THE EFFECTS OF CHANGES IN FOREST COVER
ON
INFILTRATION RATES
A Comparative Study in Forested Water Supply Catchments
by
U.SEIN WIN
A thesis submitted for the degree of Master of Science (Forestry) in the Australian National University.
Except where specific acknowledgement is given, this thesis
is my original work.
ACKNOWLEDGEMENTS
I am very grateful to the Colombo Plan Authorities in Burma and
Australia for enabling me to undertake this study. I would also like
to acknowledge and thank Professor J.D. Ovington (now Director,
Australian National Parks and Wildlife Service) and my supervisor
Mr D.M. Stodart, for their advice and sincere encouragement to change
my study programme from an ad hoc training course to academic post
graduate studies.
I am also indebted to and would like to thank the following:
. The Ministry of Agriculture and Forests, Government of the
Socialist Republic of the Union of Burma for extending my study
period to two years.
. U. Maung Gale, Director-General, Forest Department, for giving
me valuable advice.and patient encouragement throughout this
research and for providing me with facilities without which this X research could not have been concluded.
The Colombo Plan Authorities in Australia for providing me with
instruments used in the section of the research carried out in
tropical forests of Burma.
The Department of Forestry, Australian National University, for
providing me with field and laboratory facilities throughout the
part of the research carried out in Australia.
. The Forest Department, Burma, for giving me permission to continue
my research in the Tropical Catchment area at Mount Poppa.
. Dr T. Talsma and Mr K.M. Perroux, CSIRO-Pye Laboratory, for giving
me helpful advice and field demonstrations of equipment used
Mr T.S. Johnson (Australian National University), U. Aung Kyaw Myint (Forest Department, Burma) and Daw Thant Thant Tint
(Computer Centre, Rangoon) for their assistance in the comput ation of part of the results from the series of experiments conducted throughout this research.
. Mr Wissopakan for his assistance in some of the experiments carried out in Cotter Catchment, temperate forest, A.C.T., Australia.
Division of Forest Research, CSIRO, for permission to use their records.
. Mr R.T. Moreland and Mr J. B u m s for their helpful advice in the processing of rainfall data.
Mr Pierrehumbert for advice on the analysis of the rainfall records. Mrs P. Reid for assisting me in acquiring relevant literature and
for sending some of the urgently needed ones to Burma during the latter part of this research.
Department of Meteorology and Hydrology, Burma, for permission to use their records.
. Mr R. Whitty and family as an Australian host family, for hospital ity offered and received throughout my stay in Australia.
I am also grateful to the staff and colleagues of the Department of Forestry, Australian National University, for their sincere cooperation in all matters, both private and academic, during my stay in Australia.
Finally, I am deeply indebted to my wife, Hla May Win, for her courage in permitting me a long period of study away from home, and for having to cope with a family of seven during my absence.
iv
ABSTRACT
The results of field measurements to assess the effects of changes in forest cover on infiltration rates are reported.
In the Pierce's Creek forest in the Australian Capital Territory measurements were made using infiltration rings in eucalypt forest and
Pinus radiata plantation on both granite and shale soils. The results
indicate no substantial change in the infiltration characteristics as a consequence of conversion from eucalypt to Pinus radiata plantation.
In the Kyetmauk-taung catchment in central Burma measurements were made in a mixed deciduous forest, a plantation and semi-indaing forest. The plantation was established on land resumed as a catchment protection measure following deterioration by erosion under cultivation for bananas. There were significant differences between the infiltration characteristics of the three forest types. The results indicate that the infiltration characteristics of the plantation area have after seven years recovered to values comparable with the adjacent relatively undisturbed forest.
At both study areas estimates of cumulative infiltration derived from the field measurements are compared with rainfall totals for corres ponding time intervals.
The measurements of sorptivity in the Kyetmauk-taung catchment, using procedures after Talsma (1969), are related to gravimetric moisture
TABLE OF CONTENTS
Page No.
ORIGINALITY OF THESIS i
ACKNOWLEDGEMENTS ii
ABSTRACT iv
LIST OF FIGURES i*
LIST OF TABLES xii
LIST OF MAPS xiv
LIST OF PLATES xv
LIST OF APPENDICES xvi
CHAPTER I THE STUDY IN OUTLINE
1.1 INTRODUCTION 1
1.2 INFILTRATION IN FORESTED CATCHMENTS 1
1.3 INFILTRATION THEORY 2
1.4 THE STUDY SITES AND OBJECTIVES 2
CHAPTER II FOREST MANAGEMENT PRACTICES AND INFILTRATION
IN FORESTED CATCHMENTS
2.1 INTRODUCTION 3
2.2 INFILTRATION AND THE FOREST COVER 3
2.2.1 Introduction 3
2.2.2 Factors affecting infiltration 4
2.2.3 Infiltration studies in forests 6
2.2.4 Infiltration and soil properties 9
2.2.5 Infiltration and land management practices 12
2.2.6 Summary 13
CHAPTER III THE THEORY OF INFILTRATION AND THE
DEVELOPMENT OF FIELD MEASUREMENT TECHNIQUES
3.1 INTRODUCTION 15
3.2 INFILTRATION THEORY 15
3.3 THE APPLICATION OF THE PHILIP EQUATION TO THE FIELD 19
vi
Page No.
CHAPTER IV THE STUDY AREAS
4.1 INTRODUCTION 23
4.2 THE STUDY SITES 27
4.2.1 The Cotter catchment sites 27
4.2.1.1 Location 32
4.2.1.2 Climate 32
4.2.1.3 Soils 34
4.2.1.4 Vegetation 35
4.2.2 The Kyetmauk-taung catchment sites 35
4.2.2.1 Location 35
4.2.2.2 Climate 38
4.2.2.3 Topography 39
4.2.2.4 Soils 39
4.2.2.5 Vegetation 40
CHAPTER V FIELD MEASUREMENTS OF INFILTRATION
5.1 INTRODUCTION 42
5.2 FIELD MEASUREMENTS 42
5.2.1 Equipment 42
5.2.2 Field procedures 44
5.2.3 Calculation of sorptivity and hydraulic 48 conductivity
5.3 SAMPLING PROCEDURES AND RESULTS FOR INFILTRATION 52 MEASUREMENTS AT THE COTTER CATCHMENT STUDY SITES
5.3.1 Introduction 52
5.3.1.1 Sites in the Uriarra Forest 55
5.3.2 Sampling methods 56
5.3.3 Results 59
5.3.3.1 Pierce's Creek 59
5.3.3.2 Uriarra 59
5.3.4 Analysis and Statement of Results 66
5.3.4.1 Pierce’s Creek 66
5.3.4.2 Uriarra Forest 68
Page No.
5.4 SAMPLING PROCEDURES AND RESULTS FOR INFILTRATION 75 MEASUREMENTS AT THE KYETMAUK-TAUNG STUDY SITES
5.4.1 Sampling procedures 75
5.4.2 Results 79
5.4.3 Analysis and Statement of Results 79
5.4.3.1 Statistical analysis of dry season results 84 5.4.3.2 Statistical analysis of wet season results 86 5.4.3.3 Comparison of dry season and wet season 86
results
5.4.4 Discussion 87
CHAPTER VI THE DERIVATION OF RAINFALL INTENSITY-DURATION- 88 FREQUENCY DIAGRAMS FOR STATIONS IN THE LOWER
COTTER CATCHMENT AND RAINFALL INTENSITIES FOR KYETMAUK-TAUNG CATCHMENT
6.1 INTRODUCTION 88
6.2 THE ANALYSIS OF RAINFALL RECORDS IN THE COTTER 92 CATCHMENT
6.2.1 Methods of analysis 96
6.2.2 Pluviograph records in the Lower Cotter catchment 99
6.2.2.1 Introduction 99
6.2.2.2 Processing of analogue charts 102 6.2.2.3 Calculated rainfall intensities 103
6.2.2.4 Review 104
CHAPTER VII REVIEW AND SUMMARY 108
7.1 INTRODUCTION 108
7.2 THE RESULTS OF STUDIES ON THE EFFECTS OF CHANGES 108 IN FOREST COVER ON INFILTRATION
7.2.1 Change from eucalypt forest to Pinus radiata 108 plantation in the Cotter catchment, Australian
Capital Territory
7.2.2 Changes in forest cover as a consequence of 109 reafforestation in the Kyetmauk-taung catchment
in central Burma
7.3 COMPARISONS OF MEASURED INFILTRATION WITH RAINFALL 110 7.3.1 Studies in the Cotter catchment of the Australian 110
Capital Territory
viii
P a g e N o
7.4 T H E M E A S U R E M E N T O F S O R P T I V I T Y A S A C A T C H M E N T P A R A M E T E R
115
7 . 4 . 1
7 . 4 . 2 7 . 4 . 3
M e a s u r e m e n t s at P i e r c e ’s C r e e k in t h e C o t t e r c a t c h m e n t
M e a s u r e m e n t s in t h e K y e t m a u k - t a u n g c a t c h m e n t R e l a t i o n s b e t w e e n s o r p t i v i t i e s a n d g r a v i m e t r i c m o i s t u r e c o n t e n t s i n t h e C o t t e r a n d K y e t m a u k -t a u n g c a -t c h m e n -t s
115
1 1 7 1 2 3
7.5 S U M M A R Y 1 2 3
LIST OF FIGURES
FIGURE TITLE PAGE NO.
2.1 Rainfall, surface runoff and infiltration 7 on forested land and on bare abandoned land.
2.2 Rainfall, surface runoff and infiltration on 13 lands subjected to good and poor grazing
management.
3.1 The infiltration moisture profile. 16
3.2 Infiltration as a function of time. 16
3.3 Infiltration as a function of time 16
(a) in a uniform soil
(b) in a soil with a more porous upper layer (c) in a soil covered by a surface crust.
3.4 Relationship between sorptivity and conductivity. 20 3.5 Graph showing linear portion of total 21
infiltration against /time.
5.1 Calculation of sorptivity. 50
5.2 Layout of experimental plots, Cotter catchment. 53 5.3 Experimental sites in Uriarra Forest. 57
5.4 Sorptivities in eucalypt sites. 60
5.5 Sorptivities in pine plantation sites. 61 5.6 Hydraulic conductivities in eucalypt sites. 62 5.7 Hydraulic conductivities in pine plantation 63
sites.
5.8 Cumulative infiltration after 1 minute 64 in eucalypt sites.
5.9 Cumulative infiltration after 1 minute 65 in pine plantation sites.
5.10 Cumulative infiltration in high EucaJyptus 73 and high pine in the Cotter catchment.
X
F I G U R E T I T L E P A G E N O
5 . 1 2 K y e t m a u k - t a u n g c a t c h m e n t : r a n d o m p l o t l a y o u t d i a g r a m .
78
5 . 1 3 C u m u l a t i v e i n f i l t r a t i o n in m i x e d d e c i d u o u s , m a n - m a d e p l a n t a t i o n s a n d s e m i - i n d a i n g f o r e s t i n t h e K y e t m a u k - t a u n g c a t c h m e n t in t h e d r y s e a s o n .
82
5. 14 C u m u l a t i v e i n f i l t r a t i o n i n m i x e d d e c i d u o u s , m a n - m a d e p l a n t a t i o n s a n d s e m i - i n d a i n g f o r e s t
i n t h e K y e t m a u k - t a u n g c a t c h m e n t in t h e w e t s e a s o n .
83
6.1 I n f i l t r a t i o n r a t e v e r s u s t i m e - a s t e a d y r a i n f a l l r a t e .
89
6.2 T h e R e p r e s e n t a t i v e B a s i n M o d e l . 91
6. 3 V a r i a b i l i t y o f r a i n f a l l at U r i a r r a B l u e R a n g e -B u l l ' s H e a d .
9 3
6.4 V a r i a b i l i t y o f r a i n f a l l at M e i k t i l a , P o p p a , N y a u n g - o o , M y i n g y a n .
94
6.5 R a i n f a l l I n t e n s i t y - F r e q u e n c y - D u r a t i o n D i a g r a m f o r C a n b e r r a .
95
6.6 R a i n f a l l I n t e n s i t y - D u r a t i o n - F r e q u e n c i e s at B u l l ' s H e a d a n d B l u e R a n g e .
1 0 5
7.1 C o m p a r i s o n o f c a l c u l a t e d i n f i l t r a t i o n w i t h d e r i v e d r a i n f a l l d e p t h s , i n t h e C o t t e r c a t c h m e n t .
111
7.2 C o m p a r i s o n o f m e a s u r e d i n f i l t r a t i o n ( d r y s e a s o n ) w i t h e s t i m a t e d r a i n f a l l in t h e K y e t m a u k - t a u n g c a t c h m e n t .
1 1 3
7.3 C o m p a r i s o n o f m e a s u r e d i n f i l t r a t i o n (wet s e a s o n ) w i t h e s t i m a t e d r a i n f a l l i n t h e K y e t m a u k - t a u n g c a t c h m e n t .
114
7.4 S o r p t i v i t i e s a n d g r a v i m e t r i c m o i s t u r e c o n t e n t s a t P i e r c e ' s C r e e k in t h e C o t t e r c a t c h m e n t .
1 1 6
7.5 S o r p t i v i t i e s a n d g r a v i m e t r i c m o i s t u r e c o n t e n t s i n t h e K y e t m a u k - t a u n g c a t c h m e n t i n t h e d r y s e a s o n .
118
7.6 S o r p t i v i t i e s a n d g r a v i m e t r i c m o i s t u r e c o n t e n t s in t h e K y e t m a u k - t a u n g c a t c h m e n t i n t h e w e t s e a s o n .
c a t c h m e n t i n t h e d r y s e a s o n .
7 . 8 R e l a t i o n s b e t w e e n s o r p t i v i t y a n d g r a v i m e t r i c 1 2 1 m o i s t u r e c o n t e n t s i n t h e K y e t m a u k - t a u n g
c a t c h m e n t i n t h e w e t s e a s o n .
7 . 9 R e l a t i o n s b e t w e e n s o r p t i v i t y a n d g r a v i m e t r i c 1 2 2 m o i s t u r e c o n t e n t s i n t h e K y e t m a u k - t a u n g
c a t c h m e n t r e s u l t s .
xii
LIST OF TABLES
TABLE TITLE PAGE NO.
2.1 Soil characteristics influencing infiltration 11 and their effects.
3.1 Average sorptivities cm.min 2 for different 20 surface conditions of an experimental catchment.
4.1 Average rainfall in lower Cotter catchment at 32 Bull's Head, Blue Range, Uriarra 1966-1974.
4.2 Average rainfalls at stations in central Burma. 38 5.1 Field readings for sorptivity and conductivity. 49
5.2 Calculation of hydraulic conductivity. 51
5.3 Plot notations and characteristics - Pierce's Creek. 54 5.4 Classification of sample plots - Pierce's Creek. 55
5.5 Plot notations in Uriarra area. 56
5.6 Random location of measurement rings. 58
5.7 Test of significance (t test) on sorptivity, 68 Compartment 62, Uriarra Forest.
5.8 Cumulative infiltration for different types of forest in the Cotter catchment area, Australia
Eucalyptus forest 71
pine plantation. 72
5.9 Classification of randomly selected plots, 75 Kyetmauk-taung catchment: dry season measurements.
5.10 Classification of randomly selected plots Kyetmauk- 77 taung catchment: wet season measurements.
5.11 Notations and plot characteristics Kyetmauk-taung 79 catchment (wet and dry season measurements).
5.12 Cumulative infiltration for different types of 80 forests in Kyetmauk-taung catchment area, Burma
(dry season results).
5.13 Cumulative infiltration for different types of 80 forests in Kyetmauk-taung catchment area, Burma
(wet season results).
5.14 Average cumulative infiltration after 1 minute 84 by blocks and forest types - dry season,
T A B L E T I T L E P A G E N O
5 . 1 5 C o m p a r i s o n o f t h e c u m u l a t i v e i n f i l t r a t i o n ( a v e r a g e ) i n t h e d r y a n d w e t s e a s o n s .
86
6.1 P l u v i o g r a p h r a i n f a l l s t a t i o n s i n t h e l o w e r C o t t e r c a t c h m e n t .
100
6.2 R a i n f a l l - d u r a t i o n - f r e q u e n c i e s at B u l l ' s H e a d a n d B l u e R a n g e .
103
6 . 3 R a i n f a l l i n t e n s i t y - d u r a t i o n - f r e q u e n c i e s at B u l l ' s H e a d a n d B l u e R a n g e .
104
6.4 R e c o r d e d r a i n f a l l i n t e n s i t i e s d u r a t i o n s A p r i l -N o v e m b e r 1 9 7 7 K y e t m a u k - t a u n g c a t c h m e n t .
xiv
LIST OF MAPS
MAP TITLE PAGE NO.
4.1 Location of the study area and the Cotter 33 catchment.
4.2 Location of study area in Burma. 37
LIST OF PLATES
PLATE TITLE PAGE NO.
4.1 General views along the study site 27
(Cotter catchment).
4.2 Plan view of the study sites. 28
4.3 Eucalypt forest on granite soil. 29
4.4 Eucalypt forest on shale soil. 29
4.5 Pinus radiata on granite soil. 30
4.6 Revegetated pipeline track. 31
4.7 Revegetation on pipeline track. 31
4.8 Mt Poppa and Kyetmauk-taung reservoir. 36
4.9 Southern slopes of Mt Poppa. 36
5.1 Equipment for field measurement of infiltration. 43 5.2 Driving an infiltration ring Kyetmauk-taung 45
catchment, Burma.
5.3 Reading drop in water level after ponding in 46 the infiltration ring.
5.4 Measurement scale suspended in ring. 46
5.5 Reading the constant head device. 47
xvi
APPENDIX
3.1 3.2 3.3
4.1
5.1
5.2 5.3
5.4 5.5 5.6
5.7 5.8
5.9
5.10
5.11
5.12
5.13
LIST OF APPENDICES TITLE
Summary of infiltration theories.
The infiltration law of Kostiakov (1932). Comparison of methods of infiltration measurement.
Mixed deciduous forests Kyetmauk-taung catchment.
Description of field equipment for measuring infiltration.
Field measurements of infiltration in forests. Results of field measurements of soil moisture content, sorptivity, hydraulic conductivity and infiltration (Pierce’s Creek).
Cumulative infiltration: Pierce’s Creek.
Infiltration rates at 1 minute: Pierce's Creek. Results of field measurements of soil moisture content, sorptivity, hydraulic conductivity and infiltration (Uriarra Forest).
Test of significance on variation.
Results of field measurements for soil moisture content, sorptivity, hydraulic conductivity and infiltration.
Cumulative infiltration and infiltration rates Kyetmauk-taung catchment.
Kyetmauk-taung catchment studies: Analysis of Variance.
Kyetmauk-taung catchment studies. Significant differences between individual treatments. Kyetmauk-taung catchment studies: Analysis of results for sorptivity: Dry season.
Kyetmauk-taung catchment studies: Analysis of results for hydraulic conductivity: dry season.
PAGE NO.
126 128 129
130
131
134 136
140 145 150
152 153
158
174
175
176
A P P E N D I X T I T L E P A G E N O
5 . 1 4 K y e t m a u k - t a u n g c a t c h m e n t s t u d i e s : A n a l y s i s o f r e s u l t s f o r c u m u l a t i v e i n f i l t r a t i o n at
1 m i n u t e : d r y s e a s o n .
1 7 8
5 . 1 5 K y e t m a u k - t a u n g c a t c h m e n t studies.: S t a t i s t i c a l a n a l y s i s o f f i e l d m e a s u r e m e n t s : w e t s e a s o n .
179
6 . 1 R a i n f a l l i n t e n s i t y f r e q u e n c y d u r a t i o n d i a g r a m s -C a n b e r r a .
1 8 0
6 . 2 C a l i f o r n i a n m e t h o d f o r r a i n f a l l a n a l y s i s . 185
6 . 3 P r i n t o u t f r o m B 14 p r o g r a m m e . 1 8 6
6 . 4 R a i n f a l l - d u r a t i o n - f r e q u e n c i e s at B u l l ' s H e a d . 1 8 7
6 . 5 R a i n f a l l - d u r a t i o n - f r e q u e n c i e s at B l u e R a n g e 1 8 8
6 . 6 R a i n f a l l t o t a l - d u r a t i o n - r a i n f a l l i n t e n s i t i e s K y e t m a u k - t a u n g c a t c h m e n t , A p r i l - N o v e m b e r 197 7 .
1 8 9
7 . 1 R e l a t i o n s b e t w e e n s o r p t i v i t y a n d g r a v i m e t r i c : m o i s t u r e c o n t e n t s : C o t t e r c a t c h m e n t .
190
7 . 2 R e l a t i o n s b e t w e e n s o r p t i v i t y a n d g r a v i m e t r i c m o i s t u r e c o n t e n t s : K y e t m a u k - t a u n g c a t c h m e n t .
1
CHAPTER I
THE STUDY IN OUTLINE
1.1 INTRODUCTION
Changes i n f o r e s t c o v e r h av e b een a s s o c i a t e d w i t h ch a n g e s i n b o t h
th e t o t a l and t h e r a t e o f w a t e r y i e l d from c a t c h m e n t s . S e r i o u s e r o s i o n
p ro b lem s h a v e emerged i n some l o c a l i t i e s w ith t h e re m o v a l o f f o r e s t
c o v e r , p a r t i c u l a r l y w here t h e rem oval h a s b een b y b u r n i n g f o r s h i f t i n g
c u l t i v a t i o n p u r p o s e s .
The e f f e c t s o f c h a n g e s i n f o r e s t c o v e r on w a t e r y i e l d s c a n n o t be
p r e d i c t e d w i t h a c c u r a c y b u t much s t u d y i s i n h a n d to w a r d p r o v i d i n g t h e
u n d e r s t a n d i n g and i n f o r m a t i o n which w i l l e n a b l e r e l i a b l e p r e d i c t i o n s
b a s e d on know ledge o f b o t h t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e c a tc h m e n t
and t h e h y d r o l o g i c p r o c e s s e s a s s o c i a t e d w i t h r a i n f a l l - r u n o f f r e l a t i o n s .
In t h i s s t u d y t h e e f f e c t o f ch an g es i n f o r e s t c o v e r on t h e m a g n itu d e
o f i n f i l t r a t i o n r a t e s i s i n v e s t i g a t e d . I n f i l t r a t i o n r a t e s a r e com pared
w ith r a i n f a l l i n t e n s i t i e s t o e n a b l e an a s s e s s m e n t o f t h e s i g n i f i c a n c e
o f t h e c h a n g e s i n f o r e s t c o v e r i n r e l a t i o n t o t h e f r e q u e n c y o f o c c u r r e n c e
o f p o n d in g and o v e r l a n d f lo w .
1.2 INFILTRATION IN FORESTED CATCHMENTS
W isso p ak an (1977) fo u n d few r e p o r t e d s t u d i e s o f i n f i l t r a t i o n w i t h
s p e c i f i c r e f e r e n c e t o f o r e s t e d c a tc h m e n ts b u t t h e r e i s a v o lu m in o u s
l i t e r a t u r e on i n f i l t r a t i o n . A r e v ie w o f f o r e s t management p r a c t i c e s and
1.3 INFILTRATION THEORY
There has been continued development of the infiltration theory since the presentation of a general theory of water movement in porous media by Buckingham (1907). A review of infiltration theory is presented
in Chapter III together with the theoretical basis of the methods used in this study to determine estimates of infiltration rates based on field measurements.
1.4 THE STUDY SITES AND OBJECTIVES
The field measurements for this study were taken in two very differ ent situations where the changes in forest cover had been quite drastic.
In the Cotter river catchment of the Australian Capital Territory the effect on infiltration rates of a change from Eucalyptus to Pinus
radlata plantations was investigated and infiltration rates under stands
of both species compared with rainfall intensities to enable assessment of the magnitude and frequency of occurrence of rainfall excess.
In the Kyetmauk-taung catchment in central Burma infiltration rates under three different types of forest cover were investigated, viz. mixed deciduous forest, plantations (old banana plantations) and semi-indaing
forest. Rainfall intensities for a limited period were also measured to enable preliminary comparisons of infiltration rates with rainfall rates.
3
CHAPTER II
FOREST MANAGEMENT PRACTICES AND INFILTRATION IN
FORESTED CATCHMENTS
2.1 INTRODUCTION
Many workers have suggested that a suitable ground cover is of prime importance for the protection of catchments from soil erosion. Anderson and Trobitz (1960) suggested the covering mantle should preferably be woodland because it retards runoff and, in producing a steadier flow of streams, lessens the risk of damage by excessive flood* Rothacher et a l .
(1967), Rich et al. (1961) and Johnstone and Doty (1972) have suggested
that high infiltration rates on forested catchments contributed significantly to reducing flood peaks. Changes in forest cover may therefore
significantly change flood peaks and the associated rate of erosion in catchments.
Thus when planning controls on forested catchments, particularly
reservoir catchments, it is necessary to examine problems relating infiltrat ion, runoff and erosion and knowledge of infiltration rates and capacity is essential in taking steps to rehabilitate a deteriorating catchment.
2.2 INFILTRATION AND THE FOREST COVER
2.2.1 Introduction
Infiltration, or the downward entry of water into soil, is one of the more important processes in the soil phase of the hydrologic cycle. Horton
absorbed by the soil.
Infiltration is affected by vegetation and the greater the amount of vegetation cover the higher the infiltration (Musgrave, 1935; Rowe, 1955; Arend, 1942; Johnson, 1940; Packer, 1951).
The vegetation cover operates in several ways. By shielding the surface from impact it maintains the important crumb or aggregate struct ure which is essential to higher infiltration rates. The roots open channels in the soil for transmission of water. The mulch formed by
annual shedding of foliage is an important ingredient in the soil building process. Finally soil moisture is extracted by vegetation through the process of transpiration and creates a moisture deficiency or storage potential which assists infiltration when moisture is available.
Infiltration is thus of importance both for crop management in agriculture and forestry and for many purposes of catchment management. The soil and vegetation cover should be manipulated to control the entry
of water into the soil.
2.2.2 Factors affecting infiltration
Lewis and Powers (1938) listed a large number of factors affecting infiltration and divided them into two major groups,
(1) those factors influencing the infiltration rate at a given time and point such as texture, structure and organic matter, (2) those factors influencing the average infiltration rate over a
considerable area and period of time such as slope, vegetation and surface roughness.
Horton (1940) suggested the following generalized factors affecting infiltration rate,
5
b i o l o g i c and m i c r o s t r u c t u r e w i t h i n t h e s o i l
. v e g e t a l c o v e r .
While a t t h a t tim e some w o rk e rs were o f t h e o p i n i o n t h a t i n f i l t r a t i o n
r a t e was g o v e r n e d s o l e l y by t h e s o i l mass and t h e r e f o r e l a r g e l y
in d e p e n d e n t o f s u r f a c e c o n d i t i o n s o r m i c r o s t r u c t u r e a t o r c l o s e t o t h e
s o i l s u r f a c e , H o rto n c o n s i d e r e d t h a t i n f i l t r a t i o n r a t e was g o v e r n e d
m a in ly by c o n d i t i o n s a t o r n e a r t h e s o i l s u r f a c e .
W i s l e r and B r a t e r (1949) o u t l i n e d a number o f f a c t o r s a f f e c t i n g
and d e t e r m i n i n g t h e i n f i l t r a t i o n c a p a c i t y . T h ese f a c t o r s i n c l u d e ,
(1) t h e m o i s t u r e c o n t e n t o f t h e s o i l ,
(2) t h e s h r i n k i n g o r s w e l l i n g o f t h e c o l l o i d a l m a t e r i a l i n
t h e s o i l ,
(3) t h e e f f e c t p r o d u c e d by r a i n upon t h e s o i l s u r f a c e ,
(4) ch a n g e s i n m a c r o s t r u c t u r e s r e s u l t i n g from an im a l b o r i n g s ,
t h e d ec a y o f v e g e t a l r o o t s , s u n - c h e c k i n g , and t h e d i s s o l u t i o n
o f m i n e r a l s ,
(5) c o n d i t i o n o f t h e v e g e t a l c o v e r ,
(6) c u l t i v a t i o n ,
(7) c o m p re s s io n o f e n t r a p p e d a i r ,
(8) t e m p e r a t u r e c h a n g e s .
They s u g g e s t e d t h a t ' i n f i l t r a t i o n c a p a c i t y ' was a f f e c t e d by a l l t h e s e
f a c t o r s a s a r e s u l t o f t h e ch an g es p r o d u c e d i n t h e e f f e c t i v e s i z e s o f
t h e o p e n in g s t h r o u g h w hich w a t e r e n t e r s t h e s u r f a c e s t r a t a o f t h e s o i l .
The q u a l i t a t i v e d e s c r i p t i o n o f f a c t o r s by t h e a b o v e m e n tio n e d a u t h o r s
makes i t c l e a r t h a t t h e i n f i l t r a t i o n p r o c e s s i s complex i n t h e o r e t i c a l
te rm s . The t h e o r y i s d i s c u s s e d i n C h a p t e r I I I .
sections in relation to the nature of the vegetation cover and soil physical factors. These two generalized factors are subject to manipul
ation in catchment management practices.
2.2.3 Infiltration studies in forests
Linsley, Kohler and Paulus (1949) suggested that the effect of vegetation on infiltration capacity was difficult to determine directly for vegetation intercepts rainfall and thus changes the rate at which water arrives at the ground and also the disposition of the water.
However, effects of vegetation cover on infiltration have been investigated by many workers.
The importance of vegetation in promoting infiltration was shown by Michelson and Muckel (1937) in experiments on the spreading of water. The cover slowed down the movement of water over the ground surface thus giving additional time for water to enter the soil surface. The effect of ground cover vegetation on infiltration by retardation of surface flow was also studied by Arend (1942), Johnson (1940) and Rowe (1955). In general they all found that the greater the amount of vegetal cover the higher the infiltration.
While the reported stiudi.es of infiltration in forests are considerably less than those on agricultural lands there are a number of discussions and experimental investigations in connection with the effects of forests and forest management practices. As with other vegetation types forests can increase infiltration rate by (FAO, 1963),
(a) Mechanical protection of the soil from raindrop splash whereby aggregate structure is maintained and clogging of pores is prevented.
7
(c) By t h e r o o t a c t i v i t y which a c t s t o i n c r e a s e t h e
p e r m e a b i l i t y o f s o i l s .
Musgrave (1935) compared t h e r a i n f a l l , s u r f a c e r u n o f f and i n f i l t r a t
i o n on f o r e s t and abando ne d l a n d . The r e s u l t s a r e shown i n F i g u r e 2 . 1 .
F o r e s t e d l a n d B o r e a b a n d o n e d l a n d Rai nf al l 3. 9 c m / h r
Runo f f Ra i n f a l l 3 . 6 c m/ h r
R u n o f f
M i n u t e s f r o m S t a r t
F i g u r e 2 . 1 R a i n f a l l , s u r f a c e r u n o f f and i n f i l t r a t i o n on f o r e s t e d l a n d and on b a r e a bandoned l a n d .
( A f t e r Mu sgr av e, 1935)
Rowe (1941) f ound t h a t a f o r e s t s o i l c o v e r e d by P o n d e r o s a p i n e and
Douglas f i r i n an undamaged c o n d i t i o n had an i n f i l t r a t i o n r a t e o v e r
t h i r t e e n t i m e s t h a t o f denud ed a r e a s .
Woodward (1943) foun d h i g h e r i n f i l t r a t i o n c a p a c i t i e s f o r b r u s h
t h a n f o r h e r b a c e o u s t y p e s .
K i t t r e d g e (1948) i n v e s t i g a t e d i n f i l t r a t i o n i n p l a n t a t i o n s and f ou n d
g r e a t e r i n f i l t r a t i o n i n d e n s e t h a n i n open p l a n t a t i o n s , g r e a t e r i n o l d
t h a n i n young p l a n t a t i o n s and g r e a t e r i n u n t h i n n e d and l i g h t l y t h i n n e d
s t a n d s t h a n i n h e a v i l y t h i n n e d s t a n d s .
R aede r R o i t z s c h (1968) r e p o r t e d t h a t i n f i l t r a t i o n was a l s o i n f l u e n c e d
follows for different covers,
Oak forest 100%
Afforestation (pine on old field) 95% Oak forest after burning 25%
Open range land 22%
Many workers, Kittredge .(1948), Jenny (1941), Volobuev (1963), Glinker (1965), Russell (1973), Molchanov (1963), Gilmour (1965),
Hamilton (1964) and Thistlethwaite (1970) found that infiltration differs with species and age and condition of the forest stand. Penman (1963) reported many instances of the greater infiltration rates of forests relative to pastures or clean cultivated crops.
The root activity associated with trees is a significant factor in developing high infiltration rates under forests. Different tree species have different root development characteristics and produce different penetration effects into the soil although the pattern of root growth of a species also varies with the soil condition. Some roots are so
strong that they can penetrate even quite compact subsoils (Russell, 1958). However, Vasil'ev (1954) indicated that under forest vegetation only the
surface 'A' horizon is loosened whereas the 'B' horizon is compacted. Kittredge (1948) reached the following general conclusions,
(a) the infiltration rate under forest is higher than in bare soil,
9
(c) The infiltration rates in afforestation stands may be nearly as good as in the natural stands.
(d) Infiltration in dense stands is greater than in open woodland, it is often greater in old forest than in young stands, it is always better in ungrazed forest as compared with grazed forest; it is greater in unthinned or lightly thinned stands than in heavily thinned stands and always lowest in heavily grazed areas and in open fields.
(e) The difference in infiltration capacity between forest and disturbed soil can be of the order of 50:1 at the soil surface and while this is reduced in the subsoil a ratio of 2:1 may obtain at 20 cm of depth.
(f) On highly permeable soils, for example sand, the influence of vegetation on infiltration can be negligible.
2.2.4 Infiltration and soil properties
Soil formation is a function of climate, the parent material and the weathering actions. Coarse-grained soils with interaction of vegetation generally produce well structured porous soils with high infiltration potential whereas soils with fine grains usually have a less porous structure resulting in low infiltration rates.
Duley (1939) e v a l u a t e d t h e s u r f a c e f a c t o r s a f f e c t i n g t h e r a t e o f
i n t a k e o f w a t e r by s o i l s . He o b s e r v e d t h e r a p i d r e d u c t i o n i n t h e r a t e
o f i n t a k e by c u l t i v a t e d s o i l s as r a i n f e l l on t h e i r s u r f a c e s and t h a t
t h i s was a s s o c i a t e d w i t h t h e f o r m a t i o n o f a t h i n compact l a y e r a t t h e
s u r f a c e th r o u g h which w a t e r p a s s e d o n ly s l o w l y . The t h i n compact
l a y e r was t h e r e s u l t o f s e v e r e s t r u c t u r a l d i s t u r b a n c e , due i n p a r t t o
t h e b e a t i n g e f f e c t o f t h e r a i n d r o p , and i n p a r t , t o an a s s o r t i n g a c t i o n
as w a t e r flo w e d o v e r t h e s u r f a c e and t h e f i n e p a r t i c l e s w ere f i t t e d
around t h e l a r g e r ones t o form a r e l a t i v e l y n o n - p e r v i o u s s e a l . I t was
q u i t e e v i d e n t t h a t t h e compact l a y e r fo rm ed a t t h e s u r f a c e o f c u l t i v a t e d
s o i l s d u r i n g r a i n s had a g r e a t e r e f f e c t on i n t a k e o f w a t e r t h a n d i d s o i l
t y p e , s l o p e , m o i s t u r e c o n t e n t o r p r o f i l e c h a r a c t e r i s t i c s .
S o i l c h a r a c t e r i s t i c s a f f e c t i n f i l t r a t i o n i n i n t e r r e l a t e d and complex
ways. W isso p ak an (1977) sum m arized s t u d i e s on t h e e f f e c t s as shown i n
T a b le 2.1.
The t h e o r e t i c a l a s p e c t s o f i n f i l t r a t i o n w i l l be t a k e n up i n C h a p te r
I I I and r e l a t e d w i t h t h a t t h e e f f e c t s o f s o i l m o i s t u r e c o n t e n t on i n f i l
t r a t i o n and t h e s i g n i f i c a n c e o f t h e p r o p e r t y s o r p t i v i t y . However i n
d i s c u s s i n g t h e s o i l c h a r a c t e r i s t i c s i n f l u e n c i n g i n f i l t r a t i o n i t s h o u ld
be n o t e d t h a t s o r p t i v i t y , p ro p o s e d by P h i l i p (1957) a s a . p h y s i c a l
p r o p e r t y o f p o r o u s m e d ia , i n t e g r a t e s t h e d i v e r s e p h y s i c a l p r o p e r t i e s o f
s o i l and overcomes much o f t h e c o m p l e x i t y a s s o c i a t e d w i t h s e p a r a t e l y
t a k i n g i n t o a c c o u n t i n t e r r e l a t e d f a c t o r s su ch a s p a r e n t m a t e r i a l , s o i l
p r o f i l e , s o i l s u r f a c e , s o i l b u lk d e n s i t y , p o r o s i t y , t e x t u r e and
T a b le 2 .1 S o il c h a r a c t e r is t ic s in f lu e n c in g in f il t r a t io n a n d t h e ir e f f e c t s 11
t--N
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O a3 y-H •H 0 to p p p X o
0 0 4-t X o3 0 o o to aS 40
to P > O X 0 40 Py y-H P
to O 0 P P P X •H 0 X •H
0 P i—C P p a a3 CJ o O X X to
P *H 0 0 r-H 0 X to X •H aS
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40 X X X aS to P 03 X p P
P r - l X X 0 X - H 0 0 03 o X CJ
o +-> P X X P 40 P
P p X p 40 •H 0 p aS aS to •H •rH
to 0 X •H to X aS r H to 0 p X
X 0 p 0 P aS 0 P o X
O x cr w> X 0 P X p P P -H 0 X
CD x 0 0 p X 03 rH •H X X p •H
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o P X IM P X o X o X X y-H 0 0
to X 03 • H X aS P p 0 aS tO * rH X to
03 03 p r H P P P 0 P 0 P aS aS
P P o o p X X • H P 0 P P 0
0 TO o r H X •HY—C to p TS 40 X -H r>
Ph cd 1 • H *rH o aS P CJ
0 GO to p < to P o to to 40 to o p
X) p • H p 0 P X •rH •rH 0 0 •H •rH
• H X 0 to • H CJ p P to X
P T3 p X X aJ •H 0 P p 40 aS aS p
o p o 0 X 0 P X X o o 40 0 P o
•H O • H X p P • H ^ aS • H • H ctS p X •H
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a o X
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o3 a3 to p 0 to aS X aS TS p • H aS
P to P • H 0 • H to X r H P X P 0 -rH P P
X 0 X 0 aS p cd X •H X > P X
r —C O r —C 0 03 i-H 0 0 •H f-H to r H o x •rH r H
•H P •H P f—C P X •H o •H P CJ •H
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p p P X
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2.2.5 Infiltration and land management practices
Pillsbury and Richards (1954) studied the effects on infiltration rates of different amounts of ammonium sulphate and organic matter added to the soil. They found that moderate application of ammonium sulphate resulted in significantly higher infiltration rates than did urea when combined with large amounts of organic matter. They also found that infiltration rates increased progressively as the amount of surface matter increased.
Johnson (1958) tested the effectiveness of chlorination of the applied water and found that when chlorination was stopped numbers of microorganisms increased rapidly and the infiltration rate dropped sharply. Each time chlorination was resumed the number of micro organisms declined and the infiltration rate increased.
Chlorine appeared to have no lasting effect on the soil other than the reduction of soil organic matter content and coincident destruction of soil structure.
McCalla (1945) investigated the influence of products of microbial activity on soil structure and infiltration rate. He found that
microbial decomposition of products of plant residues increased the in filtration rate of these soils. Johnson (1958) also evaluated the influence of the decomposition of organic residues on infiltration rate. He noted that infiltration rates were increased owing to an improvement in soil structure.
13
i n t u m a l t e r e d t h e i n f i l t r a t i o n r a t e and m o i s t u r e s t a t u s o f t h e s o i l .
C u l t i v a t i o n a l o n g w i t h a n n u a l c o v e r c r o p s r e d u c e d t h e p e r m e a b i l i t y as
a more compact s u r f a c e l a y e r r e s u l t e d , g i v i n g r i s e t o p o o r e r i n f i l t r a t i o n
c h a r a c t e r i s t i c s .
Musgrave (1935) compared t h e r a i n f a l l , s u r f a c e r u n o f f and i n f i l
t r a t i o n on l a n d s u b j e c t t o p o o r and good g r a z i n g management. The
r e s u l t s a r e shown i n F i g u r e 2 . 2 .
G o o d g r a z i n g m a n a g e m e n t Eui nfc. l I 5 .0 c m/ h r
Infiltration
Po o r g r a z i n g m a n a g e m e n t
R a i n f a l l 3.3 e m / h r
Infi l tra t i o n
M i n u t e s f r o m S t a r t
F i g u r e 2 . 2 R a i n f a l l , s u r f a c e r u n o f f and i n f i l t r a t i o n on l a n d s s u b j e c t e d t o good and p o o r g r a z i n g management. ( A f t e r Musgrave 1 9 3 5 ) .
2 . 2 . 6 Summary
F o r e s t management p r a c t i c e s can a f f e c t i n f i l t r a t i o n r a t e s by c a u s i n g
b o t h chan ge s i n t h e v e g e t a t i v e c o v e r and i n t h e s o i l p r o p e r t i e s . The
e f f e c t s can be d r a m a t i c and even d e v a s t a t i n g i n t e r m s o f e r o s i o n i f t h e
management p r a c t i c e a l l o w s t o t a l d e s t r u c t i o n o f t h e f o r e s t .
In g e n e r a l , t h e s t u d i e s o f i n f i l t r a t i o n i n f o r e s t s h av e b e e n
e x p e r i m e n t a l i n n a t u r e and w h i l e p r o v i d i n g some b a s i s f o r q u a l i t a t i v e
s u g g e s t o r a s s i s t w i t h t h e q u a n t i t a t i v e p r e d i c t i o n o f e f f e c t s by i n c o r
p o r a t i o n i n , f o r example h y d r o l o g i c models, f o r t h e s t u d i e s r e p o r t e d
d e s c r i b e r a t h e r t h a n model t h e i n f i l t r a t i o n p r o c e s s i n f o r e s t e d
15
CHAPTER I I I
THE THEORY OF INFILTRATION AND THE DEVELOPMENT OF FIELD
MEASUREMENT TECHNIQUES
3 .1 INTRODUCTION
Many w o r k e r s h av e i n v e s t i g a t e d t h e t h e o r y o f i n f i l t r a t i o n . Buck
ingham (1907) p r o p o s e d a g e n e r a l t h e o r y o f w a t e r movement i n p o r o u s
media w h ile Green and Ampt (1911) and G a rd n e r and W id sto e (1921) made
e a r l y a t t e m p t s t o d e r i v e t h e o r e t i c a l l y t h e r a t e o r c u m u l a t i v e i n f i l t r a t
io n as a f u n c t i o n o f t i m e . S i n c e t h e s e e a r l y w o rk e rs t h e i n f i l t r a t i o n
p r o c e s s h a s b een i n v e s t i g a t e d i n a v e r y p r o g r e s s i v e way from q u a l i t a t i v e
o b s e r v a t i o n s t o , i n more r e c e n t y e a r s , t h e f o r m u l a t i o n o f a w i d e l y
a c c e p te d and a p p l i e d t h e o r y .
3 .2 INFILTRATION THEORY
H i l l e l (1971) d e s c r i b e s i n f i l t r a t i o n , and F i g u r e s 3 . 1 , 3 . 2 and 3 . 3
r e s p e c t i v e l y i l l u s t r a t e an i n f i l t r a t i o n m o i s t u r e p r o f i l e , i n f i l t r a t i o n
as a f u n c t i o n o f ti m e i n a u n i f o r m s o i l i n a more p o r o u s l a y e r and i n a
s o i l c o v e r e d by a s u r f a c e c r u s t , and i n f i l t r a t i o n as a f u n c t i o n o f ti m e
i n an i n i t i a l l y d r y and i n an i n i t i a l l y m o i s t s o i l . F i g u r e 3 . 3 show
in g t h e i n f i l t r a t i o n r a t e s as d e p e n d e n t on m o i s t u r e c o n t e n t p o i n t s t o
some o f t h e d i f f i c u l t i e s i n f o r m u l a t i n g a m a th e m a tic a l model o f t h e p r o
c e s s f o r as can be s e e n i t n o t o n l y d ep en d s on s o i l p h y s i c a l p r o p e r t i e s
b u t a l s o on m o i s t u r e c o n t e n t . The m o i s t u r e c o n t e n t a t a p o i n t c h a n g e s
T r o n t m u n o n zone
W e t t i n g z o n e
W e t t i n g f r o n t
F i g u r e 3 . 1 -The i n f i l t r a t i o n m o i s t u r e p r o f i l e . At l e f t , a s c h e m a t i c s e c t i o n o f t h e p r o f i l e ; a t r i g h t , t h e w a t e r - c o n t e n t v s . d e p t h c u r v e .
I n i t i a l l y d r y soi I
I n i t i a l l y mo i s t soi I
T i m e
Fi-gure 3 . 2 I n f i l t r a t i o n as a f u n c t i o n o f t i m e .
T i m e
F i g u r e 3 . 3 I n f i l t r a t i o n as a f u n c t i o n o f t i m e : (a) i n a u n i f o r m s o i l (b) i n a s o i l w i t h a more p o r o u s u p p e r l a y e r ; and (c) i n s o i l c o v e r e d by a s u r f a c e c r u s t .
17
determining infiltration are in a state of flux.
Wissopakan (1977) as shown in Appendix 3.1 summarized the development of infiltration theory citing Green and Ampt (1911), Horton (1933),
Philip (1954), Parlange (1971) and Farrel and Larson (1972) as making significant contributions.
The work of Kostiakov (1932), Childs (1936) and Childs and Collis- George (1950) should also be noted. Kostiakov suggested the infiltration equation, discussed in Appendix 3.2, which has been the most widely used. Childs explicitly proposed the concept of soil water movement as a
diffusion phenomenon. Childs and Collis-George (1950) and others, confirmed that Darcy's law may hold for the flow of liquid water in un saturated media. The infiltration theory of Philip (1954) was adopted to simulate the infiltration process in the mathematical model of the whole catchment process, formulated as the main analytical tool of a programme initiated by the Australian Water Resources Council, to provide
a better basis for estimating the runoff from ungauged catchments and
at the same time a better understanding of the hydrologic cycle (Australian Water Resources Council, 1969, 1974).
Philip (1957a) presented a full analysis of infiltration based on the solution of the relevant concentration dependent diffusion equation for appropriate boundary conditions. He showed that when water is applied to a uniform soil at uniform initial water content the total infiltration can be expressed by a rapidly converging power series. He formulated the equation for total infiltration as
I = S t 4 + A(th}2 + BCt^)3 + . (2.1)
where I = the cumulative infiltration flux t = the time
S = the sorptivity
The s o r p t i v i t y i s o f s p e c i a l s i g n i f i c a n c e , p a r t i c u l a r l y d u r i n g t h e e a r l y
s t a g e s o f i n f i l t r a t i o n . I t i s a te r m a f t e r P h i l i p (1954) and i s a
p r o p e r t y o f t h e medium w i t h some r e s e m b la n c e t o p e r m e a b i l i t y . I t was /
d e f i n e d i n a l e s s g e n e r a l way by S w a r tz e n d r u b e r e t a l . ( 1 9 5 4 ) .
As s t a t e d p r e v i o u s l y t h e d e f i n i t i o n o f t h e t e r m s o r p t i v i t y as t h e
i n t e g r a t i o n i n t o t h e one p a r a m e t e r o f t h e d i v e r s e p h y s i c a l p r o p e r t i e s o f
s o i l h a s overcome much o f t h e c o m p le x it y a s s o c i a t e d w i t h s e p a r a t e l y
t a k i n g i n t o a c c o u n t i n t e r r e l a t e d f a c t o r s su ch as p a r e n t m a t e r i a l , s o i l
p r o f i l e , s o i l s u r f a c e , s o i l b u l k d e n s i t y , p o r o s i t y , t e x t u r e and s t r u c t u r e .
P h i l i p (1954, 1 9 5 7 a , b , c , d , e , 1969) h a s r e p o r t e d f u n d a m e n ta l and
s y s t e m a t i c s t u d i e s o f i n f i l t r a t i o n t h e o r y and d e v e lo p e d a n u m e r i c a l
method f o r s o l v i n g t h e i n f i l t r a t i o n e q u a t i o n f o r s h o r t and i n t e r m e d i a t e
ti m e s . F o r l o n g e r t i m e s he c o n c l u d e d , as h av e o t h e r w o r k e r s , t h a t t h e
i n f i l t r a t i o n r a t e a p p r o a c h e s a q u a s i - s t e a d y s t a t e w i t h u n i f o r m m o i s t u r e
d i s t r i b u t i o n t h r o u g h a c o n s i d e r a b l e p o r t i o n o f t h e p r o f i l e . He a l s o showed
from E q u a t io n ( 2 .1 ) t h a t , when w a t e r i s a p p l i e d t o a u n i f o r m s o i l a t
u n ifo rm i n i t i a l w a t e r c o n t e n t , i t would be s u f f i c i e n t t o r e t a i n o n l y t h e
f i r s t two te r m s o f t h e s e r i e s . Thus an a d e q u a te r e p r e s e n t a t i o n o f
v e r t i c a l i n f i l t r a t i o n i s
I = S t ^ + At ( 2 . 2 )
The f i r s t t e r m o f t h e r i g h t h a n d s i d e o f t h e e q u a t i o n d e s c r i b e s t h e c o n t r i b
u t i o n t o i n f i l t r a t i o n a r i s i n g from c a p i l l a r y a c t i o n and t h e s e c o n d te r m
r e p r e s e n t s t h e c o n t r i b u t i o n a r i s i n g from g r a v i t y .
I t i s E q u a t io n ( 2 . 2 ) w hich was a d o p te d t o s i m u l a t e t h e i n f i l t r a t i o n
p r o c e s s i n t h e m a t h e m a t i c a l model o f t h e A u s t r a l i a n R e p r e s e n t a t i v e B a s in
Programme ( A u s t r a l i a n W ater R e s o u r c e s C o u n c il 1969, 1 9 7 4 ).
19
Equation (2.2), that is
i = %St + A (2.3)
where i = infiltration rate cm/sec.
3.3 THE APPLICATION OF THE PHILIP EQUATION TO THE FIELD MEASUREMENT OF INFILTRATION
Wissopakan (1977) studied aspects of infiltration in the Orroral Representative Basin and the formulation of field methods for measuring infiltration were undertaken with him. Wissopakan (op. cit.) reviewed methods for measurement of infiltration.
The Philip equation has been found useful in field studies of infiltration (Philip, 1957d; Talsma, 1969, 1974; Talsma and Parlange, 1972; Bear et al., 1968; Dunin and Costin, 1970). Philip (1957d) using laboratory data of Moore (1939) found that whereas the simplified
infiltration Equation (2.2) gave almost exact correspondence with the Horton equation (Appendix 3.1) while giving an indication of the reliab
ility at small time (t) failed badly at large t. The Kostiakov equation (Appendix 3.2) gave a moderately reliable estimate at small t. The comparisons made by Philip are summarized in Appendix 3.3.
Ta b l e 3 . 1 A ve ra ge s o r p t i v i t i e s (cm. mi n. c o n d i t i o n s o f an e x p e r i m e n t a l
( A f t e r T a l s m a , 1974)
) f o r d i f f e r e n t c a t c h m e n t
s u r f a c e
S u r f a c e c o n d i t i o n Bar e G r a s s l a n d T us s o c k s
Mo is t ( w i n t e r , 1969) 0 . 2 9 0 . 5 8 1. 0 8
Dry (summer, 1969) 0 . 6 1 0 . 8 3 1. 4 3
Tal sma (1969) h ad s t u d i e d t h e r e l a t i o n s h i p b e t w e e n s o r p t i v i t y and
h y d r a u l i c c o n d u c t i v i t y and f ound t h a t s o r p t i v i t y i n c r e a s e d as h y d r a u l i c
c o n d u c t i v i t y i n c r e a s e d as shown i n F i g u r e 3 . 4 .
1.0
0.5.
ol ___
-o 0.1 0.2 0.3
Kq c m / m i n
. Fi gu re 3 . 4 R e l a t i o n s h i p bet ween s o r p t i v i t y and c o n d u c t i v i t y ( a f t e r T a l s m a , 1 97 4) .
T al s ma (1969) m e a s u r e d i n f i l t r a t i o n u s i n g i n f i l t r o m e t e r r i n g s 30 cm
i n d i a m e t e r , 15 cm h i g h and p u s h e d 10 cm i n t o t h e s o i l . The m e a s u r e
ments a r e t h e r e f o r e e s s e n t i a l l y o f i n f i l t r a t i o n a t t h e s u r f a c e l a y e r o f
s o i l . T a l s m a and P e r r o u x ( p e r s . comm.) p r o v i d e d a d v i c e , p r a c t i c a l
e x p e r i e n c e and i n i t i a l l y l o a n e d e q u i p m e n t on t h e ' i n s i t u m e t h o d ' f o r
m e a s u r i n g i n f i l t r a t i o n and t h i s method was a d o p t e d f o r t h i s s t u d y .
y
T a l s m a f o u n d t h a t g r a p h s o f t o t a l i n f i l t r a t i o n a g a i n s t t 2 r e m a i n e d
21
r e p r e s e n t e d t h e f i r s t te r m i n t h e P h i l i p e q u a t i o n , E q u a t i o n ( 2 . 2 ) , a c c o u n t i n g
f o r n e a r l y a l l t h e flo w f o r s h o r t ti m e i n t e r v a l s .
I m e
F ig u r e 3 . 5 Graph shows l i n e a r p o r t i o n o f t o t a l i n f i l t r a t i o n a g a i n s t / t i m e . ( A f t e r Talsma)
On t h i s b a s i s t h e s l o p e o f t h e l i n e , F i g u r e 3 . 5 , w o u ld g i v e t h e
s o r p t i v i t y .
Talsm a n o t e d t h a t s o r p t i v i t y i s a f u n c t i o n o f a d d i t i o n a l w a t e r
s t o r a g e , t h e h y d r a u l i c c o n d u c t i v i t y and t h e g r a d i e n t o f c a p i l l a r y
p o t e n t i a l a c r o s s t h e w et f r o n t as i n f i l t r a t i o n t a k e s p l a c e . He showed
t h a t t h e P h i l i p e q u a t i o n ( E q u a t io n 2 . 2 ) c o u l d , b a s e d on h i s e m p i r i c a l
r e s u l t s , be r e d u c e d t o
I = St*5 + — 2 . 8
where I = t o t a l i n f i l t r a t i o n cm.
S = s o r p t i v i t y
k = h y d r a u l i c c o n d u c t i v i t y .
I t s h o u l d be n o t e d t h a t E q u a t i o n ( 2 . 4 ) i s p e r h a p s d e c e p t i v e l y s i m p l e
and becomes a ' s p e c i f i c v a l u e f o r t h e tim e a t w hich t h e f i e l d m easu re m en ts
a r e made.
The f i e l d p r o c e d u r e s u s e d t o m e asu re i n f i l t r a t i o n i n t h i s s t u d y
a l s o f o l l o w e d t h o s e d e v e l o p e d by T a ls m a P e r r o u x ( p e r s . comm.) a d v i s e d
t h a t t h e y a r e e a s i e r and f a s t e r t h a n most m e th o d s . The p r o c e d u r e s a r e
