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HYDROLOGY | Impacts of Forest Plantations on Streamflow

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FOREST HYDROLOGY
Impacts of Forest Plantations
on Streamflow
D F Scott, Okanagan University College, Kelowna,
Canada
L A Bruijnzeel, Vrije Universiteit, Amsterdam, The
Netherlands
R A Vertessy, CSIRO Land and Water, Canberra,
Australia
I R Calder, University of Newcastle upon Tyne, UK
&2004, Elsevier Ltd. All Rights Reserved.
Scott, D F
Okanagan University College
Kelowna, British Columbia VIV 1V7, Canada
Bruijnzeel, L A
Faculty of Earth and Life Sciences
Vrije Universiteit
1081 HV Amsterdam, The Netherlands
Vertessy, R A
Cooperative Research Centre for Catchment Hydrology
CSIRO Land and Water
Canberra, ACT 2601, Australia
Calder, I R
Centre for Land Use and Water Resources Research
University of Newcastle upon Tyne
Newcastle upon Tyne, NE1 7RU, UK
Characteristics of Plantation Forests
Tree plantations for the production of timber have
been established for more than a century (tropical
teak and mahogany plantations, for instance, date
back to the mid-nineteenth century), but it is mainly
in the last few decades that an exponential expansion
of this form of land use has occurred. Taking one of
the most popular tree types for plantations as an
example, eucalyptus have been planted on an
estimated 17 million ha worldwide, of which more
than 90% have been established since 1955 and
roughly 50% during the last decade. The total
plantation area around the globe is 187 million ha
of which over % are in Asia, with Europe having the
next largest share (17%). Eucalyptus and Pinus are
the dominant genera within the broadleaved and
coniferous plantations, respectively. Although forest
plantations only occupy about 5% of the world’s
forest area, they are estimated, as of 2000, to supply
35% of all roundwood, a figure that is expected to
rise to 44% by 2020 as natural forests continue to
decline and demands keep rising.
Plantations are typically established at a regular
spacing (1000–2000 stems ha
1
), and individual
stands (compartments) have the same age and are
composed of a single species or clone. Often
plantations are particularly productive because the
tree species being grown are exotic to the area and
thus free of their native pests and diseases. Generally,
a distinction is made between industrial plantations
(aimed at producing wood for commercial purposes,
including construction timber, panel products, furni-
ture timber, and paper pulpwood) and nonindustrial
plantations (aimed at fuelwood production, protec-
tion of catchment areas for soil and water conserva-
tion, provision of wind- or fog breaks, etc.).
Scope
There appears to be a significant disparity between
public and scientific perceptions of the hydrological
role of forests in general and of plantations in
particular. Arguably, the contrasts in views are
especially pronounced in tropical regions where calls
for massive reforestation programs to restore dry
season flows as well as to suppress flooding and
stream sedimentation are heard the most frequently.
Often, however, these expected hydrological benefits
are not realized and in a number of cases forest
plantations have even been observed to aggravate the
situation. This article recapitulates the current
understanding of how forest plantations affect the
hydrological functioning of catchments. Other arti-
cles outline the principles of the forest hydrological
cycle (see Hydrology: Forest Hydrological Cycle
(00206)) and indicate the hydrological effects of
various forest management activities (see Forest
Hydrology: Impacts of Forest Management on
Streamflow (00269)) and forest conversion to other
land uses. Strictly speaking, the term ‘‘reforestation’
should be used to describe the planting of trees in
areas that were once covered by natural forest
whereas ‘‘afforestation’’ applies to plantation estab-
lishment in areas that are too dry to support natural
forest vegetation. To avoid semantic problems, the
term ‘‘forestation’’ is used mostly in the following to
denote either type of planting.
The Forest Plantation Water Budget
The hydrology of tree plantations is most easily
discussed with the aid of a simple water budget
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equation, most simply expressed in equivalent units
of water depth (mm per unit of time):
P¼ET QþDS
where Pis total precipitation (mostly rainfall, some-
times also fog or snow), ET the sum of various
evaporation components (often referred to as evapo-
transpiration), Qthe surface runoff or streamflow,
and DSthe change in (subsurface) storage of water in
the catchment (soil water and groundwater reserves).
Evapotranspiration ET dominates the water bal-
ance of all but the most humid forest plantations.
Beyond an annual precipitation of c. 2000 mm and
under conditions of lowered evaporative demand
(e.g., montane or coastal fog belts) the balance
between evaporation and streamflow tips toward
streamflow. There are two main components to
forest ET: transpiration (the water which is taken
up from the soil by roots and passes through the trees
to be transpired from the stomata of the leaves, E
t
)
and interception (the water that is caught in the
canopy and evaporates directly back into the atmo-
sphere without reaching the ground, E
i
). Under
closed canopy conditions, usually rather minor
additional components of evaporation are evapora-
tion from the soil surface (E
s
), which in a forest
includes interception by the litter layer, and evapora-
tion from understory vegetation. The presence or
absence of a forest cover has a profound influence on
the magnitude of ET, and by implication, also on
streamflow Q.
Rainfall Interception
Compared to short, simple vegetation canopies
(grassland, agricultural crops), tree plantations in-
crease evaporation losses by intercepting a larger
portion of incident rainfall. Generally, annual inter-
ception totals associated with the dense canopies
typical of evergreen coniferous plantations are higher
than those of deciduous broadleaved forests. Inter-
ception is also particularly high (expressed as a
fraction of total precipitation) where rainfalls are
frequent but of low intensity, especially where the
evaporation process is aided by the influx of
relatively warm air striking over a cooler vegetation
surface as is often observed in near-coastal areas. An
example of this effect comes from the UK where
conifer plantations have been established in upland
heath and grasslands in Scotland and Wales. Here the
nature of the precipitation, proximity to the ocean,
and the change in canopy density may increase
interception losses to as much as 35–40% of annual
precipitation.
At the other end of the interception spectrum (E
i
c.
6%) are the Eucalyptus plantations of the humid,
subtropical eastern escarpment in South Africa. This
is an area of high seasonal rainfall (1200–1500 mm),
much of which falls in the form of infrequent storms
of short duration but high intensity. Interception
losses from pine plantations in the same area are
somewhat higher (13%), reflecting their denser
canopies compared to the more open canopies of
the eucalypt stands. In the cooler southwestern part
of South Africa, an area of winter rainfall of lower
intensity, interception losses from pine plantations
are higher again (18%) than in the pine plantations
in the subtropical areas. In the case of both pine and
eucalypt plantations, though, there is a net increase
in interception over the grasslands and scrub vegeta-
tion they replace because of the higher leaf area,
greater depth of canopy, and aerodynamic roughness
associated with timber plantations.
Rainfall interception in tropical tree plantations
ranges from relatively low values in eucalypt stands
(c. 12%) (Figure 1a), to c. 20% for broadleaved
hardwood species such as teak and mahogany
(Figure 1b), and 20–25% for pines (Figure 1c)and
other conifers (Araucaria, Cupressus), with the
higher values usually found in upland situations
where rainfall intensities are generally lower. Well-
developed dense stands of the particularly fast-
growing Acacia mangium, on the other hand, may
intercept as much as 30–40% of incident rain.
Typical interception values for the rainforests re-
placed by these plantations range from 10–20% in
most lowland situations to 20–35% in montane
areas.
Transpiration
Transpiration (soil water uptake) is the second large
component in the evaporation budget of forest
plantations. Usually, plantation water uptake rates
are similar to those of natural forest occurring in the
area of planting but under certain conditions water
use of the (usually exotic) newcomers may be higher,
particularly under subhumid conditions where the
natural vegetation consists of more open woodland
or scrub. Examples include the replacement of dry
forest/scrub by fast-growing plantations of Eucalyp-
tus camaldulensis and E. tereticornis in South India,
and by E. grandis in southeastern Brazil and South
Africa. Likewise, water uptake rates reported for
(vigorously growing and densely stocked) stands of
Acacia mangium in Malaysia and for various species
planted in the lowland rainforest zone of Costa Rica
are such that they must exceed the water use of the
old-growth rain forests they are replacing, possibly
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FOREST HYDROLOGY /Impacts of Forest Plantations on Streamflow
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by 100–250 mm year
1
. Even greater differences in
transpiration can be expected where plantations are
established in areas with (natural) grassland or
degraded cropland. For example, whilst forest water
uptake under humid tropical conditions typically
exceeds that of pasture by about 200 mm year
1
, the
difference may increase to as much as 500–
700 mm year
1
under more seasonal conditions.
Such differences reflect the contrasting rooting
depths of trees and grassland as well as the tendency
for natural grasslands to go dormant during ex-
tended dry periods while the (exotic) trees continue
to take up water.
F0005 Figure 1 The contrasting canopies of (a) Eucalyptus spp., (b) teak (Tectona grandis), and (c) pines (Pinus caribaea) lead to
differences in amounts of rainfall interception and in the drop size spectra (and thus eroding power) of water dripping from the canopy.
(Photographs by LA Bruijnzeel.)
FOREST HYDROLOGY /Impacts of Forest Plantations on Streamflow
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Total Evapotranspiration (ET)
It follows from the above increases in rainfall
interception and transpiration that are typically
associated with the establishment of tree plantations
in areas of (natural) grasslands or (degraded) crop-
land that overall ET totals can be much increased
after forestation. As shown in Figure 2, total ET
values for actively growing plantations may ap-
proach 1500 mm year
1
and, occasionally, as much
as 1700–1900 mm year
1
. Such very high values
must be considered the exception rather than the
rule, however, and probably reflect the advection of
warm, dry air flowing in from adjacent grassland
areas which tends to greatly enhance evaporation
rates. Nevertheless, the fear is justified that the much
increased water use of tree plantations compared to
the pastures and crops they replace will lead to
substantial reductions in catchment water yields,
particularly during the dry season, if entire catch-
ments are planted.
Effects of Tree Plantations on Streamflow
Effects of Associated Land Management
In discussing the hydrological effects of establishing
timber plantations, it is important to be clear about
the site-specific conditions and management prac-
tices associated with the land-use change and their
contribution to the effect of a change in land use.
Such background information may be very important
in assessing the overall hydrological effect of the
plantations. The following examples illustrate the
need to specify more than simply the change in
vegetation cover itself.
Firstly, a forest or plantation would be expected,
normally, to have a continuous groundcover of leaf
or needle litter and some shade-tolerant shrubs. In
parts of southern China and adjacent countries,
however, all litter and understory plants may be
collected for fuel, a practice that has a profound
influence on the occurrence of surface runoff and
erosion in the plantation (Figure 3). Elsewhere (as on
the Indian subcontinent) forests and plantations are
used to graze cattle, a practice that requires regular
burning to stimulate the growth of fresh grass shoots.
The combined effect of burning and trampling by
livestock may promote massive surface erosion,
sometimes to the extent of initiating gullies.
Contrary to popular belief, it is not the intercep-
tion of rainfall by the main tree canopy that protects
the soil underneath against the erosive impact of the
rains. Rather it is the combined protection afforded
by the understory vegetation and a well-developed
litter layer that prevents the soil from being eroded.
In fact, the erosive power of rain dripping from the
canopies of tall trees is often greater than that in the
open, because of the associated increases in drop
sizes. The largest increases in drop diameters are
observed for drip from large-leaved trees such as teak
or Gmelina, whereas those falling from eucalyptus or
pines are more modest in size (Figure 4). Such
findings underscore the importance of maintaining a
good groundcover in plantations if runoff and
erosion problems are to be avoided (cf. Figure 3).
A final example of the importance of management
comes from the wet and peaty hill country of
Scotland and the English borderlands, where surface
drains are usually excavated prior to the planting of
400
600
800
1000
1200
1400
1600
1800
2000
1000 1500 2000 2500 3000 3500 4000 4500 5000
Mean annual
p
reci
p
itation
(
mm
)
Annual evaporation (mm)
Zhang Grass Zhang Forest Pine plantation Eucalyptus plantation
Other hardwood
plantation
Fynbos Grassland n. Forest
F0010 Figure 2 Total evaporation (ET) from forest plantations and other vegetation types as a function of precipitation. Data mostly from
humid tropical (Bruijnzeel 1997) and South African plantations (courtesy of D Le Maitre, CSIR, South Africa, unpublished compilation).
The curves define average forest and grassland water use in southeastern Australia (adapted from Zhang, et al. 1999), and have been
extrapolated for rainfall 42000 mm.
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FOREST HYDROLOGY /Impacts of Forest Plantations on Streamflow
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coniferous trees (mostly Sitka spruce, Picea sitch-
ensis), to improve the success of tree crop establish-
ment. However, the influence of the drainage ditches
on streamflow has proved more important than the
vegetation change from heath and moorland to tree
plantation itself.
Forestation and Water Yield
The increased evaporation from timber plantations
replacing shorter vegetation types (Figure 2), not
unexpectedly, translates into decreases in annual
streamflow totals after plantation establishment.
Although there are no stringent (paired) catchment
experiments in the humid tropics proper, there is
overwhelming evidence to this effect from the
F0015 Figure 3 The practice of repeated removal of needle litter from coniferous forests in parts of mainland southeast Asia often leads to
dramatic increases in surface runoff and erosion. (Photographs by courtesy of C Cossalter.)
S0045
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subhumid tropics (notably India), the subtropics
(mostly South Africa), and the temperate zone
(including southeast Australia and New Zealand).
Considerable differences have been observed be-
tween species but these are not necessarily the same
in different areas. For example, in southeast Aus-
tralia and New Zealand greater reductions in flow
were observed after planting pines (Pinus radiata)on
grassland than in the case of planting eucalyptus
(Figure 5). Conversely, in South Africa, other
variables being equal, the effect of planting Euca-
lyptus grandis was more pronounced than that of P.
radiata or P. patula (see Figure 8 below). Such
contrasts mainly reflect differences in growth perfor-
mance between regions and to a lesser extent
differences in rainfall interception dynamics.
Published experimental results often represent the
maximum possible impacts on streamflow. In the real
world, variations in site characteristics and planta-
tion management may exert a moderating influence
on the hydrological impacts of forestation. Moderat-
ing factors include the fraction of the catchment
planted, planting position within the catchment
(upstream or downstream parts, close to or away
from the streams, blocks vs. strips, etc.), and
variations in stand age and productivity between
species. These factors are elaborated upon briefly
below.
Catchments are rarely completely planted with
trees because some land is usually reserved for other
uses or it may be inaccessible or otherwise unsui-
table. The classical forest hydrology literature
suggests that the magnitude of the change in
catchment water yield is linearly proportional to
the percentage of catchment planted or cleared, with
increases in flow after forest removal and reductions
after forestation (Figure 6). Hence, in the case of
plantations, one could assume that if only half of a
grassland catchment would be forested then the
estimated reduction in mean annual runoff would
also be about half of the maximum reduction
predicted by Figure 5 for a given annual rainfall
total (assuming that plantation position in the
catchment does not influence the result).
Few experimental data are available on the
influence of plantation position on catchment water
balance changes. Under humid conditions in the
eastern USA, the reverse operation (i.e., forest clear-
cutting) did not show a significant difference in
streamflow response after cutting the upper half of
the catchment or the lower half. Also, elimination of
the vegetation around streams in one experiment in
the summer-rainfall zone of South Africa did not lead
to greater increases in streamflow than when remov-
ing an equal area of forest away from the stream.
However, several other experiments in South Africa
700
600
500
400
300
200
100
0
0 10 20 30 40 50 60 70 80 90 100
Annual streamflow change (mm)
Percentage of area reforested or deforested
SEE = 89 mm
y = 3.26x
F = 66.4
S = 0.01
r2 = 0.50
F0030Figure 6 Changes in annual water yield vs. percentage forest
cover change (solid circles denote experimental data of Bosch
and Hewlett (1982); open circles those of Trimble, et al. (1987).
SEE, standard error of estimate. (After Trimble, et al. (1987).)
0
0.2
0.4
0.6
0.8
1
01 5
Drop diameter (mm)
Eucalyptus camaldulensis Tectona
g
randi
s
Pinus caribaea
Normalized cumulative
volume
234 6
F0020 Figure 4 Characteristic drop size spectra for rain dripping from
pine trees (Pinus caribaea), teak (Tectona grandis), and
eucalyptus (Eucalyptus camaldulensis) as measured in South
India. (After Calder (1999).)
0
100
200
300
400
500
600 800 1000 1200 1400 1600
Eucalypt forest
Pine forest
Mean annual runoff reduction (mm)
Mean annual rainfall
(
mm
)
Tumut
Glendhu
Waiwhiu
Lidsdale
F0025 Figure 5 Potential reduction in mean annual streamflow
estimated to result from forestation of grasslands with eucalyptus
and pines in southeast Australia. Shown (as symbols) are field
data from four pine forestation experiments in Australia and New
Zealand. (After Vertessy, et al. (2003).)
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showed that an area of plantation near streams had
roughly double the effect of the same area of mid-
slope planting.
Such contrasting results may be explained in terms
of average soil water surplus or deficit, depth to the
groundwater table and slope morphology. All these
factors influence hillslope hydrological behavior.
Where rainfall is plenty, slopes steep and convex,
and the groundwater table rather deep (say, more
than 3 m), no major spatial effect is expected. This is
because rainwater infiltrating into the soil percolates
more or less vertically to the water table, then moves
laterally as groundwater to the nearest stream
without being taken up again by the roots of the
trees. Conversely, where soil water is scarcer, slopes
gentle and concave, and depth to the water table
shallow, a more pronounced effect is possible
because trees located closer to the stream will have
more ready access to the groundwater table. As such,
they are likely to consume more water than trees
further away from the stream that have less direct
access to groundwater to supplement diminished soil
water reserves.
Furthermore, there is the intuitive notion that the
further away one gets from a stream, the smaller the
probability that water infiltrating into the soil will
actually contribute to streamflow. These ideas have
been tested in modeling experiments in the context of
southeast Australia, the results of which lend support
to the notion that plantation position could affect
catchment water yield under conditions of low
rainfall (700 mm), gentle slopes, and high water-
tables (Figure 7). Indeed, the predicted effect on
streamflow of tree planting differed strongly depend-
ing whether forestation started at the top of the
hillsides and progressively moved downslope or vice
versa. The curves of Figure 7 also suggest that under
the prevailing conditions planting of the lower 30%
of the catchment would have a much greater impact
than planting the uppermost 30%. Similarly, a
related modeling study indicated that planting trees
in strips about 40 m wide parallel to the contour with
bands of pasture in between leads to greater tree
water use and better growth than when the same
10
80
70
60
50
40
30
20
0 10080604020
Proportion with trees (%)
Annual runoff (mm)
Bottomup
Topdown
F0035Figure 7 Results from a numerical modeling experiment
showing two sets of predictions of annual streamflow after
planting trees on a catchment under pasture in central New South
Wales, Australia (mean annual rainfall 700 mm). The upper curve
(solid line) shows changes in annual flow with forestation starting
at the top of the catchment and progressing downslope. The
lower curve (dashed line) shows the comparative response when
forestation starts at the bottom of the catchment and progresses
upslope. (After Vertessy, et al. (2003).)
0
100
200
300
400
500
600
02468101214161820
Years after
p
lantin
g
Flow reduction (mm)
Westfalia, E. grandis Mok-A, E. grandis
Mok-B, P. patula CP3, P. patula
Lb-B, P. radiata
F0040 Figure 8 Reductions in streamflow as measured in five catchment afforestation experiments in South Africa. The curves are scaled
for 100% planting of the catchment and smoothed to the mean annual runoff (MAR) prior to planting. (Based on Scott and Smith
(1997).)
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number of trees are planted in a single block at mid-
slope position.
More work is needed to ascertain optimal planta-
tion positions to minimize the hydrologic impacts of
forestation under contrasting climatic and topo-
graphic conditions. Process-based, spatially distrib-
uted hydrological models can be used to assess how
different planting strategies would impact on catch-
ment flow regimes. Whilst such models are difficult
to set up and apply, the effort is surely worthwhile
given the level of investment that goes into planning
any significant forestation initiative.
Much can be learned on the effects of species,
plantation age, and vigor from a particularly
comprehensive series of long-term paired catchment
studies of the hydrological effects of afforesting
natural grasslands and scrublands in subtropical
South Africa. Ten paired catchment experiments
have studied the effects of afforestation with Pinus
radiata,P. patula, and Eucalyptus grandis within
catchments. The research sites are all in the high
rainfall zone of South Africa (mean annual precipita-
tion 1100–1600 mm). Experimental control was
provided by catchments kept under native vegeta-
tion. Although generally steep, the catchments have
deep, well-drained soils and show very low storm-
flow response to rainfall. The catchments are all in
good hydrological condition (i.e., no significant
surface erosion); thus, the experimental comparison
is between the two vegetation covers, reflecting,
ultimately, the differences in total evaporation.
The resulting streamflow reductions over time
after planting follow a sigmoidal pattern comparable
to a growth curve (Figure 8). There are clear
differences between the effects of eucalyptus and
pines, but there is also a large amount of variation
from year to year within a single experiment and
between different experiments, even in comparable
catchments in one locality. The highest flow reduc-
tions occur once the tree crop is mature, and range,
for a 10% level of planting, from 17.3 mm or 10%
year
1
in a drier catchment to 67.1 mm or 6.6%
year
1
in wetter catchments (Figure 8). As such,
relative streamflow reductions (%), for a set age, are
greater in drier catchments but absolute reductions
(mm) are greater in wetter catchments. In other
words, the reductions are positively related to water
availability. The lower of these reductions in stream-
flow are similar to results obtained after planting E.
globulus in high elevation grassland areas in the
subhumid South of India (c. 20 mm per 10% forest
year
1
) whereas the highest reductions in South
Africa rather resemble the changes observed after
planting P. caribaea on seasonal grasslands in Fiji
(50–60 mm per 10% year
1
). Similar effects on
streamflow have been recorded under the more
temperate conditions of New Zealand (see also
Figure 5), where conversion of pastures and tussock
grassland to P. radiata plantations, over a range of
climates, led to streamflow reductions of 20–45 mm
year
1
per 10% of catchment planted, the amount
again being dependent on water availability.
The timing of the first significant reductions in
flow after planting varies quite widely depending on
the rate at which catchments are dominated by the
plantation crop. The pine plantations in the high
altitude grasslands at Cathedral Peak in South Africa
(CP in Figure 8) usually took several years to have a
clear impact on streamflow. However, the same
species of pine had an earlier effect on streamflow
(within 3 years) under the drier conditions prevailing
in the Mokobulaan B catchment in Mpumalanga
Province (Mok-B in Figure 8). Other conditions
remaining the same, eucalyptus have a slightly earlier
impact on streamflows than pines in South Africa,
normally within 2–3 years.
A key factor influencing the degree of streamflow
reduction after forestation is the vigor of the trees.
Usually, there is a close link between the growth rate
of a plantation and its overall water uptake. A new
finding from the South African afforestation experi-
ments is that the flow reductions are diminishing
again during the postmaturation phase of the
plantations, both in the case of pines (after about
30 years) and in at least one of the two eucalypt
experiments (after 15 years). This undoubtedly
mirrors the gradually decreased vigor of older trees
as has also been observed in old-growth native
eucalypt forest in southeast Australia and tropical
rainforest in Amazonia. In industrial plantation
forestry, short- and medium-length tree rotations
will tend to keep the trees in their peak water use
phase, but longer rotation crops, such as those aimed
at producing good quality saw timber, are more
likely to have a smaller effect on water yield later on
in the rotation.
Forestation and Low Flows
Declines in streamflow following the establishment
of plantations are recorded in all components of the
annual hydrograph (i.e., stormflows and baseflows).
In South Africa, effects on total and low flows follow
the same pattern, but low flows are decreased more
than are total flows at the same age (Figure 9).
Similar effects have been found in the temperate zone
as well as in Fiji, India (even more so after coppicing
and resprouting), and Malawi. The effect of foresta-
tion on low flows in subhumid areas has two
supposed sources. First, exotic plantations, in con-
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trast to the native grasses or scrub vegetation they
replace, do not go dormant in the dry season. The
second cause, though less easily quantified, is that of
steadily reducing soil water reserves as the trees
mature. Low flows are a reflection of the amounts of
soil water and groundwater stored in the catchment
and as these are steadily depleted by tree water
uptake so will low flows diminish accordingly. It is
clear from the South African experiments that total
water use by the tree crop can exceed annual rainfall
in many years and that, once dry season flow has
ceased altogether, the occurrence of rainstorms may
not easily cause the streams to flow again.
Strongly reduced baseflows after forestation of
(nondegraded) grassland or scrubland can thus be
expected to be a generally occurring phenomenon.
The magnitude of this effect is probably related to
the capacity of the soils to store water and to the
extent that this water can be accessed by the roots of
the tree crop. Thus, where the new trees are able to
occupy a much greater volume of soil through their
deeper roots, reductions in baseflows following
forestation can be expected to be proportionately
larger than in situations where rooting volume is
restricted.
Finally, it is important to bear in mind that the
above examples concern situations where the soil is
not degraded and rainfall infiltration generally
proceeds unimpaired. Under such conditions, stream-
flow amounts will simply reflect the change in
vegetation water use and low flows will be thus
(much) reduced (see Figures 2 and 6). However, in
areas with degraded, compacted soils where much of
the rain may run off along the surface as overland
flow (and therefore not contribute to soil water
reserves), the planting of trees can be expected to
ultimately have a positive effect on infiltration.
Theoretically, the extra water entering the soil
through improved infiltration after forestation may
moderate or, in extreme cases, even reverse the
adverse effect of the larger water use of the trees on
streamflow. In all cases, the net effect of tree planting
on the baseflow from degraded areas will reflect a
trade-off between these two effects. Where infiltra-
tion is already sufficient to accommodate most of the
rainfall, any further improvements by forestation
will not tip the balance. Rather, water yield will be
reduced even further (Figure 6). However, where
soils are deep but overland flow during rainfall is
rampant and much is to be gained from improved
infiltration (Figure 10a), it cannot be excluded that a
net positive effect on low flows may occur. The
experimental evidence for this contention is only
indirect, however, and based on a comparison of
observed reductions in stormflow response (having a
positive effect on soil water reserves) (Figure 10a) vs.
increases in vegetation water use (having a negative
effect on soil water reserves) (Figure 2).
Forestation and Stormflows
Forest hydrological research has shown that the
influence of vegetation cover or type on catchment
runoff response to rainfall (‘‘stormflows’’) is inversely
related to the size of the rainfall event that generates
the flows. This can be explained as follows: in small
to medium storm events the combined water storage
capacity of vegetation layers, litter, surface depres-
sions, and the soil mantle will be considerable
relative to the amount of rain delivered by the storm.
As a result, the associated catchment response will be
much reduced in the case of a good forest cover. The
soil mantle is potentially the largest water store, but
its capacity to accommodate additional rain varies as
a function of soil wetness. Where previous uptake by
the vegetation has depleted soil water reserves (as is
often the case during summer), storage capacities,
and thus stormflow reduction, will be relatively high
(Figure 10a). However, once the soil has become
thoroughly wetted by previous rains (typically during
winter or the main rainy season), very little
opportunity to store additional water will remain,
regardless of vegetation type (Figure 10b). In
addition, as rainfall events increase in size, so does
the relatively fixed maximum storage capacity of the
soil become less important in determining the size of
the stormflows. In other words, the presence or
absence of a well-developed forest cover has a
significant effect in the case of small events but this
typically makes very little difference (less than 10%)
0
10
20
30
40
50
60
70
80
90
100
-1 1 3 5 7 9 11 13 15
Years after afforestation
Reduction in streamflow (%)
Total flow
Low flow
F0045 Figure 9 Pooled results from two catchment eucalyptus
afforestation experiments in South Africa, showing the pattern
of flow reductions as a function of plantation age, and illustrating
the greater and earlier effects on the low flow component. (After
Scott and Smith (1997).)
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FOREST HYDROLOGY /Impacts of Forest Plantations on Streamflow
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in the case of truly large events (floods) generated by
extreme and prolonged rainfall (Figure 10b). Under
such conditions, runoff response is governed almost
entirely by the capacity of the soil to accommodate
and transfer the rain.
However, where degradation of a catchment’s soils
has produced strong reductions in canopy and
groundcover (including litter), and above all in
infiltration capacity and soil depth through contin-
ued erosion (and thus overall soil water storage
opportunity), reforestation could clearly lead to an
improvement of most or all these factors over time.
These ideas are conceptualized in Figure 11.
Concluding Remarks
Catchment experiments all over the world have
demonstrated convincingly that total amounts of
streamflow emanating from catchments where forest
plantations have replaced (natural) grassland or
scrubland, or (degraded) cropland, are invariably
much reduced. In addition, the reductions in base-
flows during the dry season are relatively greater
than during the wetter season. Small to medium-
sized stormflows are also reduced significantly by
forestation but the effect on occurrence and size of
flood peaks associated with truly large rainfall events
is very limited.
These observations differ strongly from the pop-
ular view held by many foresters, policy-makers, and
the public at large that forestation will lead to (more
or less rapidly) increased streamflows and the
High
Low
Shallow Deep
Soil depth: Available storage
Storm response (QS/P)
Degraded grassland
Grassland
Old forest
Immature forest
F0055Figure 11 Postulated generalized relationship between catch-
ment storage capacity and stormflow response to rainfall, as
affected by vegetation cover. (After Scott, et al. (2004).)
180
160
140
120
100
20
40
60
80
Peak discharge in cubic feet per second
Annual frequency-equaled or exceeded
024 681012
1958
1945
1937
1935
180
160
140
120
100
20
40
60
80
Peak discharge in cubic feet per second
Five-
y
ear frequenc
y
-equaled or exceeded
0 5 10 15 20 25 30
(a) (b)
193536 to 193940
195152 to 195556
F0050 Figure 10 Frequency distributions for peak discharges during (a) summer and (b) winter in the While Hollow catchment, Tennessee,
USA before (1935) and after (1937–1958) reforestation. (Modified from Tennessee Valley Authority (1961) Forest Cover Improvement
Influences upon Hydrologic Characteristics of White Hollow Watershed,1935–1958. Report no. 0-5163A. Knoxville, TN: Tennessee
Valley Authority.)
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FOREST HYDROLOGY /Impacts of Forest Plantations on Streamflow
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elimination of flooding. Although the establishment
of forest plantations on degraded land will improve
the soil’s capacity to absorb rainfall, this is likely to
take at least several decades. However, because water
use by the trees is much increased within a few years
already compared to that of the former vegetation,
the balance of probability is that low flows will also
be reduced in this case. Establishing the precise
hydrological effects of reforesting areas in various
stages of soil degradation constitutes a prime
research need.
List of Technical Nomenclature
See also:Forests, Tree Physiology and Climate
(00114). Hydrology: Forest Hydrological Cycle (00206);
Forests and Streamflow (00208); Forests and Water
Quality (00209); Impacts of Forest Management on
Streamflow (00269). Silviculture: Forest Plantations
(00212); Short Rotation Forestry for Biomass Production
(00236). Soils and Site: Forests and Soil Development
(00239); Soil and its Relationship to Forest Productivity
and Health (00240); Water Storage and Movement
(00249). Tree Physiology: A Whole Tree Perspective
(00098); Root System Physiology (00102).
Further Reading
Bridges EM, Hannam ID, Oldeman LR, et al. (2001)
Response to Land Degradation. Enfield, NH: Science
Publishers Inc.
Bruijnzeel LA (1997) Hydrology of forest plantations in the
tropics. In: Nambiar EKS and Brown AG (eds) Manage-
ment of Soil, Nutrients and Water in Tropical Plantation
Forests, pp. 125–167. Canberra, Australia: ACIAR/
CSIRO/CIFOR.
Bruijnzeel LA (2004) Tropical forests and environmental
services: not seeing the soil for the trees? Agriculture,
Ecosystems and Environment (in press).
Calder IR (1999) The Blue Revolution: Land Use and
Integrated Water Resources Management. London:
Earthscan Publications.
Calder IR, Rosier PTW, Prasanna KT, and Parameswar-
appa S (1997) Eucalyptus water use greater than rainfall
input: a possible explanation from southern India.
Hydrology and Earth System Science 1: 249–256.
Dye PJ (1996) Climate, forest and streamflow relationships
in South African afforested catchments. Commonwealth
Forestry Review 75: 31–38.
Fahey B and Jackson RJ (1997) Hydrological impacts of
converting native forests and grasslands to pine planta-
tions, South Island, New Zealand. Agricultural and
Forest Meteorology 84: 69–82.
Gilmour DA, Bonell M, and Cassells DS (1987) The effects
of forestation on soil hydraulic properties in the Middle
Hills of Nepal: a preliminary assessment. Mountain
Research and Development 7: 239–249.
Johnson R (1998) The forest cycle and low river flows: a
review of UK and international studies. Forest Ecology
and Management 109: 1–7.
McJannet DL, Silberstein RP, and Vertessy RA (2001)
Predicting the water use and growth of plantations on
hillslopes: the impact of planting design. In Proceedings
of MODSIM2001, International Congress on Modelling
and Simulation, December 2001, Canberra, vol. 1, pp.
455–460.
Scott DF (1999) Managing riparian zone vegetation to
sustain streamflow: results of paired catchment experi-
ments in South Africa. Canadian Journal of Forest
Research 29: 1149–1157.
Scott DF and Smith RE (1997) Preliminary empirical
models to predict reductions in total and low flows
resulting from afforestation. Water South Africa 23:
135–140.
Scott DF, Bruijnzeel LA, and Mackensen J (2004) The
hydrological and soil impacts of forestation in the
tropics. In: Bonell M and Bruijnzeel LA (eds) Forests–
Water–People in the Humid Tropics, pp. 00–00. Cam-
bridge, UK: Cambridge University Press.
Sikka AK, Samra JS, Sharda VN, Samraj P, and Lakshma-
nan V (2003) Low flow and high flow responses to
converting natural grassland into bluegum (Eucalyptus
globulus) in Nilgiris watersheds of South India. Journal
of Hydrology 270: 12–26.
Trimble SW, Weirich FH, and Hoag BL (1987) Reforesta-
tion and the reduction of water yield on the southern
Piedmont since circa 1940. Water Resources Research
23: 425–437.
Vertessy RA, Zhang L, and Dawes WR (2003) Plantations,
river flows and river salinity. Australian Forestry 66: 55–
61.
Waterloo MJ, Bruijnzeel LA, Vugts HF, and Rawaqa TT
(1999) Evaporation from Pinus caribaea plantations on
former grassland soils under maritime tropical condi-
tions. Water Resources Research 35: 2133–2144.
Zhang L, Dawes WR, and Walker GR (2001) Response of
mean annual evapotranspiration to vegetation changes at
catchment scale. Water Resources Research 37: 701–
708.
Zhou GY, Morris JD, Yan JH, Yu ZY, and Peng SL (2001)
Hydrological impacts of reafforestation with eucalyptus
and indigenous species: a case study in southern china.
Forest Ecology and Management 167: 209–222.
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... As interception tends to be higher in forests, evaporation from the canopy also increases [10]. Afforestation with pine lowers the total flow [11] and reduces water retention in soils owing to organic carbon matter losses [12,13]. According to Balthazar et al. [14], changing grassland to pine forests is associated with negative impacts, such as a decrease in soil water content, soil organic matter, water retention capacity, and potentially irreversible provision of ecosystem services. ...
Article
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To gain insights into the impact of afforestation on the hydrology of a mountain river basin, the Mpinos catchment was modeled to obtain parameters that represent pine land use. Thereafter, the parameters were applied in the Zhurucay catchment to answers the following questions: 1) How can the parameters of hydrological models subject to land use change be calibrated? 2) What is the impact on peaks, total flow, and baseflow when land use gradually changes from tussock grass to pine plantations? 3) Is the impact different when land use changes gradually from upstream to downstream (U-D) or downstream to upstream (D-U)? Based on our results, the total flow, baseflow, and peaks were reduced by 21%, 66%, and 21%, respectively. Overall, this study presents a calibration approach to predict the effects of land-use change prior to its occurrence.
... Ei depended on the precipitation and canopy status. Compared to Ec and Es, it accounted for a little proportion of ET, while it was an indispensable component of surface water balance, particularly for the vegetated areas with higher vegetation coverage and leaf area index [22]. ...
Article
Full-text available
Identifying the spatiotemporal variations and influencing climate factors of evapotranspiration (ET) and its components (vegetation transpiration (Ec), soil evaporation (Es), and canopy interception evaporation (Ei)) can greatly improve our understanding of water cycle, carbon cycle, and biogeochemical processes in a warming climate. As the world′s largest hydropower project, the construction of the Three Gorges Project (TGP) coupled with the significant land use/land cover change affected the regional water and energy exchange in the Three Gorges Reservoir Area (TGRA). This study aimed to reveal the spatiotemporal variations and influencing climate factors in ET and its components using PML-V2 products in TGRA during 2000–2020. Results showed that the mean annual ET, Ec, Es, and Ei in TGRA were 585.12, 328.49, 173.07, and 83.56 mm, respectively. The temporal variation of ET was dominated by Ec, with no significant change in the time trend. Es decreased (2.92 mm/y) and Ei increased (1.66 mm/y) significantly mainly in the cultivated land. ET, Ec, and Ei showed a similar seasonal variation pattern with a single peak, while Es presented a bimodal pattern. From the pre-impoundment to the first impoundment period, ET and Ec mainly increased in the head of TGRA, meanwhile, Es in urban area increased significantly by 27.8%. In the subsequent impoundment periods, ET and Ec changed slightly while Es sharply decreased. The Ei increased persistently during different impoundment period. The dominant climate factors affecting changes in Ec and Es were air temperature, vapor pressure deficit, and sunshine hours, while the variation of Ei was mainly affected by air temperature, vapor pressure deficit, and precipitation.
... A detailed review of these studies is given in the papers (Bosch and Hewlett 1982;Brown et al. 2005;Gu et al. 2013). Analysis of the results of hydrological studies on forest watersheds over the past two decades (Chang 2003;Scott et al. 2004;Sun et al. 2006;Wei et al. 2013;Li et al. 2017) showed that forest hydrologists have no consensus on the effect of forest cover on the total river flow. Estimates of the hydrological role of boreal forests and the characteristics of their water cycles are likely the most controversial regarding their influence on the annual runoff volume. ...
Article
Full-text available
Background Assessment of the reasons for the ambiguous influence of forests on the structure of the water balance is the subject of heated debate among forest hydrologists. Influencing the components of total evaporation, forest vegetation makes a significant contribution to the process of runoff formation, but this process has specific features in different geographical zones. The issues of the influence of forest vegetation on river runoff in the zonal aspect have not been sufficiently studied. Results Based on the analysis of the dependence of river runoff on forest cover, using the example of nine catchments located in the forest-tundra, northern and middle taiga of Northern Eurasia, it is shown that the share of forest cover in the total catchment area (percentage of forest cover, FCP) has different effects on runoff formation. Numerical experiments with the developed empirical models have shown that an increase in forest cover in the catchment area in northern latitudes contributes to an increase in runoff, while in the southern direction (in the middle taiga) extensive woody cover of catchments “works” to reduce runoff. The effectiveness of geographical zonality in regards to the influence of forests on runoff is more pronounced in the forest-tundra zone than in the zones of northern and middle taiga. Conclusion The study of this problem allowed us to analyze various aspects of the hydrological role of forests, and to show that forest ecosystems, depending on environmental conditions and the spatial distribution of forest cover, can transform water regimes in different ways. Despite the fact that the process of river runoff formation is controlled by many factors, such as temperature conditions, precipitation regime, geomorphology and the presence of permafrost, the models obtained allow us to reveal general trends in the dependence of the annual river runoff on the percentage of forest cover, at the level of catchments. The results obtained are consistent with the concept of geographic determinism, which explains the contradictions that exist in assessing the hydrological role of forests in various geographical and climatic conditions. The results of the study may serve as the basis for regulation of the forest cover of northern Eurasian river basins in order to obtain the desired hydrological effect depending on environmental and economic conditions.
... Management practices increase growth by maximising leaf area and growth efficiency (Waring 1982). Increased leaf area can increase water demand by trees (Scott et al. 2004). Fastgrowing plantation forests may, under certain conditions, have greater water use efficiency (WUE) than unmanaged vegetation (Gyenge et al. 2008). ...
... Management practices increase growth by maximising leaf area and growth efficiency (Waring 1982). Increased leaf area can increase water demand by trees (Scott et al. 2004). Fastgrowing plantation forests may, under certain conditions, have greater water use efficiency (WUE) than unmanaged vegetation (Gyenge et al. 2008). ...
Chapter
This chapter reviews how global change affects forest-water interactions and water availability to ecosystems and people and synthesises current understanding of the implications of present and anticipated changes to forests and tree cover for local and global hydrology. Forest cover has declined in the past half-century, despite an increase in plantation forestry. Natural and human disturbances affect forest components (eg canopy and leaf area, litter and soil surface, rooting depth, and soil porosity) that in turn affect hydrological processes (eg interception, evapotranspiration, infiltration, soil moisture storage, and percolation). Many of these changes result from several influential natural disturbance processes including insects and pathogens, wildfire, ice storms, and windthrow, and human disturbances including establishment and harvest of forests, plantations, agroforestry areas, and urban/peri-urban forests …
... have reduced storm runoff responses but with responsiveness and fluctuation values still higher than those for primary forest, ( Figure 5, Table 9) in line with findings for regenerated forests elsewhere (Bruijnzeel and Vertessy, 2004;Kuraji, 2013, 2012;Qazi et al., 2017;Walsh et al., 2011). ...
Article
Full Text: <https://authors.elsevier.com/a/1XqAQ_WGi1y8q> Although erosional impacts of rainforest logging are well established, changes in hydrological dynamics have been less explored especially in the post-logging recovery phase following repeat-logging cycles and mature phase of oil palm plantation cycles. This study addresses this gap by comparing hydrological characteristics of five catchments in a steep land area of Sabah, Malaysian Borneo on a gradient of disturbance and recovery – twice-logged forest, 22 years recovery (LF2); multiple-logged forest, 8 years recovery (LF3); mature oil palm, 20 years old (OP); and two primary forests (PF and VJR) as controls. Each catchment was instrumented with water depth (converted to discharge), conductivity, temperature, and turbidity sensors, and a raingauge connected to a solar-powered datalogger recording data at 5-minute intervals from November 2011 to August 2013. Data were analysed via the flow-duration curve (FDC) supplemented by the runoff coefficient (RR) and coefficient of variation in discharge (QVAR) for aggregated characteristics, as well as via a combination of the Dunn's test and multiple-regression at the storm event scale for focused hydrological dynamics. Results show that OP is characterised by a relatively low RR (0.357) but with high responsiveness during storm events and very low baseflow (38.4% of total discharge). Discharge in the LF3 (RR = 0.796) is always the highest while having an intermediate level of responsiveness. LF2 with longer-term recovery shown a reduction in terms of discharge (RR = 0.640). Being the benchmark, the undisturbed forest (PF) has the most buffered storm response with the highest baseflow (67.9% of total discharge). Stormflow and baseflow are anomalously high and low respectively in the near-primary VJR catchment, but this probably reflects the shallow soils and short-stature rainforest associated with its igneous and metamorphic lithology. From a management aspect, although hydrological recovery is more advanced in the 22 years than in the 8-years post-logging catchment, full recovery is yet to be achieved and might be hastened by enrichment planting of the degraded forest. The low baseflow and flashy nature of the mature oil palm have major implications for downstream water supply in ENSO periods and flooding in La Nina periods. Steep lands in the humid tropics are best avoided from any form of landscape disturbance.
... Valuation studies of watershed ecosystem services and management or policy decisions about PES are not always scientifically sound. Various misconceptions about the role of ecosystems in regulating the flow of water persist among managers and decision‐makers, despite the publication of many scientific papers on this issue (e.g., Bosch and Hewlett, 1982; Bruijnzeel, 1990; Critchley and Bruijnzeel, 1996; Sahin and Hall, 1996; Bonell, 1998; Calder, 2002; Best et al., 2003; Andreassian, 2004; Bruijnzeel, 2004; Scott et al., 2004; Bonell and Bruijnzeel, 2005; Farley et al., 2005; Guillemette et al., 2005; Scott et al., 2005). A challenge facing managers and decision‐makers is the complexity of the effect of ecosystems on water flows (Bruijnzeel, 1990; Fujieda et al. 1997; Bonell, 1998; van Noordwijk et al., 2004; Waage et al., 2008).). ...
Thesis
An increasing production of scientific literature addresses the past, current and future importance of ecosystems for the provision of goods and services to society. However, pressure from climate change, national policies and globalization are increasingly degrading this capacity of ecosystems. Typically, the design of societal responses to ecosystem services degradation requires considering the decisionmaking of multiple actors from the local scale where ES are provided to regional, national and international scales where decisions on rules and resources are defined and distributed. This is the case of SRS degradation which requires consideration of decision processes also considering the on‐ and off‐site benefits of SRS. Indeed, soil erosion affects upstream land users and downstream user as hydropower dam due to siltation. Moreover, in a watershed, these actors directly benefiting from and making decisions regarding SRS are embedded in decision processes happening at scales that go beyond the watershed and beyond the direct use of SRS services (e.g., decisions to design technical solutions to soil erosion, decision on the provision of incentives and their types, etc.). The goal of the thesis is to analyze the multi‐hierarchy human decision processes characterizing SRS provision considering the human and environment system as two different, complementary and interrelated systems. In this thesis I analyze the case study of Soil Regulation Services (SRS) in the Birris watershed Costa Rica, a country pioneer in policies to protect ecosystem services. The Birris watershed is a relatively small territory with an area of 5800 has. It is characterized by intensive and marketoriented agricultural production activities with high fragmented farm areas and steep slopes. Soil erosion has been increasing since several decades due to inadequate land use practices and increasing extreme precipitation events whose intensity and frequency is expected to grow under climate change. The Birris watershed represents a learning case for many national and regional actors due to its commonality with other watersheds in the region. The first of four articles of this thesis analyzes the environmental dimension focusing the role of forest ecosystem cover on hydrological responses. In the following three papers I address the human dimension focusing on decision processes occurring at different scales: i) from the individual farmers decision‐making on soil conservation; ii) to the watershed scale where actors are directly affected by on‐site and off‐site provision of SRS; and, finally, iii) to the national cross‐scale information‐sharing network of multiple actors involved directly or indirectly (scientists, regulators, farmers’ associations, users of SRS, etc.) in the management of watersheds and of SRS. More specifically, the goal of the first paper is to synthesize the findings of several paired‐catchment experiments from Africa, Asia and Latin America that analyze the effect of forest cover change on hydrological responses. We use meta‐analysis as a tool used in ecological studies to synthesize results from different studies. This help us overcoming the small “n” problem associated to many of these types of forest hydrology studies in watersheds especially in developing world. The second paper addresses the complexity of farmers’ decision‐making in respect to soil conservation decisions. This is analyzed through a multi‐dimensional decision 2 model including economic cognitive and territorial variables influencing farmers’ decision‐making regarding soil conservation efforts. The structured survey included items built through previous meetings with farmers to capture their perspectives and understanding on soil erosion and its solutions. The goal of the third paper is to analyze aspects that are relevant to build a collaborative mechanism among users of on‐site and off‐site provision of SRS. Specific methods from decision science and negotiation analysis are applied implying consultations with both users and providers of SRS. In separate meetings, consultations allowed structuring their fundamental objectives and identifying key aspects of composing a desired mechanism such as: i) how to select farmers; ii) what type of contract should be used; iii) who should intermediate and, finally, iii) the type of possible incentives for farmers’ soil conservation. In order to identify negotiation space for key aspects of a mechanism, preferences of both stakeholders in respect to different alternative aspects of a mechanism were elicited in two separate focus groups. The goal of the fourth paper is to identify cross‐scale institutional mismatches arising from formal policies and mandates and constraining SRS provision and use. We use “betweeness centrality” algorithm (common to social network science) to test how structural analysis of information‐exchange network can identify boundary organizations which are potentially strategic to overcome cross‐scale institutional mismatches. We analyze actors’ official mandates contributing directly or indirectly to SRS provision and their interaction in information‐sharing network. The analysis of the environmental dimension proves the usefulness and methodological limitations of using meta‐analyses to synthesize findings from pairedcatchments experiments studies on the hydrological effects of changes in natural and planted forests cover. Overall results from experiments from Asia, Africa and Latin America show that forest cover can play an important role in diminishing the base flow in watershed but its effect on storm‐flow control (i.e. water runoff causing erosion) depends more on local characteristics. Some methodological limitations from this use of quantitative meta‐analysis can also be outlined. We found a relatively small number of paired‐catchment experiment studies from Asia, Africa and Latin America thus limiting the capacity to analyze the interacting effect of important factors for water flow regulation, such as soil, geology, topography, or land management practices. As for the capacity of forests to control storm flows (related to increase in erosion) data found in the scientific articles used in the meta‐analysis did not allow accounting for the effects of frequent and intense extreme precipitation events. This also limits the capacity to compare the provision of SRS under climate change from natural forest ecosystem vs non‐forest land uses such as conservation agriculture. At the local scale, farmers’ awareness of their exposure level to soil erosion combines with other variables to determine their level of soil conservation efforts. The decision model includes socioeconomic, territorial and cognitive variables such as beliefs, values and risk perception and clearly separates three groups of farmers based on their soil conservation efforts. Most farmers are aware of the risk of erosion although socioeconomic aspects such as type of production and farm size indicate that 3 perceived opportunity cost given the farm production context might hinder their conservation efforts. Farmers with low perception of erosion risk might also be expressing “availability heuristic” paradigm due to their daily experience with erosion in the watershed. At the watershed scale, the design of collaborative efforts for the on‐ and off‐site provision of SRS requires agreement on the fundamental objectives of a mechanism for collaborative efforts for soil conservation. Consulted farmers and hydropower agree on the importance of the promotion of learning through technical assistance and monitoring of soil conservation programs and the fair distribution of incentives. Direct payment for soil conservation is only limitedly considered as a desired incentive alternative. Consistent with the fundamental goal of promoting learning, technical assistance is seen as a more desirable alternative than direct payments. The national cross‐scale analysis of governance structure for SRS highlights that important regulatory mismatches affect the definition of societal responses at the local level (i.e. where direct actions to promote adequate provision and use of ES happen). Network analysis helps us identifying the information‐bridging characteristics of actors in informal information‐sharing networks. This analysis outlines the boundary role of the watershed agricultural‐extension office helping diffusing information on impacts as well as social and technical feasibility of responses to SRS degradation across‐scales and policy areas. Overall the thesis’ results show that soil conservation policies to support the provision of SRS would benefit from the use of mixed policies. This might include programs to raise awareness on current and future soil erosion risks, promote learning among farmers, and institutionalize the boundary role of agricultural extension offices for their importance to promote learning and adaptive management of SRS. This is especially valuable in the context of areas highly exposed to increasing frequency of extreme precipitation events such as Central America. Moreover, in the face of high uncertainties and scarcity of data (e.g. on the impacts of land use/management and climate change), mechanisms to update and disseminate information over time on impacts on soil erosion and correspondent solutions are required. In this respect, strengthening the boundary role of agricultural‐extension office can potentially help updating information available to scientists, regulators and farmers on impacts and social and technical feasibility of solutions. This might prove a strategy to address some of the regulatory mismatches that hinder responses to SRS degradation at local level and promote adaptive management of soil regulation services.
Article
Ecosystem service approaches to watershed management have grown quickly, increasing the importance of understanding the streamflow response to realistic land-cover change. Previous work has investigated the relationship between watershed characteristics and streamflow in catchments around the world, but little has focused on systematic relationships between watershed characteristics and streamflow change after land-cover restoration. To address this gap, we simulate streamflow responses to restoring 10% of watershed area from agricultural land to forest and natural pasture in 29 watersheds around the world. This change is consistent with that performed in watershed-service programs. We calculate the change in a broad array of streamflow indices for each site and use a graph-connectedness approach to cluster the sites based on the sign of the index value changes. We find three primary clusters with distinct responses to restoration. Permutation tests and effect size demonstrate the difference in watershed characteristics and streamflow indices across clusters. The low-flow intensifying sites have shallower soils and smaller saturated soil volume. After restoration, simulated streamflow in these sites increases during relatively dry periods and declines during high-flow periods. The high-flow intensifying sites have larger saturated soil volume. After restoration, simulated dry-season flow in these sites decreases. The high-flow enhancing sites have larger soil hydraulic conductivities than the high-flow intensifying sites. After restoration, simulated dry-season flow in these sites decreases less than in high-flow intensifying sites. The soil depth and hydraulic conductivity appear to be the characteristics that determine clusters, as clusters are not statistically related to climate, watershed location, proximity, size and shape, elevation, or pre-existing land cover. This study provides valuable understanding of land-cover restoration and the watershed characteristics that most impact streamflow change.
Chapter
Considerable advances have been made since the first estimates of the impacts of invasive alien plants on water resources in the early 1990s. A large body of evidence shows that invasive alien plants can increase transpiration and evaporation losses and thus reduce river flows and mean annual runoff. Riparian invasions, and those in areas where groundwater is accessible, have 1.2–2 times the impact of invasions in dryland areas. The magnitude of the impacts is directly related to differences between the invading species and the dominant native species in size, rooting depth and leaf phenology. Information on the impacts has been successfully used to compare the water use of invasive plants and different land cover classes, to quantify the water resource benefits of control measures, and to prioritise areas for control operations. Nationally, the impacts of invasive alien plants on surface water runoff are estimated at 1.44–2.44 billion m³ per year. The most affected primary catchments (>5% reduction in mean annual runoff) are located in the Western and Eastern Cape, and KwaZulu-Natal. If no remedial action is taken, reductions in surface water runoff could increase to 2.59–3.15 billion m³ per year, about 50% higher than current reductions. This review illustrates the importance of measuring water-use over as wide a range of species as possible, and combining this with information from remote sensing to extrapolate the results to landscapes and catchments. These methods will soon provide much more robust estimates of water use by alien plants at appropriate spatial and temporal scales. The results of these studies can be used in water supply system studies to estimate the impacts on the assured yields. This information can also be used by catchment water resource managers to guide decision-makers when prioritising areas for clearing and rehabilitation, and for targeting species for control measures.
Article
Increased water yield and baseflow and decreased peak flow are common goals of watershed service programs. However, is the forest management often used in such programs likely to provide these beneficial watershed services? Many watershed service investments such as water funds typically change less than 10% of watershed land cover. We simulate the effects of 10% forest-cover change on water yield, low flow, and high flow in hydrologic models of 29 watersheds around the world. The forest-cover changes considered are: forest restoration from degraded natural lands or agriculture, forest conversion to agriculture, and forest conversion to urban cover. We do not consider grassland restoration by removal of alien tree species from riparian zones, which does increase water yield and low flow. Forest restoration from locally-predominant agricultural land resulted in median loss in annual water yield of 1.4%. Forest restoration reduced low flow and high flow by ∼3%. After forest restoration, low flow increased in ∼25% of cases while high flow and water yield declined in nearly all cases. Development of forest to agriculture or urban cover resulted in a 1–2% median increase in water yield, a 0.25–1% increase in low flow, and a 5–7% increase in high flow. We show that hydrologic responses to forest cover changes are not linearly related to climate, physiography, initial land cover, nor a multitude of watershed characteristics in most cases. These results suggest that enhanced streamflow watershed services anticipated from forest restoration or conservation of 10% or less of a watershed are generally modest.
Article
The reductions in streamflow associated with timber plantations are of particular concern in South Africa and, as a means of sustaining flows, permits granted by the state for the establishment of plantations have required that plantings should be no closer than 20-50 m from streams and other waterbodies. This paper presents the results of three catchment experiments, analysed by the paired catchment method, that aimed to provide a quantitative evaluation of the water yield savings attributable to this practice. These experiments show conclusively that, for South African conditions, riparian vegetation is a more liberal user of water than vegetation in other parts of a catchment and that the clearing of indigenous forest or exotic trees in the riparian zone of the catchment will result in disproportionately greater gains in water yield than would result from clearing similar vegetation elsewhere in the catchment. First year flow increases from clearing of tall woody vegetation in the riparian zone ranged from 55 to 110 mm (9-44%) per 10% of catchment cleared. In the same catchments, clearing of similar vegetation in upslope (nonriparian positions) led to flow increases ranging from 27 to 35 mm (2.5-14%) per 10% of catchment cleared.
Article
Claims are frequently made that deforestation in this heavily populated region has negative effects on downstream flooding and sedimentation. This study examines soil hydraulic properties of a typical area in the Middle Hills were a forestation programme has been in operation for about 12 years. The general conclusion is that infiltration of most of the monsoon season rainfall would not be impeded by the soils measured in this study and very little overland flow could be expected in the heaviest of the season rain events. -from Authors
Article
South African forest plantations, by virtue of their presence in very limited areas of high rainfall, and their increased water use relative to preafforestation vegetation, have caused significant reductions in streamflow from afforested catchments. The demand for both water and wood products is expected to increase strongly in the future, challenging the forestry industry to increase efficiency, both in timber production and in minimising the impacts of forest plantations on water supplies. Climate change adds a degree of uncertainty to future scenarios. As GCM predictions improve and regional predictions of climate change become more certain, the consequences to forests and water supplies must be comprehended and anticipated. This paper reviews aspects of these relationships with potential significance for the minimisation of water use of plantations and for the prediction of climate change consequences.
Article
The southern Piedmont has undergone extensive cropland reversion during the twentieth century with row crops being replaced by forest and pasture. Ten continguous river basins with a total area of 54,020 km 2 had 10 to 28% of their respective areas reforested during the period 1919-1967. During the same period, water yield decreased 3 to 10 cm according to both regression and double-mass analysis. These reductions in water yield constituted a 4 to 21% decrease in annual stream discharge and were statistically significant for a majority of the basins. The reduction of water yields by forests tends to be greater for dry years than for wet years. There was little or no relation between the degree of reforestation and reductions of water yield at the scale of this study, but when our data are included with the universe of data, the variance of our data from the overall model is much less than in the universal set. The inclusion of our results extends the range and predictive power of the universal model, giving it greater utility for water yield planning.
Article
Changes in water yield, flood hydrology, and low flows caused by replacing indigenous forests and grasslands with commercial softwoods have been investigated in New Zealand since the mid-1970s. The long-term results of two of these studies are discussed here.The first deals with the conversion of mixed evergreen forest to pine plantation in the northwestern South Island. After a 2-year calibration period one catchment was left as the control (DC2) and the other two catchments were harvested in 1981, one by skidder (DC1) and the other by hauler (DC4), and planted in pines shortly thereafter. For the first 4 years after harvesting the average annual difference in water yields between DC1 and DC2 was 352 mm (69%), and between DC4 and DC2 it was 463 mm (90%), which equates to an annual increase of 312 mm (61%) and 344 mm (68%), respectively, when compared with the calibration period. Planting the harvested areas caused the water yield from both catchments to return to pre-harvesting levels within 8 years, and an estimated reduction in runoff of 340 mm within 5 years at DC4. Mean flood peaks increased after harvesting, especially for small and medium storms on the skidder-logged catchment (75–100%). The response of the storm quickflows to harvesting was similar but much more subdued. Low flows also increased after harvesting. Tree growth brought storm peak flows, quickflows, and low flows back to the levels of those in the original beech forest within 10 years.The second study examines the impact of converting tussock grasslands to pine plantations using data collected from two catchments in the eastern uplands of southern New Zealand. After a 3-year calibration period (1980–1982) one catchment was planted in pines over 67% of its area and the other was left in tussock. By 1989 the difference in annual water yield from the planted catchment was 130 mm, and between 1991 and 1994 it averaged 260 mm (27% of total runoff from the control). Differences in low flows (represented by the minimum annual 7-day mean) showed a similar trend, and suggest that in dry periods, afforestation of tussock grasslands can reduce water yields by 0.18 mm day−1. Higher interception losses from increased canopy evaporation is believed to be the main reason for the reduction in water yield. After 10–12 years of tree growth mean flood peaks had fallen between 55 and 65%, and quickflows had decreased by about 50%.
Article
The modification of the drop size spectra of natural and simulated rainfall by the canopies of three tropical plantation tree species was measured using a disdrometer. Contrary to previously published results, large differences were found between species in the degrees of modification. The median-volume drop diameters measured were 2.3, 2.8, and 4,2 mm for Pinus caribaea, Eucalyptus camaldulensis, and Tectona grandis, respectively. The characteristic drip spectra for the different species are equivalent to the corresponding drop size spectra for rainfall with approximate intensities of 50, 100, and 3000 mm h-1 (essentially infinite) for P. caribaea, E. camaldulensis, and T. grandis, respectively. These results have implications for the choice of the best tree species for areas susceptible to soil erosion.
Chapter
'Blue Revolution upturns some environmental applecarts - not for the hell of it, but so we can manage our environment better.' Fred Pearce, New Scientist This updated and revised edition of The Blue Revolution provides further evidence of the need to integrate land management decision-making into the process of integrated water resources management. It presents the key issues involved in finding the balance between the competing demands for land and water: for food and other forms of economic production, for sustaining livelihoods, and for conservation, amenity, recreation and the requirements of the environment. It also advocates the means and methodologies for addressing them. A new chapter, 'Policies, Power and Perversity,' describes the perverse outcomes that can result from present, often myth-based, land and water policies which do not consider these land and water interactions. New research and case studies involving ILWRM concepts are presented for the Panama Canal catchments and in relation to afforestation proposals for the UK Midlands.