Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations in Victoria, Australia

Journal of Hydrology (2007) 336, 416– 429

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Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations in Victoria, Australia Leon Bren

a,*

, Peter Hopmans

b

a

School of Forest and Ecosystem Science, Faculty of Land and Food Resources, University of Melbourne, Creswick, Vic. 3363, Australia b Timberlands Research Pty Ltd, 1 Rodney Place, Carlton, Vic. 3053, Australia Received 14 September 2005; received in revised form 11 January 2007; accepted 15 January 2007

KEYWORDS Eucalypt; Radiata pine; Stream flow; Afforestation; Water yield; Australia

Results from a paired catchment project measuring the water balance of native eucalypt forest and the change caused by conversion of one catchment to radiata pine are presented. The results are from 1975 to 1987 and from 1997 to 2006. The project measured the water yield from three small, steep, contiguous, forested catchments carrying similar native eucalypt vegetation in south-eastern Australia. One of the catchments was converted to radiata pine in 1980. Data from the untreated catchment showed that annual water yield was substantially a function of annual rainfall, and that annual rainfalls below 900 mm generated little runoff. The conversion to radiata pine led to an increased water yield of up to 300 mm per annum immediately after clearing. This response has declined but in general the runoff from the pine catchment is still higher than that of the eucalypt catchment except for ‘‘drought’’ years. Combination of these results with other Australian results gave a simple model of radiata pine water use (relative to the native eucalypt vegetation) as a function of age and annual rainfall. Older trees and low-rainfall years are associated with a higher water use by the radiata pine than the native forest, but conversely younger trees and higher rainfalls give lower water use. Extrapolation of the results to grassland showed that reforestation of grassland sites would lead to diminished yields. The relationships generated may be useful for estimation of the net change in streamflow caused by afforestation with radiata pine in larger catchments. ª 2007 Elsevier B.V. All rights reserved.

Summary

* Corresponding author. Tel.: +61 3 53 214 117; fax: +61 3 53 214 194. E-mail address: [email protected] (L. Bren). 0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2007.01.018

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations

417

List of Symbols a A B c c* C C* e E ET m P

regression constant age of trees, years annual yield of Betsy catchment, mm daily average flow from Clem Creek, Ls1 estimated daily average flow from Clem Creek, Ls1 annual yield of Clem catchment, mm estimated annual yield of Clem catchment, mm daily average flow from Ella Creek, Ls1 annual yield of Ella Creek catchment, mm evapotranspiration from the community, mm regression slope of C regressed on E or c regressed on e annual precipitation, mm

Introduction The work reported in this paper resulted from community concern about the changes in water use and nutrient impacts caused by conversion of native eucalypt forest to radiata pine (Pinus radiata D. Don) plantations in southern Australia. In particular these alleged that pine plantations created on eucalypt forest sites led to ‘‘excessive runoff.’’ In 1975 the Croppers Creek paired catchment project in north-east Victoria was initiated to determine the hydrologic impact of intensively-managed radiata pine on water flows and water quality compared to the native (eucalypt) forest. One catchment (Clem Creek) was converted from native forest to plantation in 1980 after a five year ‘‘calibration’’ period. Adjoining ‘‘control’’ catchments (Ella and Betsy Creek) consist of native, mixed species eucalypt forest. Measurement on the initial Croppers Creek project ceased in 1987 because of Government reorganisation. The present phase involved a rehabilitation of the defunct catchment project in 1997 to allow resumption of the previous measurement regime. The results reported in this paper (until April 2006) are a continuation of the water yield results reported by Leitch and Flinn (1986); Bren and Papworth (1991), and Bren (1997). Specifically this paper reports (1) the water yield of the catchments carrying native eucalypt forest and (2) the changes in water yield associated with the growth of radiata pine. By inclusion of results from two similar paired catchment projects, a more general model of radiata pine water use relative to eucalypt forest is derived, and extrapolated to grassland sites.

The Croppers Creek hydrology project A full description of the project is given in Bren et al. (1979). The project is located in steep foothill country in the Black Range, about 22 km south-west of Myrtleford, Victoria. The area has substantial commercial radiata pine plantations. Fig. 1 shows a computer-generated view of the three catchments. The project area comprises:

R

residual (mm) – i.e. observed – expected streamflow s standard error of difference between regression coefficients SGrass streamflow from a grassed catchment, mm SForest streamflow from a forested catchment, mm DSEuctopine Change in streamflow (mm) when a catchment is converted from eucalypt to pine forest. DSGrasstopine Change in streamflow (mm) when a catchment is converted from grass to pine forest ‘‘t’’ ‘‘Student’s t’’ T Change in streamflow (mm) attributable to the catchment treatment e Error in model, mm.

• Clem Creek catchment (46 ha). Measurement commenced on the catchment in 1975. This was converted from native eucalypt forest to Pinus radiata D. Don plantation in 1980 by clearing of the mid and upper slopes with heavy tractors. These were then planted at approximately 1000 trees per hectare. A 30 m undisturbed ‘‘buffer’’ of native vegetation was retained on either side of the stream. The plantation was fertilised and thinned to 600 stems ha1 in May/June 1998. This noncommercial thinning removing approximately one third of the basal area (50% of stems). The area was scheduled for complete harvesting about 2008 but at the time of writing (January 2007) is being salvage-logged because of a wild-fire in December 2006. • Ella Creek catchment (113 ha). This is the control catchment, and has had no forest disturbance since initial flow measurement in 1975. This is a mature, native mixed species eucalypt forest with no known history of forest cutting. • Betsy Creek catchment (44 ha). This is an auxiliary control, with a similar vegetation cover and history to Ella Creek. • Rainfall measuring stations at each of the weirs and stations (now defunct) at Black Range, Hazle Spur, and Candlebark Spur. All the catchments are steep with hill-slopes commonly in excess of 20. All three catchments are gauged by passing water over 120 V-notch weirs. Water levels are recorded on analogue (‘‘Leupold-Stevens’’) recorders. In our 1997 instrumentation there is also electronic recording of flow, rainfall, pH, conductivity, temperature, and turbidity at Clem and Ella Weirs. Base flow water samples were taken at weekly intervals for full chemical analysis. In addition, automated samplers collect frequent samples during storm flows. Each site has a storage rain-gauge to give an accurate weekly measure of rainfall. Rainfall is currently measured at cleared sites near each measurement weir using 203 mm storage gauges read fortnightly. In this area rainfall averaged 1412 mm per year

418

L. Bren, P. Hopmans

Betsy Creek

Ella Creek

*

*

Cr op pe Cr eek r

*

Clem Creek Research Catchments Radiata Pine/ Native Forestt

Pine Plantations

* Figure 1

Rain Gauge Sites

Computer-generated view of the project area, showing the three catchments. The displayed surface is 3 km · 4 km.

(median 1456 mm, standard deviation 363 mm per year), with the bulk of rainfall occurring in the winter–spring period. Winter storm rainfalls tended to be long periods of lowintensity (5–20 mm h1) rain. Summer storms tended to be short-duration, convective thunderstorms of higher intensity (20–60 mm h1). Bren and Leitch (1986) examined the accumulation of rainfall at upland and lower sites using six gauges around the rim of the catchments. The results showed that the occurrence of rainfall was very highly correlated at all locations, that the gauge catch on the upper sites tended to be slightly less than the lower sites, and that differences between sites was quite consistent. An up-dated analysis of rainfall variation is given below. The parent material of the catchment is shale with some metamorphism from adjacent igneous intrusions. This material is strongly oriented, with the rock splitting along preferred directions, and is viewed by drillers as ‘‘weak’’. Although erosion is not a problem, our measurements have shown that the highest flows experienced in the streams do have the ability to significantly erode the stream channels and lower the stream bed by perhaps 0.2 m in 25 years. The rock is exposed only on the ridges and along the streams. The common soil type found on the catchments is a red-brown loam. This is highly friable in its upper layers, but with depth becomes increasingly cohesive. Free rock becomes a more important constituent with increasing

depth. Soils are commonly 2 m deep on the lower slopes, but become skeletal on the upper slopes and may even be non-existent on the ridge. At the stream the A horizon may be shallow or non-existent, with the soil passing into a whitish, structureless clay associated with decomposing shale. This same clay (‘‘saprolite’’) can be found at depths of 2 m or more in mid-slope positions. Studies of Bren and Turner (1979) showed that overland flow on the slopes is virtually unknown. This reflects both the porosity and the depth and roughness of organic matter. The native forest is a mature, mixed species, unevenaged eucalypt forest. The predominant tree species is narrow-leaf peppermint (Eucalyptus radiata Sieber), with manna gum (E. viminalis Labill) along the stream, and brittle gum (E. mannifera Mudie) on the spurs. The forest shows changes in the site quality by changes in the tree height, form, and understorey composition rather than by changes in the tree species. Thus the drier northern aspects carry smaller peppermints and shrub species understorey, the midslope and southern aspects carry well-developed peppermint with a bracken (Pteridium esculentum Nakai) understorey, and the gully vegetation is large peppermints and manna gums, and an understorey of bracken or false bracken (Culcita dubia Maxon). The trees in the riparian zone can achieve heights of 50 m, while those on the slopes rarely achieve 30 m. Although it has undergone burning from

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations time to time, there is no evidence of commercial logging in the area. The radiata pine plantation created on Clem Creek catchment has been moderately productive and is representative of a large area of similar plantation in this steep foothill country. Growth on the northern (exposed) side of the catchment has been less than that on the sheltered southern side. There have been some issues with wind damage, and the plantation has required both fertilisation with sulphur and nitrogen and thinning to stimulate growth. Typically growth has been about 15 m3 ha1 year1. This is viewed as being reasonable for the area. Over the years various hydrologic studies on the area have been made. Fig. 2 gives daily flow from the three streams for the 2005/2006 year; this could be considered as ‘‘typical’’ of the flow. In particular the streamflows show a strong winter-spring maximum followed by a long recession over summer and autumn. Bren et al. (1979) showed that groundwater inflow from the slopes was the dominant streamflow-generating process at all times, with hydrograph properties being dominated by the shape of the groundwater system. Rainfall caused fast recharge of the groundwater close to the stream. Bren and Leitch (1986) looked at

419

rainfall variations on the catchment and also examined water loss due to ‘‘deep seepage.’’ In particular all three streams have similar behaviour in wet periods but have quite different low flow properties. Ella Creek usually starts flowing in early winter (June and ceases flowing in February or March the following year. Betsy Creek will typically flow only in late winter to early summer. Clem Creek, both before and after treatment, has always had some streamflow. This variation reflects stream ephemerality, so common in this Australian environment. It is hypothesised that the freely draining sandstones pass water to deep aquifers, or that perhaps some of the groundwater flows laterally under the catchments to the deeper Croppers Creek valley to the north. The overall impact on error in estimating annual flows due to the ‘‘controls’’ drying up is small. However it has precluded us from making accurate estimates of the impact of the catchment conversion on low flows. Continuing work on the catchment has shown the importance and complexity of groundwater variations in controlling the rate of inflow of water into the streams and also in determining the stream temperature variations. Bren and Papworth (1991) reported the results of the eucalypt to pine conversion on annual streamflow for seven

Instananeous Flow Rate, Ls -1 Ella Creek

120

80 Flow Starts

40

Flow Stops

0

60

Clem Creek

40

20

0

Betsy Creek

60 40 Flow Starts

20

Flow Stops

0

May

J

J

S

O 2005

N

A

D

J

F

M

A

2006

Figure 2 Annual stream flow as a function of annual rainfall (May–April basis) for the native forest. The solid lines shown are from are the ‘‘forest’’ and the ‘‘grassland’’ line given by Zhang et al. (2001).

420 years after conversion from eucalypt forest to pine plantation. These results showed that: (i) initial conversion led to substantial increases in streamflow in the early winter storms, but no detectable change in summer streamflow. Peak flows from major storms were unaffected, and (ii) the impact of conversion was not detectable in years of low rainfall. Bren (1997) examined the impact of the land use change on the summertime diurnal variation in streamflow exhibited by Clem Creek before and after conversion, and showed that the destruction of slope vegetation caused no detectable change on this. The diurnal variation encountered was similar to that experienced in forested catchments in Mediterranean climates in other parts of the world (e.g. Lundquist and Dettinger, 2005). The observed variation appears to be associated with transpiration by the riparian trees (Bren, 1997).

Method In the initial phase, measurement ran from May 1975 to August 1987. In December 1979, 94% of Clem Creek was cleared. In April 1980 the clearing debris was burnt and the catchment slopes planted to radiata pine (approximately 1000 stems per hectare). Measurement ceased in 1987 and recommenced in 1997. Rehabilitation of the project involved refurbishment of the weirs, clearing of vegetation on roads, and reinstallation of recording equipment. Over the period from May 1997 to April 30th, 2006 (and continuing) the streamflow emanating from each of the small catchments was again measured using the original Vnotch weirs, using similar equipment specifications to those described in Bren and Papworth (1991). Quality control of data were achieved by a fortnightly service. The data were integrated to give daily data (midnight to midnight). Further integration gave monthly and annual flows. Rainfall was measured at three storage gauge sites associated with the weirs. Rainfall recorders allowed the daily rainfall at each weir site to be computed.

Estimation of catchment rainfall The catchment rainfall was initially measured at up to seven sites, but in recent years measurement has been at three sites. Basic measurement was periodic reading of 203 mm rain-gauges mounted in a clearing with a 45 cone of clearance from the gauge. The rainfall data was compared by use of double-mass plots to derive a ‘‘weighting factor’’ for each of the rain gauges relative to the Ella Creek gauge. This was then used to compute an ‘‘average scaling factor’’ to give the best estimate of overall catchment rainfall for each of the catchments and for the project area based on Ella Creek gauge readings.

Water yield under eucalypt forest For each of the catchments the annual water balance is expressed as:

L. Bren, P. Hopmans P ¼ S þ ET þ e

ð1Þ

where P = annual precipitation (mm), S = streamflow (mm), ET (mm) is evapotranspiration (mm), and e (mm) is an error in the annual water balance. This includes errors in measurement and changes in soil water storage. Annual water yields have been computed using a year running from May 1st to April 30th of the subsequent year. This period, towards the end of the long recession in flows after spring rainfalls, has the most predictable flow of any period (very low). Hence it is reasonable to assume that this subdivision gives similar levels of soil moisture storage on the catchment at the start and end of each period, thereby minimising errors associated with changes in storage levels. The annual yield from Clem Creek (pretreatment data only), Ella Creek, and Betsy Creek were plotted as a function of annual rainfall (Ella Creek gauge) and compared with the wellknown relationships of Zhang et al. (2001) for runoff from native forest and grassland. These postulates that evapotranspiration can be expressed thus: ! 1 þ 2820 P ETForest ¼ P ð2aÞ P 1 þ 2820 þ 1410 P ! 1 þ 550 P P ð2bÞ ETGrass ¼ P 1 þ 550 þ P 1100 Hence it follows that if e is taken as zero, then ! 1 þ 2820 P SForest ¼ P  ETForest ¼ P  P P 1 þ 2820 þ 1410 P ! 1 þ 550 P P SGrass ¼ P  ETGrass ¼ P  P 1 þ 550 þ P 1100

ð3aÞ ð3bÞ

These relationships (referred to as ‘‘Zhang Curves’’) form a useful comparison to our observed data and also give a basis for extrapolation of treatment effects from plantations developed on native forests to plantations developed on grasslands. The curves were developed using results from over 250 catchments world-wide. The parameters given have both a systematic and an empirical component developed on the basis of potential evapotranspiration (see Zhang et al., 2001).

Regression analysis of the conversion effects on daily and annual flows Previous analysis (e.g. Bren and Papworth, 1991) had established that there was a good linear relationship between the average daily flow rate in Clem and Ella Creeks. Using daily flow data, linear regression lines were derived for the entire pre-treatment period, and then for each year after treatment. These were of the form: c ¼ a þ me

ð4Þ 1

where c* is the estimated daily average flow rate (Ls ) from Clem Creek, e is the observed daily average flow rate (Ls1) from Ella Creek, and a and m are the regression constants. The Null Hypothesis was that the slopes of the set of regression equations were the same for the pre and post data. This was tested using the ‘‘method of comparing more than two slopes’’ (analysis of covariance) as explained by Zar (1984).

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations This included use of ‘‘Student’s t’’ test to compare the slope of lines derived from different years. In this, Student’s t was defined by m1  mpre t¼ ð5Þ sm1 mpre in which ‘‘t’’ is the value of Student’s t, m1 and mpre are slopes of the regression for a given year and the pretreatment regression respectively, and sm1 mpre is the standard error of the difference between the two regression coefficients.

Estimation of annual treatment effects By summation of Eq. (4) over a year, with numeric transformations to reflect the changes in units and areas of the catchments, an equivalent relation to Eq. (4) for dealing with annual data was derived. From this direct estimates of the annual residual (i.e. treatment effect plus error) were derived. This can be expressed as C ¼ a0 þ m0 E

ð6Þ

where C* is the estimated annual yield (mm) of Clem Creek, E is the measured yield of Ella Creek (mm), and a 0 and b 0 are transformed versions of a and b in Eq. (4) above. From this the ‘‘treatment residual’’, R (mm) was derived by R ¼ C  C

ð7Þ

where C is the observed annual yield of Clem Creek (mm) and C* (mm) is the expected yield based on the regression of Eq. (6). This was viewed as being composed of a treatment effect, T, and an error experimental error e: R¼T þe

ð8Þ st

By summation over a hydrologic year (May 1 to April 30th) the value of the error was minimised. By definition, for the pre-treatment period, T = 0. From the pre-treatment data, assuming hydrologic stationarity, an estimate of the magnitude of e in estimating annual flows was made for the pretreatment period. Then by comparison of the magnitude of the (T + e) values for the pines and the pre-treatment e values, the effect of the treatment was discerned.

Table 1

421

Results Flow measurement error The rating equation for the weirs was derived from a laboratory calibration of the prototype weir blade, and this was checked in the field using a ‘‘drawdown method’’ (see Bren et al., 1979). Analysis of the likely flow measurement error and conversion to flow rates suggested that our percentage error would be around 5%. Other flow errors relate to occasional missing data and the impact of stream temperature changes on the weir rating due to viscosity. We believe that our measurements are of high quality with good quality control and that the error level would be below 10% over most of the flow regime. An issue in data analysis has been that the control catchment, Ella Creek, dries up in late summer or early autumn. Commonly there is a 3 month period where Ella Creek may have little or no flow. Notwithstanding this, Ella Creek has been a good control catchment but this has limited our ability to accurately assess the impact of the treatment on low flows. However the error involved in this is relatively small because the low flows themselves are a small component of the local catchment hydrology. The analysis of Bren and Leitch (1986) quantifies this ‘‘deep seepage’’ loss on the catchments; presumably this passes into regional or national aquifers passing far to the north of our area. If our measurements of flow have left one impression on us, it is the ephemerality of stream flows in the Australian environment and our catchments certainly show this.

Estimation of catchment rainfall Virtually all precipitation experienced was rainfall; snow is a rare occurrence and at best a fraction of a mm averaged over the years. With the exception of Clem Creek gauge, we have been able to maintain a 45 cone of clearance from the horizontal. Clem Creek gauge has usually achieved a 60 (from the horizontal) cone; in recent years clearance has been limited by the size of the gauge clearing that would be necessary. Table 1 shows the relative rainfall (total gauge catch/total gauge catch of Ella gauge for all rainfall measurements for two separate periods.

Depths of accumulated catches of rainfall at six stations relative to the catch at Ella Creek gauge for two periods

Gauge

Period of measurement

Relative rainfall

Ella Creek

May 1975–July 1979 May 1997–April 2006 May 1975–July 1979 May 1975–July 1979 Weighted average May 1975–July 1979 May 1975–July 1979 Weighted average May 1975–July 1979 May 1975–July 1979 May 1975–July 1979

1.00* 1.00* 1.026 0.979 1.000 1.026 0.995 1.009 1.000 1.000 0.929

Clem Creek

Betsy Creek

Candlebark Station Hazle Station Black Range Station *

Defined as 1.0 for analysis.

422

L. Bren, P. Hopmans

The findings suggested that the gauge catch was consistent over time, with perhaps a small area of lower rainfall adjacent to the catchment ridge (this effect was noted by Corbett, 1967); this reflects ridge-top turbulence. There are some changes in relativity of the gauges, probably associated with micro-effects of the adjacent forest vegetation. After consideration of the likely relative areas it was concluded that the reading of the Ella Creek gauge was the best representation of average catchment rainfall for each of the catchments (i.e. a weighting factor of 1.00). Curtis and Burnash (1996) reviewed ‘‘indavertent rain gauge inconsistencies and their effect on hydrologic analysis’’ and concluded that ‘‘the best that can be hoped for is that the gauging equipment will operate close to the scale of reality and with a degree of consistency which will provide a stable index to the rainfall-runoff process’’. We believe that this has been achieved in our rainfall measurement, although we are conscious of the ever-present ‘‘crowding’’ of our gauges by the heavy adjacent forest cover. In general we have been surprised at the consistency of our rainfall records in this steep, upland country. Occasionally there will be a marked spatial variability in summer thunderstorms but overall this has been a very minor factor.

est and grassland are also shown. Regression analysis of annual yield of the native forest catchments against annual rainfall generated the following equations for annual flow: C ¼ 0:9134  P  861:33

ð9Þ

R2 ¼ 0:99; 5 observations E ¼ 0:642  P  584:94

ð10Þ

2

R ¼ 0:84; 21 observations B ¼ 0:610  P  576:82

ð11Þ

2

R ¼ 0:81; 21 observations Annual Catchment Yield, mm 1500

Grassland Clem Creek Ella Creek Betsy Creek

1000

Forest

500

Water yield under eucalypt forest Table 2 gives the observed annual rainfall and stream yields for each of the catchments whilst under native forest. Fig. 3 shows this yield as a function of the annual rainfall (May– April basis); for comparison the ‘‘Zhang’’ curves (Zhang et al. (2001)) of water yield as a function of rainfall for for-

Table 2

a

0

500

1000 1500 Anuual Rainfall, mm

2000

2500

Figure 3 Annual sequence of flow from the three streams for the year from May 2005 to April 2006.

Annual rainfall (Ella Creek gauge) and catchment yield from catchments under native forest

Year May May May May May May May May May May May May May May May May May May May May May

0

1975–April 1976–April 1977–April 1978–April 1979–April 1980–April 1981–April 1982–April 1983–April 1984–April 1985–April 1986–April 1997–April 1998–April 1999–April 2000–April 2001–April 2002–April 2003–April 2004–April 2005–April

1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1998 1999 2000 2001 2002 2003 2004 2005 2006

Clem Ck (mm)

Ella Ck (mm)

Betsy Ck (mm)

Annual rainfall (mm)

951.9 79.4 147.9 485.4 621.1a

783.5 39.9 99.3 348.0 485.5 330.2 919.5 2.8 445.6 350.0 213.4 679.3 28.0 254.3 185.9 448.8 214.0 49.0 253.1 278.4 325.4

639.1 10.8 134.5 279.3 448.8 272.9 879.4 0.0 398.6 333.9 169.8 651.0 10.9 188.7 121.7 524.0 109.0 28.2 267.5 190.1 303.3

1996.0 1058.0 1083.0 1529.0 1551.7 1476.0 2101.4 800.4 1627.8 1176.8 1310.1 1868.6 955.0 1166.0 1456.0 1769.0 1107.0 1088.0 1464.3 1657.1 1384.7

Clearing of native vegetation from December 1979 on slopes.

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations where C, E, and B are the annual stream-flows of Clem, Ella, and Betsy Creeks (in mm) respectively, and P is the annual rainfall (mm). R2 is the coefficient of determination of the equations. Given these equations it is of interest to derive the minimum amount of annual rainfall to generate some annual streamflow. These are: Table 3 Slope of the post-treatment regression of mean daily flow rate of Clem Creek as a function of mean daily flow rate of Ella Creek, and the computed ‘‘Student’s t’’ value of the difference in regression slope Regression

n

Slope m

‘‘Student’s t’’

Pretreatment 1980/81 1981/82 1982/83 1983/84 1984/85 1985/86 1986/87 1997/98 1998/99 1999/2000 2000/01 2001/02 2002/03 2003/04 2004/05 2005/06

1699 365 365 365 366 365 365 296 365 365 366 365 365 365 366 365 365

0.4783 0.7512 0.5401 0.8810 0.5859 0.6423 0.6768 0.5766 0.3448 0.5292 0.5901 0.4524 0.6308 0.3202 0.5121 0.5510 0.4433

23.4 12.6 0.95 17.4 43.0 34.1 26.3 6.7 6.5 17.5 9.3 24.3 7.5 8.5 15.3 8.6

May May May May May May May May May May May May May May May May May May May May May

Clem Creek catchment 943 mm rainfall Ella Creek catchment 914 mm rainfall Betsy Creek catchment 945 mm rainfall Use of a straight line relationship for Eqs. (9)–(11) is somewhat unreliable in that the properties of the line are set by points from high rainfall/high streamflow periods whereas we are extrapolating this line to periods of low rainfall and low streamflow. However, even given this, by Australian standards there is a high ‘‘loss’’ reflecting the overhead of evapotranspiration by the native eucalypt forest. The magnitude of such a ‘‘loss’’ is of substantial political interest in Australia because of recent drought. Hawkins (1992) examined storm rainfall-runoff data collected between 1975 and 1980 on Clem and Ella Creeks. He characterised the response of Clem Creek as ‘‘standard’’ (defined as a ‘‘traditional’’ response), and that of Ella Creek as ‘‘complacent’’ response passing into a ‘‘violent’’ response (i.e. an abstraction threshold precedes a sudden high response). The concepts are more formally defined in Hawkins (1993). Certainly the annual data suggests that once the ‘‘loss threshold’’ is passed the catchments respond efficiently and consistently to rainfall. He also noted that these type of responses were quite common in his data set of 100 small watersheds from North America, Australia, and South Africa.

Regression analysis of the conversion effects on daily and annual flows The pre-treatment regression equation derived for daily flow values was

All values cited are significant at p = 0.01.

Table 4

423

Observed flows in Clem and Ella Creek, the ‘‘residual’’ (treatment effect plus error) and Ella Creek rainfall

1975–April 76 1976–April 77 1977–April 78 1978–April 79 1979–April 80a 1980–April 81 1981–April 82 1982–April 83 1983–April 84 1984–April 85 1985–April 86 1986–April 87 97–April 98 98–April 99 99–April 2000 2000–April 01 2001–April 02 2002–April 03 2003–April 04 2004–April 05 2005–April 06

Clem Ck (mm)

Ella Ck (mm)

Residual (mm)

Rainfall (mm)

951.9 79.4 147.9 485.4 621.1 797.4 1336.7 78.6 894.4 608.3 459.7 963.4 66.0 470.0 390.2 757.5 377.0 83.0 412.6 488.7 416.7

783.5 39.9 99.3 348.0 485.5 330.2 919.5 2.8 445.6 350.0 213.4 679.3 28.0 254.3 185.9 448.8 214.0 49.0 253.1 278.4 325.4

7.0 14.0 14.6 33.3 9.0 366.1 219.5 28.4 328.7 154.0 164.4 125.8 13.6 127.0 126.8 188.2 81.0 21.0 71.0 117.7 9.0

1996.0 1058.0 1083.0 1529.0 1551.7 1476.0 2101.4 800.4 1627.8 1176.8 1310.1 1868.6 955.0 1166.0 1456.0 1769.0 1107.0 1088.0 1464.3 1657.1 1384.7

The shaded portion is pretreatment data. a A small impact of clearing in the January–March period is present in this year.

424

L. Bren, P. Hopmans

c ¼ 0:4783e þ 0:6917 2

R ¼ 0:99;

ð12Þ

n ¼ 1691

where c and e are the average daily flow rate (Ls1) in Clem Creek and Ella Creek respectively. The Null Hypothesis was that the slopes of the regression of daily flows in Clem Creek as a function of daily flows in Ella Creek were unchanged by the treatment. Using the analysis of ‘‘comparing more than two slopes’’ (Zar, 1984) for the pre and post treatment data, the value of ‘‘F’’ was computed as 70.06 (highly significant). Thus the null hypothesis was rejected and the alternative hypothesis – that there is a real treatment effect – is accepted. Table 3 shows the slopes of the regressions together with the numbers of degrees of freedom and the value of ‘‘Student’s t’’ in testing the significance of difference between the annual regressions and the pre-treatment regression. All post treatment regression coefficients were significantly different from the pre-treatment regression, and generally were higher. In ‘‘drought years’’ such as 1982/83 and 1997/98, the regressions were still significantly different but the slope was less than the pretreatment effects; however the range of flows tended to be quite small. Close study of the data from these years suggests that the radiata pine is more effective at gaining water in dry years than the native eucalypt forest.

Because Ella Creek usually dries up for a few months we can not be very specific about the impact of the plantation conversion on the lowest flow range. We have used (and are continuing to use) a variety of techniques based on hydrologic models and autoregressions to see if we can establish a clear impact of the conversion on low flow data. Other than establishing that if there are effects on this flow range, they are not large, the studies have been inconclusive.

Estimation of annual treatment effects When summed over 1 year and with the constants adjusted to take account of differing units and catchment areas, Eq. (12) can be expressed as C ¼ 1:1639E þ 46:9695

ð13aÞ

C ¼ 1:1639E þ 47:0982

ð13bÞ

in which C and E are the annual yield of Clem Creek and Ella Creek in mm. Eq. (13a) is for non-leap years, and 13b is for years including leap days. Using this relation Table 4 was derived. This shows the observed flows in Clem Creek and Ella Creek, and the annual rainfall. From this the ‘‘residual’’ (Treatment + error) was computed. Fig. 4 shows this data over the course of the project.

Annual Flow, mm 1200

Ella Creek

800 400

No Data 0

Annual 1200 Flow, mm

Clem Creek

800 400

No Data

0

Increased 400 Flow, mm 300

Treatment Effect

200 100

No Data

0 -50

Annual Rainfall, 2000 mm

Rainfall

1000

Figure 4

05/06

1999/2000

94/95

84/85

79/80

1975/76

Pretreatment

89/90

No Data 0

Annual flow in Ella and Clem Creek, estimated treatment effect, and annual rainfall. The pretreatment period is shaded.

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations Use of the pretreatment data suggests that our error in our model of the conversion from eucalypt to pine is reasonably viewed as about 30 mm. It can be seen that there is a clear treatment effect which appears to be slowly dying away as the radiata pine ages. The treatment effect is linked to the rainfall in the sense that the very dry years (e.g. 1997/98; 2005/06) in general have no apparent treatment effect and tend to show a small diminishment of flow relative to the eucalypt forest; however the magnitude of such an effect is small relative to the error of measurement. It is possible that radiata pine is more efficient at extracting water in such circumstances than the native forest it has replaced. The results were: (i) The effect of the conversion to radiata pine appears to have diminished substantially with the passing of time. At the time of clearing this led to increases above the native eucalypt yield of in excess of 300 mm. Periods where increased flows were detectable were in the late winter and early spring periods. (ii) the water yield increases led to increases in streamflow in the wetter periods. No change could be detected in the low flow periods. (iii) there was no conclusion about changes in flow associated with the non-commercial thinning of the catchment. Because there is no replication of the catchment with and without thinning it is not possible to separate the potential effects of thinning from other influences (particularly low rainfall) happening at the same time.

stemflow, throughflow, storage capacity of the forest floor litter, and ultimately streamflow. Fig. 5 shows the increase in annual streamflow associated with the conversion to radiata pine as a function of the years since treatment for each of these data sets. The Clem Creek and the Stewarts Creek data show substantial similarities. The Lidsdale data shows a reduced response compared to these, which possibly reflects the lower rainfall. The data (age, change in water yield, annual rainfall) from these three Australian experiments were pooled and a regression of increase in water yield as a function of age and rainfall was computed. The equation derived was DSEuctopine ¼ 12:9612A  141:539A0:5 þ 12:0903P 0:5 R2 ¼ 0:83;

The Croppers Creek project represents an attempt to determine the impact on water yields of a change from eucalypt forest to pine plantation on a reasonably large scale. Partial results from two other paired catchment hydrology projects (both terminated) have become available in recent years. The Stewarts Creek project was located near Daylesford (Victoria). Results from this have been reported in Nandakumar and Mein (1993); Mein et al. (1988) and, more recently, Lane et al. (2005). Treatments examined the impact of conversion from mixed species eucalypt forest to radiata pine (Catchment 5, 17.6 ha). Results available cover the period from 1961 to 1990. The eucalypt to pine conversion occurred in 1969. Rainfall in the area is given as ‘‘1000– 1280 mm’’ (Tsykin et al., 1982). The mixed eucalypt cover was of similar appearance, maturity, and density to that of Croppers Creek. However the catchments tended to be much flatter (average slope around 6). Putuhena and Cordery (2000) published an account of the ‘‘Lidsdale Project’’ (near Bathurst, NSW) which also involved measurement of changes in water yield when a dry sclerophyll eucalypt forest was cleared and converted to Pinus radiata. This area had an average annual rainfall of 755 mm. The catchment had an area of 9.4 ha, with an average slope of 12% (approximately 7). Measurement commenced in 1959. In 1978 the catchment was cleared and planted with pine. The results presented examine the impact of the conversion on

ð14Þ

n ¼ 59

where DSEuctopine is the change in water yield from plantation relative to that expected from native eucalypt forest, mm; P is the annual rainfall, mm, and A is the age of the radiata pine, years. The coefficient of determination, R2, was viewed as reasonable given the many different factors between the three sites. This relationship is illustrated in a

Increased Yield, mm 400

Clem Creek

300 200

No Data

100

Formation of a model using other Australian catchment data

425

0 -100 0

5

10

15

400

20

25

Stewarts Creek

300 200 100 0 -100 0

5

10

15

20

400

25

Lidsdale

300 200 100 0 -100 0

5

10

15

20

25

Age of Plantation, years Figure 5 Increased yields of clearing of eucalypts for Pinus radiata as a function of the age of the plantation for Clem Creek, Stewarts Creek, and Lidsdale catchment.

426

L. Bren, P. Hopmans

contour plot in Fig. 6a. The ‘‘zero line’’ marks the line of equal catchment yield from both the pine and the native eucalypt forest. Catchments carrying radiata pine of a given age and rainfall will be one side or the other of this line. Thus for instance the plot suggests that 15 year-old radiata pine receiving 1200 mm of rainfall will generate approximately 65 mm more runoff than would native forest on the same site. In contrast, if the rainfall was only 800 mm annually then the runoff would be 12 mm less than the same catchment under eucalypt. A physiological interpretation would be that, in dry conditions, the radiata pine

Plantation Age, 25 Years

-200 mm

-100 mm

is more effective at extracting water from the catchment than the native eucalypt forest, but under less-dry conditions it does not have the same water use needs. The results are also in accordance with the claims of ‘‘excess runoff from young plantations’’ that led to the Croppers Creek project being initiated (after a run of unusually wet years). The relationship is empirical and takes no account of thinning effects, site quality, etc. However it does offer a means of estimating the net effect of many small catchments of different plantation ages on outflow from a larger catchment.

0 mm

100 mm

20

15

200 mm

10

5 300 mm

0 0

500

1000

1500

2000

Annual Rainfall, mm Plantation Age, Years

-200 mm

-200 mm

25

20

15

10

-100 mm

5 0 mm 100 mm

0 0

500

1000

1500

Annual Rainfall, mm

2000

Increased yields domain

Figure 6 Contour plots showing computed change in annual streamflow as a function of the plantation age and annual rainfall: (a) is the change from eucalypt forest and (b) is the change from pasture. The shaded area shows where there is increased yield relative to the original vegetation.

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations Although the above result is of interest, there is greater concern about the impact of radiata pine on water yields when it replaces grassland in southern Australia. Use of the ‘‘Zhang Curves’’ defining the difference between mature forest and grassland runoff as a function of annual rainfall gives a method of extrapolation of results to pasture sites. This has been computed as DSGrasstopine ¼ DSEuctopine  ðSGrass  SForest Þ

ð15Þ

When expanded out this relationship is " # " # ð1 þ 2820 ÞP ð1 þ 550 ÞP P P þ P DSGrasstopine ¼  P þ 1 þ 2820 þ 1 þ 550 1410 P 1100 P þ 12:090P0:5 þ 12:961A  141:539A0:5

427

essy (2001) findings on the impact of radiata pine forestry on grassland land in the Tumut region of NSW. This showed a small increase in streamflow immediately after pine establishment and a continued decrease in streamflow for the subsequent eleven years after planting. It is reiterated that the Cropper Creek project did not involve a grassland component and the result is obtained by extrapolation and therefore caution should be exercised in the interpretation of these results.

Comparison with New Zealand and South African data ð16Þ

in which DSGrasstopine(P, A) is the potential change in annual yield (mm) associated with annual rainfall P mm for trees of age A for a plantation on grassland, and the term (SGrass  SForest) represents the difference between the ‘‘grassland yield’’ and the ‘‘eucalypt yield’’ in Fig. 3. A contour plot of Eq. (16) is shown in Fig. 6b. The results suggest that only for very young trees (and perhaps high rainfall) would runoff exceed that of grassland, and that as the trees age the runoff diminishes. This is in agreement with Vert-

Rowe and Pearce (1994) presented results generated by harvesting native forest catchments and establishing radiata pine plantations in New Zealand. The catchments were small (1.6–4.6 ha) and in a high-rainfall (2000–2500 mm). They found that ‘‘in the year after treatment streamflow generally increased by 200–250 mm. This was followed by rapid colonisation of the newly planted catchments with bracken and other species. This led to a rapid decline in streamflow which returned to pretreatment levels after an average of about 5 years. Streamflow yields then continued to decline for another 2–3 years before stabilising at a level

100 Change in Annual Yield, 0 mm

Lambrechtsbos A

-100 -200 -300 0

5

10

100

15

20

Lambrechtsbos B

0 -100 -200 -300 -400 -500 0

5

10

15

100

20

Biesievlei

0 -100 -200 -300 -400 0

5

10

15

20

Age Modelled Data

Figure 7 (16).

Observed Data

Changes in flow observed at three catchments at Jonkershoek compared to the values estimated using the model of Eq.

428 about 250 mm year1 lower than the pre-treatment level. At this time the catchments had a dense bracken/honeysuckle understorey beneath 5 m tall pine trees. The small size of the catchments and the high rainfall make direct comparisons difficult but the results did show an increase in streamflow after clearing and a diminishment of streamflow as the trees aged. One of the catchments exhibited increases in streamflow of 550 mm – a magnitude of response far beyond what might be expected at Croppers Creek. van Wyk (1987) examined the impact of radiata pine on streamflows at Jonkershoek in the Western Cape Province of South Africa. The catchments were converted from fynbos (a proteaceous scrubland) to radiata pine. van Wyk (1987) provided a streamflow–rainfall relationship for fynbos which is quite similar to that of Zhang et al. (2001) for grassland so the model of Eq. (16) has been used. Data sets of rainfall, change in flow, and age of the radiata pine were available for three catchments within the Jonkershoek project – Lambrechtsbos A (31 ha), Lambrechtsbos B (66 ha), and Biesievlei (201 ha). The data were sourced from Scott et al. (2000). Fig. 7 shows the model applied to this data. The post-treatment range of this data exceeded that of the Croppers Creek model but the model reproduced the general form of change well. To the authors, at least, the fit is about as reasonable as one might get from a simple model, remembering that the error levels alone in both the source data and the modelled data are not insubstantial, and that thinning, wind-throw, and other factors influencing growth will also influence water yield.

Future of the project It was intended to clearfall Clem Creek catchment in 2007/8 with measurements of the nutrients and streamflow properties being made for at least some years after clearfalling. However on December 10th 2006, a major Victorian bushfire started after a year of record low rainfall. This burnt all the catchments and destroyed recording equipment. At the time of finalisation of this paper (January 2007) salvage logging of the burnt pine catchments and rebuilding burnt weirs is proceeding. Thus the annual data presented in this paper will be the last for this phase of the project. Future options may include following through the development of the new plantation and examining the impact of the burn but there is no clear control.

Conclusion The project examined the water yield of two small catchments carrying native eucalypt forest and one catchment which was converted from native forest to radiata pine plantation. The natural yield of these catchments under eucalypts was a more or less linear function of rainfall with an annual rainfall of around 900 mm being required before any runoff was generated. There was some variation in the yield between catchments, which probably represents a deep seepage loss to regional groundwater. The water yield of these native eucalypt catchments as a function of rainfall was below the expected value for Victorian catchments. Comparison with results of two other paired-catchment experiments in Australia produced a useful estimate

L. Bren, P. Hopmans of change in water use (relative to native forest) as a function of both radiata pine age and rainfall. This relationship has been extrapolated to estimate the effects of plantation pines replacing grassland.

Acknowledgement The project was funded by the Forests and Wood Products Research and Development Corporation, Hancock Victorian Plantations Pty Ltd (formerly Victorian Plantations Corporation), the CRC for Catchment Hydrology, the Centre for Forest Tree Technology, and the University of Melbourne Department of Forestry. Particular thanks are due Hugh Stewart and David Flinn for their efforts in restarting measurement at Croppers Creek and for Mike McCormick and John Costenaro for their servicing of field instrumentation.

References Bren, L.J., 1997. The effects of a slope vegetation change on the diurnal variations of a small mountain stream. Water Resources Research 33 (2), 321–330. Bren, L.J., Flinn, D.W., Hopmans, P., Leitch, C.J., 1979. Hydrology of small forested catchments in north-eastern Victoria. 1: Establishment of the Croppers Creek Project. Bulletin No. 27, Forests Commission Victoria, p. 48. Bren, L.J., Leitch, C.J., 1986. Rainfall and water yields of three small, forested catchments in north-east Victoria, and relation to flow of local rivers. Proceedings of the Royal Society of Victoria 98 (1), 19–29. Bren, L.J., Papworth, M., 1991. Early water yield effects of conversion of slopes of a Eucalypt forest catchment to radiata pine plantation. Water Resources Research 27, 2421–2428. Bren, L.J., Turner, A.K., 1979. Overland flow on a steep, forested, infiltrating slope. Australian Journal of Soil Research 30, 43–52. Corbett, E.S., 1967. Measurement and estimation of precipitation in experimental watershedsin Forest Hydrology, Proceedings of the International Symposium on Forest Hydrology, Pennsylvania State University, August 1965. Pergamon Press, New York. Curtis, D.C., Burnash, R.J., 1996. Inadvertent rain gauge inconsistencies and their effect on hydrologic analysis. In: 1996 California-Nevada ALERT Users Group Conference, Ventura, CA, May 15–17, 1996. Hawkins, R.H., 1992. Variety, classification, and association in rainfall-runoff response. Report to the US Environmental Protection Agency, Corvallis, Oregon, USA (Contract #813651-01). Hawkins, R.H., 1993. Asymptotic determination of runoff curve numbers from data. ASCE Journal Irrigation and Drainage Engineering 119, 334–345. Lane, P.N.J., Best, A.E., Hickel, K., Zhang, L., 2005. The response of flow duration curves to afforestation. Journal of Hydrology 310, 253–265. Leitch, C.J., Flinn, D.W., 1986. Hydrological effects of clearing native forest in northeast Victoria; the first three years. Australian Forest Research 16, 103–116. Lundquist, J., Dettinger, M., 2005. How snowpack heterogeneity affects diurnal streamflow timing. Water Resources Research, 41, W05007, doi:10.1029/2004WR0003649. Mein, R.G., Bieniaszewska-Hunter, H., Papworth, M., 1988. Land use changes and the hydrologic water balance – Stewarts Creek Experimental Area. In: Hydrology and Water Resources Symposium, Canberra, Institute of Engineers Australian National Conference Publication No. 88/1, pp. 129–134.

Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations Nandakumar, N., Mein, R.G., 1993. Analysis of paired catchment data to determine the hydrologic effects of changes in vegetative cover on yield. Project UM010, Department of Civil Engineering, Monash University, Melbourne, Australia. Putuhena, W.M., Cordery, I., 2000. Some hydrological effects of changing forest cover from eucalypts to Pinus radiata. Agricultural and Forest Meteorology 100, 59–72. Rowe, L.K., Pearce, A.J., 1994. Hydrology and related changes after harvesting native forest catchments and establishing Pinus radiata plantations. Part 2. The native forest water balance and changes in streamflow after harvesting. Hydrological Processes 8, 281–297. Scott, D.F., Prinsloo, F.W., Moses, G., Mehlomakulu, M., Simmers, A.D.A., 2000. A re-analysis of the South African catchment afforestation experimental data. WRC Report No. 810/1/00. Water Research Commission, Pretoria, 138 pp.

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Tsykin, E., Laurenson, E.M., Wu, A.K.Y., 1982. Hydrologic effects of replacement of eucalypt forest by pasture and pines. In: First National Symposium on Forest Hydrology Melbourne, 11th–13th May, 1982, The Institute of Engineers, Australia, pp. 124–131. van Wyk, D.B., 1987. Some effects of afforestation on streamflow in the Western Cape Province, South Africa. WaterSA 13 (1), 31– 36. Vertessy, R.A., 2001. Impacts of plantation forestry on catchment runoff. Proceedings of National Workshop, July 2000. Water and Salinity Issues in Agroforestry No. 7, Rural Industries Research and Development Corporation, Publication No. 01/20. Zar, J.H., 1984. Biostatistical Analysis, 2nd ed. Prentice-Hall, New Jersey, pp. 718. Zhang, L., Dawes, W.R., Walker, G.R., 2001. Response of mean annual evapotranspiration to vegetation changes at catchment scale. Water Resources Research 37 (3), 701–708.