

Geographical Information System (GIS), a computer-based tool has become an essential

means in watershed management. Burrough (1 986) defined GIS as "a powerful set of tools

for collecting, storing, retrieving at will, transforming and displaying spatial data from the

I

real world for a particular purpose". GIS is a specialized computer software for handling,

displaying and analyzing spatial data. The use of GIS can reduce technical constraints

faced in field data acquisition and analysis process. With its high efficiency, numerous

databases can be used to integrate and generate information that would have taken much

time and difficulty in the past.

1.2 Problem Statement

Soil erosion has been traditionally computed based on plot studies of which the Universal

Soil Loss Equation (USLE) is the most popular. This equation, composed after Wishmeier

and Smith (I958), has been used in many parts of the world. The USLE has been widely

used around the world because it seems to give acceptable results moreover, of its

simplicity and flexibility in modification. Before computers were used extensively, soil

erosion estimations were made based on manually derived topographic variables from

topographical maps. With increasing availability of computer software, particularly in

spatial analysis, USLE has been applied in conjunction with GIS software in some studies.

While GIS has received great interest, there is much more to be done in spatial analysis

and predictive work in the country. This study was conceived to estimate soil erosion

based on topographical information and hydrology data for Sg. Hijau watershed, Fraser's

Hill.

2

1.3 Objectives

The general objective of this study is to determine soil erosion under primary forest

condition in the Sg. Hijau watershed, Fraser's Hill, Pahang using the integration of

Universal Soil Loss Equation (USLE) and Geographical Information System (GIS).

The specific objectives of this study are:

1. To estimate the soil erosion rates of the study watershed under natural forest

and thus generating digital soil erosion map.

11. To compare soil erosion rates of other watershed studies, namely of hill forest

watersheds with this study watershed which is of lower montane watershed.

3

2.1 Introduction

CHAPTER TWO

LITERATURE REVIEW

Since the general public is ignoring the capacity of our biological system,

deterioration that once took centuries is now being compressed into years by inexorable

population growth. Forests have proved to be one of the most heavily exploited. Erosion is

undermining soil productivity. A natural process, soil erosion as such is neither new nor

necessarily alarming but when it out places the formation of new soil, inherent fertility

declines. Soil loss under different land uses have been recorded and studied by many

researchers i.e. Morgan (1974), Wischmeier ( 1978), Hudson ( 1981), De Roo ( 1993).

The increasing costs of soil loss in both environmental and monetary terms have

encouraged both data collection and model development efforts, at a range of spatial and

temporal scales. Soil loss, have been represented by models at many different levels of

complexity and at varying spatial and temporal scales. They have been used widely as

generators of quantitative predictions with the Universal Soil Loss Equation (USLE), for

all its acknowledged limitations forming basis of many models. Recently, a more spatially

distributed approach has been adopted in modeling work. The move to a more distributed

approach has been supported by a better understanding of the variability of erosion

processes, as well as increasing resources of digital map and sensor data. Computing

technology in the form of Geographical Information System (GIS) has been developed

specifically for handling spatial data. The integration of soil erosion models with the

technology of GIS can effectively store, manipulate, and analyze the large amounts of

spatial data demanded by soil erosion models and can effectively display spatial

information, which is very useful for analyzing the findings.

2.2 Soil Erosion

Soil erosion is a physical process and refers to the wearing-away of the land surface

by water and/or wind as well as to the reduction in soil productivity due to physical loss of

topsoil, removal of plant nutrients, and loss of water.

Soil erosion is particularly problematic in tropical countries. It threatens developing

tropical countries where agriculture is primary to development and the majority of the rural

population depends on it for their livelihood (Enters, 1998). Soil erosion affects both

upstream and downstream areas. Upstream, soil erosion leads to loss of productivity and

loss of water storage capacity on eroded sites. Downstream sedimentation results in

damage to downstream fields, river channels, and capital infrastructure such as dams,

water system and irrigation channels, thereby imposing costs to downstream water users.

5

Soil erosion includes a range of phenomena such as sheet erosion (the removal of

thin layers of topsoil from an eroded site), gully erosion (the formation of incised gullies in

a hillside) and mass wasting (the structural failure of part of a hillside as in the case of a

landslide). Although all of these types of erosion do occur naturally, man's use of

watershed lands, particularly steep areas, can greatly increase the extent and rate of

erosion. Activities such as agriculture, logging, and construction can lead to increased soil

erosion. Soil erosion is therefore a complex phenomenon influenced by natural and socio

economic factors.

2.3 Types of Erosion

Erosion in all forms involves the dislodgement of soil particles, their removal and

eventual deposition away from the original position. Soil erosion is a naturally occurring

process on all land. The agents of soil erosion are water and wind, each contributing a

significant amount of soil loss each year. Soil erosion may be a slow process that continues

relatively unnoticed, or it may occur at an alarming rate causing serious loss of topsoil.

Large areas of the earth are affected by accelerated erosion by water and wind.

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Table 2. 1 : The global extent of land affected by wind and water erosion (Oldeman,

1 99 1 ).

Land Area Affected by Erosion ( lOt> ha)

Region Water Erosion Wind Erosion Africa 227 1 86 Asia 441 222 South America 1 23 42 Central America 46 5

North America 60 35 Europe 1 14 42 Oceana 83 16 World 1094 548

2.3.1 Wind Erosion

Wind erosion is a serious problem in many parts of the world. It is worse in arid

and semiarid regions. Areas most susceptible to wind erosion on agricultural land include

much of North Africa and the Near East, parts of southern, central, and eastern Asia, the

Siberian Plains, Australia, northwest China, southern South America, and North America.

Wind erosion, unlike water, cannot be divided into such distinct types. Unlike

water erosion, wind erosion is generally not related to slope gradient. Occurring mostly in

flat, dry areas and moist sandy soils along bodies of water, wind erosion removes soil and

natural vegetation, and causes dryness and deterioration of soil structure. Wind erosion

physically removes the lighter, less dense soil constituents such as organic matter, clays,

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and silts. Thus it removes the most fertile part of the soil and lowers soil productivity

(Lyles, 1975).

The main factor in wind erosion is the velocity of moving air. Soil movement is

initiated as a result of wind forces exerted against the surface of the ground. For each

specific soil type and surface condition there is a minimum velocity required to move soil

particles. This is called the threshold velocity. Once this velocity is reached, the quantity of

soil moved is dependent upon the particle size, the cloddiness of particles, and wind

velocity itself.

Suspension, saltation and surface creep are the three types of soil movement, which

occur during wind erosion. While soil can be blown away at virtually any height, the

majority (over 93%) of soil movement takes place at or below one meter.

2.3.2 Water Erosion

Water erosion is intense in most countries due to their erosive climatic conditions.

Water erosion results from the removal of soil material by flowing water. The extent of

erosion by water is affected by several factors, the length and steepness of slope, soil

texture, the amount and timing of severe rains.

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A part of the water erosion process is the detachment of soil material by the impact

of raindrops. The soil material is suspended in runoff water and carried away. Generally,

three kinds of accelerated water erosion are commonly recognized: sheet, rill and gully.

Sheet erosion is soil movement from raindrop splash resulting in the breakdown of

soil surface structure and surface runoff. It occurs rather uniformly over the slope and may

go unnoticed until most of the productive topsoil has been lost. Sheet erosion can be

serious on soils that have a slope gradient of only 1 or 2 percent. However, it is generally

more serious as slope gradient increases.

Rill erosion is intermediate between sheet and gully erosion. This erosion feature

occurs as rainwater accumulates after rainfall event and it concentrates in depressions, than

flows along the irregularities, causing the formation of rills. Due to the acceleration of the

velocity of moving water, the detachment and transport of soil particles are both greater in

rill erosion than in sheet erosion.

Gully erosion is the consequence of water that cuts down into the soil along the line

of flow. Gullies form in exposed natural drainage way, in plow furrows, in animal trails, in

vehicle ruts, between rows of crop plants, and below broken man-made terraces. In

contrast to rills, they cannot be obliterated by ordinary tillage. Their presence is a strong

indicator that erosion is out of control and that land is entering a critical phase that

threatens its productivity (Laflen et ai, 1997).

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Roose ( 1 994) described three processes of gully formation: (i) the formation

of a V -shape gully where the weathered material from the gully sides is moved from

the gully bottom and additional material is moved from the gully bottom due to

hydraulic shear; (ii) a U-shaped gully due to gully wall failure due to the pressure of

a watertable; and (iii) tunneling in soluble material or because of burrowing

animals. Bradford et al. ( 1 973) described gully erosion has having three phases: (i)

failure of gully head and G u l l y banks, (ii) cleanout of the debris by streamflow,

and (iii) degradation of the channel.

2.3.3 Water and Wind Erosion

There are significant similarities and differences between the processes

involved in water and wind erosion. Both are understood as rate processes that

depend on the speed of the wind or overland flow. While these speeds are measured

in experimental studies, since lengthy time series are not generally available,

extrapolation to longer time periods poses similar weather generation problems for

both wind and water erosion prediction. Water erosion does not have the highly

sensitive dependence on surface water content that is characteristic of water erosion.

However, both types of erosion are strongly reduced by contact cover.

In water erosion, cover III such close contact with the soil surface that it

impedes overland flow is generally much more effective than aerial cover, which

provides protection only against rainfall impact (Rose, 1 993). In wind erosion aerial

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cover is very effective, and the resulting substantial reductions in wind speed at the

soil surface can lead to threshold wind speeds not being exceeded.

There is no direct analogy in wind erosion to rainfall detachment, though the

surface bombardment by wind-driven saltating particles conjures up vague

similarities. Deposition under gravity is the cause of return of saltating particles to

the soil surface in both media. However, the speed of the particles involved in this

return is much lower in the case of water erosion than for wind-driven sand, and is

thought not to be a cause of erosion. Though terminal velocity is achieved with much

less fall distance in water compared to air, the fall velocity on return may not be

terminal in either medium except for small particles.

The distinction made between entrainment and re-entrainment in describing flow -

driven water erosion has some relevance to wind erosion in that there is resistance to be

overcome to remove dust particles against attractive forces, and even very low levels of

moisture increase the attraction between sand size particles that threshold velocities may

not be achieved. If saltation is prevented by capillary and matric forces due to water, then

the main source of energy for the ejection of dust particles is unavailable.

2.4 Erosion Models

In attempts to better manage soil and water resources, computer modeling has

received increasing attentio� including development of numerous soil erosion mOOels_

1 1

These erosion models have been used in investigative, evaluative, predictive and learning

modes. These models have been developed to extend the understanding of physical process

as well as for generating quantitative predictions of losses. The resulting models differ

widely in their representation of processes, temporal and spatial scales. On the temporal

scale, an erosion model can be classified as:

� A long term average annual model, such as the USLE (Wischmeier and Smith,

1978) and RUSLE (Renard et aI., 1991);

� A seasonal model, such as GAMES (Rudra et al., 1986) and GAMESP (Rousseau

et al., 1988);

� An event model, such as WEPP (Laflen et al., 1991), EPIC (Jones et al., 1991) and

ANSWERS (Beasly et al., 1980) or

� A continuous simulation model, such as WEPP (Laflen et al., 1991), EPIC (Jones

et al., 1991), CREAMS (Knisel et al., 1980) and GLEAMS (Leonard et al., 1987).

Most of the continuous models can be used for an event.

On the spatial scale, an erosion model can be classified as:

� A hill-slope model, such as WEPP (Laflen et al., 1991);

� A field scale model which aims to consider both hill slope and valley bottom, such

as CREAMS (Knisel, 1980), GLEAMS (Leonard et al., 1987) and EPIC (Jones et

al., 1991); or

� A watershed or grid model, such as GAMES (Rudra et al., 1986), GAMESP

(Rousseau et al., 1988), ANSWERS (Beasly et al., 1980) and AGNPS (Young et

al., 1985).

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in soil erosion prediction: a case study of the sg

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