Download WATER USE BY FORESTS, LIMITS AND CONTROLS

Tree Physiology 18, 625--631
© 1998 Heron Publishing----Victoria, Canada
Water use by forests, limits and controls
IAN R. CALDER
Centre for Land Use and Water Resources Research, The University of Newcastle upon Tyne, NE1 7RU, U.K.
Received July 24, 1997
Summary Based on a review of several studies that have
been carried out to determine the water use of forests in relation
to other crops in different regions of the world, it is shown that
the principal controls on evaporation from forests and shorter
crops vary markedly between the temperate and tropical regions and between the wet and dry zones of these regions.
Although there are detailed physical and physiological models
available that allow calculation of forest water use, these models are not always readily applicable. It is proposed that a
knowledge of the limits on evaporation can be used to devise
models of varying complexity for estimating water use of
forests in different regions and for predicting differences in
water use between forests and shorter crops. Limits on evaporation may be related to radiation, advection, tree physiology,
soil water, tree size or drop size. Examples are given of the use
of models based on the limits concept for solving forest related
water resource problems in Malawi and the U.K.
Keywords: advection, drop size, physiology, radiation, soil
water, tree size.
Introduction
A new understanding of forest evaporation has been gained
from a variety of experimental studies conducted by physiologists and forest hydrologists using a range of experimental
techniques and methodologies. ‘‘Natural’’ lysimeters have been
used to measure total evaporation. One of its components,
transpiration, has been determined by soil water measurements
(Calder 1990), micrometeorological and eddy correlation
methods (Dyer 1961), plant physiological studies and tree
cutting studies (Roberts 1977, 1978, Jarvis and Stewart 1979),
heat (Cohen et al. 1981), and radioactive (Kline et al. 1970,
Luvall and Murphy 1982) and stable isotope tracing methods
(Calder 1991). Evaporation of intercepted precipitation has
been determined by several techniques, including interception
gauges (Calder and Rosier 1976), gamma ray (Olsycka 1979,
Calder and Wright 1986) and microwave attenuation methods
(Bouten et al. 1991), ‘‘wet lysimeters,’’ and rainfall simulators
(Calder et al. 1996).
With equations for calculating evaporation based on solutions of the energy balance and aerodynamic transport equations by Penman (1948), and the incorporation in these
equations of physiological controls, through the use of a sur-
face resistance term (Monteith 1965), a conceptual framework
was established for the estimation of evaporation from any
surface. This framework has been developed by later workers,
using ever more complex sub-models of the interactions between the environment and physiological processes, to determine values for the surface resistance term (Whitehead 1998).
This approach has increased our understanding of plant water
systems. However, the models are often difficult to apply,
especially when the meteorological driving variables are not
available at short-time intervals and the model parameters, as
they relate to, for example, the spatial distribution of canopy
leaf area and species differences in stomatal response to environmental conditions such as vapor pressure, light and temperature, remain largely unknown. This raises the question of
whether the pursuit of ever greater detail in the understanding
of the processes and their interactions is the only approach that
should be adopted or whether other scales should also be
considered.
Historically, evaporation models reflected best the wet temperate climates, because they are primarily meteorological
demand driven models with moderating functions built on
physiological controls. However, in dry environments, evaporative systems are limited more by supply, and where demand
greatly exceeds supply, detailed measurement or modeling of
the meteorological demand in the evaporative equations becomes largely irrelevant.
This paper explores an alternative modeling approach for
estimating forest water use that focuses on supply limitations,
and seeks to determine whether a knowledge of the principal
controls and limits on forest evaporation can be used to derive
robust estimates of forest evaporation for different regions of
the world. Examples of the use of this modeling approach for
answering water resource questions related to forests are described.
The limits concept
Results from studies carried out in both wet and dry climates
of temperate and tropical regions of the world are interpreted
in terms of six types of limit on the evaporation process:
advection, radiation, tree physiology, soil water, tree size and
raindrop size (Table 1).
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CALDER
Table 1. Effects of land use and climate on the principal limits and
controls on evaporation.
Land use
Temperate climate
Tall crop
Short crop
Tropical climate
Tall crop
Short crop
Dry climate
Wet climate
Physiological
Soil water
Advection
Soil water
Physiological
Radiation
Physiological
Soil water
Tree size
Drop size
Physiological
Soil water
Radiation
forest covers. The high aerodynamic roughness of forest allows
the transport of heat to the forest surface from the air, and the
transport of water vapor from the surface to the atmosphere, to
occur at rates up to ten times higher than those possible from
shorter vegetation. The utilization of advected energy is therefore much higher for forests and is the principal reason for the
much higher evaporative losses from upland forests compared
with shorter vegetation types. In the U.K. upland forests,
advection can probably be regarded as not only a major source
of energy for forest evaporation but as the principal limit on the
evaporative process (Table 1).
Radiation and physiological limits Compared with forests,
shorter vegetation is less able to draw on advective energy to
augment evaporation rates. For shorter vegetation, aerodynamic roughness is less, and evaporation rates are more closely
linked to the supply of radiant energy, i.e., they are radiation
limited. Stomatal controls, i.e., physiological controls on transpiration, also become more important. Soil water deficits
recorded under heather (Calluna vulgaris (L.) Hull), grass and
coniferous forest at the Balquhidder and Crinan sites are shown
in Figure 1. Modeling studies indicate that, at these wet upland
sites (annual rainfall > 1500 mm), with vegetation growing on
generally deep peaty soils, soil water availability does not
usually limit evaporation (Calder 1990). The differences in soil
water deficits under heather and grass are principally a reflection of the increased physiological controls on transpiration
imposed by heather compared with grass. The much higher soil
water deficits recorded under forest than under heather or grass
provide another demonstration of the overriding importance of
interception in determining forest evaporation in these climates.
Evaporation studies in upland forest and moorland in the
U.K.----short and tall crops of wet temperate regions
Results from studies carried out at the Plynlimon experimental
catchments in mid-Wales, the Balquhidder experimental
catchments in central Scotland and at the Crinan Canal catchments in west Scotland, illustrate some of the important
controls on evaporation from vegetation growing in wet temperate climates (Calder 1990, Kirby et al. 1991, Calder 1992,
Johnson 1995).
Advection limit The Plynlimon ‘‘natural’’ plot lysimeter, containing 26 spruce trees that were hydraulically isolated by
containing walls and impermeable clay subsoil, demonstrated
the importance of the interception process for upland forest.
On an annual basis, forest interception losses, determined by
large plastic-sheet net-rainfall gauges, were about twice those
arising from transpiration, determined from lysimeter and neutron probe measurements. Total evaporative loss, from both
transpiration and interception, required a latent heat supply,
supplied by large-scale advection, which exceeded the radiant
energy input to the forest (Table 2). The uplands of the U.K.,
subject to a maritime climate typified by high rainfall, a large
number of rain days per year, and high windspeeds, is an
example of a region where large-scale advection of energy
routinely occurs from moving air masses as they pass over wet
Evaporation studies in lowland broadleaf forest in the
U.K.----tall and short crops of dry temperate regions
Physiological and soil moisture limits Measurements of evaporative characteristics and soil water deficits under ash and
beech forests in southern England indicated that, at these sites,
soil water deficits were strongly influenced by physiological
controls on transpiration but were also influenced by soil water
availability and, to a lesser extent, interception (Harding et al.
1992). Differences in soil water availability to the ash and
Table 2. Annual water use and energy balance of wet temperate and wet tropical forests.
Site
Rain
(mm)
Transpiration
(mm)
Interception
(mm)
Total evaporation
(mm equivalent)
Net radiation
(mm equivalent)
Wet temperate1
2013
335
864
617
Wet tropical2
2835
886
1481 ± 12%
1543 ± 10%
Wet tropical3,4
2593
1030
529
(26%)
595
(21%)
363
(13%)
1393
1514
1
2
3
4
Plynlimon, Wales, 1975 (Calder 1978).
West Java, Indonesia, August 1980--July 1981 (Calder et al. 1986).
Reserve Ducke Forest, Amazonia, Brazil, 1984 (Shuttleworth 1988).
The Amazonian site experiences dry periods that may limit transpiration.
TREE PHYSIOLOGY VOLUME 18, 1998
WATER USE BY FORESTS, LIMITS AND CONTROLS
Figure 1. Soil water deficits determined from neutron probe measurements made with access tube networks located to the full rooting depth
beneath mature spruce forest, heather and grass moorland at Crinan
and Balquhidder. Adapted from Calder (1990).
beech forests were strongly related to soil type. For ash and
beech growing on soil overlying chalk, the available soil water
was essentially infinite, whereas at the clay soil site the value
of the available soil water was about 280 mm. From measurements of stomatal conductance, Harding and colleagues (Harding et al. 1992) concluded that the physiological controls on
transpiration from the beech forest were sufficient to reduce
total annual water use to less than that expected for grass. This
result has recently been questioned and is the subject of the
study described later in this paper.
Radiation and soil water controls For grassland and other
short crops it is generally recognized that radiation and soil
water controls are the major determinants of evaporation. The
Penman approach (Penman 1948), which takes account of
radiation control within the calculation of potential evaporation, and soil water control within a ‘‘root constant’’ function,
has been shown to be adequate for calculating evaporation from
short crops in dry temperate regions.
Evaporation studies in forest and agricultural crops in
India----short and tall crops in dry tropical regions
Soil water and tree size limits Comparative studies of the
evaporative characteristics and soil water deficits in a Eucalyptus plantation, an indigenous forest and agricultural crops in
the dry zone of Karnataka, in southern India, indicated that soil
water availability was a major limit on evaporation for both
agricultural crops and trees. For the annual agricultural crop
studied, finger millet (Eleusine coracana (L.) Gaertn.), the
rooting depth was less than 2 m and the available soil water
was 160 mm. This compares with 390 mm for most of the
forest sites. At one of the Eucalyptus sites, the Hosakote
experimental site of the Karnataka Forest Department, the roots
are known to extend to a depth of 8 m (Calder et al. 1997).
Furthermore, eucalypt roots reach this depth within two to three
years of being planted. Thus, in addition to evaporating essentially all of the rainfall that enters the soil, these trees are able
to extract approximately 100 mm of additional water from each
meter depth of soil the roots penetrate. Therefore, the concept
627
of soil water availability cannot readily be applied at this site
(Figures 2 and 3).
Reduced soil water availability was the principal reason
why annual evaporation from agricultural crops was generally
about half that from either plantation or indigenous forest.
These Indian studies also demonstrated a linear, scaling relationship between transpiration rate and basal cross-sectional
area for the relatively young plantation trees investigated (less
than 7 years old) (Figure 4). Higher correlation coefficients
were obtained when transpiration rate was regressed against
basal cross-sectional area than against leaf area index (Calder
et al. 1992); however, it is not known whether this is because
the basal cross-sectional area represents a more fundamental
controlling property such as root extent or because of the high
experimental error asociated with the measurement of leaf area
index. For older plantations (> 10 years old) where self-thinning was occurring, the relationship was less well defined
(Calder et al. 1992).
It would appear that, although evaporative demand is the
driving mechanism for evaporation, for most of the year, it is
not limiting in dry tropical regions; the primary controls are
soil water availability, and, for tree crops, some factor relating
Figure 2. Soil water deficits (SWD) recorded beneath Eucalyptus
camaldulensis Dehnh. and Eleusine coracana (Finger millet) at the
Hosakote site, India.
Figure 3. Neutron probe measurements of profile volumetric water
contents beneath E. camaldulensis at the Hosakote site, India, from
day of planting in August 1992. Adapted from Calder et al. (1997).
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628
CALDER
Figure 4. Measured transpiration rates as a function of tree basal area
(g) at the Puradal site before and after the monsoon (July--September
1989). Adapted from Calder (1992). Measurements were taken on
February 18, 1989: q = (1.06 ± 0.07)g, r 2 = 0.90 (⋅⋅⋅s⋅⋅⋅); May 24,
1989: q = (0.66 ± 0.03)g, r 2 = 0.90 (− − d− − ); Νοvember 8, 1989: q
= (6.90 ± 0.20)g, r 2 = 0.99 (----m----) and January 3, 1990: q = (2.25 ±
0.22)g, r 2 = 0.63 (--⋅⋅--j--⋅⋅--).
to tree size. At the dry zone sites in India, which experience an
extended dry season, interception losses, amounting to less
than 13% of the annual 800 mm of rainfall are not important
in determining soil water deficits and are not a major component of the total evaporation. The results from semiarid Karnataka (Calder et al. 1993), indicating that evaporation is
limited principally by soil water availability and plant physiological controls, are therefore in direct contrast to the observations from the wet uplands of the U.K., where evaporation is
principally limited by atmospheric demand through advection
and radiation controls.
Evaporation studies in forests in Indonesia and Sri
Lanka----tall crops in wet tropical regions
Raindrop size controls In the wet tropics, interception is a
significant component of the annual forest evaporation. Studies
carried out in the rain forests of Indonesia (Calder et al. 1986)
and Sri Lanka (Hall et al. 1996) have revealed the importance
of raindrop size in determining interception losses from tropical forests. Application of a two-layer stochastic interception
model (Calder 1996b, Calder et al. 1996), which explicitly
takes drop size into account, more accurately describes the
initial wetting-up phase of the storm, and hence the overall
interception loss, than conventional interception models (Rutter et al. 1971). The stochastic interception model shows that
up to ten times as much rain is required to achieve the same
degree of canopy wetting for tropical convective storms, with
large drop sizes, than is necessary for the range of smaller drop
sizes usually encountered for frontal rain in the U.K. Similarly,
studies with rainfall simulators show that the final degree of
canopy saturation also varies with drop size, being greater for
drops of smaller size (Figure 5). Thus, drop size dependence
of canopy wetting provides part of the reason why forest
interception varies worldwide, and why interception losses
from coniferous temperate forests are much higher than from
tropical forests (Table 2).
Figure 5. Wetting-up response of Eucalyptus camaldulensis subject to
simulated rainfall with median volume raindrop sizes of 0.065, 0.27
and 73 mm3. The storage of water on the sample, C, is shown normalized by the maximum storage obtained with fine drops, Cmx, and the
depth of simulated rainfall applied, P1, is also normalized by Cmx. Also
shown as continuous lines are the wetting functions predicted by the
stochastic interception model for these drop sizes and with model
parameter values: ve = 4 mm3, vc = 11.5 mm3, L*p = 0.799. Adapted
from Calder et al. (1996).
Radiation limit The wet, evergreen forests of the tropics represent another region where climatic demand is likely to limit
forest evaporation. However, climate circulation patterns in the
wet tropics do not favor large-scale advection of energy to
support evaporation rates and so evaporation rates are likely to
be closely constrained by the availability of solar radiation
(Table 2). Because humid rain forest is able to convert, on an
annual basis, the equivalent of virtually all the net radiation into
evaporation, it is unlikely that any other land use will evaporate
at a higher rate. Therefore, conversion of forest to annual crops
in these areas, as well as in most other areas of the world, is
likely to result in increased annual streamflows.
Application of the limits concept----a lowland England
case study
Models based on the limits concept have been used to assess
forest impacts on water resources in wet climates where interception loss predominates: the uplands of Scotland (Price et al.
1995), and in New Zealand (Calder 1996a) on the Otago
catchments (Fahey and Watson 1991, Fahey and Jackson 1997)
on the South Island. They have also been applied in the relatively dry climate of Malawi, in southern Africa (Calder et al.
1995, D.J. Price, Institute of Hydrology, Stirling, Scotland,
unpublished observations), where increased transpiration
through deep rooting is largely responsible for increased evaporative losses from forests. Although not ideally suited to the
application of the limits concept, these models have also been
applied to the intermediate climatic conditions of lowland
U.K. to obtain information on the possible hydrological impacts of the proposed doubling of lowland forests in England
(Rural England White Paper 1995).
Assessment of the impacts of lowland forest on hydrology
is difficult because processes are at work that can either increase or decrease forest evaporation. In the lowlands of the
TREE PHYSIOLOGY VOLUME 18, 1998
WATER USE BY FORESTS, LIMITS AND CONTROLS
U.K., both the effects of higher interception losses in wet
periods and greater transpiration during dry periods operate,
but neither effect predominates. Furthermore, during non-rainfall periods, when soils are sufficiently wet to offer no restriction to the availability of soil water to either short crops or
forests, transpiration rates from forests are usually about 10%
less than from shorter crops. Consequently, in the present
British lowland climate, the prediction of forest evaporation,
unlike that for short crop evaporation, is uncertain.
Information on evaporative differences among different tree
species × soil type combinations in the lowlands of the U.K. is
limited, and for some important tree species × soil type combinations, it is nonexistent. A study carried out at Thetford
forest in the east of England in the early eighties (Cooper 1980)
showed that recharge under a pine forest was reduced by as
much as 50% compared with grassland. The pine trees were
able to root into and extract water from the underlying chalk,
whereas the grass, because of a shallower rooting system, had
limited access to water. In contrast, based on a study at Black
Wood, Harding et al. (1992) concluded that beech afforestation
of grassland overlying chalk would increase recharge by 18%.
A review of published results on the water use of forests in
lowland England revealed a range of impacts, but only the
results from Black Wood indicated increases in runoff or recharge as a result of afforestation. The conclusions of Harding
et al. (1992) concerning the evaporative losses from the forest
at the Black Wood site were partly based on results of soil
water accounting models which included drainage functions
that predicted significant drainage (~150 mm) taking place
from the soil/chalk profile during conditions of soil water
deficit. (The drainage function was required to give consistency with the transpiration values calculated from diffusion
porometer measurements of stomatal conductance.) To investigate whether an alternative interpretation of the Black Wood
results was possible, the conclusions of Harding et al. (1992),
based on their drainage function for chalk of ~150 mm, were
compared with a new scenario based on a drainage of ~25 mm
(Calder et al. 1997). The modeling approach adopted by Calder
et al. (1983), for modeling soil water deficits in relation to soil
water availability using a ‘‘step length’’ to represent freely
available water, was used. The model includes a transpiration
fraction, β, to relate actual transpiration to potential evaporation under conditions of adequate water, together with a twoparameter exponential interception model (Calder 1990),
where parameter γ represents the maximum interception loss
that can occur in a day and δ defines the rate at which, with
increasing precipitation, it is achieved.
The model predictions of soil water deficits due to evaporation alone (no drainage function has been incorporated) obtained with model parameters relating to the original
hypothesis of Harding et al. (1992) and the new hypothesis are
shown in Figure 6. For comparison, the measured soil water
deficits are also shown in Figure 6. The model parameters
relating to the original scenario (Harding et al. 1992) and the
new scenario, which were obtained partly from default values
(Calder 1990) and partly by adjusting the interception model
parameters to give a better fit to the observed soil water
deficits, are given in Table 3. The new scenario predicted that,
629
Figure 6. Model predictions of soil water deficits resulting from
evaporation at the Black Wood site for beech (original (---- ----) and new
scenarios (--------)), and for grass (fine line) together with the observed
soil water deficits under beech (m).
as a long-term mean, the annual evaporation from broadleaf
forest (beech) is 105 mm higher than from grassland and the
average recharge (assuming no runoff) would be reduced by
Table 3. Evaporation model parameters for the different vegetation
covers on different soils. Step length relates to available soil water, β
is the transpiration fraction and γ and δ are interception parameters.
Soil type
Vegetation
type
Original
scenario
New
scenario
Chalk
Step length
β
γ
δ
Winter γ
Winter δ
Grass1
160
1
0
-0
--
Beech2
1000
0.75
2.23
0.21
1.84
0.108
Beech3
1000
0.9
4.46
0.099
3.68
0.099
Sand
Step length
β
γ
δ
Winter γ
Winter δ
Grass1
53
1
0
-0
--
Pine4
83
0.9
4.6
0.099
4.6
0.099
Broadleaf 5
83
0.9
4.46
0.099
3.68
0.099
Pine + broadleaf
83
0.9
4.6
0.099
3.68
0.099
Clay-loam
Step length
β
γ
δ
Winter γ
Winter δ
Grass1
75
1
0
-0
--
Pine4
200
0.9
4.6
0.099
4.6
0.099
Broadleaf 5
200
0.9
4.46
0.099
3.68
0.099
Pine + broadleaf
200
0.9
4.6
0.099
3.68
0.099
1
2
3
4
5
Calder et al. (1983).
Harding et al. (1992).
Calder et al. (1997).
Cooper and Kinniburgh (1993).
Cooper and Kinniburgh (1993); model parameters based on new
scenario.
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630
CALDER
38% as a result of broadleaf afforestation of grassland. This
predicted reduction of recharge of 38% contrasts with the
increase in recharge of 15% predicted by the study of Harding
et al. (1992).
The same models and model parameters used by Harding
et al. (1992) were applied to a site overlying an aquifer, the
Nottingham Triassic sandstone (Cooper and Kinniburgh
1993), and again it was predicted that broadleaf afforestation
on previously grassland areas would increase recharge. However, if the reservations concerning the use of these parameters to describe the evaporative response of forest on chalk are
well founded, the same reservations must apply to the conclusions drawn of reduced evaporation from broadleaf forest, as
compared with grass, on sandstone. To investigate the range
of impacts, a modeling study using a GIS-based version of the
HYLUC model (Calder 1996a) was carried out using the new
scenario model parameters for broadleaf forest but adjusted to
take into account the differing soil water availabilities expected on the sand and clay-loam soils in the Greenwood
Community Forest in Nottinghamshire (Table 3). The new
scenario predicted that the long-term mean annual evaporation from broadleaf forest on sandy soils is 69 mm higher than
from grassland and the mean recharge plus runoff would be
reduced by 51% as a result of broadleaf afforestation of
grassland (Figure 7). For broadleaf afforestation on clay-loam
soils, the predicted reduction in recharge plus runoff would be
62%.
The calculated cumulative recharge plus runoff from the
Greenwood Community Forest assuming the present forest
cover is shown in Figure 8. Also shown is the calculated
cumulative recharge plus runoff for a future forest cover sce-
Figure 8. Calculated recharge plus runoff for the Greenwood Community Forest for the period 1969--1993 assuming the present forest cover
throughout the period and for threefold increase in forest cover.
nario representing a proposed threefold increase in forestry,
where it is assumed that the increases occur in proportion to
the present distribution of forestry on the different soil types.
Over the 24-year period from 1969 to 1993, the calculated
mean reduction in recharge plus runoff from the Greenwood
Community Forest, as a result of a threefold increase in forest
cover, from the existing 9 to 27%, is 14 mm (11%).
The model predictions obtained, based on the two scenarios,
differ greatly with respect to the hydrological impact of increased lowland afforestation of broadleaf forest. Earlier studies indicated that broadleaf afforestation of grassland
overlying chalk or sandstone bedrock would have beneficial
impacts on water resources by increasing recharge. The alternative scenario indicates the opposite. Of primary concern, if
this scenario is correct, are the local implications of increased
forestry in areas where water resources are already utilized to
the limit or where low flows in rivers are causing environmental concerns.
Conclusions
Figure 7. Predicted cumulative evaporation for different land uses at
the Greenwood Community Forest. Mean annual evaporation:
grass/sand, 494 (− −d− −); conifer/sand, 581 (− −j− −); broadleaf/sand, 563 (− −r− −); mixed/sand, 564 (− −m− −); grass/loam, 503
(−−d−−); conifer/loam, 600 (−−j−−); broadleaf/loam, 577 (−−r−−);
mixed/loam, 579 mm (−−m−−). Mean annual rain is 628 mm.
The search for increasing realism in general biophysical models has led tree physiologists to build complex models that are
becoming increasingly difficult to apply to real-world waterresource problems. The approach outlined here presents an
alternative or supplementary methodology for assessing forest
water use. It incorporates knowledge gained from detailed
studies of forest water use, and seeks to use this knowledge to
identify the limits that may be controlling water use in a
specific environment. The approach has led to the identification of some of the important controls on the water use of
forests in different regions of the world and it has also has been
used in GIS-based evaporation models for assessing the hydrological impacts of forests for the purpose of integrated land and
water resources management.
TREE PHYSIOLOGY VOLUME 18, 1998
WATER USE BY FORESTS, LIMITS AND CONTROLS
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