Catchment response and watershed management in the tropical rainforests in north-eastern Australia

Forest Ecology and Management, 10 (1985) 155--175 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

155

CATCHMENT RESPONSE AND WATERSHED MANAGEMENT IN THE TROPICAL RAINFORESTS IN NORTH-EASTERN AUSTRALIA

D.S. CASSELLS', D.A. GILMOUR 2 and M. BONELL 3

'Department of Ecosystem Management, University of New England, Armidale, N.S. W. 2351 (Australia) 2Forest Research Centre, Queensland Department o f Forestry, Gympie, Qld. 4350 (Australia) 3Department of Geography, James Cook University of North Queensland, Townsville, Qld. 4811 (Australia) (Accepted 2 August 1984)

ABSTRACT Cassells, D~S., Gilmour, D.A. and Bonell, M., 1985. Catchment response and watershed management in the tropical rainforests in north-eastern Australia. For. Ecol. Manage., 10: 155--175. An active program of hydrological research has been undertaken in the rainforests of the wet tropical coast of north-eastern Australia since the mid-1960's. The program initially involved a series of trials which were designed to monitor the impact of a number of forest management and land clearing practices. In the mid-1970's the program was expanded to include investigations of important hydrological processes acting in these wet tropical environments. In contrast with the results reported from studies in undisturbed forested catchments in the humid temperate latitudes, studies in an undisturbed rainforest catchment near Babinda demonstrated the widespread presence of overland flow during monsoon and post-monsoon storms. The results of investigations of key soil physical properties at representative sites outside the experimental catchments indicate that similar hydrological behaviour can be expected in many other rainforest environments in north-eastern Australia. To reduce the impact of commodity utilization of the rainforests in this wet tropical environment, management controls need to be rigorously applied. Appropriate land capability classifications in these environments will need to be more exclusive than capability classifications commonly applied in temperate latitudes. It is considered that long-term environmental monitoring should be seen as an integral part of land management in the tropics.

INTRODUCTION C o n s i d e r a b l e advances have b e e n m a d e in the past few decades in t e r m s of developing an understanding of the hydrological behaviour of forest e c o s y s t e m s in the h u m i d t e m p e r a t e l a t i t u d e s ( K i r k b y , 1978). However,

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156 there is still relatively little quantitative information on the hydrologic behaviour of tropical forest ecosystems. Fournier (1978, p. 259) has noted that for tropical forests, "Complete and coherent studies of the hydrological dynamics beneath different types of forest and under different climatic regions are lacking." Similarly, in reporting the results of a recent international workshop on tropical forest influences, Hamilton and King (1983, p. ix) have noted that "In attempting this state-of-knowledge synthesis on tropical forest influences and the effects of forest alterations, the workshop participants and editors were dismayed at the paucity of reliable data ... sorely needed are more studies on entire small and medium sized watersheds having ... good instrumentation ... and records over periods of five to twenty-five years. Such long term experiments are extremely rare in developing countries where most of the world's tropical forests are f o u n d . " Hamilton and King (1983) have also pointed out the limitations of simply extrapolating research results and management experiences from temperate forest ecosystems directly to watershed management problems in tropical forest areas, without the validation of regional research programs. The tropical forests of north-eastern Australia have been the focus of the hydrological research program of the Queensland Department of Forestry since the late 1960's. This program began with a series of experiments designed to monitor the effects of logging and land clearing practices. The program was expanded in the mid-!970's to investigate the important hydrological processes operating in the wet rainforest environments of north Queensland. These process studies were designed to trace the movement of water into, through and out of the soil profile in an undisturbed rainforest catchment. They were undertaken to help elucidate the causal mechanisms behind the hydrological responses to land use change that were recorded in the earlier monitoring experiments, and to provide a basis for the extrapolation of the results from the specific catchment studies to the wider rainforest environment within the north Queensland region. This paper aims to give a brief outline of the hydrologic regime described by these studies and to discuss the implications of this regime for watershed forest management in this particular tropical forest region. The differences with accepted hydrological process descriptions developed from catchment studies in humid temperate forest environments are highlighted, and the relevance of these studies to watershed management and research elsewhere in the tropics is discussed. PHYSICAL ENVIRONMENT

OF THE STUDY AREA

The study area is located on the seasonally wet tropical coast of northeast Queensland (Fig. 1). The dominant physiographic feature of this region is the series of mountain ranges immediately inland from and running parallel to the coast. These ranges culminate in the two tallest peaks in the

157 State, Mt. Bartle Frere (1622 m a.s.1.) and Mt. Bellenden Ker (1561 m a.s.1.). Orographic uplift of the prevailing moist easterly and south-easterly winds results in annual rainfall totals in excess of 2500 mm over large sections of the region. Particularly high rainfall totals are experienced on the coastal mountain ranges themselves, with records from the top of Mt. Bellenden Ker showing an average annual total of 9 1 4 0 mm. ioo

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The major focus of the research program has been the Wyvuri paired catchment experiment which is located near Babinda on the eastern slopes of the Graham Range at latitude 17°20'S. The catchment soils are derived from an even-grained plagioclase hornblende amphibolite (De Keyser, 1964). They are deeply weathered, kaolinitic clay rich soils that have been classified as red podzolics (Stace et al., 1968; Gn 3.11, 3.14, Northcote, 1979), equivalent to an intergrade between Ultisols and Inceptisols in Soil T a x o n o m y (Soil Survey Staff, 1975) (R.F. Isbell and G.G. Murtha, CSIRO Div. Soils, Pers. Commun. 1981). The vegetation that covered the catchments at the beginning of the research program is classified as mesophyll vine forest (Tracey and Webb, 1975). It is typical of the rainforest vegetation that covers much of the lower foothills along the seasonally wet tropical coast. During the 8 years from 1970 to 1977, the Wyvuri catchments received an average annual rainfall of 4293 mm. Bonell and Gilmour {1980) have described their short-term rainfall intensity regime in relation to the synoptic climatological environment of the wet tropical coast. The most hydrologically active period occurs during the summer monsoon season from December to March when over 60% of the annual rainfall occurs. Daily totals in excess of 250 mm frequently occur during this season and peak 6-min rainfall depths range between 7 and 15 mm (70--150 mm h -~, hourly rate equivalent). Peak discharges recorded from the 25.7 ha undisturbed catchment have exceeded 5000 1 s-~ during this summer period. Daily rainfall totals greater than 100 m m can also occur in the post-monsoon 'transitional' season from April to June which accounts for an additional 20% of the mean annual rainfall. The hourly equivalent of the peak 6-min rainfall depths range from 25 to 65 mm h -~ during this season. The remaining rainfall between June and December comes mainly from low intensity events associated with the south-easterly trade winds. The two catchments at Wyvuri were instrumented in 1969 with compound V-notch weirs and stilling ponds, water level recorders and banks of rising stage streamwater samplers. A meteorological station was established between the two catchments to record a range of meteorological variables, m a n y of them automatically. Both catchments were kept in an undisturbed condition until June 1971, when the northern catchment was logged on an unconstrained salvage basis prior to clearing in July 1973 for the establishment of tropical pastures. Fig. 1 also illustrates the location and soil types of eleven external sites where soil investigations were undertaken as an initial test of the relevance of the results of catchment studies to the wider rainforest environment. HYDROLOGICAL

REGIME OF THE UNDISTURBED

RAINFOREST

In the initial stages of the experiment before the North Creek catchment was treated, both undisturbed rainforest catchments displayed an intensely

159

active hydrological regime. In the 4 years 1970--1973, the North Creek catchment produced an average annual streamflow of 2637 mm while the South Creek catchment produced an average annual streamflow of 2619 mm. The hydrographs of both catchments closely paralleled the seasonal rainfall distribution with in excess of 60% of t h e annual streamflow occurring during the wet season months from November to March. The high daily rainfall totals that frequently occurred during these wetter months gave rise to high daily runoff totals, and a high proportion of the total annual streamflow occurred on a relatively few days. Details of daily streamflow events greater than 50 mm during the period 1970--1973 are given in Table I. TABLE I N u m b e r o f d a y s w h e n daily s t r e a m f l o w e x c e e d e d 5 0 mrn for t h e N o r t h and S o u t h C r e e k catchments, 1970---1973 North Creek

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Percent of annual streamflow

No. o f d a y s with streamf l o w :> 5 0 m m

Volume of streamflow (ram)

Percent of annual streamflow

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367 639 1190 1338

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310 587 992 1149

15.3 27.4 34.7 33.3

The hydrographs from both catchments also displayed a rapid response to rainfall (Fig. 2) with a high proportion of streamflow appearing as quickflow (stormflow). Using the streamflow separation techniques of Hibbert and Cunningham (1967), it was determined that for the 4 years prior to the clearing of North Creek catchment, an average of 47.8% of the annual streamflow from North Creek and an average of 46.3% of the annual streamflow from South Creek appeared as quickflow. These high values contrast strongly with values of 15--20% reported from forested catchments in humid temperate regions in the United States (Hewlett and Nutter, 1969, p. 93). These striking differences p r o m p t e d the detailed investigations of the catchment drainage processes operating in the undisturbed control catchment, and investigations to determine whether similar hydrological regimes operated in other tropical rainforest environments in northern Queensland. STORM RUNOFF CATCHMENT

GENERATION

P R O C E S S E S IN T H E U N D I S T U R B E D

CONTROL

The storm runoff generation studies aimed at assessing the mechanisms contributing to the rapid storm hydrograph response. The studies were designed to monitor the movement of natural rainfall at selected depths in the soil profile under undisturbed rainforest.

160

Three sites were selected in the South Creek slopes adjacent to the main stream channel (Site ha; Site l b -- slope 26.5 °, area 0.006 ha; Fig. 1) where the first order streams originate (Site 2 6] "1

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ha; Fig. 1). At each site, a bank into the face of a profile trench and at the 0.25 m, 0.50 m, and trough was led into a 3-1 tipping

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electronically by a Rimco digital event recorder on a 6 min time base. Rainfall was recorded using a similar instrument in the nearby catchment meteorological station (Fig. 1). A network of tensiometers, piezometers and deep wells was established above the upper slope site and one of the lower slope sites to provide information on soil moisture conditions and saturated zones. At all sites, detailed investigations of soil physical characteristics were made, notably near-saturated hydraulic conductivity (K) as measured by the methods developed by Talsma (Talsma, 1969; Talsma and Hallam, 1980). Table II summarises the volumes collected from the r u n o f f troughs during the 1976 wet season. Clearly, substantial surface flow occurred at all sites. However, differences between the r u n o f f response of the upper and lower slope sites were apparent, with subsurface flow making a proportionally greater contribution to the total volume of lateral flow on the lower slope sites, Sites l a and lb. During the more intense storms of the summer monsoon, the hourly equivalent of the peak 6 min rainfall intensities usually ranges between 70 and 150 mm h -1. Under these conditions, surface flow was the dominant storm r u n o f f generation process at each site. However, during post-monsoonal storms, rainfall intensity levels are usually lower with the m a x i m u m 6 min rainfall intensities ranging between 25 and 65 mm h -1. Under these conditions, surface flow remained the d o m i n a n t r u n o f f generation process on the upper slope site. However, on the lower slope sites surface r u n o f f was confined to the intensity peaks of storms and subsurface flow became a much more important process. Fig. 2 illustrates the range of r u n o f f generation responses at the three sites with details of their response to two post-monsoon storm events which occurred on April 6 1977. The response to the first storm is shown in Fig. 2.1. This storm yielded 51.30 mm rainfall in 6 h with a m a x i m u m 6 min rainfall of 5.25 mm and a m a x i m u m 60 min rainfall of 16.51 mm. Allowing for differences in plot area, proportionately much greater volumes of surface flow were recorded at Site 2 t h a n either Site l a or Site l b . Site 2 also responded almost instantaneously, generating surface flow in response to the first rainfall pulse of the storm. By contrast, subsurface flow was the d o m i n a n t r u n o f f process at the two lower slope sites, though both these sites also generated surface flow in response to the storm's second and third intensity peaks. Fig. 2.2 illustrates the response to the second storm event which occurred some 9 h after storm 1. Storm 2 had slightly more intense rainfall yielding 41.26 mm in 4.5 h with a m a x i m u m 6 min rainfall of 6.56 mm and a m a x i m u m 60 min rainfall of 17.76 mm. With this increase in rainfall intensity, surface flow became the d o m i n a n t r u n o f f generation process at all sites. Investigations of soil moisture conditions, saturated zones and soil physical properties have clarified the mechanisms behind these response patterns.

164 The network o f piezometers and deep wells installed above Site l a and Site 2 did not provide any evidence of a shallow water table which might influence t h e generation of surface flow. The piezometers remained dry at both sites except during and immediately after rainfall events. A semipermanent saturated zone was present at Site 2, but the water level did not rise above 2.6 m from the surface during the 1976 wet season and for the most part it was more than 3.5 m below the surface. Even during the period of heaviest rainfall during February 1977 the water level did not rise above 1.1 m below the surface. Due to the high levels of biological activity in the soil and the continual incorporation of organic matter, all three sites have extremely permeable surface soils. The near-saturated hydraulic conductivity (K) of the upper 0.10 m of the soil profile in the South Creek catchment ranges from 2.75 m d a y - ' to 170.67 m d a y - ' with a catchment log mean value of 20.13 m day-'. The hourly equivalent of this value is in excess of 800 m m h-' so clearly, the surface soils are transmissive to even peak monsoonal rainfall intensities. However, there is a marked decline in K with depth, and K values at depths between 0.20 and 0.50 m in the profile show distinct spatial patterns within the catchment. The K values at Site 2 for the 0.20--0.50 m layer range between 0.003 and 0.050 m d a y - ' with a log mean value of 0.020 m day-'. The hourly equivalent of this mean value is less than 1 mm h-' and both monsoon and post-monsoon rainfall intensities frequently exceed this value. Consequently, ponding might be expected to occur above this layer, and continued rainfall could result in the development of a temporary saturated zone which is continually recharged by 'pipe flow' infiltration through surface macropores. During the prolonged wet season, the soil profile remains at or close to saturation, limiting the moisture deficit of the upper 0.20 m of the soil profile. As a result, a temporary perched water table rises to the surface during intense rainfall events, generating widespread surface runoff. This surface r u n o f f is the equivalent of 'saturation overland flow' as defined by Kirkby and Chorley (1967). It is considered to be a mixture of 'surface stormflow' and 'subsurface stormflow' as defined by Hewlett (1974). This particularly applies during the intensity peaks of storms, when water collected in the surface trough resulted from a combination of throughfall that failed to enter the soil profile and exfiltrated water (Hewlett and Troendle, 1975) from upslope areas. The lower slope K values for the 0.20--0.50 m layer are an order of magnitude higher t h a n those at Site 2. At Site l a , the K values for this layer range between 0.009 and 1.920 m d a y - ' with a log mean value of 0.16 m day-', an hourly equivalent of just less than 7 mm h-'. At Site l b , the K values for this layer range between 0.20 and 0.570 m day -1, with a log mean value of 0.11 m day -~, and hourly equivalent of 5 m m h - ' . During the intense monsoonal storms, saturation overland flow is generated in a similar manner to the upper slope site. However, in the reduced rainfall

165 intensity regime of the post-monsoon storms, the 0.20--0.50 m layer offers less impedance to the prevailing rainfall intensities and subsurface flow is more likely to become the dominant process. More complete descriptions of these studies, including descriptions of a statistical model o f the runoff generation process, have been given else: where (Bonell and Gilmour, 1978; Bonell et al., 1979, 1981; Gilmour and Bonell, 1977, 1979; Gilmour et al., 1980). GROUNDWATER RECHARGE MECHANISMS IN THE UNDISTURBED CONTROL CATCHMENT Groundwater recharge mechnisms in the lower soil profile were studied b y tracing the m o v e m e n t of a layer of water isotopically labelled with tritium through u n b o u n d e d plots established on b o t h the upper and lower slopes of the control catchment. In both these plots, arrays of soil moisture extractors were installed at depths ranging from 0.25 to 1.50 m in line transects across the plots to intercept any vertical or lateral movement of the tracer. Nests o f tensiometers were installed around each plot to provide information a b o u t soil moisture conditions. Samples were extracted immediately after injection and then on a weekly basis with additional samples being collected after major storm events. The samples were subsequently analysed at a laboratory equipped with a liquid scintillation spectrometer. Tritium injections were made during b o t h the 1979 and 1980 wet seasons and both the m e t h o d o l o g y and results of these experiments have been reported in detail elsewhere (Bonell et al., 1982, 1983a). Briefly the experiments provided evidence of a general layered vertical and lateral movement of soil water, with both the activity peaks and centre of mass of the tracer being displaced down the profile and down the slope with time. This general soil moisture advance was slow and there was evidence of an increasing soil moisture transit time with depth. For example, 2 weeks after injection a t the lower slope plot in 1979, the centre of mass of the tracer had only been displaced some 0.20 m d o w n the profile following some 260 mm of rainfall. Four months later, the centre of mass of the tracer had only been displaced some 0.20--0.30 m further d o w n the profile after an additional 460 mm rainfall. This general layered soil water advance accounted for some 30--39% o f rainfall. However, the tracer profiles also provided evidence o f a mechanism, as suggested by Beven and Germann (1982), for by-passing this general advance through rapid soil moisture movement in the soil macropores. RELATIONSHIP OF THE UNDISTURBED CONTROL CATCHMENT TO OTHER RAINFOREST ENVIRONMENTS IN NORTH QUEENSLAND The storm r u n o f f generation studies discussed earlier in this paper demonstrated the importance of the near-saturated hydraulic conductivity

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(K) characteristics of the soil profile below 0.20 m in terms of explaining the widespread generation of saturation overland flow under undisturbed rainforest at Wyvuri. As an initial test of whether similar runoff generation processes would be likely to occur in other rainforest environments in north Queensland, the K characteristics of a number of rainforest sites outside the Wyvuri experimental catchments were also investigated. In conjuction with CSIRO Division of Soils, eleven sites were selected as representative samples of the major soil groups that support rainforest in the region. The K data from these sites were compared with the K data from two of the detailed runoff sites in the control catchment. The known hydrologic behaviour of the t w o catchment sites was then used to infer the likely hydrologic behaviour of the eleven other sites on the basis of the soil hydraulic properties. Like the Wyvuri catchments, all the external sites had extremely permeable surface soils, and profiles that displayed a marked decline in K with depth. Table III summarises the K data for the profile between 0.20 and 1.00 m depth for three of the external sites (which cover the range of values measured) and the two catchment sites. These data have been subjected to a variety of statistical and numerical analyses to establish differences and similarities between the sites. The results of these analyses have been described in detail elsewhere (Bonell et al., 1983b). Briefly, the analyses suggest that for similar topographic situations, a similar range of r u n o f f generation responses to those found in the Wyvuri catchments could be expected at the external sites given similar rainfall intensity patterns. This initial investigation of likely runoff generation processes in rainforest environments in north Queeensland outside the experimental catchments has depended on the assumption that rainfall intensities experienced in the Wyvuri catchments prevail throughout the region. In addition, time constraints prevented any detailed investigation of the variability of soil physical properties within each soil system, including the effects of changing slope angles. Clearly, there is a need for a much greater understanding of these factors before anything approaching a complete understanding of the area's hydrology is developed. SIMILARITIES W I T H H Y D R O L O G I C A L ENVIRONMENTS

P R O C E S S E S IN T E M P E R A T E

The groundwater recharge mechanisms described b y the tracing studies at Wyvuri are similar to the process descriptions of soil water movement that have been developed in temperate environments. In these environments, a process of d o w n w a r d displacement of previously stored water was described by H o r t o n and Hawkins (1965) in experiments which used tritium to trace soil water movement under field conditions. Zimmermann and co-workers (Zimmermann et al., 1966; Blume et al., 1966; Zimmermann

168

et al., 1967) subsequently developed and validated a theoretical model of tritiated soil water movement. The basis of this model is the rapid molecular exchange between 'stationary' and 'mobile' water which causes a layered downward movement of soil moisture despite some blurring of the boundaries and broadening of the peaks due to molecular diffusion and streamline convection. Such a flow mechanism is very slow. It has been termed 'downward displacement' (Horton and Hawkins, 1965), 'translatory flow' (Hewlett and Hibbert, 1967), 'piston type flow' (Goel et al., 1977) and 'interstitial piston flow' (Foster and Smith-Carrington, 1980). Work by Blake et al. (1973) emphasised the preferential downward movement of free tritiated water in the fine cracks of 'dry' clay soils under spruce forest. Other writers have termed this type of flow as 'short circuiting' (Bouma and Dekker, 1978; Bouma et al., 1981). Observations by Foster and Smith-Carrington (1980) have also suggested the presence of rapid 'by-pass flow' in the larger fissures (> 0.2 mm) in the unsaturated zone of chalk and its associated soil under conditions of intense rainfall and very low matrix potentials in the rock matrix. Thus, evidence from a variety of sources suggest that both interstitial piston flow and rapid by-pass flow can act simultaneously, with their relative importance dependent both on the structure and the moisture status of the soil or rock. The results of the tracing studies at Wyvuri suggest the action of similar processes in this wet tropical environment. The main difference is that in the Wyvuri catchment, the high frequency of moderate to heavy rainfall events results in the same processes displacing soil water over greater distances per unit time. DIFFERENCES WITH HYDROLOGICAL ENVIRONMENTS

P R O C E S S E S IN T E M P E R A T E

The processes that generated the widespread saturation overland flow under the undisturbed rainforest at Wyvuri differ markedly from the storm r u n o f f processes that have been described in forest environments in the temperate latitudes. In temperate environments, most writers favour some version of the Hewlett variable source area concept of overland flow generation (Hewlett, 1961; Hewlett and Hibbert, 1967). Kunkle (1974, p. 10) notes: "The variable source area concept recognises that for many forest lands having a good vegetation cover -- especially in humid areas -- the following runoff process occurs. During a typical storm only a small part of the basin around the channel actually yields surface runoff, whereas on the upper reaches of the basin the rain infiltrates becoming subsurface flow and does n o t appear in the streams until long after the 'storm hydrograph' is finished." During monsoon and post-monsoon storms at Wyvuri, wet areas are not confined to slowly expanding wedges above the main channels. Instead, w e t areas are widespread throughout the catchments during storm events

169 and, with the frequency of rainfall events maintaining the catchment soils at or near field capacity for extended periods between December and May, such w e t areas can redevelop almost instantaneously with the onset of intense storms. The South Creek catchment at Wyvuri has a drainage density of 0.23 mm -2. Observations during the runoff generation studies indicated that this network expands considerably during storm events when numerous shallow swales become ephemeral streams. This enables surface water from the widespread areas of saturation overland flow to move quickly into the stream network, resulting in a large proportion of total streamflow occurring as quickflow. IMPLICATIONS FOR W A T E R S H E D M A N A G E M E N T FORESTS OF N O R T H Q U E E N S L A N D

IN THE TROPICAL

The variable source area concept provides the conceptual model on which many of the watershed protection provisions commonly applied to forest management problems in temperate regions are based (Kunkle, 1974; Cornish, 1975). Bren and Turner (1980, p. 117) indicate the t y p e of management implications usually drawn from the concept: " .... activities close to the stream are more likely to be detectable than the same activity further from the stream ... In situation in which hydrological change is to be prevented, care should be taken to ensure that all overland flow generated from compacted areas is allowed to infiltrate to groundwater." While the runoff source areas in the Wyvuri catchments vary b o t h spatially and temporally in relation to variations in rainfall intensity, it is clear that the widespread occurrence of saturation overland flow places these catchments in a somewhat different situation to that described b y Bren and Turner (1980). In these more intense hydrological situations, forest managers cannot assume that hydrological impacts will be avoided simply b y restricting harvesting and other management activities away from what are conventionally accepted as 'sensitive areas', i.e. the 'wet' areas surrounding stream channels. During most wet seasons, whole catchments will become 'hydrologically sensitive areas' during the more intense storms and, in areas with less transmissive subsoils, large sections of catchments will become 'hydrologically sensitive areas' during most storm events. However, the techniques available to forest managers to reduce the impact of management activities in the tropics are essentially the same as those available to forest managers in the temperate latitudes: control of the timing of forest operations, retention of streamside buffer strips, effective drainage of the extraction roading network, and pre-logging planning to reduce disturbance and achieve optimal extraction network locations. What does differ in the tropical situation is the degree of rigor with which such controls need to be implemented. In terms of controlling the impact of forest harvesting on water quality, the importance of retaining undisturbed streamside buffer strips is increased.

170 The prime function of these strips in this environment is to prevent harvesting operations creating soil disturbance in the stream bed or along the immediate stream banks. With the intense hydrologic conditions of an average wet season, even streams with small catchment areas produce high discharges. This 'flashy' behaviour means that any streambed disturbance can contribute large quantities of stream sediment to downstream areas. The regular occurrence of stream rises ensures that areas of streambed disturbance have little chance to revegetate before receiving another flood event and t h e y therefore remain active contributors of stream sediment over extended periods. With the capacity of small catchments to generate high discharge levels during wet season storms, buffer strip protection needs to be extended to many streams with catchment areas far smaller than 100 ha. Appropriate location and drainage of the harvesting road network is also particularly important as it is this network which will potentially have the greatest effect on erosion generation and sediment transport. In routine selection logging operations, the extraction network can cover up to 25% of any particular logging area. Pre-logging planning offers the opportunity to reduce this area and hence the proportion of any catchment where bare soil is directly exposed to raindrop impact. Regular road and snig track drainage will reduce water velocities at sites where the soil is exposed. It will also help ensure that overland flow carrying suspended sediment loads has the o p p o r t u n i t y of passing through undisturbed or largely intact vegetation before reaching the organised drainage channels. Reduction of water velocities at exposed soil sites is particularly important where rainforest soils have highly dispersive, deeply weathered subsoil horizons. The value of strict watershed management controls in the wet tropical environment of north Queensland has been demonstrated by the monitoring studies of this research program. Gilmour (1971, 1977a, b, c) has reported the effectiveness of watershed management controls in reducing stream sedimentation in the Freshwater Creek basin near Cairns. Similar controls based on modifications of the environmental guidelines of Cameron and Henderson (1979) are now routine practice on all timber harvesting operations in State Forests in north Queensland. IMPLICATIONS FOR LAND USE PLANNING AND MANAGEMENT

Where rainforest lands are cleared for alternative uses the occurrence of widespread surface r u n o f f will have a major influence on the landscape stability of the new regime. The monitoring studies at Wyvuri indicated that due to the intensely active hydrological regime of the undisturbed forest, clearing had little or no impact on wet season storm runoff generation (Gilmour, 1977b, 1977c). However, the same studies also indicated that forest disturbance and clearing had a major influence on the erosion

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generation response of the treated catchment (Gilmour et al., 1982; Cassells et al., 1982). The intensity of the prevailing rainfall along the wet tropical coast of north Queensland makes it difficult to design economic mechanical soil conservation systems that will n o t fail at unacceptably short intervals. Soil stability problems are apparent whenever cultivation is extended onto even moderate slopes. Soil erosion is particularly noticeable when heavy rainfall occurs on areas where trash protection has not been maintained and where crop cover is not well developed. As a consequence, land capability classifications appropriate for the north Queensland environment will need to be more exclusive than capability classifications commonly applied in temperate latitudes. To maintain economic levels of productivity, agricultural systems in the tropics o f t e n rely on high levels of management inputs such as the extensive use of agricultural chemicals. Many of these chemicals would be leached into the stream systems with the annual wet season flush of surface runoff. However, the long soil moisture transit time d o c u m e n t e d during the tracing studies at the Wyvuri catchments suggest that despite the intense surface hydrological activity, there is some potential for che~ micals to accumulate in the lower soil horizons. With this potential problem in addition to the more easily recognizable problem of soil and nutrient losses, the authors consider that long-term environmental monitoring should be seen as an integral part of land management for sustained c o m m o d i t y production in this region. RELEVANCE OF THE NORTH TROPICAL ENVIRONMENTS

QUEENSLAND

STUDIES T O O T H E R

The forests of the tropical world are facing increasing pressures for development and alienation (U.S. Interagency Task Force on Tropical Forests, 1980; Meyers, 1981). In this context, the paucity of data on the hydrologic behaviour of these forests noted in the beginning of this paper must be viewed with concern. Hydrological research work in the tropical forests of northern Australia is n o w well developed. However, like the research work from temperate forest areas, the results of this work cannot simply be transferred to other tropical areas without the validation of regional research programs. On rainfall intensity criteria alone, the tropical world is far from a uniform environment (Jackson, 1977). Catchment research is inevitably an expensive, long term undertaking and planners and land managers in the tropical world must make decisions n o w rather than in the distant future. However, better decisions and better management can occur if managers have an understanding of the hydrologic regime with which they are working. The northern Australian work would seem to indicate that shorter term research to define the rainfall intensity regime and the soil hydraulic properties in particular regions can aid the

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development of such an appreciation, before the results of longer term catchment studies come to hand. CONCLUSIONS

The hydrological research program in the tropical forests of northern Australia has revealed an intensely active hydrological regime with widespread surface runoff occurring in undisturbed rainforest environments. To reduce the impact of commodity utilization in these forests, management controls need to be applied with a rigor appropriate to the intensity of the wet tropical environment. Land capability classifications appropriate to these environments will need to be more exclusive than capability classifications commonly applied in temperate latitudes. Short-term research to describe the rainfall intensity regime and the soil hydraulic properties of other tropical regions should allow planners and land managers to develop an appreciation of the type of hydrologic environment in which they must make land use decisions. ACKNOWLEDGEMENTS

The investigations on which this paper is based form part of the research programmes of the Queensland Department of Forestry and the Department of Geography, James Cook University of North Queensland. Additional support and assistance h a s been provided by the Queensland Water Resources Commission, the Queensland Department of Primary Industries, the Australian Water Resources Council, CSIRO Division of Forest Research, CSIRO Division of Soils, CSIRO Division of Computing Research and CSIRO Division of Mathematics and Statistics. REFERENCES Beven, K. and Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res., 18: 1311--1325. Blake, G., Schlichting, E. and Zimmermann, V., 1973. Water recharge in a soil with shrinkage cracks. Soil Sci. Soc. Am. Proc., 37 : 669--672. Blume, H.P., Munnich, K.O. and Zimmermann, V., 1966. Das Verhalten des Wassers in einer Loess -- Parabraunerde unter Laubwald. Z. Planzenern//hr. D~/ng. Bodenkd., 112: 156--168. Bonell, M. and Gilmour, D.A., 1978. The development of overland flow in a tropical rainforest catchment. J. Hydrol., 39: 365--382. BoneU, M. and Gihnour, D.A., 1980. Variations in short-term rainfall intensity in relation to synoptic climatological aspects of the humid tropical north-east Queensland coast. Singapore J. Trop. Geogr., 1 (2): 16--30. Bonell, M., Cassells, D.S. and Gilmour, D.A., 1982. Vertical and lateral soil water movem e n t in a tropical rainforest catchment. In: E.M. O'Loughlin and L. Bren (Editors), The first National Symposium on Forest Hydrology. 11--13 May, 1982, Melbourne, National Conference Publication No. 82/6, The Institution of Engineers, Canberra, pp. 30--38.

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175 Tracey, J.G. and Webb, L.J., 1975. A Key to the Vegetation of the H u m i d Tropical Region of North Queensland -- Bartle Frere i : 100,000 sheet. C.S.I.R.O. Rainforest Ecology Unit, Division of Plant Industry, Long Pocket Laboratories, Indooroopilly, Brisbane, Qld. U.S. Interagency Task Force on Tropical Forests, 1980. The World's Tropical Forests: A Policy, Strategy and Program for the United States. Report to the President by a U.S. Interagency Task Force on Tropical Forests. Department of State Publication 9117, U.S. Government Printing Office, Washington, DC, pp. 15)19. Zimmermann, U., Munnich, K.O. and Roether, W., 1967. Downward movement of soil moisture traced by means of hydrogen isotopes. In: G.E. Stout (Editor), Isotope Techniques in the Hydrological Cycle. Geophysical Monograph Series, No. 11, Am. Geophys. Union, Washington, DC, pp. 28--36. Zimmermann, U., Mruetz, W., Schubach, K. and Siegel, O., 1966. Tracers determine movement of soil moisture and evaporation. Science, 152: 346--347.