Hydrologic Effects of Changes in
Forest Structure and Species
Composition
Travis Idol
Dept. Nat. Res. & Env. Manage.
CTAHR, Univ. Hawaii-Manoa
Effects of Forests on Hydrologic Functioning
Forest Floor
Canopy
1.
2.
3.
1.
2.
3.
4.
5.
intercepts precipitation,
especially during low rainfall
events
changes drop size and reduces
velocity
“throughfall”
throughfall” alters rainfall
chemistry
ET generally maximum in forest
canopy
Leaf structure and water use
greatly influence canopy effects
depth and surface roughness
greatly increase infiltration of
precipitation
highly variable ability to hold
water
in riparian zones, litter slows
overland flow, traps sediments,
and sequesters nutrients
Roots
1.
2.
3.
roots stabilize soil aggregates
and stream banks
increase macropore space and
preferential flow/infiltration
take up water and nutrients from
deep in the soil profile
Example of Forest Floor Effects on Hydrologic Function
• Removing the forest floor can reduce infiltration rates not only for the
litter layer but also for underlying mineral soil layers
• Also note that the native IR for the litter layer is much higher than for
the mineral soil
• In large storm events or rainy seasons, total infiltration and storage
capacity are ultimately limited by these lower soil horizons
Hamilton and King (1983)
Soil
Horizon
H
A1
A2
B
Infiltration Rate (mm/min)
Litter Intact
Litter Removed
120
60
14
5
2
0
4
3
Forest Harvesting Effects
• Reduction in leaf area and crown closure reduce canopy
effects
• Reduction in root mass and live biomass reduce water
and nutrient uptake
• Addition of litter to forest floor leads to pulse of OM and
nutrient “inputs”
• Soil disturbances: compaction, removal of forest floor,
creation of skid trails are much more harmful than
removal of trees in degrading hydrologic functioning
• Road-building can potentially greatly increase erosion
and sedimentation
• Rapid forest regrowth typically mitigates effects in a few
years (independent of disturbance effects)
Forest Harvesting Effects: Case Studies
Harvest/Deforestn
80-27% forest cover
IF
(mm/h)
OF
(mm)
SF
(mm/yr)
Sed
(kg/ha)
N
(kg/ha)
N
+
N
N
N
N
56
560
Waterloo et al. 1997
1x
10 x
2x
Zulkifli and Suki (1994)
Trop Forest
Selective Harvest
Trop Forest Reserve
Commcl Harvest
Consvtn Harvest
Trop Rain Forest
Comml Harvest
Trop Rain Forest
Harv For (12 yrs)
Tractor tracks
88
73
15
N
+
1x
4x
18 x
3.6 x
Trop Second. Forest
7% convn to ag
+400
+1100
Malmer and Grip (1994)
Douglas et al. (1992)
3%
7%
Cumul
Increase
in Runoff
Wilk et al. 2001
Plas and Bruijnzeel (1993)
Trop Forest
Road disturb
Harv+Rd (5 mo)
Harv+Rd (1 yr)
Tropical Forest
During Harv (4 mo)
After Burn (5 mo)
Plant. Est. (2 Yrs)
↑10- 100
x
↑400-1200
Reference
Period.
Sedim.
and
N-tot
Lal (1997)
11
50
57
7.5
1.5
10
19
Malmer (1996)
IF = infiltration rate; OF = overland flow; SF = stream flow; Sed =
Sedimentation; N = nitrogent loss
Harvesting Effects cont.
As would be expected, changes in hydrologic function are typically a
function of the proportion of forest cover or area removed
Annual stream flow increase after harvesting (mm)
% Cover
Reduction
20
40
60
80
100
Forest Type
Conifer
Deciduous
80
80
150
120
220
150
300
200
390
250
Hamilton and King (1983)
Scrub
40
50
70
90
100
Forest Harvesting Effects: Case Studies
• Rapid vegetation regrowth in harvested forests can quickly mitigate impacts
on forest and watershed hydrology
• However, those first few years can lead to large differences when
considered cumulatively over time
Harvest/Deforestn
Temp clearcut Yr 0
turbidity (ppm) Yr 1
Yr 2
Temp sel. cut Yr 0
Yr 1
Yr 2
Temp Forest
Clearcut Yr 1
Clearcut Yr 2
IF
(mm/h)
OF
(mm)
SF
(mm/yr)
Sed
(kg/ha)
N
(kg/ha)
490
38
1
897
6
0
Reference
Hornbeck and Reinhart (1964)
0.51
1.75
8.25
Hamilton and King (1983)
IF = infiltration rate; OF = overland flow; SF = stream flow; Sed =
Sedimentation; N = nitrogent loss
Harvesting vs. Soil & Vegetation Disturbance
• Even clearcut harvesting may not have dramatic effects on forest and
watershed hydrology due to the rapid vegetation regrowth in most temperate
and tropical forests.
• However, soil and vegetation disturbances associated with harvesting
activities, post-harvest site preparation, and control of competing vegetation
often have large negative effects on soil properties and vegetation processes
that influence hydrologic function.
Harvest/Disturbance
Runoff
(m3/ha/mo)
Erosion
(t/ha/mo)
NO3(mg/L)
Reference
Temp Forest
45% clear-cut
45% c-c+herbic.
0.03
0.10
2.09
Corbett et al. (1978)
Temp Forest
Clear-cut
C-C+Herbicide
0.23
8.67
11.94
Likens et al. (1970)
Undist. Trop For
Active Skid Roads
New, unused SR
2-yr aband SR
3-yr aband SR
2
189
148
43
19
0
13
11
6
3
Hamilton and King (1983)
Harvesting vs. Disturbance cont.
In some cases, harvesting or disturbance effects become minimal after a few
years, but the cumulative change can be quite high during those few years.
Cumulative Changes in Hydrology After Harvesting and Site Preparation
Yrs After
Harvest
1
2
3
4
1
2
3
4
-----------------Harvest Type----------------------Control
Clearcut (diff)
CC+Shear (diff)
Cumulative Stream Flow (mm)
0
30
(30)
300
(300)
80
150
(40)
900
(520)
100
200
(30)
1300
(380)
150
250
(0)
1700
(350)
Cumulative Sedimentation (kg/ha)
200
400
500
600
800 (600)
1700 (700)
1800
(0)
1800 (-100)
2500
5300
6000
6200
McBroom et al. (2002)
East Texas mixed-wood forest
Shear = removal and windrowing of logging slash +
shearing of stumps at ground level
(diff) = difference between harvest type and control on a year-to-year basis
(2300)
(2600)
(600)
(100)
More on Harvesting Effects
•
A conceptual model of harvesting effects
for southern US forests shows increasing
effect on hydrology with increasing
• topography
• management intensity (aka
disturbance severity)
but decreasing PET
Wet
High PET
•
Dry
Low PET
•
some forests are highly resistant to alterations in soil properties and thus hydrologic
function
a study of Texas bottomland forest showed no significant loss of hydraulic
conductivity, macroporosity, or infiltration with both harvesting and soil disturbance
(Messina et al. 1997)
ts
f ec
f
E
gic
o
l
dro
y
H
Climate
•
from Sun et al. (2001)
hing
Ditc king
ra
ing
ootedd ut
R
B
l
a
i
c
Part
lear
Management
C
t
Cu
BottomBottomland
Wet
flat
Isolated
wetland
Topography
Uplands
Land Use Conversion
• in the tropics, commercial logging is relatively new
– unfortunately, illegal logging makes up most of the harvesting
activity in many tropical countries
• traditional land uses include
– swidden agriculture
– pasture conversion
– small farm clearings
– fuelwood gathering
– non-timber forest product gathering
• thus, semi-permanent alteration of forest structure is perhaps more
common traditionally than periodic large-scale harvesting
• specific alterations of hydrology depend upon
– type of conversion
– intensity and frequency of activities
– extent and location within watersheds
• human activity: tillage, cattle movement, etc. tends to coincide with
land cover change, confounding vegetation effects with disturbance
Partial Conceptual Model of
Deforestation Effects on Hydrology
King and Hamilton (1983)
Primary
Effects
Secondary
Effects
Deforestation
Reduced
Evapotranspiration
Reduced
Interception
Reduced
Infiltration
Red. Tree
Root Strength
Increased
Raindrop
Impact
Increased
soil detachment
Higher soil water
storage
Increased
surface
runoff
Increased
mass
wasting
Increased
surface
erosion
Increased
stormflow
Increased sedimentation
Final Effects
Increased streamflow
™ note the interaction of effects
within and across levels, e.g.,
increased surface runoff
Land Use Comparisons
Land Use Cfn
IF
(mm/hr)
Forest/Agric (Tr
(Tr))
For/Plant/Past (Tr
(Tr))
For/Ag/Swid
/Past (Tr
For/Ag/Swid/Past
(Tr))
Trop Forest
Rubber/oil palm
Tea
Veget
OF
SF
(m3/sec)
Sed
(t/ha)
Godsey and Elsenbeer 2002
-/-
Chandler and Walter 1998
+
Ann. soil
loss
3
(m /km2)
Eros
(g/d)
g/d)
Prim Trop For
Softw fallow
Grassland
1010-yr abaca plant
1212-yr corn swidd
Ungrazed Pine
Light Grazing
Heavy Grazing
200
49
45
Upl Dry Agric
25% Reforest
100% Reforest
Water
Discharge
Reference
Krishnaswamy et al. 2001
+
-
Nut
2525-30
4545-85
488
732
Daniel and Kulasingam (1974)
0.20
0.29
0.40
0.59
120
Hamilton and King (1983)
Hamilton and King (1983)
38.3
23.3
48.8
20.2
13.6
8.1
Hamilton and King (1983)
*Although cumulative annual differences may not seem dramatic, the majority of the differences
usually occur within pulses of extreme events (e.g., storms)
Long-Term Changes in Land Use and Erosion
a long-term (1880-1980) study of forest conversion to agriculture or pasture
within an English watershed showed a simple linear correlation between
amount of land cover in forest and estimated 10-year erosion rates (Lowrance
et al. 1986).
Erosion Rate (Mg/yr)
Erosion Rate vs Forest Cover
25000
R2 = 0.7851
20000
15000
10000
5000
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Forest Cover (proportion)
0.7
0.8
0.9
Native vs. Exotic Forest
• Plantation forests are becoming common in tropical countries due to their
often rapid growth, uniformity of wood production, and ease of silvicultural
management
• There is concern over their effects not just on biodiversity but also on
hydrology
• Certain eucalyptus species are sometimes blamed for “drying out” the soil
because of their deep roots and fast growth (and thus high water use).
• Such claims are rarely borne out for exotics in general, but specific species
comparisons can yield significant differences in water use and stream flow
(e.g., Verchot et al. 1997)
• “Afforestation” of semi-arid or seasonally-dry grasslands or savannas with
closed-canopy forests can reduce stream flow and “dry out” watersheds (Lill
et al. 1980, Waterloo et al. 1999)
Native Vs. Exotic Forest
Forest Type
ET
(mm/yr)
Native sal (India)
Exotic teak
compacted
Tropical semi-decid
Exotic eucalyptus
Temp exotic pine
Temp native euc
Native Temp For
Exotic Pine Forest
Infil Rate
(mm/min)
Water
Storage
(mm)
NO3(mg/L)
4.5
4.2
1.5
Reference
Ram and Jana (1997)
Ram and Patel (1992)
Ram and Patel (1992)
15.6%
14.2%
(can. E)
Leite et al. (1997)
Brazil: Eucalyptus grandis
4.66
2.98
Putuhena and Cordery
(1996) Australia
0.04
0.08
Hamilton and King (1983)
New Zealand
• For most exotic-native comparisons, small differences exist
• Major differences generally occur during the conversion process due to
•cutting
•site preparation
•planting
• The expected increase in management intensity and frequency, however, must
also be considered in the total impact of exotic forests (e.g., more frequent
harvesting, thinning, herbiciding, etc.)
Species-Specific Effects on Hydrology
• Putuhena and Cordery (1996) studied the water storage and infiltration
characteristics of the leaf and small branch litter of several Australian
forest species (native and exotic). By estimating soil cover under
different forest mixtures, they were able to determine the effects of
different species compositions on infiltration rates and water storage.
• In another study, Calder (2001) determined the effects of the canopy of
different species on the distribution of raindrop diameter size under
different rainfall intensities (see below).
Leaf Effects on Raindrop Diameter (mm)
Cumulative
Raindrop
Volume
0.2
0.4
0.6
0.8
-------------------Species--------------------Pinus
Eucalyptus
Tectona
carribaea camaldulensis
grandis
1.2
1.5
2.2
1.8
2.5
3.7
2.5
3.1
4.4
3.0
3.5
4.7
Riparian Buffers
• these are becoming common in soil and water
conservation guidelines for both agriculture and forestry
operations
• standard buffer widths and vegetation layers are often
recommended for specific soils, slopes, and farm or
forestry activities
• the conventional wisdom is that grass, shrub, and tree
cover will
–
–
–
–
slow overland flow
increase infiltration
trap incoming sediments
transpire through flow water and take up nutrient inputs.
• how effective are they?
Buffer Effects on Water and Nutrient Flows
• A review by Osborne and Kovacic (1993) of studies from the temperate
zone show that both forest and grass buffers typically remove 50-90% of the
N and P contained in both surface runoff and subsurface through flow
(Osborne and Kovacic 1993)
• A typical study cited in their
review (Lowrance et al.
1984)
• Buffers can also effectively
trap and remove pesticides
from runoff and subsurface
flow, but results are
variable by soil type
(kg/ha)
N
P
Ca
Mg
K
Cl
Tot
Input
51.2
5.3
52.6
19.5
23.4
104.9
Pesticide
Atrazine
Cyanazine
Metolachlor
Metribuzin
from NRCS (2000)
Min
11
30
16
50
Tot
Output
13.0
3.9
31.8
15.0
22.2
97.0
Percent
Max
100
100
100
97
Buffer Effects cont.
• Not all buffers are created equal. In this study, variable amounts of runoff
were infiltrated in the forest buffer zones of adjacent areas. Surface nutrients
were little retained (Verchot et al. 1997a, b).
Runoff (mm)
Input Output
Buffer 1
Buffer 2
3271
1029
3924
1019
Infil
(% RO)
5
40
Total N (kg)
Input Output
8.77
4.60
10.35
4.49
• Sometimes results are even contrary to expectations! A New Zealand study
showed that afforestation of a riparian pasture reduced soil plant cover and
increased runoff and nutrient export (Smith 1992).
Low Flow
Pasture
Pine Buffer
Storm Flow
Pasture
Pine Buffer
Susp
Sedim
Tot N
(mg/m3)
Tot P
(mg/m3)
15
35
730
1350
180
320
61
410
1515
4500
488
1300
Riparian Buffers:
How Wide Should They Be?
•
The hydrologic benefits of riparian buffers
obviously increase with greater width
•
But, what kinds of trade-offs are associated
with keeping riparian zones in undisturbed
forest?
•
First, there’s the loss of manageable land
•
Second, wider buffers create “islands” of
manageable land surrounded by buffer
forest
•
Ironically, thinner buffers are more irregular
in shape and perhaps more difficult to lay out
properly
from Bren (1995)
GIS and Forest Buffers
•
•
•
•
constant-width buffers are generally recommended because of
convenience
the width is determined by what is to be “buffered” (sediments,
nutrients, water) and general characteristics of the watershed
(climate, soils, topography)
the complex topography of most watersheds, however, means that a
single width doesn’t provide a constant “buffer capacity”
using GIS tools and understanding of upstream load upon various
sections of streams, variable-width but constant load riparian buffers
can be constructed
Constant-Load Riparian Buffers
Facet
•
Mapping the “facets” of a
watershed allows for reasonablyscaled variable width buffers along
the entire stream length
constant
proportion
buffer
stream
•
the hydrological benefits of such
systems have to be balanced
against
– access to sites
– lost income from unharvested
timber
– inherent complexity
constant
width buffers
from Bren (1998)
constant
proportion buffer
Research in Hawaii
„
„
„
„
„
„
Some work has been done on forest cover and hydrology
Anecdotal evidence is strong for a role of forests in improving water
quality and maintaining more consistent water flows
Early work focused on soil properties under different land uses
(Wood 1971)
A comparison of forest cover types showed that soil hydrologic
properties are not related to native vs. exotic status (Yamamoto and
Anderson 1967)
At the watershed scale, land use, topography, and soils interact to
create a complex system that does not follow easily predictable
patterns with respect to average or peak stream discharge rates
(Doty et al. 1981); ~90% of total sediment loading on Oahu occurs
during infrequent storm events (<2%)
Recent work on Kahoolawe has included using GIS to predict
hydrologic processes (Wahlstrom 1999); model outputs, however,
vary widely based on assumptions of soil properties and vegetative
cover effects
Research in Hawaii
Aggregate Stability
Aggregates < 0.25 mm
(percent frequency)
10 20 30 40 50
60
Forest
Pineapple
Site A
Forest
Pineapple
B
Forest
Sugarcane
C
Forest
Pasture
D
80
Site A
B
Hydraulic Conductivity (in/hr)
60
40
20
0
40
C
D
20
0
0- 3
Depth (in)
120- 3
1212-15
12-15
6-9
186-9
1818-21
18-21
Forest
Other (pineapple, sugar, or pasture)
Small aggregates occur
with higher frequency
under various
agricultural systems
compared to forests.
Forests also stabilize
larger aggregates
relative to agriculture
% aggregates retained (by weight)
50
Forest
40
Dry
Wet
30
20
10
0
Combined with lower
bulk densities and
greater porosity, forests
have much higher
infiltration rates and
surface hydraulic
conductivites.
Sugarcane
Dry
Wet
<0.105 0.105 0.25 0.50 1.0
Sieve size (mm)
1
Forest
Pineapple
2.0
Infiltration Rate (in/hr)
2
3
4
5
4.8
6
Site A
Forest
Pineapple
B
Forest
Sugarcane
C
Forest
Pasture
D
from Wood (1971)
Research in Hawaii
As seen in other studies, trees that are native or Polynesian introductions
do not necessarily sustain more favorable soil properties than exotic
species.
In this study, the influence of vegetative cover on soil aggregate size and
stability was relatively low compared to soil type.
Yamamoto and Anderson (1967)
Species
Koa-ohia
Uluhe
Eucalyptus
Grass
Guava
Silk-oak
Paperbark
Kukui
Aggregates
<0.25 mm (%)
11.9
8.3
8.0
7.2
6.3
5.3
3.4
2.4
Suspension
of sm. agg. (%)
6.4
7.0
10.9
6.1
5.9
6.7
3.5
5.9
Invasive Species Effects
• Certain weedy or invasive species may have
significant effects on the hydrology of intact or
disturbed forests
• As with forest plantations or land cover change,
the specific characteristics of the invading
vegetation, rather than invasion itself, are what
give rise to hydrologic effects
• Because invasion often takes place under acute
or chronic disturbance, disturbance effects may
interact with vegetation effects
Invasive Species: Hawaii Examples
• Andropogon virginicus invasion of wet forests on
windward Oahu (Mueller-Dombois 1973)
– this grass is active during the “summer” which
coincides with the dry season of the forest
– during the winter wet season, the grass dies back,
reducing transpiration
– standing dead litter also holds water at the surface,
reducing evaporation
– thus, the forest essentially becomes water-logged
because of the invasion of an alien non-adapted
grass
Invasive Species: Hawaii Examples
•
Miconia understorey invasion
– the broad, thick, dark leaves capture light and prevent regeneration
underneath the plants
– this leads to a reduction in herb/shrub cover and increased soil exposure
– this may lead to problems with decreased infiltration and increased
runoff and erosion
Invasive Species: Hawaii Examples
• Spread of albizzia (Falcataria moluccana)
– although planted to help reforest watersheds, albizzia
spreads in wet forests, invading native forest as well
as disturbed sites
– its high N-fixation rate increases soil nutrient
availability and facilitates invasion of other exotics
– nitrate production, soil pH reduction, and cation
leaching (accompanying the nitrate anions) may
reduce water quality in nearby streams
– evidence for increased nutrient availability and cycling
exist; effects on hydrology are unknown
The Myths of forests, watersheds, and water
availability
•
Forests increase overall water availability in
watersheds
•
the rationale for this is that forests
•
•
•
•
increase total precipitation
increase dry-season stream flow
increase infiltration and thus shallow through flow
studies suggest that
•
the balance between infiltration and ET is critical
– e.g., reforestation can increase infiltration and stream
flow on degraded ag lands (Hamilton and King 1983)
•
spatial scale is important for evaluating land use
change or management effects (e.g., stream flow vs.
precipitation)
Forests and Water Availability
Rural people in tropical countries often see a strong link between forests and
their quality of life, including conservation and availability of water
Percentage of respondents mentioning a link between forests and rainfall amounts or patterns
Precipitation
Amount
Pattern
Village
India
Kunjapani
MK Pudur
Thailand
Non Sawand
Khok Sawand
Living Standard
Low
High
Own Land?
No
Yes
Education
Low
High
59
92
3
11
67
76
71
46
53
70
65
67
73
89
100
60
81
93
16
20
77
96
84
85
78
100
95
91
86
91
82
93
from Wilk (2000)
• Some common beliefs:
• trees increase infiltration and long-term water supply to streams
• trees increase humidity and thus precipitation
• trees increase evaporation and thus cloud formation
• trees “capture” passing clouds and increase rainfall
Do Forests Increase Total Precipitation?
General Climate Models suggest that massive
deforestation on a regional to continental scale
may decrease rainfall, esp. in semi-arid regions.
A study of 30-year changes in rainfall patterns across
tropical Africa showed reasonable agreement between
observed changes and changes predicted by the effects
of deforestation (Calder et al. 2001).
Most changes in rainfall are < 10% of the annual total (e.g.,
Ataroff and Rada 2000). Although not dramatic, small
changes may be important in marginal environments.
The Myths of forests, watersheds,
and water availability
•
Forests prevent or reduce the impact of flooding and
landslides
• most watershed-scale studies suggest that forest
infiltration capacity is overwhelmed by large storm
events or monsoonal rains
• topography, soils, and geological characteristics are
more important during these events
• land slides and mass wasting due tend to occur
more frequently in non-forested areas
• human occupation and alteration of watersheds
(especially upper reaches) increase perception and
severity of flooding and erosion in floodplain areas
Summary
• Forest canopy, roots, and the forest floor reduce rainfall
impact, improve infiltration rates, and maximize
evapotranspiration
• Harvesting typically leads to small, short-duration
impacts
• Disturbance associated with harvesting can greatly
increase harvesting impacts
• Forest conversion to agriculture leads to variable effects,
depending upon
– type of vegetation change
– frequency and intensity of soil disturbance
– longevity of land use (e.g., swidden agriculture)
Summary
• Replacement of native with exotic forest species
does not necessarily impair hydrologic function
– the disturbance associated with conversion and
management, however, may be significant
• Afforestation of semi-arid grasslands typically
reduces water yield and stream flow, especially
during the dry season
• Prediction of species replacement effects can be
done in a straight-forward manner from an
understanding of canopy, litter, and root effects
of different species and mixtures on soil
properties and evapotranspiration
Summary
• Riparian forest buffers have varying effectiveness at
reducing overland flow, sedimentation, and removing
nutrients
• Constant-width buffers typically ignore topographical
changes in watersheds
• Variable-width buffers require more sophistication but
can provide more effective protection
• Hydrology and land use cover research in Hawaii should
be a high priority given our reliance on ground water and
our island setting
• Some work has shown the effects of different land use
types and species cover on soil properties
• Other work is attempting to model and predict hydrology
of severely degraded lands like Kahoolawe
Summary
• Finally, “conventional wisdom” about forests and
hydrology mix local observations, empirical evidence,
and cultural perspectives to create myths that may or
may not be true for specific regions or forest types
• Unfortunately, these myths often provide a convenient
and popular rationale for making land-use policy, despite
scientific skepticism
• Modern GIS and modeling tools can help provide a more
comprehensive framework for testing the validity of these
myths and creating more rational public policy
Acknowledgments
•
•
•
I thank Chittaranjan Ray and Phillip Moravcik of the Water
Resources Research Center for providing me with the opportunity to
gather and share the information in this presentation.
I give a special thanks to Michael Robotham of the NRCS Tropical
Technology Consortium and Ali Fares in the department of Natural
Resources and Environmental Management for providing me with
information and resources on this topic.
I also thank J.B. Friday in NREM for making this presentation
available on the web to all those who are interested in this topic.
May the decades of work summarized in this presentation provide
you with the inspiration and foundation needed to protect our
forests, streams, and watersheds in Hawaii and throughout the
tropics.
References
•
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