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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). 626 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). TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com 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. TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com 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 References Bouten, W., P.J.F. Smart and E. De Water. 1991. Microwave transmission, a new tool in forest hydrological research. J. Hydrol. 124:119-130. Calder, I.R. 1978. Transpiration observations from a spruce forest and comparison with predictions from an evaporation model. J. Hydrol. 38:33--47. Calder, I.R. 1990. Evaporation in the uplands. John Wiley & Sons Ltd., Chichester, 166 p. Calder, I.R. 1991. Implications and assumptions in using the total counts and convection-dispersion equations for tracer flow measurements--with particular reference to transpiration measurements in trees. J. 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Swaminath, G.S. Kariyappa, N.V. Srinivasalu, K.V. Murty and J. Mumtaz. 1992. Deuterium tracing for the estimation of transpiration from trees. 3. Measurements of transpiration from Eucalyptus plantation, India. J. Hydrol. 130:37--48. Calder, I.R., R.L. Hall and K.T. Prasanna. 1993. Hydrological impact of Eucalyptus plantation in India. J. Hydrol. 150:635--648. Calder, I.R., R.L. Hall, H.G. Bastable, H.M. Gunston, O. Shela, A. Chirwa and R. Kafundu. 1995. The impact of land use change on water resources in subSaharan Africa: a modelling study of Lake Malawi. J. Hydrol. 170:123--135. Calder, I.R., R.L. Hall, P.T.W. Rosier, H.G. Bastable and K.T. Prasanna. 1996. Dependence of rainfall interception on drop size: 2. Experimental determination of the wetting functions and twolayer stochastic model parameters for five tropical tree species. J. Hydrol. 185:379--388. Calder, I.R., P.T.W. Rosier, K.T. Prasanna and S. Parameswarappa. 1997. 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Meteorol. 84:69--82. Fahey, B.D. and A.J. Watson. 1991. Hydrological impacts of converting tussock grasslands to pine plantation, Otago, New Zealand. J. Hydrol. 30:1--15. Hall, R.L., I.R. Calder, E.R.N. Gunawardena and P.T.W. Rosier. 1996. Dependence of rainfall interception on drop size: 3. Implementation and comparative performance of the stochastic model using data from a tropical site in Sri Lanka. J. Hydrol. 185:389--407. Harding, R.J., R.L. Hall, C. Neal, J.M. Roberts, P.T.W. Rosier and D.K. Kinniburgh. 1992. Hydrological impacts of broadleaf woodlands: implications for water use and water quality. Institute of Hydrology, British Geological Survey Project Report 115/03/ST and 115/04/ST for the National Rivers Authority. IH, Wallingford, U.K. Jarvis, P.G. and J.B. Stewart. 1979. Evaporation of water from plantation forest. In The Ecology of Even Aged Forest Plantations. Eds. E.D. Ford, D.C. Malcolm and J. Atterson. Inst. Terrestial Ecology, NERC, Cambridge, pp 327--350. 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