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Estimation of interception capacity of the forest floor
_•
Journal of
Hydrology
ELSEVIER
Journal of Hydrology 180 (1996) 283-299
Estimation of interception capacity of the forest floor W i l l i a m M. P u t u h e n a * , I a n C o r d e r y School of Civil Engineering, The University of New South Wales, Sydney, N.S.W. 2052, Australia
Received 14 June 1995;revision accepted 26 July 1995
Abstract
Methods of measuring interception capacity of the understorey (grasses) and litter layer have been developed to estimate the forest floor interception capacity of a 15-year-old Pinua radiata plantation and a native dry sclerophyll eucalypt forest at Lidsdale State Forest, Australia. In this study, interception by various types of forest floor have been measured in the laboratory using a technique of applying artificial rain to undisturbed samples of the forest floor. These laboratory experiments separately measure the interception storage capacity of the pine needle mat, the leaf/twig/bark debris mat in the eucalypt forest, and of the understorey (grasses). The results indicate that the interception storage capacity of all components of the forest floor of both vegetation types were proportional to the mass per unit area of forest floor cover. It was also shown that the interception storage capacity of the pine needle mat and the leaf/twig/bark debris mat under eucalypt were proportional to the thickness of the surface debris. For standing grasses the capacity was proportional to the percentage of ground cover. These laboratory results were then used to estimate the forest floor interception storage capacity of two experimental catchments each covered by one of the two forest types. In each case the forest floor was extremely heterogeneous, and so a large number of undisturbed samples were examined. Approximate forest floor interception capacity of the pine catchment was 2.8 mm and of eucalypt was 1.7 mm. The contribution of leaf litter, stem and branch litter, and grass vegetation to the overall interception capacity was similar for both catchments at 47%, 8% and 45%, respectively.
1. Introduction In a forest catchment, where forest floor litter has developed on the soil surface, the surface litter will intercept both throughfall and stemflow. Miller (1977) reported that typically, 1-3 kg m -2 o f liquid water can be stored on forest vegetation, and a like
* Corresponding author. 0022-1694/96/$15.00 © 1996- Elsevier Science B.V. All rights reserved SSDI 0022-1694(95)02883-8
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amount can be retained on the forest floor. The hydraulic mechanisms of the forest floor interception process are similar to the canopy interception process. Thus the amount of rainfall intercepted is also similarly related to the water storage capacities of the surface components. Helvey and Patric (1965) reported that the water storage capacity of the forest floor of US Eastern hardwoods, is 135-170% of the weight of the forest floor litter. However, there have been few other studies of the storage capacity of forest floor litter. In determining water storage capacity of the canopy vegetation, conventional rainfall gauges and trough gauges can be installed under the canopy. For forest floor litter, however, this straightforward method presents obvious mechanical and spatial difficulties. The mechanical difficulty is due to lack of space between the litter and the mineral soil or the grading of the litter into the soil. In a catchment, the thickness of the litter layer can vary from a few millimetres to a few centimetres. The type and composition of the litter on the forest floor in a catchment also varies greatly from spot to spot and the components of the forest floor litter are so intermixed that identification of boundaries between distinct forest floor components is difficult. The spatial variability of the amount and composition of litter layer makes it difficult to measure forest floor interception for a whole catchment. The inherent difficulties in making measurements of forest floor interception probably accounts for the relative scarcity of information. In this study an attempt has been made to develop equipment and techniques to investigate the forest floor interception capacity in eucalypt and pine forest catchments by taking into account the spatial variability of the composition of forest floor in the catchment. To provide information of more widespread utility, the interception storage capacity of each forest floor component was determined independently and the relationships between litter amount and water storage capacity of each forest floor litter type was also investigated. Furthermore, contrary to some other operational definitions of the forest floor, which only include organic debris shed by vegetation upon the surface of the soil, the definition adopted in this study includes green and dead standing grasses. These form an integral part of the forest floor, standing, dying and decaying in situ rather than being shed like the leaves of trees or shrubs. An arbitrary upper height limit of 50 cm was adopted to include most of the blade grass on the study catchments.
2. Site and forest floor description 2.1. Site
The study was carried out within the Lidsdale State Forest, (Lidsdale S.F.), approximately 130 km west-northwest of Sydney, Australia at latitude 33°241 south and longitude 1500041 east. Two adjacent catchments were selected for the present study. The area of the catchments selected are 6.2 ha and 9.4 ha with surface slopes of 13% and 12%, respectively. The first catchment is in a natural forest of native eucalypt, the principal species being Eucalyptus rossii and Eucalyptus mannifera and this
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association forms an open forest with Eucalyptus dives and Eucalyptus dalrympleana, the trees growing more closely on the ridge tops than in the valleys. The understorey is not well developed. The average projective canopy cover is 34%, and the trees are generally 8-14 m tall. The second catchment is in 15-year-old Pinus radiata plantation. It was a first-rotation stand planted on the site of a cleared native eucalypt forest. The stand was pruned in 1986 when the stand was 8 years old. The average projective canopy cover is 67%, and the trees are generally 16 m tall. Total basal area of the eucalypt catchment was 21.1 m 2 ha -1 with a density of 960 trees ha -1 (> 1'0 era diameter at breast height) whereas basal area of pine catchment was 27.4 m 2 ha -1 with a density 1024 trees ha -l ( > 10 cm diameter at breast height). Based on continuous measurement of gross rainfall and throughfall for the period of two years (1993-1994) on both eucalypt and pine catchments, it was found (by the method of Leyton et al., 1967) that the canopy storage capacities were 0.63 mm and 1.31 mm for eucalypt and pine catchments, respectively. The study area has subtropical climate; winters (April-October) are moderate with short cold periods and the summers (November-March) are mild to hot. Based on the data period of 1964-1993, the mean annual rainfall of 890 mm is fairly well distributed throughout the year with an average of 128 raindays per year. 2.2. Forestfloor Htter
In Lidsdale S.F., there are some clearly observable differences on the forest floor of pine and eucalypt catchments. In the eucalypt catchment, litter is patchily distributed and consists of fragmented remnants of woody plants and leaves; while in the pine catchment there are frequently areas where the forest floor is a mat of pure pine needles and no undergrowth occurs. A characteristic of the pine catchment forest floor is a sharp boundary between the litter and the mineral soil. The litter layer of this catchment may be lifted as a carpet. This may be contrasted with the characteristics of the eucalypt catchment forest floor. Eucalypt forest has larger leaves than pine, has low biomass and often there is no clear dividing line between the litter layers and their underlying mineral soil. However, very little is known about the nature and development of the forest floor under these two different vegetation covers and with the exception of the work done by Stafford (1976), no suitable measurements have been made for Lidsdale S.F. Stafford (1976) reported that the largest component of the litter fall in eucalypt forest per year was leaves, 0.145 k[g m -2, followed by bark, 0.085 kg m -2, and then branches and twigs, 0.050 kg m-L In the pine plantation, the litter fall per year consists of leaves as the largest component, 0.229 kg m -2, followed by branches and twigs, 0.014 kg m -2, and then bark, 0.001 kg m -2. She also reported that the litter fall in the eucalypt forest reflects the average long term situation under the present management regime. In contrast to the eucalypt forest, the data collected in the pine forest does not necessarily reflect the average conditions throughout the duration of the rotation. Marked changes in rates of litter fall can be expected, especially during the establishment phase of a new crop, and as a result of management practices (thinning and pruning). Knights (1983), in a botanical investigation of
W.M. Putuhena, L Corder), / Journal of Hydrology 180 (1996) 283-299
286
the Pinus radiata plantation at Lidsdale S.F. showed that the pine developed exponentially during the early years. Knights estimated that by 8.5 years the root and canopy system had almost completely occupied the available space so that conditions remained stable thereafter. These suggest that the forest floor of the 15-year-old Pinus radiata plantation can be assumed to be at a steady state.
3. Method and experimental procedure In this study, the interception capacity of the forest floor of the study catchments were estimated using a combination of field and laboratory measurements. As shown in Fig. 1, the procedure involved three major activities. First, field measurement to produce the maps of the forest floor litter for each study catchment (forest floor mapping). The forest floor mapping produced a set of single feature choropleth maps that demonstrate the spatial distribution of each particular component of the forest floor. Second, laboratory measurements to determine the water storage capacity of the forest floor samples which were taken from each classified subgroup of the forest floor. Third, as shown in Fig. 1, the results of the laboratory measurement were then extrapolated to the study catchment by using the series of single feature choropleth maps to estimate the water storage capacity of the forest floor litter of the catchment. 3.1. Forest floor mapping In this study, the forest floor mapping was concerned with the spatial recording of the categorised visible features of the forest floor litter of each of the study catchments A set of single feature
Base map + data collected by
chotop~thmaps~ the
detailed ground survey
forest fl¢~¢ ¢~the
catchment
A set ~ single feature chompleth maps of the i n t e r ' o n storage capaaty A choropleth map of the ]
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[ll Forest Floor Mapping
- Classification of the forest floor liuer components - Detailed groond sm'vey
[21 Laboratory Determhmtlen ef W a t e r Storage C a l ~ d t y d F o r m Floor ~ Components - Sample collection and preparation - San']pietest ron using an artifida] rainfall
[3] Extrapolalion of the
Laboratory Results to the Catchment
- Data pre.~entation
Fig. 1. Procedure to estimate interception storage capacity o f the spatial variability o f the forest floor litter in a catchment.
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287
and has two purposes. The first is to present the spatial variability of the forest floor of each catchment. The second is to extrapolate the laboratory water storage capacity measurements of the forest floor litter samples to the study catchments.
3.1.1. Classification of the data to be mapped The forest floor components in the study catchments have been classified into two main groups. The first group is litter which includes organic debris shed by vegetation. The second group is grass vegetation including green grass, standing dead grass and their organic debris. The litter layer in the study catchments consists of leaves, twigs, bark, flowers, and fruits or seeds (small items), as well as fallen stems and branches (large items). For the purpose of this study the litter layer was divided into two groups, namely, leaf litter, including all small items, and non leaf litter, consisting of large items. The grass vegetation was also divided into two groups, namely tussock grass and blade grass. Both grass groups include their standing dead material and organic debris. Based on the classification of the forest floor components described above, the forest floor of the pine catchment, comprises four groups, namely leaf litter (pine needle, flowers, and fruits or seeds), non-leaf litter (stems and branches), tussock grass and blade grass while the eucalypt forest comprises only three groups, namely, leaf litter (leaves, bark, flowers, and fruits or seeds), non-leaf litter (stems and branches) and tussock grass. Furthermore, the heterogeneous nature of each group of forest floor components within the stand has led to subclassifications of each group based on the thickness for the leaf litter, the density index (scale: 1 to 5; 1 is very sparse and 5 is very dense) for the stems and branches, and percentage of ground cover for the grass. 3.1.2. Discretisation of the study catchments A reference grid was set out on each catchment. The chosen grid spacing needs to be sufficientlyclose to minimise the variability of components within each grid square. At the same time the total number of squares has to be small enough for data to be collected for each square within the time and financial constraints that are operative. In this study, the size of the grid has been governed by the capability of visual observation to provide a reasonable estimate of the average component within each grid square. Thus the size of the grid should be easily covered by the eyes from one position. In this study a grid of size 25 m x 25 m was selected giving a total breakdown of the eucalypt catchment into 113 and the pine catchment into 171 squares. 3.1.3. Detailed fieM survey In this survey, a sampling procedure was not employed because the area of the study catchments to be mapped was small (6.2 and 9.4 ha), so that the total area could be covered by the detailed field survey. In the survey or field data collection, the values estimated are the percentage of grass cover, the thickness of the leaf litter, and the density index of the stems and branches in each grid square. The most effective estimation was made by Using categories rather than a continuous scale. For example, when estimating percentage cover of grass vegetation, the
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ranges 1-20, 20-40, 40-60, 60-80 and 80-100% were used. Estimating a specific percentage, such as 46%, is probably not realistically possible. To estimate the percentage of grass cover within the grid, the area was observed through a combined visual and mental process. All grass of the same species was placed together, and the portion of the plot they covered was ascertained by mentally dividing the area in half, and halves into quarters, and asking the question, 'is more than a quarter or more than half of the plot covered by that species?'. Through this technique, repetitive, comparable measures of grass cover were achieved. Other measurements depend on the subject matter and are easily made after gaining some field experience. For example, the thickness of the leaf litter was estimated initially by randomly sampling at a number of points. Leaf litter for each sampling point was exposed and its thickness was measured. This was done separately only for the first three or four grids to mentally fix the general thickness ranges of objects and was estimated thereafter. The collection of information necessary for mapping of the forest floor component was dependent on the detailed field survey described above. To reduce the subjectivity of this estimation, two people worked independently on each grid square during the survey.
3.1.4. Data presentation As the data collected are in the form of the average values per unit area (grid of 25 m x 25 m) it is more suitable to present the spatial variability of the forest floor by using a choropleth map rather than an isoline map. A choropleth map is a map in which quantitative spatial distributions are depicted from data that have been calculated from mean values per unit area. It is produced by dividing an area into squares or hexagons, and finding a mean value for each. A range of shading, stippling or colouring is used to show successive orders of mean values. The complexity of the forest floor commonly makes it difficult and impractical to represent the variability of all forest floor components on a single map. For example, in many grids, three or four components of the forest floor are mixed together and underlie each other. A single map showing the thicknesses and densities together with the percentage of the grass cover would be practically illegible. It was decided, therefore, that a series of single-feature choropleth maps (Fig. 1) are needed to present the various components of the forest floor in each study catchment. A set of the single feature choropleth maps of the pine catchment is presented in Fig. 2. The maps of individual groups of the forest floor components can then be directly overlaid both for computational purposes and for comparison of the spatial distribution of the different forest floor components. 3.2. Laboratory determination of the interception storage capacity The laboratory determination of the interception storage capacity for each group of forest floor components involved two distinct processes: sample collection and preparation, and laboratory test runs. In the following section, before the two processes are described, the sampling technique used in this study is described.
W.M. Putuhena, I. Corder), / Journal of Hydrology 180 (1996) 283-299
weir m
I. . . . . . . . .
289
I
.,,,q-
w ir LEAF LITTER GRID (25m x 25m)
[] [] []
[a]
thickness (cm)
STEM & BRANCH [b] density index
none
none
none
[
o ,,ss I
TUSSOCK GRASS [el ground cover (%)
BLADE GRASS [dl ground cover (%)
none
none
1
very sparse
1
< 20
< 20
2
sparse
2
20 - 40
20 - 40
3
medium
3
40-60
40-60
4 5
dense very dense
4 5
60 - 80 > 80
60 - 80 > 80
Fig. 2. A set of single-feature ehoropleth maps of the forest floor litter of the pine catchment.
3.2.1. Sampling technique T h e spatial variability i n a m o u n t o f litter o n the forest floor has been well docum e n t e d . F o r example, Blow (1955) r e p o r t e d t h a t 40 plots, each m e a s u r i n g 0,4 o f a s q u a r e metre, were n e e d e d to m a i n t a i n a s a m p l i n g error o f 5 % o f the total litter weight. Similarly Helvey (1964), o n 77 occasions, collected 12 litter samples, each 0.18 o f a square metre, a n d the coefficient o f v a r i a t i o n averaged 2 0 % for each g r o u p
290
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
of 12 samples. These two studies showed that owing to the spatial variability of the amount of the forest floor litter in a catchment, a large number of random samples were needed to give an accurate assessment. This makes it necessary to resort to purposive or stratified sampling. In this study, purposive sampling was adopted after the forest floor litter had been grouped and classified. The classification described in Section 3.1.1 gave a total of 20 groups of forest floor litters under pine stands and 15 groups of forest floor litters under eucalypt stands. By using the purposive sampling technique, each sample was selected by visual observation to represent one group of the forest floor litter. In this study, each group of the forest floor components was represented by at least one sample which had been selected from the field in an appropriate location.
3.2.2. Sample collection and preparation The method used to collect and prepare a particular sample depended on the characteristics of the sample.
3.2.2.1. Pine leaf litter sample In order to get an undisturbed sample, the pine leaf litter sample was collected within an area larger than the area of the sample needed for the experiment. The area of the sample for the laboratory experiment was 0.54 m x 0.60 m, while the pine leaf litter sample collected had an area of 0.70 m x 0.80 m. On the forest floor, the area selected for the sample plot was marked by removing the leaf litter around the border of the sample area. A 1 m x 1 m thin sheet of steel was forced along the top of the mineral soil at the base of the litter (at ground level). The thin sheet of steel with the sample was lifted and then the sample was slid into a tray and transported to the laboratory. In the laboratory, the size of the sample collected (70 cm x 80 cm) was reduced to 54 cm x 60 cm (to fit the sample tray) by trimming the damaged edge of the sample.
3.2.2.2. Eucalypt leaf litter sample The same procedure as that used for the pine leaf litter sample collection is difficult to apply to the eucalypt leaf litter sample because the boundary between mineral soil and the leaf litter under eucalypt stands is not as sharp as under the pine stands. The eucalypt leaf litter sample was collected by the following procedure; a sharp-edge galvanised steel frame with the inside dimension the same as the area of the sample needed for experiment was driven into the ground. The material contained within the frame was removed by hand in layers. First, a layer of fresh leaves and bark was collected and put into a plastic bag. Secondly, a layer of decaying fragmented leaves and bark was picked up and put into a separate plastic bag. In the laboratory, the sample of the decaying layer components were spread across the sample tray in its natural position based on the photographs taken in the field before the sample was collected. This was followed by the fresh leaf litter, these components also being placed on the sample tray as naturally as possible, based on the photograph taken in the field.
W.M, Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
291
3.2.2.3. Grass sample The grass sample was collected and prepared by the following procedure; a sharpedged galvanised steel frame with the inside dimension the same as the area of the sample needed for the experiment was driven into the ground. The decaying grasses and their organic debris were cut along the frame edge and horizontally at ground level, with minimum disturbance to the sward structure, and lifted out carefully. The decaying grass was put in the same position into a tray which had been covered by a plastic sheet. The green grass and the standing dead material were collected by digging up their roots and these were also put into the tray. In the laboratory, the frame was placed in a 1 m x 1 m tray. Inside the frame, grass vegetation was rearranged as naturally as possible based on the photographs taken before the sample was collected in the field. To support the standing grass and to provide a base wet clay was spread on the frame. A dry clay was poured to cover the whole surface of the wet clay. Melted paraffin wax was carefully poured on to the dry clay and allowed to solidify making a continuous seal over the dried clay. Before the paraffin wax solidified, aluminium tubes, 1 cm diameter, were pushed into the base to provide drainage holes. In the time between the paraffin wax drying and solidifying completely, the aluminium tubes were pulled out. When the wax had solidified completely the steel frame was removed and the sample with the paraffin wax was lifted, leaving the clay in place. The other parts of the sample (the dead standing grass and the fallen dead grass) were then rearranged on the sample as naturally as possible based on the field photographs. 3.2.3. Laboratory test run The flow of intercepted rainwater on samples was observed using the rainfall simulator described by Mutchler and Hermsmeier (1965). As shown in Fig. 3, uniform, artificial raindrops with mean diameter of 2.0-3.0 mm diameter were produced ~6~:~.~:~\\~\~ 0 0 . . . . . . o . 0 ° . . . o o o ,
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Fig. 3. Experimental set-up for the measurement o f simulated rainfall draining from the forest floor litter sample.
292
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
from the rain drop maker. The falling drops hit a splash-screen, suspended at approximately 2 m above the sample tray. The splash-screen is a plastic wire-netting composed of 0.4 mm diameter strands constituting a 3.1 mm square mesh. At the suspension level, this splash-screen 'reworks' the 2.0-3.0 mm drops to give a range of smaller drop sizes. Rainfall intensity was controlled by maintaining a constant head of water in the raindrop maker. Quantitative observations of water filtered through the forest floor litter sample and flowing down through a drain collector were made using a tipping bucket recorder. The tipping bucket, purpose built for this project, was designed to tip every 0.2 mm depth of rainfall for the area of the sample tray (54 cm × 60 cm). The tipping bucket recorder was then connected to a data logger. For each test run, the rate of rainfall was assessed by catching the 'rain' in the drain collector without any litter sample in place. If the desired intensity had been achieved the movable protection screen was closed and the forest floor sample was put into the drain collector. Before placement the sample was carefully weighed. To start a test run, the movable protection screen was opened and the timer was started. The rainfall continued until the rate of the draining water was constant. To stop the rainfall, the movable protection screen was dosed and the clock time was recorded. The test run still continued until water ceased draining from the sample. The wet sample was then weighed again. To measure variation of water detention on a stem and branch sample using this equipment is very difficult because water detained by this type of sample is very small. Instead water storage capacities for this type of sample were determined by weighing the sample before and after being subjected to simulated rainfall. Each sample was weighed and then subjected to simulated rainfall (90-100 mm h-l). Immediately artificial rainfall had stopped the sample was put in a plastic bag and weighed to determine the maximum water storage capacity. After 10-20 min the sample was removed from the bag and reweighed to determine the minimum water storage capacity.
4. Results and discussion
In this study, 55 samples of the forest floor litter were tested in the laboratory. The relation between mass per unit area and physical characteristics (estimated percentage of ground cover for the grass samples, thickness for the leaf litter samples and density index for stem and branch samples) is presented in Fig. 4 while Table 1 shows a summary of the resulting linear regressions. The derived regression equation is Y = r n X where Y is mass of forest floor litter per unit area (kg m-2), X is their estimated physical characteristic and rn is the regression coefficient. The direct linear relations show that the estimated physical characteristic can be used to indicate the amount of forest floor litter. In the laboratory, most of the samples were tested twice with different rates of artificial rain. An example of the relationships between water detention on pine leaf litter samples (thickness 2, 4 and 6 cm) and time during the test run and with rainfall amount are presented in Fig. 5. The curves in Fig. 5 were typical of all other types of
W.M. Putuhena, I. Cordery / Journal of Hydrology 180 (1996) 283-299
293
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40
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Fig. 4. (a) Relationships between m a s s (kg m -2) a n d thickness (cm) for leaf litter samples and between m a s s (kg m -2) a n d density index for stem and branch samples. (b) Relationships between mass (kg m -2) and percentage g r o u n d cover for grass samples.
samples tested in the laboratory and as indicated earlier, the artificial rainfall was stopped when the steady-state condition had been achieved while the measurement continued until the drainage had ceased. From a visual inspection of Fig. 5 and also for other samples the following points may be noted. (I) The test run made it possible to calculate the proportions of rainfall retained and drained after various amounts of simulated rain. The saturation of the interception storage is not achieved in the early stages of a storm event. Filling of the interception storage continued for some time into each storm event. (2) The rate of accumulation of stored water increases with increasing rainfall intensity. (3) Immediately after the simulated rain had ceased there was a maximum of water detention which was related to the thickness of the litter sample. Slightly higher maximum water detention was observed for the higher rainfall intensity. Although the difference was very small, consistently larger maximum water detention was observed with higher rainfall intensities for all samples. (4) When rainfall ceases there is rapid initial drainage which then slows and ends at Table 1 Details o f linear regression analysis o f the relation between m a s s o f the forest floor litter per unit area (kg m =2) a n d their estimatedphysical characteristic Sample type
Regression coefficient m 4- t0.05 SE
N u m b e r of sample (n)
Correlation coefficient (r)
Physical characteristic
Pine leaf litter Euc. leaf litter Tussock g r a s s - P i n e Tussock g r a s s - E u c . Blade grass Stem a n d branch
0.659 0.620 0.065 0.057 0.070 0.409
7 6 7 6 6 23
0.979 0.979 0.994 0.978 0.983 0.968
Thickness (cm) Thickness (cm) G r o u n d cover (%) G r o u n d cover (%) G r o u n d cover (%) Density index
4- 0.027 -4-0.024 4- 0.002 + 0.003 4- 0.003 4- 0.011
W.M. Putuhena, L Cordery I Journal of Hydrology 180 (1996) 283-299
294
s[ /
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40
60
80
100
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Fig. 5. Change in water detention of three different thicknesses of pine leaf litter with time (a) and with rainfall (b). Note: 1 and 2 indicate first and second test runs; other numerals e.g. 6(68) indicate sample thickness 6 cm and rainfall rate applied 68 mm h -l.
a lower storage. When drainage ceased there was still a large amount of water stored within the litter. (5) Good replication was recorded when a sample was tested twice. The differences between the curves of the successive tests were mainly due to differences in rainfall intensities between tests.
4.1. Interception storage capacity of the forest floor litter components Based on the data obtained from the laboratory experiment, two different interception storage capacities of the sample were determined for each test run. (1) Cma~, is the maximum interception storage capacity of the forest floor litter and it was taken as the amount of water detained in the sample immediately before the artificial rainfall ceased. (2) Cmi~, is the minimum interception storage capacity of the forest floor litter and it 7
5
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(4) + (3)
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3 2 1
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6
8
0
2
4
6
8
Mus t'kam"z)
Fig. 6. Relationships between Cmi~ (ram) (a); and Cm~ (nun) (b) and mass per unit area (kg m -2) for the six components of the forest floor fitter samples.
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Table 2 Details of linear regression analysis relating Cminand Cma~to the mass of forest floor litter per unit area Sample type
n
Cmin and C ~ vs. mass
Cmln and Cma~vs. physical characteristic a
a q- to.o5 SE
b 4- to.o5 SE
r2
a q- to.o5 S E
b 4- to.o5 SE
Pine leaf litter Cm~n 14 Cmax 14
0.011 4- 0.150 0.5964- 0.140
0.9624- 0.042 0.977 1.2494- 0.040 0.988
-0.348 4- 0.266 0.7194- 0.057 0.109 4- 0.283 0.9394- 0.061
0.930 0.953
EUC. leaf litter Cn~n 11 Cm~, 11
0.018 4- 0.180 0.5294- 0.148
1.1254- 0.068 1.3654- 0.056
0.969 0.985
-0.060 4- 0.188 0.7274- 0.046 0.451 4- 0.210 0.8784- 0.051
0.966 0.971
Tussock grass (Pine) Cmln 14 0.0694- 0.128 Cmax 14 0.3784- 0.170
0.5104- 0.022 0.7054- 0.029
0.978 0.979
0.013 4- 0.149 0.0344- 0.002 0.295 4- 0.185 0.0474- 0.002
0.970 0.976
Tussock grass (Euc.) Cm~n 11 0.1224- 0.165 (?max 11 0.4614- 0.215
0.5574- 0.032 0.7644- 0.042
0.971 0.974
-0.072 4- 0.178 0.0354- 0.002 0.200 4- 0.249 0.0484- 0.003
0.967 0.966
Blade grass Cmin Cmax
r2
12 12
0.3384- 0.114 0.985 4- 0.116
0.2964- 0.019 0.428q- 0.019
0.960 0.980
0.238 4- 0.116 0.0234- 0.001 0.849 4- 0.148 0.0324- 0.002
0.960 0.968
Stem and branch Cmin 23 Crnax 15
0.0764- 0.021 0.103 4- 0.023
0.2084- 0.010 0.2214- 0.011
0.950 0.967
0.012 4- 0.026 0.1014- 0.006 0.024 4- 0.030 0.1134- 0.007
0.924 0.953
a Physical characteristic is thickness (cm) for leaf litter, percentage ground cover for grass and density index for stem and branch.
w a s t a k e n as the a m o u n t o f w a t e r d e t a i n e d o n the s a m p l e after d r a i n a g e h a d ceased (Fig. 5). T h i s a m o u n t o f w a t e r c a n be r e m o v e d o n l y b y e v a p o r a t i o n . T h e values o f Cram a n d Cmax for e a c h test r u n s h o w a c o n s i s t e n t r e l a t i o n s h i p to either the e s t i m a t e d p h y s i c a l c h a r a c t e r i s t i c s (thickness for the l e a f litter sample, perc e n t a g e g r o u n d c o v e r for the grass s a m p l e o r d e n s i t y index f o r the stem a n d b r a n c h s a m p l e ) o r m a s s p e r u n i t a r e a o f the samples. T h e r e l a t i o n s h i p s b e t w e e n m a s s p e r u n i t a r e a (kg m -2) o f the forest floor litter a n d Cmin (ram), a n d Cmax (ram) a r e s h o w n in Fig. 6. A s expected, a l i n e a r r e l a t i o n s h i p w i t h c o r r e l a t i o n coefficient a b o v e 0.95 ( T a b l e 2) w a s f o u n d for all c o m p o n e n t s o f the forest floor litter. T h e d e r i v e d e q u a t i o n f o r this r e l a t i o n s h i p is Y = a + b X ; w h e r e Y is Cmin o r Cmax (mm), X is m a s s o f the forest f l o o r litter p e r u n i t a r e a (kg m -2) o r p h y s i c a l c h a r a c t e r i s t i c o f the forest floor litter, a is the i n t e r c e p t ( m m ) a n d b is the slope ( m m k g - I m2). A similar linear r e l a t i o n s h i p b e t w e e n Cn~n a n d m a s s p e r u n i t a r e a was also f o u n d b y P i t m a n (1989) f o r b r a c k e n litter while in c a n o p y i n t e r c e p t i o n studies, A s t o n (1979), H e r w i t z (1985) a n d P i t m a n (1989) f o u n d t h a t c a n o p y s t o r a g e capacities h a v e a n excellent linear r e l a t i o n s h i p w i t h l e a f a r e a index. I n the p r e s e n t study, even w i t h the m a s s p e r u n i t a r e a r a n g i n g f r o m 0.5 to 6 k g m -2, the s t o r a g e c a p a c i t y was linearly r e l a t e d t o the m a s s o f litter p e r unit area, b u t h o w far this can be e x t r a p o l a t e d is u n c e r t a i n . This
296
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
uncertainty occurs because as the litter becomes more fragmented, its absorption rate increases (Chen and Lindley, 1981, as reported by Pitman, 1989). The slope of the regression between storage capacities and mass of litter per unit area varies among forest floor litter components and it is apparent that there are four different groups of responses. In terms of Cmin, eucalypt leaf litter and pine leaf litter stored the largest quantity of water per increment of unit mass (kg m-2), 1.13 mm and 0.97 mm, respectively. Stem and branch litter held the least, 0.25 mm per increment of unit mass. The other types of forest floor litter (tussock grass and blade grass) made up intermediate groups. Tussock grass collected from pine and eucalypt catchments held an average 0.53 mm per increment of unit mass, whereas blade grass stored 0.59 mm per increment of unit mass. Table 2 also presents a summary of the linear regression analysis relating water storage capacities (Cminand Cmax)to their estimated physical characteristics• Taking into account estimation error and subjectivity relevant to the estimation of the physical characteristics it is understandable that the relationship between Cmin or Cmax and mass per unit area gives a better fit than the relationship between these storage capacities and the estimated physical characteristic. Extrapolation of the physical characteristic to zero is incorrect since some of the intercept values in Table 2 are less than zero and the linearity of the relationship is uncertain from the small data sample. It may serve, however, as an approximation until other data are available. In terms of mass of the forest floor litter per unit area, minimum storage capacities (Craig) for pine and eucalypt leaf litter reported here are smaller than previously reported • • values for other forest stands. Cminof pine leaf litter reported here Is 0.97 mm kg - 1 m 2 / 2 while eucalypt leaf litter is 1.13 mm kg- m . Pitman (1989) reported that Cmhaof bracken litter is 1.63 mm kg -1 m 2 while Dabral et al. (1963) and Pradham (1973) reported a value of 1.3 4- 0.32 mm kg -1 m 2. However, comparisons with previously reported values for forest floor litter are difficult owing to differences in forest stands. Bracken litter as reported by Pitman (1989) has a large surface area with highly fragmented pinnae and numerous broken tubular petioles, so that the rate of water absorption is relatively high when compared with litter of other species.
4.2. Interception storage capacity of the forest floor of the study catchments The spatial variability of the forest floor minimum interception capacity (Cmin), obtained by extrapolation of the laboratory results are presented in Fig. 7. The forest floor storage capacity in each grid is the weighted mean of the water storage capacity of all components of the forest floor. The average values of the minimum storage capacity of the forest floor of each study catchment and the contribution of each component are also presented in Fig. 7, while the results of the estimation of the maximum storage capacity of the forest floor of the study catchments are presented in Table 3. The stem and branch litter has relatively little influence on the forest floor storage capacity of these catchments; 8% for pine and 7% for eucalypt catchments. For the pine catchment, the contributions of the leaf litter and grasses to the storage capacity of the forest floor are 47% and 45%, respectively, while for the eucalypt forest they are 47% and 46%.
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
weir l a LEGEND Grid Storage (25m x 25m) capacity (ram)
[] [] [] m
none <1
297
[Euc. Catchment
Minimum Contribution of each forest floor storage component to the overall interception Catchment capacity of storage capacity of the catchment (nun) the catchment leaf stem & tussock blade (mm) litter branch grass grass
1-2 2-3
Pine
2.78
1.32
0.22
0.85
3-4
Eucalypt
1.69
0.79
0.12
0.78
0.39
>4 Fig. 7. Minimumstorage capacity (Craig)of the forest floor of the study catchments and the contributionof each forest floor component to the overall interception storage capacity of each catchment.
5. Conclusion A detailed p r o c e d u r e has been o u t l i n e d for e s t i m a t i o n o f the spatial variability o f the forest floor i n t e r c e p t i o n capacity o f two c a t c h m e n t s with different vegetative cover. This a p p r o a c h e n a b l e d assessment o f the effect o n forest floor i n t e r c e p t i o n storage capacity o f the c o m p o s i t i o n a n d spatial variability o f the forest floor Table 3 Maximum storage capacity ((?max)of the forest floor of the study catchments and the contribution of each component Catchment
Pine Eucalypt
Maximum storage capacity of the catchment (ram)
Contribution of each forest floor component to the overall interception storage capacity of the catchment (mm) Leaf litter
Stem and branch
Tussock grass
Blade grass
4.66 2.89
2.28 1.38
0.25 0.14
1.44 1.37
0.69 -
298
IV.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283:299
components. By relating Cmin and Cmax to the amount o f forest floor litter per unit ground area for each forest floor component, a method is provided which can be extrapolated to a catchment with the same plant species at different locations, possibly at different stages o f development and with different forest management practices. The only requirement to use this relation is an adequate measure o f the amount of forest floor litter in the catchment. Furthermore, the application o f the relationships between storage capacities and leaf litter thickness, densities o f stems and branches, and percentage ground cover of grass offers a promising method o f determining the interception capacity of forest floor litter in a catchment where the mass per unit area o f the forest floor litter is unavailable. The difference between Cmin and Cmax is an indication o f the water which actively drains from the forest floor litter after the cessation o f rainfall and may be regarded as a dynamic storage. This dynamic water storage is in reality not a fixed value but will vary with the intensity and droplet size of the rainfall and with the forest floor litter structure. F o r this reason the value of Cmax derived from this work should only be taken as an indication of the possible magnitude o f Cmax. The contribution o f this dynamic storage to interception losses is likely to be small unless the evaporation rate is very high since the forest floor litter samples were observed to drain down to Cmin within 6-30 min o f the cessation of rainfall. To the hydrologist, therefore, Cmin is a more important interception parameter than Cmax, as this represents the effects of the forest floor litter on the difference between catchment rainfall and the sum of soil water accretion and runoff. When calculating a water balance the term Cmin should be used as the storage capacity o f the forest floor litter.
Acknowledgements The authors thank Sabina Hamaty, Jim Tilley and Alfred Wojcik for their help during field and laboratory work. The authors also acknowledge the Australian Agency for International Development (AusAID) for scholarship funding.
References Aston, A.R., 1979. Rainfall interception by eight small trees. J. Hydrol., 42: 383-396. Blow, F.E., 1955. Quantity and hydrologic characteristics of litter under upland oak forests in eastern Tennessee.J. For., 53: 190-195. Chen, L.-Z. and Lindley,D.K., 1981.Primaryproduction, decompositionand nutrient cyclingin a bracken grassland ecosystem.MerlewoodRes. Dev. Pap., 80. Institute of TerrestrialEcology,Cambridge,66 pp. Dabral, B.G., Premnath, and Ramswarup, 1963.Some preliminaryinvestigationson the rainfall interception by leaf litter. Indian For., 89:112-I 16. Helvey, J.D., 1964. Rainfall interceptionby hardwood forest litter in the southern Appalachians. US For. Serv. Southeast. For. Exp. Stn. Res. Pap. 8. Helvey, J.D. and Patric, J.H., 1965. Canopy and litter interception of rainfall by hardwoods of eastern United States. Water Resour. Res., 1: 193-206.
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299
Herwitz, S.R., 1985. Interception storage capacities of tropical rainforest. J. Hydrol., 77: 237-252. Knights, P.C., 1983. Hydrologic significance of pine forest development at Lidsdale, NSW. Ph.D. Thesis, University of New South Wales. I_~yton, L., Reynolds, E.C.R. and Thompson, F.B., 1967. Rainfall interception in forest and moorland. In: W.E. Sopper and H.W. Lull (Editors), Prec. Int. Symp. Forest Hydrology, Pennsylvania, 29 August-10 September 1965. Pergamon, Oxford, pp. 163-178. Miller, D.H., 1977. Water at the Surface of the Earth, 2nd ¢dn. Academic, New York. Mutchler, C.K. and Hermsmcicr, L.F., 1965. A review of rainfall simulators. Trans. ASAE, 8: 67-68. Pitman, J.I., 1989. Rainfall interception by bracken litter - - Relationship between biomass, storage and drainage rate. J. Hydrol., 111: 281-291. Pradham, I.P., 1973. Prelirmnary study of rainfall interception through lcaflittcr. Indian For., 99: 440-445. Stafford, R.M., 1976. Litter fall in forest of Pinus radiata and mixed eucalypt species at Lidsdale State Forest, NSW. M.Sc. Thesis, University of New South Wales.
Journal of
Hydrology
ELSEVIER
Journal of Hydrology 180 (1996) 283-299
Estimation of interception capacity of the forest floor W i l l i a m M. P u t u h e n a * , I a n C o r d e r y School of Civil Engineering, The University of New South Wales, Sydney, N.S.W. 2052, Australia
Received 14 June 1995;revision accepted 26 July 1995
Abstract
Methods of measuring interception capacity of the understorey (grasses) and litter layer have been developed to estimate the forest floor interception capacity of a 15-year-old Pinua radiata plantation and a native dry sclerophyll eucalypt forest at Lidsdale State Forest, Australia. In this study, interception by various types of forest floor have been measured in the laboratory using a technique of applying artificial rain to undisturbed samples of the forest floor. These laboratory experiments separately measure the interception storage capacity of the pine needle mat, the leaf/twig/bark debris mat in the eucalypt forest, and of the understorey (grasses). The results indicate that the interception storage capacity of all components of the forest floor of both vegetation types were proportional to the mass per unit area of forest floor cover. It was also shown that the interception storage capacity of the pine needle mat and the leaf/twig/bark debris mat under eucalypt were proportional to the thickness of the surface debris. For standing grasses the capacity was proportional to the percentage of ground cover. These laboratory results were then used to estimate the forest floor interception storage capacity of two experimental catchments each covered by one of the two forest types. In each case the forest floor was extremely heterogeneous, and so a large number of undisturbed samples were examined. Approximate forest floor interception capacity of the pine catchment was 2.8 mm and of eucalypt was 1.7 mm. The contribution of leaf litter, stem and branch litter, and grass vegetation to the overall interception capacity was similar for both catchments at 47%, 8% and 45%, respectively.
1. Introduction In a forest catchment, where forest floor litter has developed on the soil surface, the surface litter will intercept both throughfall and stemflow. Miller (1977) reported that typically, 1-3 kg m -2 o f liquid water can be stored on forest vegetation, and a like
* Corresponding author. 0022-1694/96/$15.00 © 1996- Elsevier Science B.V. All rights reserved SSDI 0022-1694(95)02883-8
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W.M. Putuhena, L Corder), / Journal of Hydrology 180 (1996) 283-299
amount can be retained on the forest floor. The hydraulic mechanisms of the forest floor interception process are similar to the canopy interception process. Thus the amount of rainfall intercepted is also similarly related to the water storage capacities of the surface components. Helvey and Patric (1965) reported that the water storage capacity of the forest floor of US Eastern hardwoods, is 135-170% of the weight of the forest floor litter. However, there have been few other studies of the storage capacity of forest floor litter. In determining water storage capacity of the canopy vegetation, conventional rainfall gauges and trough gauges can be installed under the canopy. For forest floor litter, however, this straightforward method presents obvious mechanical and spatial difficulties. The mechanical difficulty is due to lack of space between the litter and the mineral soil or the grading of the litter into the soil. In a catchment, the thickness of the litter layer can vary from a few millimetres to a few centimetres. The type and composition of the litter on the forest floor in a catchment also varies greatly from spot to spot and the components of the forest floor litter are so intermixed that identification of boundaries between distinct forest floor components is difficult. The spatial variability of the amount and composition of litter layer makes it difficult to measure forest floor interception for a whole catchment. The inherent difficulties in making measurements of forest floor interception probably accounts for the relative scarcity of information. In this study an attempt has been made to develop equipment and techniques to investigate the forest floor interception capacity in eucalypt and pine forest catchments by taking into account the spatial variability of the composition of forest floor in the catchment. To provide information of more widespread utility, the interception storage capacity of each forest floor component was determined independently and the relationships between litter amount and water storage capacity of each forest floor litter type was also investigated. Furthermore, contrary to some other operational definitions of the forest floor, which only include organic debris shed by vegetation upon the surface of the soil, the definition adopted in this study includes green and dead standing grasses. These form an integral part of the forest floor, standing, dying and decaying in situ rather than being shed like the leaves of trees or shrubs. An arbitrary upper height limit of 50 cm was adopted to include most of the blade grass on the study catchments.
2. Site and forest floor description 2.1. Site
The study was carried out within the Lidsdale State Forest, (Lidsdale S.F.), approximately 130 km west-northwest of Sydney, Australia at latitude 33°241 south and longitude 1500041 east. Two adjacent catchments were selected for the present study. The area of the catchments selected are 6.2 ha and 9.4 ha with surface slopes of 13% and 12%, respectively. The first catchment is in a natural forest of native eucalypt, the principal species being Eucalyptus rossii and Eucalyptus mannifera and this
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285
association forms an open forest with Eucalyptus dives and Eucalyptus dalrympleana, the trees growing more closely on the ridge tops than in the valleys. The understorey is not well developed. The average projective canopy cover is 34%, and the trees are generally 8-14 m tall. The second catchment is in 15-year-old Pinus radiata plantation. It was a first-rotation stand planted on the site of a cleared native eucalypt forest. The stand was pruned in 1986 when the stand was 8 years old. The average projective canopy cover is 67%, and the trees are generally 16 m tall. Total basal area of the eucalypt catchment was 21.1 m 2 ha -1 with a density of 960 trees ha -1 (> 1'0 era diameter at breast height) whereas basal area of pine catchment was 27.4 m 2 ha -1 with a density 1024 trees ha -l ( > 10 cm diameter at breast height). Based on continuous measurement of gross rainfall and throughfall for the period of two years (1993-1994) on both eucalypt and pine catchments, it was found (by the method of Leyton et al., 1967) that the canopy storage capacities were 0.63 mm and 1.31 mm for eucalypt and pine catchments, respectively. The study area has subtropical climate; winters (April-October) are moderate with short cold periods and the summers (November-March) are mild to hot. Based on the data period of 1964-1993, the mean annual rainfall of 890 mm is fairly well distributed throughout the year with an average of 128 raindays per year. 2.2. Forestfloor Htter
In Lidsdale S.F., there are some clearly observable differences on the forest floor of pine and eucalypt catchments. In the eucalypt catchment, litter is patchily distributed and consists of fragmented remnants of woody plants and leaves; while in the pine catchment there are frequently areas where the forest floor is a mat of pure pine needles and no undergrowth occurs. A characteristic of the pine catchment forest floor is a sharp boundary between the litter and the mineral soil. The litter layer of this catchment may be lifted as a carpet. This may be contrasted with the characteristics of the eucalypt catchment forest floor. Eucalypt forest has larger leaves than pine, has low biomass and often there is no clear dividing line between the litter layers and their underlying mineral soil. However, very little is known about the nature and development of the forest floor under these two different vegetation covers and with the exception of the work done by Stafford (1976), no suitable measurements have been made for Lidsdale S.F. Stafford (1976) reported that the largest component of the litter fall in eucalypt forest per year was leaves, 0.145 k[g m -2, followed by bark, 0.085 kg m -2, and then branches and twigs, 0.050 kg m-L In the pine plantation, the litter fall per year consists of leaves as the largest component, 0.229 kg m -2, followed by branches and twigs, 0.014 kg m -2, and then bark, 0.001 kg m -2. She also reported that the litter fall in the eucalypt forest reflects the average long term situation under the present management regime. In contrast to the eucalypt forest, the data collected in the pine forest does not necessarily reflect the average conditions throughout the duration of the rotation. Marked changes in rates of litter fall can be expected, especially during the establishment phase of a new crop, and as a result of management practices (thinning and pruning). Knights (1983), in a botanical investigation of
W.M. Putuhena, L Corder), / Journal of Hydrology 180 (1996) 283-299
286
the Pinus radiata plantation at Lidsdale S.F. showed that the pine developed exponentially during the early years. Knights estimated that by 8.5 years the root and canopy system had almost completely occupied the available space so that conditions remained stable thereafter. These suggest that the forest floor of the 15-year-old Pinus radiata plantation can be assumed to be at a steady state.
3. Method and experimental procedure In this study, the interception capacity of the forest floor of the study catchments were estimated using a combination of field and laboratory measurements. As shown in Fig. 1, the procedure involved three major activities. First, field measurement to produce the maps of the forest floor litter for each study catchment (forest floor mapping). The forest floor mapping produced a set of single feature choropleth maps that demonstrate the spatial distribution of each particular component of the forest floor. Second, laboratory measurements to determine the water storage capacity of the forest floor samples which were taken from each classified subgroup of the forest floor. Third, as shown in Fig. 1, the results of the laboratory measurement were then extrapolated to the study catchment by using the series of single feature choropleth maps to estimate the water storage capacity of the forest floor litter of the catchment. 3.1. Forest floor mapping In this study, the forest floor mapping was concerned with the spatial recording of the categorised visible features of the forest floor litter of each of the study catchments A set of single feature
Base map + data collected by
chotop~thmaps~ the
detailed ground survey
forest fl¢~¢ ¢~the
catchment
A set ~ single feature chompleth maps of the i n t e r ' o n storage capaaty A choropleth map of the ]
•
s_>
_>
[->
->
o
R
[ll Forest Floor Mapping
- Classification of the forest floor liuer components - Detailed groond sm'vey
[21 Laboratory Determhmtlen ef W a t e r Storage C a l ~ d t y d F o r m Floor ~ Components - Sample collection and preparation - San']pietest ron using an artifida] rainfall
[3] Extrapolalion of the
Laboratory Results to the Catchment
- Data pre.~entation
Fig. 1. Procedure to estimate interception storage capacity o f the spatial variability o f the forest floor litter in a catchment.
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
287
and has two purposes. The first is to present the spatial variability of the forest floor of each catchment. The second is to extrapolate the laboratory water storage capacity measurements of the forest floor litter samples to the study catchments.
3.1.1. Classification of the data to be mapped The forest floor components in the study catchments have been classified into two main groups. The first group is litter which includes organic debris shed by vegetation. The second group is grass vegetation including green grass, standing dead grass and their organic debris. The litter layer in the study catchments consists of leaves, twigs, bark, flowers, and fruits or seeds (small items), as well as fallen stems and branches (large items). For the purpose of this study the litter layer was divided into two groups, namely, leaf litter, including all small items, and non leaf litter, consisting of large items. The grass vegetation was also divided into two groups, namely tussock grass and blade grass. Both grass groups include their standing dead material and organic debris. Based on the classification of the forest floor components described above, the forest floor of the pine catchment, comprises four groups, namely leaf litter (pine needle, flowers, and fruits or seeds), non-leaf litter (stems and branches), tussock grass and blade grass while the eucalypt forest comprises only three groups, namely, leaf litter (leaves, bark, flowers, and fruits or seeds), non-leaf litter (stems and branches) and tussock grass. Furthermore, the heterogeneous nature of each group of forest floor components within the stand has led to subclassifications of each group based on the thickness for the leaf litter, the density index (scale: 1 to 5; 1 is very sparse and 5 is very dense) for the stems and branches, and percentage of ground cover for the grass. 3.1.2. Discretisation of the study catchments A reference grid was set out on each catchment. The chosen grid spacing needs to be sufficientlyclose to minimise the variability of components within each grid square. At the same time the total number of squares has to be small enough for data to be collected for each square within the time and financial constraints that are operative. In this study, the size of the grid has been governed by the capability of visual observation to provide a reasonable estimate of the average component within each grid square. Thus the size of the grid should be easily covered by the eyes from one position. In this study a grid of size 25 m x 25 m was selected giving a total breakdown of the eucalypt catchment into 113 and the pine catchment into 171 squares. 3.1.3. Detailed fieM survey In this survey, a sampling procedure was not employed because the area of the study catchments to be mapped was small (6.2 and 9.4 ha), so that the total area could be covered by the detailed field survey. In the survey or field data collection, the values estimated are the percentage of grass cover, the thickness of the leaf litter, and the density index of the stems and branches in each grid square. The most effective estimation was made by Using categories rather than a continuous scale. For example, when estimating percentage cover of grass vegetation, the
288
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
ranges 1-20, 20-40, 40-60, 60-80 and 80-100% were used. Estimating a specific percentage, such as 46%, is probably not realistically possible. To estimate the percentage of grass cover within the grid, the area was observed through a combined visual and mental process. All grass of the same species was placed together, and the portion of the plot they covered was ascertained by mentally dividing the area in half, and halves into quarters, and asking the question, 'is more than a quarter or more than half of the plot covered by that species?'. Through this technique, repetitive, comparable measures of grass cover were achieved. Other measurements depend on the subject matter and are easily made after gaining some field experience. For example, the thickness of the leaf litter was estimated initially by randomly sampling at a number of points. Leaf litter for each sampling point was exposed and its thickness was measured. This was done separately only for the first three or four grids to mentally fix the general thickness ranges of objects and was estimated thereafter. The collection of information necessary for mapping of the forest floor component was dependent on the detailed field survey described above. To reduce the subjectivity of this estimation, two people worked independently on each grid square during the survey.
3.1.4. Data presentation As the data collected are in the form of the average values per unit area (grid of 25 m x 25 m) it is more suitable to present the spatial variability of the forest floor by using a choropleth map rather than an isoline map. A choropleth map is a map in which quantitative spatial distributions are depicted from data that have been calculated from mean values per unit area. It is produced by dividing an area into squares or hexagons, and finding a mean value for each. A range of shading, stippling or colouring is used to show successive orders of mean values. The complexity of the forest floor commonly makes it difficult and impractical to represent the variability of all forest floor components on a single map. For example, in many grids, three or four components of the forest floor are mixed together and underlie each other. A single map showing the thicknesses and densities together with the percentage of the grass cover would be practically illegible. It was decided, therefore, that a series of single-feature choropleth maps (Fig. 1) are needed to present the various components of the forest floor in each study catchment. A set of the single feature choropleth maps of the pine catchment is presented in Fig. 2. The maps of individual groups of the forest floor components can then be directly overlaid both for computational purposes and for comparison of the spatial distribution of the different forest floor components. 3.2. Laboratory determination of the interception storage capacity The laboratory determination of the interception storage capacity for each group of forest floor components involved two distinct processes: sample collection and preparation, and laboratory test runs. In the following section, before the two processes are described, the sampling technique used in this study is described.
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weir m
I. . . . . . . . .
289
I
.,,,q-
w ir LEAF LITTER GRID (25m x 25m)
[] [] []
[a]
thickness (cm)
STEM & BRANCH [b] density index
none
none
none
[
o ,,ss I
TUSSOCK GRASS [el ground cover (%)
BLADE GRASS [dl ground cover (%)
none
none
1
very sparse
1
< 20
< 20
2
sparse
2
20 - 40
20 - 40
3
medium
3
40-60
40-60
4 5
dense very dense
4 5
60 - 80 > 80
60 - 80 > 80
Fig. 2. A set of single-feature ehoropleth maps of the forest floor litter of the pine catchment.
3.2.1. Sampling technique T h e spatial variability i n a m o u n t o f litter o n the forest floor has been well docum e n t e d . F o r example, Blow (1955) r e p o r t e d t h a t 40 plots, each m e a s u r i n g 0,4 o f a s q u a r e metre, were n e e d e d to m a i n t a i n a s a m p l i n g error o f 5 % o f the total litter weight. Similarly Helvey (1964), o n 77 occasions, collected 12 litter samples, each 0.18 o f a square metre, a n d the coefficient o f v a r i a t i o n averaged 2 0 % for each g r o u p
290
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of 12 samples. These two studies showed that owing to the spatial variability of the amount of the forest floor litter in a catchment, a large number of random samples were needed to give an accurate assessment. This makes it necessary to resort to purposive or stratified sampling. In this study, purposive sampling was adopted after the forest floor litter had been grouped and classified. The classification described in Section 3.1.1 gave a total of 20 groups of forest floor litters under pine stands and 15 groups of forest floor litters under eucalypt stands. By using the purposive sampling technique, each sample was selected by visual observation to represent one group of the forest floor litter. In this study, each group of the forest floor components was represented by at least one sample which had been selected from the field in an appropriate location.
3.2.2. Sample collection and preparation The method used to collect and prepare a particular sample depended on the characteristics of the sample.
3.2.2.1. Pine leaf litter sample In order to get an undisturbed sample, the pine leaf litter sample was collected within an area larger than the area of the sample needed for the experiment. The area of the sample for the laboratory experiment was 0.54 m x 0.60 m, while the pine leaf litter sample collected had an area of 0.70 m x 0.80 m. On the forest floor, the area selected for the sample plot was marked by removing the leaf litter around the border of the sample area. A 1 m x 1 m thin sheet of steel was forced along the top of the mineral soil at the base of the litter (at ground level). The thin sheet of steel with the sample was lifted and then the sample was slid into a tray and transported to the laboratory. In the laboratory, the size of the sample collected (70 cm x 80 cm) was reduced to 54 cm x 60 cm (to fit the sample tray) by trimming the damaged edge of the sample.
3.2.2.2. Eucalypt leaf litter sample The same procedure as that used for the pine leaf litter sample collection is difficult to apply to the eucalypt leaf litter sample because the boundary between mineral soil and the leaf litter under eucalypt stands is not as sharp as under the pine stands. The eucalypt leaf litter sample was collected by the following procedure; a sharp-edge galvanised steel frame with the inside dimension the same as the area of the sample needed for experiment was driven into the ground. The material contained within the frame was removed by hand in layers. First, a layer of fresh leaves and bark was collected and put into a plastic bag. Secondly, a layer of decaying fragmented leaves and bark was picked up and put into a separate plastic bag. In the laboratory, the sample of the decaying layer components were spread across the sample tray in its natural position based on the photographs taken in the field before the sample was collected. This was followed by the fresh leaf litter, these components also being placed on the sample tray as naturally as possible, based on the photograph taken in the field.
W.M, Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
291
3.2.2.3. Grass sample The grass sample was collected and prepared by the following procedure; a sharpedged galvanised steel frame with the inside dimension the same as the area of the sample needed for the experiment was driven into the ground. The decaying grasses and their organic debris were cut along the frame edge and horizontally at ground level, with minimum disturbance to the sward structure, and lifted out carefully. The decaying grass was put in the same position into a tray which had been covered by a plastic sheet. The green grass and the standing dead material were collected by digging up their roots and these were also put into the tray. In the laboratory, the frame was placed in a 1 m x 1 m tray. Inside the frame, grass vegetation was rearranged as naturally as possible based on the photographs taken before the sample was collected in the field. To support the standing grass and to provide a base wet clay was spread on the frame. A dry clay was poured to cover the whole surface of the wet clay. Melted paraffin wax was carefully poured on to the dry clay and allowed to solidify making a continuous seal over the dried clay. Before the paraffin wax solidified, aluminium tubes, 1 cm diameter, were pushed into the base to provide drainage holes. In the time between the paraffin wax drying and solidifying completely, the aluminium tubes were pulled out. When the wax had solidified completely the steel frame was removed and the sample with the paraffin wax was lifted, leaving the clay in place. The other parts of the sample (the dead standing grass and the fallen dead grass) were then rearranged on the sample as naturally as possible based on the field photographs. 3.2.3. Laboratory test run The flow of intercepted rainwater on samples was observed using the rainfall simulator described by Mutchler and Hermsmeier (1965). As shown in Fig. 3, uniform, artificial raindrops with mean diameter of 2.0-3.0 mm diameter were produced ~6~:~.~:~\\~\~ 0 0 . . . . . . o . 0 ° . . . o o o ,
, . . 0 . . ° °
,
o 0 . ° ° ° • . . . . . . . , • ° ) ° . . . . °
° ° .
~<.xN~.~:~ ~.~,~.~:~;~:~':: maker
0 ° •
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0 .
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. .
• ° . . 0 .
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.
. , .
•
• 0 °
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.
. . 0 0 0 . . . o
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• ° . .
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::::::::::::::::::
2m
:
splash
• ~'e~__~_Jonscreen sample
. . . . ./. . . . .
~ ' ~ - - tray ~lllll ..........................
sample
collec~
bucket re~¢~¢~
data loggex
Fig. 3. Experimental set-up for the measurement o f simulated rainfall draining from the forest floor litter sample.
292
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
from the rain drop maker. The falling drops hit a splash-screen, suspended at approximately 2 m above the sample tray. The splash-screen is a plastic wire-netting composed of 0.4 mm diameter strands constituting a 3.1 mm square mesh. At the suspension level, this splash-screen 'reworks' the 2.0-3.0 mm drops to give a range of smaller drop sizes. Rainfall intensity was controlled by maintaining a constant head of water in the raindrop maker. Quantitative observations of water filtered through the forest floor litter sample and flowing down through a drain collector were made using a tipping bucket recorder. The tipping bucket, purpose built for this project, was designed to tip every 0.2 mm depth of rainfall for the area of the sample tray (54 cm × 60 cm). The tipping bucket recorder was then connected to a data logger. For each test run, the rate of rainfall was assessed by catching the 'rain' in the drain collector without any litter sample in place. If the desired intensity had been achieved the movable protection screen was closed and the forest floor sample was put into the drain collector. Before placement the sample was carefully weighed. To start a test run, the movable protection screen was opened and the timer was started. The rainfall continued until the rate of the draining water was constant. To stop the rainfall, the movable protection screen was dosed and the clock time was recorded. The test run still continued until water ceased draining from the sample. The wet sample was then weighed again. To measure variation of water detention on a stem and branch sample using this equipment is very difficult because water detained by this type of sample is very small. Instead water storage capacities for this type of sample were determined by weighing the sample before and after being subjected to simulated rainfall. Each sample was weighed and then subjected to simulated rainfall (90-100 mm h-l). Immediately artificial rainfall had stopped the sample was put in a plastic bag and weighed to determine the maximum water storage capacity. After 10-20 min the sample was removed from the bag and reweighed to determine the minimum water storage capacity.
4. Results and discussion
In this study, 55 samples of the forest floor litter were tested in the laboratory. The relation between mass per unit area and physical characteristics (estimated percentage of ground cover for the grass samples, thickness for the leaf litter samples and density index for stem and branch samples) is presented in Fig. 4 while Table 1 shows a summary of the resulting linear regressions. The derived regression equation is Y = r n X where Y is mass of forest floor litter per unit area (kg m-2), X is their estimated physical characteristic and rn is the regression coefficient. The direct linear relations show that the estimated physical characteristic can be used to indicate the amount of forest floor litter. In the laboratory, most of the samples were tested twice with different rates of artificial rain. An example of the relationships between water detention on pine leaf litter samples (thickness 2, 4 and 6 cm) and time during the test run and with rainfall amount are presented in Fig. 5. The curves in Fig. 5 were typical of all other types of
W.M. Putuhena, I. Cordery / Journal of Hydrology 180 (1996) 283-299
293
8
ra :piaeleaflitte¢(l) I t :ouc. I ~ limit (2)
I,, :~o~zO) ]
4
0 : amock gnue - pine (1)
ra
j
: t u ~ k gra~ - euc. (2) • :bladegra~0) "~
•
•
1
6
~
~2 2
1 A
0
,
~
0 (a)
i
1
i
i
i
2 3 4 5 Thickness (cm) or Density Index
t 6
0 7
i
i
i
t
20
40
60
80
(b)
100
Ca'oundCover (%)
Fig. 4. (a) Relationships between m a s s (kg m -2) a n d thickness (cm) for leaf litter samples and between m a s s (kg m -2) a n d density index for stem and branch samples. (b) Relationships between mass (kg m -2) and percentage g r o u n d cover for grass samples.
samples tested in the laboratory and as indicated earlier, the artificial rainfall was stopped when the steady-state condition had been achieved while the measurement continued until the drainage had ceased. From a visual inspection of Fig. 5 and also for other samples the following points may be noted. (I) The test run made it possible to calculate the proportions of rainfall retained and drained after various amounts of simulated rain. The saturation of the interception storage is not achieved in the early stages of a storm event. Filling of the interception storage continued for some time into each storm event. (2) The rate of accumulation of stored water increases with increasing rainfall intensity. (3) Immediately after the simulated rain had ceased there was a maximum of water detention which was related to the thickness of the litter sample. Slightly higher maximum water detention was observed for the higher rainfall intensity. Although the difference was very small, consistently larger maximum water detention was observed with higher rainfall intensities for all samples. (4) When rainfall ceases there is rapid initial drainage which then slows and ends at Table 1 Details o f linear regression analysis o f the relation between m a s s o f the forest floor litter per unit area (kg m =2) a n d their estimatedphysical characteristic Sample type
Regression coefficient m 4- t0.05 SE
N u m b e r of sample (n)
Correlation coefficient (r)
Physical characteristic
Pine leaf litter Euc. leaf litter Tussock g r a s s - P i n e Tussock g r a s s - E u c . Blade grass Stem a n d branch
0.659 0.620 0.065 0.057 0.070 0.409
7 6 7 6 6 23
0.979 0.979 0.994 0.978 0.983 0.968
Thickness (cm) Thickness (cm) G r o u n d cover (%) G r o u n d cover (%) G r o u n d cover (%) Density index
4- 0.027 -4-0.024 4- 0.002 + 0.003 4- 0.003 4- 0.011
W.M. Putuhena, L Cordery I Journal of Hydrology 180 (1996) 283-299
294
s[ /
e : rai,',c~ed ] O: drainageceased I
s [O : rainc.eued /O: drainageceased
+
/
+l-
+:
::
2
+(-,+>
.......
,
+ 2
+
I'
L
~ 2
20
00
40
60
80
100
001
210
Tkne (minute+)
4l0 Rain (nun)
Fig. 5. Change in water detention of three different thicknesses of pine leaf litter with time (a) and with rainfall (b). Note: 1 and 2 indicate first and second test runs; other numerals e.g. 6(68) indicate sample thickness 6 cm and rainfall rate applied 68 mm h -l.
a lower storage. When drainage ceased there was still a large amount of water stored within the litter. (5) Good replication was recorded when a sample was tested twice. The differences between the curves of the successive tests were mainly due to differences in rainfall intensities between tests.
4.1. Interception storage capacity of the forest floor litter components Based on the data obtained from the laboratory experiment, two different interception storage capacities of the sample were determined for each test run. (1) Cma~, is the maximum interception storage capacity of the forest floor litter and it was taken as the amount of water detained in the sample immediately before the artificial rainfall ceased. (2) Cmi~, is the minimum interception storage capacity of the forest floor litter and it 7
5
R
8
6
4
0 : piae leaf litter (1)
5
-B
+ : euc. leaf litter (2)
(4) + (3)
o : tu~ock grim - pine (3) • :tuuockgrass-cuc. (4) • : blade grass (5) A : ste.m & branch (6)
3 2 1
1
0
0 0
2
4
Mm O~ m2)
6
8
0
2
4
6
8
Mus t'kam"z)
Fig. 6. Relationships between Cmi~ (ram) (a); and Cm~ (nun) (b) and mass per unit area (kg m -2) for the six components of the forest floor fitter samples.
295
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
Table 2 Details of linear regression analysis relating Cminand Cma~to the mass of forest floor litter per unit area Sample type
n
Cmin and C ~ vs. mass
Cmln and Cma~vs. physical characteristic a
a q- to.o5 SE
b 4- to.o5 SE
r2
a q- to.o5 S E
b 4- to.o5 SE
Pine leaf litter Cm~n 14 Cmax 14
0.011 4- 0.150 0.5964- 0.140
0.9624- 0.042 0.977 1.2494- 0.040 0.988
-0.348 4- 0.266 0.7194- 0.057 0.109 4- 0.283 0.9394- 0.061
0.930 0.953
EUC. leaf litter Cn~n 11 Cm~, 11
0.018 4- 0.180 0.5294- 0.148
1.1254- 0.068 1.3654- 0.056
0.969 0.985
-0.060 4- 0.188 0.7274- 0.046 0.451 4- 0.210 0.8784- 0.051
0.966 0.971
Tussock grass (Pine) Cmln 14 0.0694- 0.128 Cmax 14 0.3784- 0.170
0.5104- 0.022 0.7054- 0.029
0.978 0.979
0.013 4- 0.149 0.0344- 0.002 0.295 4- 0.185 0.0474- 0.002
0.970 0.976
Tussock grass (Euc.) Cm~n 11 0.1224- 0.165 (?max 11 0.4614- 0.215
0.5574- 0.032 0.7644- 0.042
0.971 0.974
-0.072 4- 0.178 0.0354- 0.002 0.200 4- 0.249 0.0484- 0.003
0.967 0.966
Blade grass Cmin Cmax
r2
12 12
0.3384- 0.114 0.985 4- 0.116
0.2964- 0.019 0.428q- 0.019
0.960 0.980
0.238 4- 0.116 0.0234- 0.001 0.849 4- 0.148 0.0324- 0.002
0.960 0.968
Stem and branch Cmin 23 Crnax 15
0.0764- 0.021 0.103 4- 0.023
0.2084- 0.010 0.2214- 0.011
0.950 0.967
0.012 4- 0.026 0.1014- 0.006 0.024 4- 0.030 0.1134- 0.007
0.924 0.953
a Physical characteristic is thickness (cm) for leaf litter, percentage ground cover for grass and density index for stem and branch.
w a s t a k e n as the a m o u n t o f w a t e r d e t a i n e d o n the s a m p l e after d r a i n a g e h a d ceased (Fig. 5). T h i s a m o u n t o f w a t e r c a n be r e m o v e d o n l y b y e v a p o r a t i o n . T h e values o f Cram a n d Cmax for e a c h test r u n s h o w a c o n s i s t e n t r e l a t i o n s h i p to either the e s t i m a t e d p h y s i c a l c h a r a c t e r i s t i c s (thickness for the l e a f litter sample, perc e n t a g e g r o u n d c o v e r for the grass s a m p l e o r d e n s i t y index f o r the stem a n d b r a n c h s a m p l e ) o r m a s s p e r u n i t a r e a o f the samples. T h e r e l a t i o n s h i p s b e t w e e n m a s s p e r u n i t a r e a (kg m -2) o f the forest floor litter a n d Cmin (ram), a n d Cmax (ram) a r e s h o w n in Fig. 6. A s expected, a l i n e a r r e l a t i o n s h i p w i t h c o r r e l a t i o n coefficient a b o v e 0.95 ( T a b l e 2) w a s f o u n d for all c o m p o n e n t s o f the forest floor litter. T h e d e r i v e d e q u a t i o n f o r this r e l a t i o n s h i p is Y = a + b X ; w h e r e Y is Cmin o r Cmax (mm), X is m a s s o f the forest f l o o r litter p e r u n i t a r e a (kg m -2) o r p h y s i c a l c h a r a c t e r i s t i c o f the forest floor litter, a is the i n t e r c e p t ( m m ) a n d b is the slope ( m m k g - I m2). A similar linear r e l a t i o n s h i p b e t w e e n Cn~n a n d m a s s p e r u n i t a r e a was also f o u n d b y P i t m a n (1989) f o r b r a c k e n litter while in c a n o p y i n t e r c e p t i o n studies, A s t o n (1979), H e r w i t z (1985) a n d P i t m a n (1989) f o u n d t h a t c a n o p y s t o r a g e capacities h a v e a n excellent linear r e l a t i o n s h i p w i t h l e a f a r e a index. I n the p r e s e n t study, even w i t h the m a s s p e r u n i t a r e a r a n g i n g f r o m 0.5 to 6 k g m -2, the s t o r a g e c a p a c i t y was linearly r e l a t e d t o the m a s s o f litter p e r unit area, b u t h o w far this can be e x t r a p o l a t e d is u n c e r t a i n . This
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W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
uncertainty occurs because as the litter becomes more fragmented, its absorption rate increases (Chen and Lindley, 1981, as reported by Pitman, 1989). The slope of the regression between storage capacities and mass of litter per unit area varies among forest floor litter components and it is apparent that there are four different groups of responses. In terms of Cmin, eucalypt leaf litter and pine leaf litter stored the largest quantity of water per increment of unit mass (kg m-2), 1.13 mm and 0.97 mm, respectively. Stem and branch litter held the least, 0.25 mm per increment of unit mass. The other types of forest floor litter (tussock grass and blade grass) made up intermediate groups. Tussock grass collected from pine and eucalypt catchments held an average 0.53 mm per increment of unit mass, whereas blade grass stored 0.59 mm per increment of unit mass. Table 2 also presents a summary of the linear regression analysis relating water storage capacities (Cminand Cmax)to their estimated physical characteristics• Taking into account estimation error and subjectivity relevant to the estimation of the physical characteristics it is understandable that the relationship between Cmin or Cmax and mass per unit area gives a better fit than the relationship between these storage capacities and the estimated physical characteristic. Extrapolation of the physical characteristic to zero is incorrect since some of the intercept values in Table 2 are less than zero and the linearity of the relationship is uncertain from the small data sample. It may serve, however, as an approximation until other data are available. In terms of mass of the forest floor litter per unit area, minimum storage capacities (Craig) for pine and eucalypt leaf litter reported here are smaller than previously reported • • values for other forest stands. Cminof pine leaf litter reported here Is 0.97 mm kg - 1 m 2 / 2 while eucalypt leaf litter is 1.13 mm kg- m . Pitman (1989) reported that Cmhaof bracken litter is 1.63 mm kg -1 m 2 while Dabral et al. (1963) and Pradham (1973) reported a value of 1.3 4- 0.32 mm kg -1 m 2. However, comparisons with previously reported values for forest floor litter are difficult owing to differences in forest stands. Bracken litter as reported by Pitman (1989) has a large surface area with highly fragmented pinnae and numerous broken tubular petioles, so that the rate of water absorption is relatively high when compared with litter of other species.
4.2. Interception storage capacity of the forest floor of the study catchments The spatial variability of the forest floor minimum interception capacity (Cmin), obtained by extrapolation of the laboratory results are presented in Fig. 7. The forest floor storage capacity in each grid is the weighted mean of the water storage capacity of all components of the forest floor. The average values of the minimum storage capacity of the forest floor of each study catchment and the contribution of each component are also presented in Fig. 7, while the results of the estimation of the maximum storage capacity of the forest floor of the study catchments are presented in Table 3. The stem and branch litter has relatively little influence on the forest floor storage capacity of these catchments; 8% for pine and 7% for eucalypt catchments. For the pine catchment, the contributions of the leaf litter and grasses to the storage capacity of the forest floor are 47% and 45%, respectively, while for the eucalypt forest they are 47% and 46%.
W.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283-299
weir l a LEGEND Grid Storage (25m x 25m) capacity (ram)
[] [] [] m
none <1
297
[Euc. Catchment
Minimum Contribution of each forest floor storage component to the overall interception Catchment capacity of storage capacity of the catchment (nun) the catchment leaf stem & tussock blade (mm) litter branch grass grass
1-2 2-3
Pine
2.78
1.32
0.22
0.85
3-4
Eucalypt
1.69
0.79
0.12
0.78
0.39
>4 Fig. 7. Minimumstorage capacity (Craig)of the forest floor of the study catchments and the contributionof each forest floor component to the overall interception storage capacity of each catchment.
5. Conclusion A detailed p r o c e d u r e has been o u t l i n e d for e s t i m a t i o n o f the spatial variability o f the forest floor i n t e r c e p t i o n capacity o f two c a t c h m e n t s with different vegetative cover. This a p p r o a c h e n a b l e d assessment o f the effect o n forest floor i n t e r c e p t i o n storage capacity o f the c o m p o s i t i o n a n d spatial variability o f the forest floor Table 3 Maximum storage capacity ((?max)of the forest floor of the study catchments and the contribution of each component Catchment
Pine Eucalypt
Maximum storage capacity of the catchment (ram)
Contribution of each forest floor component to the overall interception storage capacity of the catchment (mm) Leaf litter
Stem and branch
Tussock grass
Blade grass
4.66 2.89
2.28 1.38
0.25 0.14
1.44 1.37
0.69 -
298
IV.M. Putuhena, L Cordery / Journal of Hydrology 180 (1996) 283:299
components. By relating Cmin and Cmax to the amount o f forest floor litter per unit ground area for each forest floor component, a method is provided which can be extrapolated to a catchment with the same plant species at different locations, possibly at different stages o f development and with different forest management practices. The only requirement to use this relation is an adequate measure o f the amount of forest floor litter in the catchment. Furthermore, the application o f the relationships between storage capacities and leaf litter thickness, densities o f stems and branches, and percentage ground cover of grass offers a promising method o f determining the interception capacity of forest floor litter in a catchment where the mass per unit area o f the forest floor litter is unavailable. The difference between Cmin and Cmax is an indication o f the water which actively drains from the forest floor litter after the cessation o f rainfall and may be regarded as a dynamic storage. This dynamic water storage is in reality not a fixed value but will vary with the intensity and droplet size of the rainfall and with the forest floor litter structure. F o r this reason the value of Cmax derived from this work should only be taken as an indication of the possible magnitude o f Cmax. The contribution o f this dynamic storage to interception losses is likely to be small unless the evaporation rate is very high since the forest floor litter samples were observed to drain down to Cmin within 6-30 min o f the cessation of rainfall. To the hydrologist, therefore, Cmin is a more important interception parameter than Cmax, as this represents the effects of the forest floor litter on the difference between catchment rainfall and the sum of soil water accretion and runoff. When calculating a water balance the term Cmin should be used as the storage capacity o f the forest floor litter.
Acknowledgements The authors thank Sabina Hamaty, Jim Tilley and Alfred Wojcik for their help during field and laboratory work. The authors also acknowledge the Australian Agency for International Development (AusAID) for scholarship funding.
References Aston, A.R., 1979. Rainfall interception by eight small trees. J. Hydrol., 42: 383-396. Blow, F.E., 1955. Quantity and hydrologic characteristics of litter under upland oak forests in eastern Tennessee.J. For., 53: 190-195. Chen, L.-Z. and Lindley,D.K., 1981.Primaryproduction, decompositionand nutrient cyclingin a bracken grassland ecosystem.MerlewoodRes. Dev. Pap., 80. Institute of TerrestrialEcology,Cambridge,66 pp. Dabral, B.G., Premnath, and Ramswarup, 1963.Some preliminaryinvestigationson the rainfall interception by leaf litter. Indian For., 89:112-I 16. Helvey, J.D., 1964. Rainfall interceptionby hardwood forest litter in the southern Appalachians. US For. Serv. Southeast. For. Exp. Stn. Res. Pap. 8. Helvey, J.D. and Patric, J.H., 1965. Canopy and litter interception of rainfall by hardwoods of eastern United States. Water Resour. Res., 1: 193-206.
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299
Herwitz, S.R., 1985. Interception storage capacities of tropical rainforest. J. Hydrol., 77: 237-252. Knights, P.C., 1983. Hydrologic significance of pine forest development at Lidsdale, NSW. Ph.D. Thesis, University of New South Wales. I_~yton, L., Reynolds, E.C.R. and Thompson, F.B., 1967. Rainfall interception in forest and moorland. In: W.E. Sopper and H.W. Lull (Editors), Prec. Int. Symp. Forest Hydrology, Pennsylvania, 29 August-10 September 1965. Pergamon, Oxford, pp. 163-178. Miller, D.H., 1977. Water at the Surface of the Earth, 2nd ¢dn. Academic, New York. Mutchler, C.K. and Hermsmcicr, L.F., 1965. A review of rainfall simulators. Trans. ASAE, 8: 67-68. Pitman, J.I., 1989. Rainfall interception by bracken litter - - Relationship between biomass, storage and drainage rate. J. Hydrol., 111: 281-291. Pradham, I.P., 1973. Prelirmnary study of rainfall interception through lcaflittcr. Indian For., 99: 440-445. Stafford, R.M., 1976. Litter fall in forest of Pinus radiata and mixed eucalypt species at Lidsdale State Forest, NSW. M.Sc. Thesis, University of New South Wales.