performance of grass and eucalyptus riparian buffers in a pasture catchment, western australia, part...

HYDROLOGICAL PROCESSESHydrol. Process. 20, 2327–2346 (2006)Published online 11 May 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.6054

Performance of grass and eucalyptus riparian buffers in apasture catchment, Western Australia,

part 2: water quality

Lucy A. McKergow,1,2* Ian P. Prosser,2 David M. Weaver,3 Rodger B. Grayson1

and Adrian E. G. Reed3

1 Cooperative Research Centre for Catchment Hydrology, Department of Civil and Environmental Engineering, University of Melbourne,VIC 3010, Australia

2 Cooperative Research Centre for Catchment Hydrology, CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia3 Department of Agriculture, Western Australia, 444 Albany Highway, Albany, WA 6330, Australia

Abstract:

Declining water quality on the south coast of Western Australia has been linked to current agricultural practices.Riparian buffers were identified as a tool available to farmers and catchment managers to achieve water qualityimprovements. This study compares 10 m wide regenerating grass and Eucalyptus globulus buffer performance.Surface and subsurface water quality were monitored over a 3-year period. Nutrient and sediment transport wereboth dominated by subsurface flow, in particular through the B-horizon, and this may seriously limit the surface-runoff-related functions of the riparian buffers. Riparian buffer trapping efficiencies were variable on an event basisand annual basis. The grass buffer reduced total phosphorus, filterable reactive phosphorus, total nitrogen and suspendedsediment loads from surface runoff by 50 to 60%. The E. globulus buffer was not as effective, and total load reductionsin surface runoff ranged between 10 and 40%. A key difference between the grass and E. globulus buffers was theseasonality of sediment and nutrient transport. Surface runoff, and therefore sediment and nutrient transport, occurredthroughout the year in the E. globulus buffer, but only during the winter in the grass buffer. As a consequence ofhigh summer nutrient and sediment concentrations, half the annual loads moving via surface runoff pathways throughthe E. globulus buffer were transported during intense summer storms. This study demonstrates that grass and E.globulus riparian buffers receiving runoff from pasture under natural rainfall can reduce sediment and nutrient loadsfrom surface runoff. However, in this environment the B-horizon subsurface flow is the dominant flowpath for nutrienttransport through the riparian buffers, and this subsurface flow pathway carries contaminant loads at least three timesgreater than surface runoff. Copyright 2006 John Wiley & Sons, Ltd.

KEY WORDS riparian buffers; water quality; grass buffer; Eucalyptus globulus; tree buffer; phosphorus; nitrogen;sediment

INTRODUCTION

Riparian buffers perform several key roles in minimizing the impacts of agriculture on stream water quality.Buffers can: (1) stabilize stream channel morphology (Thorne, 1990); (2) protect streams from upland sourcesof pollution by physically filtering and trapping sediment, nutrient and chemicals in surface runoff (e.g.Dillaha et al., 1989; Vought et al., 1994); (3) provide suitable subsurface conditions for plant uptake andchemical transformations, such as denitrification (e.g. Peterjohn and Correll, 1984; Haycock and Burt, 1993);(4) displace sediment and nutrient-producing activities away from streams (Wenger, 1999).

Despite being an accepted water quality mitigation tool, there are limited data evaluating riparian bufferperformance under field conditions. Experimental studies with simulated rainfall or runoff on cropland

* Correspondence to: Lucy A. McKergow, National Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, New Zealand.E-mail: [email protected]

Received 24 June 2003Copyright 2006 John Wiley & Sons, Ltd. Accepted 22 March 2005

2328 L. A. MCKERGOW ET AL.

dominate the riparian literature (e.g. Dillaha et al., 1989; Magette et al., 1989; Barfield et al., 1998). Thesestudies are valuable for investigating processes, but the optimized experimental conditions often do notaccurately represent natural rainfall and runoff conditions. Field studies with natural rainfall and at a largerscale than confined experimental plots may provide a more realistic picture of riparian buffer potential. Naturalbuffers receiving runoff from cropland have been effective at trapping both nutrients and sediment (Lowranceet al., 1983; Peterjohn and Correll, 1984; Jordan et al., 1993; Daniels and Gilliam, 1996; Robinson et al.,1996). In contrast, riparian buffers receiving runoff from pasture have received little attention.

Riparian buffers have been evaluated at the plot and catchment scales on pastoral land. Smith (1989)measured surface runoff from both pasture strips and retired pasture strips on a sheep and dairy farm inNew Zealand. Sediment, phosphorus (P), particulate nitrogen and nitrate concentrations in runoff exitingretired pasture were substantially lower than concentrations in runoff at grazed pasture sites (Smith, 1989).Several studies have used a catchment-scale approach to evaluate riparian buffers and have assessed changesin stream water quality either using a paired or nested catchment approach (e.g. McColl, 1978; Smith, 1992)or a before-and-after approach (e.g. Owens et al., 1996; Williamson et al., 1996; Line et al., 2000). Thesecatchment-scale assessments concluded that riparian buffers can mitigate some of the adverse effects ofpastoral farming.

Declining water quality on the south coast of Western Australia (WA) has been linked to current agriculturalpractices, particularly P losses. Eutrophication of coastal water bodies has encouraged algal growth and ledto noxious algal blooms in many south-coast harbours and estuaries, e.g. the Peel–Harvey Estuary (Hodgkinand Hamilton, 1993). Subsidies have increased adoption of riparian buffers as a water quality tool in thisregion and provide the impetus for this study. In addition, over the past decade, many blocks of riparian landhave been planted with Eucalyptus globulus Labill. subsp. globulus for pulpwood production. On good sites,E. globulus can be ready to harvest when only 8 to 10 years old (DAWA, 2001). It was unclear from theliterature what impact these plantation buffers would have on stream water quality and whether plantationE. globulus buffers performed differently to regenerating grass buffers.

This study compares the performance of grass and E. globulus riparian buffers under natural rainfallconditions in southern WA. Three key questions are examined in this paper: (1) Which flowpath is the majorcontributor of pollutants to streamflow? (2) Under what conditions are riparian buffers effective in reducingsediment and nutrient delivery to streams? (3) Are grass buffers more effective at improving water qualitythan E. globulus buffers?

In this paper, the term surface runoff is used for a visible flow of water over the ground surface, howeverit is produced (see ‘overland flow’ in Goudie et al. (1994)). Surface runoff may be either infiltration-excessoverland flow (IEOF; Hortonian) or saturation overland flow (SOF), including exfiltration or return flow(Chorley, 1978). The term subsurface flow is used for all water moving through soil horizons, and mayinclude macropore flow and displacement of soil water.

STUDY SITE DESCRIPTION

A 307 ha agricultural catchment 40 km north of Albany, WA (Figure 1) was chosen for this study. It is asubcatchment of the Wilson Inlet catchment, a focus catchment in the National Eutrophication ManagementProgram. A complete description of the study site can be found in McKergow et al. (2006) and only a summaryis presented here. The area has a Mediterranean climate, and the majority of rain falls between April andOctober (see Figure 2). Catchment soils are duplex yellow sands and the topography is low rolling hillswith gentle slopes. Land use is predominantly improved pasture (annual subterranean clovers and ryegrass)and stocking rates are generally 10 sheep per hectare. Stocking rates on the experimental hillslope were fivesheep hectare. Pasture re-establishment is a common practice in this region, particularly after dry summers.It was expected that pasture re-establishment would be undertaken on the study hillslope; however, owing toa change in land ownership, this did not eventuate.

Copyright 2006 John Wiley & Sons, Ltd. Hydrol. Process. 20, 2327–2346 (2006)

GREEN AND EUCALYPTUS BUFFERS, PART 2 2329

Figure 1. Monitoring layout on Farm 3a. Study catchment showing the monitoring site, current catchment vegetation and farm boundaries.Accompanying maps place the catchment within the Wilson Inlet catchment and Australia



0

10

20

30

40

50D

aily

rai

nfal

l tot

al (

mm

)

0

60

120

180

1Jul97 1Jan98 1Jul98 1Jan99 1Jul99 1Jan00 1Jul00

Date

Mea

n da

ily fl

ow (

l s -1

)

Figure 2. Daily rainfall and streamflow for the period July 1997 to October 2000

Small blocks of E. globulus were planted on all farms in the catchment between 1992 and 1995 (Figure 1,Table I), and in 1999 a vineyard was established (Figure 1). Riparian margins were fenced when theE. globulus were planted, and consequently stock has not had access to any stream reaches in the catchmentsince the mid 1990s.

Fertilizer type, timing, and rates vary between farms, and limited soil nutrient testing is carried out as awater quality management tool. Fertilizer is broadcast in autumn (March–May), at the break of season. Farms1 and 2 have similar fertilizer histories (Table I). Farm 3 received 9Ð1 kg ha�1 of P in May each year until1998. In 1998, it was sold and split into two properties (Farms 3a and 3b). Farm 3a is no longer fertilizedannually, although it received 10Ð9 kg ha�1 of P in 1999 and in March 2000 the experimental hillslopereceived 18Ð2 kg ha�1 of P. Farm 3b, the vineyard, was fertilized with blood and bone in mid 1999.

METHODS

Intensive monitoring of hillslope runoff and streamflow was conducted over a 3-year period. A completedescription of the site layout and monitoring equipment can be found in McKergow et al. (2006).

Two adjacent planar hillslopes were instrumented with surface runoff plots and subsurface flow troughsover a total hillslope width of 275 m. One hillslope drained towards a grass riparian buffer, and the otherdrained to an adjacent E. globulus buffer (Figure 1). The grass riparian buffer was fenced in 1997 and theE. globulus buffer was fenced at planting in 1992 (890 trees per hectare; 4Ð5 m row ð 2Ð5 m tree spacing).The buffers were evaluated over a 10 m width, measured perpendicular to the stream. This is a standard bufferwidth commonly considered by land managers, and it also allowed installation of the monitoring infrastructurein the available distance between the fence and stream.

The experimental hillslopes were approximately 350 m long with a predominantly uniform slope, althoughthe microtopography was uneven and lumpy. Soils were shallow and rocky at the top of the slope. In theriparian buffers, three characteristic soil horizons were identified. A dark grey loamy sand (0 to 8 cm depth;90% sand, 6% silt and 4% clay) gradually changes to a light brown sandy gravel A-horizon (8 to 40 cm depth),scattered with roots, small gravel (1–3 cm diameter) and macropores (4–16 mm diameter). The B-horizon(40 to 80 cm depth) is yellowish gravelly clay with pockets of lateritic gravels (3–8 cm diameter).



Tabl

eI.

Cat

chm

ent

land

use

and

fert

iliz

erhi

stor

y(s

eeFi

gure

1fo

rfa

rmlo

cati

ons)

Farm

1Fa

rm2

Farm

3aFa

rm3b

Farm

4Fa

rm5

Are

a(h

a)44

2510

460

3626

Cle

ared

1975

1948

–63

1968

1968

Unk

now

nU

nkno

wn

Prim

ary

land

use

Gra

zing

10sh

eep/

haG

razi

ng10

shee

p/ha

Gra

zing

10sh

eep/

haV

iney

ard

55ha

(199

9)G

razi

ng10

shee

p/ha

Gra

zing

10sh

eep/

haSe

cond

ary

land

use

Can

ola

C6

haE

.glo

bulu

s,19

921

haE

.glo

bulu

s,19

9246

haE

.glo

bulu

s,19

93–

95—

1ha

E.g

lobu

lus,

1992

—

Pfe

rtil

izer

pres

ent

(kg

ha�1

)12

Ð712

Ð7M

inim

alB

lood

and

bone

Unk

now

nU

nkno

wn

past

(kg

ha�1

)16

169Ð1

9Ð1(w

hen

graz

ed)

Unk

now

nU

nkno

wn



Rainfall was measured with two tipping-bucket rain gauges located in the grass buffer (Figure 1). Anygaps in the site rainfall record were filled using data from the Mt Barker station (Figure 1) operated by theWestern Australian Department of Agriculture.

Surface runoff, subsurface flow and streamflow measurement

Surface runoff was measured at 20 locations in the riparian buffers with runoff plots (Figure 1). In eachriparian buffer, five plots were positioned immediately below the fence, i.e. input from paddock to riparianbuffer, and five plots were placed 10 m into the riparian buffer, i.e. output from a 10 m buffer (Figure 1). Ofthe hillslope width, 10% was monitored in each buffer: runoff was sampled from 10 m (5 ð 2 m plots) ofthe total width of 100 m. Multiple plots were constructed in each riparian buffer, as the spatial variability inrunoff volumes was expected to be large, and aggregated concentrations and loads are presented, rather thanindividual plots values.

Surface runoff was intercepted by 2 m wide PVC troughs with concrete aprons to provide smooth contactwith the soil surface (see McKergow et al. (2006: figure 2a)). Each plot was covered to eliminate directprecipitation inputs. Runoff flowing into the plots was directed through a splitter and 10% was diverted to205 l storage drums. The remaining 90% of runoff was returned to the buffer as dispersed flow (nine of tensplitter box outlets). The volume of runoff collected was measured on each visit.

Subsurface flow was measured at two depths (A- and B-horizons) in both the grass and E. globulus buffers(Figure 1). Whipkey-style troughs (Whipkey, 1965), 6 m wide, were installed in February 1998. Subsurfaceflow collected in each trough was piped under gravity to a 50 mm RBC flume (Clemmens et al., 1984)lower down the slope, where water levels were measured with pressure transducers and stored on dataloggers (see McKergow et al. (2006: figure 2b)). Water levels were converted to discharge using theoreticalstage–discharge rating curves (Bos et al., 1991), which were verified with manual measurements.

Streamflow was measured continuously from July 1997 onwards at a Parshall flume (Parshall, 1950) locateddownstream of the buffers (Figure 1). Water levels were measured every 15 min and converted to instantaneousflow using a theoretical stage–discharge rating (Bos, 1989).

Water quality sampling

Grab and automated samples were collected for water quality analysis. Grab samples were collected fromall flumes and storage drums on each site visit. The time between site visits varied between 1 and 7 days,depending on weather conditions. Therefore, surface runoff samples were a composite of single or multipleevents. Automatic water samples (ISCO 1680) were collected from the subsurface trough flume pools daily.Rising-stage samplers (Guy and Norman, 1970) supplemented grab and automatic sample collection at thestream flume. Four rising-stage samplers were installed at the flume at staggered depths. An automatic sampler(ISCO model 6300) collected stream event samples and entered ‘storm mode’ after a predetermined stage risein the previous hour.

The use of automatic samplers to overcome the logistical difficulties of manual sampling has disadvantages.A key disadvantage is that samples must be stored before analysis. During storage, chemical, biological andphysical processes may change the water quality of the sample (e.g. Maher and Woo, 1998; Gardolinskiet al., 2001). These changes may be particularly important for samples analysed for filterable reactive P(FRP). Several studies have examined the changes in unfiltered FRP samples stored under field conditionsfor up to a week and found little or small percentage changes in concentration (Klingaman and Nelson, 1976;Allen-Diaz et al., 1998; Kotlash and Chessman, 1998). In this study, the storage of samples for a few dayswithout preservation was deemed acceptable, especially considering the cool air temperatures (BOM, 2002),brackish waters and potential sources of uncertainty in differing sampling and load calculation methods.Despite the development and use of guidelines for sampling nutrients, the techniques are not absolute (Maherand Woo, 1998; Allen-Diaz et al., 1998, Jarvie et al., 2002). The impacts of possible storage effects on FRPconclusions are discussed later.



Water quality analysis

All samples were refrigerated on return to the laboratory and subsamples filtered (0Ð45 µm Millipore)for FRP determination. All samples were analysed for total P (TP), FRP, total nitrogen (TN), electricalconductivity (EC) and suspended sediment (SS). Methods of analysis are outlined in Table II. The term FRPis used here, rather than dissolved or soluble, as the filtrate could be a mixture of dissolved P and P attachedto colloidal material that passes through the <0Ð45 µm filter.

Statistical analysis

Non-parametric tests were used, as the data had non-normal distributions and no manipulation of databelow the detection limit (DL) was required (Helsel and Hirsch, 1992). Mann–Whitney (MW) rank sum testswere used to compare two datasets. Kruskel–Wallis (KW) one-way analysis of variance on ranks was used

Table II. Methods of water quality and soil analysis

Parameter Units and DL(if applicable)

Method and method code (APHA, 1989; S codes fromACL (1995))

Lab

Water samplesTP 0Ð01 mg l�1 Unfiltered sample; colorimetric analysis following digestion

with 1 : 1Ð5 K2S2O8 to NaOH (APHA 4500-P)CCWAa

TN 0Ð02 mg l�1 Unfiltered sample; colorimetric analysis following digestionwith 1 : 1Ð5 K2S2O8 to NaOH (APHA 4500-N)

CCWA

FRP 0Ð01 mg l�1 Filtered through 0Ð45 µm Millipore filter, colorimetric analysisfollowing addition of molybdate and ascorbic acid reagents(APHA 4500-P)

CCWA

EC mS m�1 Unfiltered sample, conductivity meter reading corrected to25 °C (APHA 2510-B)

CCWA

SS 1 mg l�1 Gravimetric, 1Ð2 µm GF/C filter paper, 105 °C for 24 h (APHA2540-D)

WADAb

Soil samplespH — Measured by pH meter using a glass electrode in a 1 : 5

suspension of soil in 0Ð01 M CaCl2 (S03)CCWA

Organic carbon %C Metson’s colorimetric modification of Walkley and Blackmethod: oxidation of soil organic matter in a finely groundsample (<0Ð15 mm) by dichromate in the presence ofsulphuric acid (S09)

CCWA

Bicarbonate-extractableP, P-HCO3

mg kg�1 Colorimetric analysis following extraction in 0Ð5 M NaHCO3 atpH 8Ð5; soil solution ratio of 1 : 100 (S12)

CCWA

TP mg kg�1 Kjeldahl digestion with P measured by colorimetry (S14) CCWAPhosphorus retention

index, PRImg l�1 Involves equilibrating soil with a 10 µg ml�1 of P in 0Ð02 M

KCl and 0Ð25% chloroform at a soil : solution ratio of 1 : 20.PRI is the ratio of Pads (P adsorbed from the solution, µg g�1

soil) to Peq (concentration of P remaining in solution atequilibrium, g ml�1) (S15)

CCWA

Oxalate extractable iron,Fe-AmOx

mg kg�1 Atomic absorption spectrophotometry after extraction in 0Ð2 M

ammonium oxalate, pH 3Ð25 (S29)CCWA

Oxalate-extractablealuminium, Al-AmOx

mg kg�1 Atomic absorption spectrophotometry after extraction in 0Ð2 M

ammonium oxalate, pH 3Ð25; soil solution ratio 1 : 33 (S29)CCWA

Particle size analysis — Plummet method (sand 2–0Ð02 mm, silt 0Ð02–0Ð002 mm, clay<0Ð002 mm) (S06)

CCWA

a Chemistry Centre of Western Australia.b Western Australian Department of Agriculture, Albany.



to compare more than two groups, and Dunn’s method (DM) of pairwise comparisons was used to identifysignificantly different pairs (Jandel Software, 1995).

Many FRP samples and a few TP samples were below the DL, but only the E. globulus buffer B-horizonflow had more than 50% of FRP data below the DL. A simple substitution of 0Ð5 DL was made for valuesbelow the DL for plotting, and FRP : TP ratio and load calculations. A sensitivity analysis was completed forthe grass buffer B-horizon trough to assess the likely impact of different substitution levels on the FRP : TPratio. No significant difference in the FRP : TP ratios was evident between 0Ð5 DL and 0Ð99 DL (MW,p D 0Ð36) and calculated loads using the two substitution levels were also similar.

Load calculations

For each surface runoff plot, the load was calculated as the product of runoff volume and water qualityconcentration for each event (or multiple events). Some storage drums overflowed during large events.Consequently, the loads on all overflowing drums were calculated using a total runoff volume of 2050 land, therefore, are underestimates.

The subsurface trough and stream discrete time series chemistry were converted into a continuous variableby linear interpolation and instantaneous loads were calculated directly at each instantaneous water levelmeasurement. Flow-weighted mean concentrations (FWMCs) were calculated by dividing the total load bythe total flow volume for the specified period.

Soils

Surface soil samples (<10 cm depth) were collected on three occasions: December 1998, March 2000 andAugust 2000 (see McKergow et al. (2006)). The March 2000 sampling preceded application of 200 kg ha�1

of standard superphosphate. The samples were bulked, air-dried and the <2 mm fraction was used for analysisof soil properties. The Chemistry Centre of Western Australia conducted all soil sample analyses and Table IIsummarizes the analytical methods.

RESULTS

Soil chemical propertiesSurface soils in the riparian buffers have low P status and moderate to high P sorption (Table III). The

P retention index (PRI) varied temporally and spatially; however, the majority of samples were in the moderate(5–20) to strongly (20–70) absorbing range (Allen et al., 1991). Using the Fe–AmOx classification of Weaverand Reed (1998), all of the surface soils are in the moderate–high sorption range in comparison with othersouth-coast soils. The upper section of the hillslope had moderate to high P status on all sampling occasions,which may be a sampling artefact. This area had shallow (<3 cm deep) and rocky soil. If P is concentratednear the surface, then inadvertently collecting soil samples to a shallower depth can lead to high soil-testP values and thus lead to large errors in estimating the current P status of the soil (Bolland, 1998). The threesubsurface soil samples suggest that the subsurface soil P status is low and that the P sorption ability is low(Fe–AmOx) but may increase with depth (Weaver et al., 1999; Table III).

Flowpath chemistry and loadsNutrient concentrations in surface runoff, subsurface flow and streamflow were generally similar (Figure 3).

Stream EC and SS concentrations were typically higher than those in surface runoff and subsurface flow.TP concentrations were generally low, with medians around 0Ð1 mg l�1 (Figure 3). P concentrations were

similar in subsurface flow, surface runoff and streamflow with the exception of the E. globulus B-horizonflow, which had significantly lower TP and FRP concentrations than all other sites (Figure 3; DM, p < 0Ð05).The stream median raw TP concentration was 0Ð05 mg l�1 (Figure 3) and the FWMC was 0Ð11 mg l�1. Incontrast, stream FRP concentrations were low (Figure 3, 18% of samples <DL) and the FRP FWMC was0Ð016 mg l�1. Stream FRP concentrations remained fairly constant regardless of flow condition or season.



Tabl

eII

I.Su

mm

ary

stat

istic

sfo

rsu

rfac

eso

il(<

10cm

)an

dsu

bsur

face

soil

sam

ples

Part

icle

size

(%)

pH(C

aCl 2

)O

rgC

(W/B

)(%

)P-

HC

O3

(mg

kg�1

)T

P(m

gkg

�1)

P(P

RI,

ml

g�1)

Fe-A

mO

x(m

gkg

�1)

Al-

Am

Ox

(mg

kg�1

)P

stat

usa

Sand

Silt

Cla

y

Gra

ssR

B(n

D5)

Med

ian

90Ð5

5Ð54

4Ð72Ð3

38

160

24Ð2

1100

1000

Low

IQR

01Ð5

0Ð50

0Ð26

010

2620

060

0E

.glo

bulu

sR

B(n

D5)

Med

ian

87Ð5

6Ð55Ð5

4Ð62Ð9

66

199

7779

021

00L

owIQ

R1Ð5

11

0Ð10Ð1

82

106

210

400

Hil

lslo

pe(n

D20

)M

edia

n89

6Ð54Ð5

4Ð93Ð7

720

Ð528

090

Ð577

532

00M

oder

ate

IQR

1Ð75

12

0Ð12Ð1

817

210Ð5

122Ð1

157Ð5

2340

Subs

urfa

ce(D

ec98

)0

–0Ð5

m92

3Ð54Ð5

4Ð90Ð2

236

4Ð918

017

0L

ow0Ð5

–1

m88

Ð52Ð5

95Ð1

0Ð1<

244

1217

030

0L

ow1

–1Ð5

m88

48

4Ð90Ð4

258

2535

035

0L

ow

aP

stat

usba

sed

onst

atus

base

don

amm

oniu

mox

alat

e-ex

trac

tabl

eir

onan

dbi

carb

onat

e-ex

trac

tabl

eP

rang

es(W

eave

ran

dR

eed,

1998

).



TP

(m

g l -1

)

FR

P (

mg

l -1)

TN

(m

g l -1

)S

S (

mg

l -1)

0.001

0.01

0.1

1 1

10

0.001

0.01

0.1

10

FR

P:T

P r

atio

0.0

0.2

0.4

0.6

0.8

1.0

0.1

1

10

100

0.1

1

10

100

1000

10000

EC

(m

S m

-1)

1

10

100

1000

10000

GA GB EA EB GU GL EU ELStream




GA GB EA EB GU GL EU ELStreamGA GB EA EB GU GL EU ELStream

Figure 3. Box plots of the complete chemistry dataset, including stream, subsurface trough (GA: grass A-horizon; GB: grass B-horizon; EA:E. globulus A-horizon; EB: E. globulus B-horizon) and surface runoff concentrations (GU: grass upper; GL: grass lower; EU: E. globulusupper; EL: E. globulus lower). Box represents the median with 25th and 75th percentiles, whiskers are 10th and 90th percentiles and outliers

are dots

TN concentrations were generally lower in surface runoff and streamflow than subsurface flow (Figure 3).Statistical analysis of TN concentration data creates two groups (the first being grass A-horizon (GA),grass B-horizon (GB) and E. globulus A-horizon (EA) and the second being E. globulus B-horizon (EB),surface runoff and streamflow; KW p < 0Ð001 and DM p < 0Ð05). The subsurface flow group had medianconcentrations around 5 mg l�1, whereas median concentrations were <2Ð5 mg l�1 in the predominantlysurface runoff group. The stream FWMC was 2Ð23 mg l�1 and the median raw concentration was 2Ð1 mg l�1

(Figure 3). ECs were generally lower in surface runoff than subsurface flow and streamflow (Figure 3).



Median ECs were lower in the grass buffer subsurface troughs compared with the E. globulus buffer troughs(Figure 3). Surface runoff ECs were elevated (plots 6, 9 and 14, see Figure 1 for plot locations) to valuessimilar to subsurface flows, suggesting that the runoff had flowed through the soil. Streamflow ECs weregenerally highest during summer and were statistically different from subsurface and surface runoff (KWp < 0Ð001, DM p < 0Ð05). Waters with EC values greater than 270 mS m�1 are classified as brackish (Georgeet al., 1996), and the stream was nearly always brackish. Baseflow samples (identified using HYBASE;Hydsys, 1999) had higher EC values than event flows (MW p < 0Ð001), with medians of 597 mS m�1 and420 mS m�1 respectively. ECs dropped considerably during some events, suggesting that rainwater contributedto streamflow.

Surface runoff and subsurface flow SS concentrations were generally less than 10 mg l�1 and surfacerunoff SS concentrations were higher in the E. globulus buffer than the grass buffer (Figure 3). Stream SSconcentrations were higher and more variable (Figure 3), and the majority of sediment moved during events.The event FWMC was 91 mg l�1, well above the raw median concentration of 14 mg l�1. Under baseflowconditions the median SS concentration was 7 mg l�1, compared with 81 mg l�1 for event samples. Duringsome events, large quantities of sand moved downstream and the flume and instrumentation were covered withsand. Downslope of the grass buffer the stream has incised 2 to 3 m into the unconsolidated sandy soil andhas a sandy bed with occasional bedrock outcrops. Below the E. globulus buffer the channel is considerablynarrower, as rock outcrops prevent headcutting.

Whereas the concentration data reveal few differences between surface and subsurface runoff, subsurfacenutrient and sediment loads were typically an order of magnitude higher than surface runoff loads (Figure 4).Between June 1998 and August 2000 the total grass buffer B-horizon losses per metre width of trough were2 g m�1 for TP, 90 g m�1 for TN and 190 g m�1 for SS. In contrast, nutrient and SS exports via the surfacerunoff pathway were low, and for the grass-lower plots the average losses were 0Ð1 g m�1 for TP, 3 g m�1

for TN and 18 g m�1 for SS. The dominance of B-horizon flow in nutrient loss reflects its dominance overother flowpaths (McKergow et al., 2006). Very small nutrient and SS loads were transported through theA-horizon troughs compared with the B-horizon troughs (Figure 4), as they flowed just a few days a season(see McKergow et al. (2006: table V)).

Annual total loads varied with runoff volume and nutrient concentrations (Figure 4). The 1998 surfacerunoff loads are likely to be underestimates of the actual load, as monitoring began in June 1998, so nolate summer or early winter data were collected. In addition, the storage drums on several plots repeatedlyoverflowed, underestimating the runoff volume for these plots (see McKergow et al. (2006)). Variability inloads is also due to interannual variability in nutrient concentrations. For example, at the E. globulus B-horizontrough the TP FWMC increased from 0Ð02 mg l�1 in 1998 to 0Ð3 mg l�1 in 1999. Similarly, the 1998 TNFWMC in the grass B-horizon trough was 2Ð1 mg l�1 compared with 8Ð0 mg l�1 for 1999.

In summer (November–March) IEOF occurred in the E. globulus buffer during high-intensity storms.Summer concentrations of SS, TP and TN in the E. globulus buffer were elevated compared with the winter(April–October) concentrations (e.g. 1999 in Table IV). Summer events, therefore, have a disproportionateinfluence on the surface runoff loads (Table IV). Summer storms moved similar loads of pollutants to all ofthe winter events combined, although the runoff volume was smaller (Table IV).

Riparian buffer effectiveness

Riparian buffer effectiveness can be evaluated using either pollutant concentrations or loads, and trappingefficiencies can be reported for either individual events or entire monitoring periods. In this study, emphasis isplaced on the reductions in total loads and trapping efficiency for surface runoff filtering was calculated using

Trapping D Upper load � Lower load

Upper load

The variability in trapping between events is examined to gain insight into uncertainty in the total trappingefficiency.



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Figure 4. Total annual runoff volumes and nutrient and sediment loads for surface plots (GU–EL) and subsurface troughs (GA–EB) for theperiod June 1998–August 2000



Table IV. Surface runoff concentrations (median and FWMCs) and loads for all plots in the E. globulus buffer for summer(12 samples, two events) and winter (18 samples, 13 events) 1999

Runoff TP FRP TN SS

Summer Median conc. (mg l�1) (IQR) 0Ð43 (0Ð37) 0Ð07 (0Ð33) 4Ð50 (2Ð3) 28Ð4 (22Ð9)FWMC (mg l�1) 0Ð49 0Ð109 9Ð29 38Ð7Load (g) 1Ð05 0Ð233 20Ð0 83Ð3Total runoff (l) 2154

Winter Median conc. (mg l�1) (IQR) 0Ð16 (0Ð12) 0Ð065 (0Ð30) 2Ð9 (3Ð15) 12Ð3 (10Ð2)FWMC (mg l�1) 0Ð10 0Ð05 3Ð57 7Ð9Load (g) 0Ð79 0Ð33 25Ð9 51Ð2Total runoff (l) 7253

Table V. Total loads, runoff volumes and trapping efficiencies for surface runoff passingthrough grass and E. globulus riparian buffers

Parameter Buffer Upper load(g)

Lower load(g)

Trapping(%)

TP Grass 2Ð6 1Ð2 54E. globulus 3Ð1 1Ð0 37

FRP Grass 1Ð2 0Ð6 50E. globulus 0Ð9 0Ð8 11

TN Grass 71 30 58E. globulus 74 43 42

SS Grass 310 184 64E. globulus 606 479 21

Runoff (l) Grass 40 060 18 930 53E. globulus 25 520 26 250 �3

Overall, the grass buffer reduced nutrient and SS concentrations in surface runoff by 50–60% (Table V).Surface runoff trapping in the E. globulus buffer was lower, particularly for SS and FRP, where total loadreductions were less than 20% (Table V). Trapping was generally positive and highest during 1998, thewettest year monitored, whereas low and negative trapping was typical in 2000, the driest year monitored(Figures 5 and 6). The high trapping during 1998, the most testing conditions monitored, with high loadsand widespread saturation shows that the buffers can perform under such conditions. It is worth noting that,despite reasonable trapping of contaminants in surface runoff pathways, the contaminant loads in surfacerunoff are small compared with subsurface contaminant loads.

The grass buffer consistently trapped SS and TP in surface runoff. For example, during 80% of events theTP loads at the upper plots were higher than those measured at the lower plots (Figure 5). In the grass buffer,FRP loads were dominated by a small number of events (Figure 5) and it was an FRP source during 5 of the20 events. The pattern of TN loads closely follows the runoff volumes for the majority of individual events.

On an event basis, SS load reductions were typically higher in the grass buffer (Figures 5 and 6). Forexample, similar runoff volumes were measured during Event 1 at the grass upper and lower plots, but thesediment load was reduced by 50%. In contrast, the E. globulus buffer during the same event was an SSsource area with five times more sediment leaving than entering the buffer (Figure 6). Overall, the tree bufferhad limited impact on incoming SS loads.

Riparian trapping calculations have been influenced by storage drum overflow, particularly during 1998.For example, during 1998 nearly 18 m3 of surface runoff was measured at plot 7 and 92% of this volumewas affected by overflow (McKergow et al., 2006). All runoff volumes greater than 2050 l in Figures 5 and 6



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were affected by drum overflow problems, with the exceptions of Event 1 at EU (Figure 6) and Event 24 atGU (Figure 5). The majority of drums that overflowed were on upper plots, suggesting that the upper loadsare underestimated and, therefore, that actual trapping efficiencies may be higher than estimated (Table V).

Despite the overall trapping figures for nutrients, there was large variability in the event-to-event results,which suggests that limited trapping occurs under certain conditions. In contrast, a clear SS picture emerged,with the grass buffer trapping significant quantities of SS, whereas the tree buffer’s performance was morevariable.



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Figure 6. Eucalyptus buffer surface runoff volumes and TP, FRP, TN and SS loads plotted by event number

DISCUSSION

Which flowpath is the major contributor of pollutants to streamflow?

Nutrient and sediment transport are dominated by flow through the B-horizon at this site. This is not entirelysurprising given that B-horizon flow is the dominant flowpath for water movement through the riparian buffersat this site, with up to 20 times more runoff moving via the subsurface than the surface (McKergow et al.,2006).



It is a commonly held belief that most P lost from agricultural catchments will be transported via surfacerunoff, and in some catchments this may be the case (e.g. Nash and Murdoch, 1997; Fleming and Cox, 1998).However, in other catchments, subsurface pathways can dominate P transport (e.g. Chittleborough et al., 1994;Stevens et al., 1999; Cox and Pitman, 2001). P transport in this catchment is dominated by B-horizon flow.Phosphorus transport via subsurface pathways can occur either by the movement of soil solution through thesoil or by preferential flow paths (e.g. macropores).

The surface runoff and subsurface flow P concentrations at this site are similar to those reported in theliterature for pasture grazed by sheep. Tham (1981) used 0Ð5 m wide surface runoff collectors in a Victoriancatchment and reported raw TP concentrations <2 mg l�1. Ridley et al. (2003) measured FWMCs up to0Ð92 mg l�1 of TP in surface runoff and up to 0Ð42 mg l�1 in A-horizon subsurface flow in Victoria. Surfacerunoff concentrations are considerably lower than those measured under dairy pastures in Victoria and SouthAustralia (e.g. Nash and Murdoch, 1997; Fleming and Cox, 1998). Despite the low P concentrations in surfacerunoff and subsurface flow, stream TP concentrations are comparable to several other agricultural Wilson Inlettributaries and considerably higher than the pristine Mitchell River catchment (Donohue et al., 1999).

The seasonality of P concentrations follows the annual P loss cycle from sandy soils in a Mediterraneanclimate, postulated from laboratory and field experiments in the Peel–Harvey catchment (Weaver et al., 1988a;Ritchie and Weaver, 1993). The high summer P concentrations can be explained by mineralization caused bywetting and drying, leading to high availability of rapidly released P (Weaver et al., 1988a). As the winterrains progress, the availability of P is reduced by losses to drainage and plant uptake (Weaver et al., 1988a).

Sediment concentrations were similar in surface and subsurface flow, but more sediment moved through thesoil than over it. Sediment can be readily transported below the soil surface at this site as SS is dispersed, theproportion of clay increases with depth and macropores may transport sediment either from the soil surfaceor from within the profile rapidly.

George (2001) reported on erosion rates for different land uses throughout WA using 137Cs techniques, andthe majority of annual pasture sites had soil loss rates less than 1 t ha�1 yr�1. Soil loss rates at this site weresmall by comparison, and low rates are most likely due to good surface cover during the winter combinedwith low winter rainfall intensities. Summer sediment concentrations may be higher due to detachment byraindrops and grazing sheep (Coles and Moore, 1998).

TN concentrations were generally higher in subsurface flow (median >4Ð3 mg l�1) than surface runoff(median <2Ð5 mg l�1). Nitrogen moves through the landscape in many different forms, and potential sourcesin this catchment may include groundwater nitrate, stock excretion, decomposition of organic matter, andnitrogen fixation by pasture legumes (Deeley et al., 1999). High summer and early winter concentrations canmost likely be traced back to back to decomposition of legume herbage accumulated during the previousspring (Peoples and Baldock, 2001). Comparison of stream TN concentrations with the near-pristine MitchellRiver (median 0Ð5 mg l�1 of TN; Donohue et al., 1999) shows that TN concentrations are clearly abovenatural background levels.

Annual variability in nutrient concentrations was larger than anticipated. We can only speculate on thereason for the elevated nutrient concentrations in 1999; several explanations exist, including a pronouncedflushing effect, fertilizer application, vineyard establishment or a different subsurface source. It may be thatall of them played some role.

Sample ECs decreased in a similar manner to TN at the start of the 1999 winter (correlation coefficientr D 0Ð98 for the first 16 samples collected from the grass buffer B-horizon), suggesting that the two parametershad the same source. High EC values are associated with groundwater at this site, so it is possible that thesubsurface flow in 1999 was from a groundwater source and was increasingly diluted with rainfall throughtime.

Any changes to sample FRP concentrations before sample filtration are unlikely to alter the key conclusionsof this study. If FRP concentrations did decrease due to storage, then the FRP concentrations, FRP loads andFRP : TP ratios could be incorrect. However, the dominance of subsurface FRP loads over surface runoffloads would almost certainly remain, given the high subsurface flow volumes. Several samples taken 2–4 h



apart by different sampling and storage methods have similar nutrient concentrations and EC. For example,stage height and grab samples taken on 9 August 2000 at the stream had concentrations of 0Ð09 mg l�1 and0Ð05 mg l�1 of TP, 0Ð01 mg l�1 and 0Ð01 mg l�1 of FRP, 2Ð9 mg l�1 and 2Ð6 mg l�1 of TN and ECs of264 mS m�1 and 265 mS m�1 respectively, despite the stage height sample being collected from the field5 days later. This suggests that field storage of samples (up to 1 week) may not change sample concentrations,but does not exclude other sources of error (see Jarvie et al. (2002)).

Despite the uncertainty in the measurement of flow volumes (e.g. storage drum overflow and interruptionof natural subsurface flow patterns) and possible sample storage effects, the order of magnitude differencebetween the B-horizon troughs and surface runoff plot loads means that we can have confidence in theseconclusions. The dominance of subsurface flowpaths, coupled with the presence of macropores, presents adifficult management dilemma. Source control is one of the only options available. Reassessment of thetype, timing, rate and method of fertilizer application may be a suitable option for reducing catchment Pexports. For example, Weaver et al. (1988b) suggest that the application of coastal superphosphate, whichcontains only 20% water-soluble P (compared with 84% for standard superphosphate), could reduce P lossesby 30%. Reductions in the frequency or rate of application may also decrease P leaching losses (Weaveret al., 1988b).

Grass versus eucalyptus buffers

Incoming surface runoff loads were extremely low and did not test either buffer. Higher pollutant loads mayhave been generated if pasture re-establishment had proceeded as originally planned. Despite this, the bufferswere valuable, particularly for displacing sediment and nutrient-producing activities away from the stream.Removal of fertilizer application from riparian areas can slow the transport between pasture and stream andreduces the likelihood of direct fertilizer application to the stream. Direct application may be particularlyimportant where fertilizer is broadcast or top-dressed (e.g. Cooke, 1988). Removal of livestock from riparianareas is also a valuable water quality tool. Stock, in particular cattle, can severely degrade riparian areas (seereviews by Mosley et al. (1997) and Belsky et al. (1999)). Sheep are not attracted to water and, therefore,are less likely to damage streams (Platts, 1989), unless they are forced to graze riparian areas heavily (Platts,1981). However, sheep tracks through a riparian buffer may increase the risk of pollutants reaching streams.

An additional riparian buffer function is to stabilize stream banks and reduce channel erosion. In thiscatchment the stream has incised and the remnant trees and grasses in the riparian area are not able to controlthe stream bed and bank erosion. The riparian areas have been fenced since the mid 1990s, and interventionmay be required to re-establish a healthy understorey.

A key difference between the two buffers at this site was the summer hydrologic behaviour. No surfacerunoff was measured in the grass riparian buffer during any of the summer storms, so nutrient and sedimenttransport was limited to winter in this buffer. In 1999, IEOF transported half the annual sediment and nutrientload during two intense summer storms in the E. globulus buffer. Loads were particularly high during thesummer because of higher concentrations.

SS concentrations were typically higher in the E. globulus buffer, and on several occasions it was an SSsource area. The E. globulus buffer had no understorey vegetation, and so the lack of surface cover is the mostlikely cause of higher SS concentrations. It is normal practice to control weeds and understorey vegetationin E. globulus plantations to minimize fire risk and maximize water and nutrient use by trees. Tree riparianbuffers have been sediment source areas in other locations, with more sediment leaving the riparian bufferthan entering (Smith, 1992; Jordan et al., 1993; Daniels and Gilliam, 1996; McKergow et al., 2004).

This study evaluated the riparian buffers over a 10 m width, rather than the entire riparian zone. A 10 mwidth was chosen as this is a standard buffer width commonly considered by land managers and it alsoallowed for installation of the monitoring infrastructure. Given that the pollutant loads in surface runoff werelow and the hydrology was extremely variable across the riparian zone, extrapolation of the results to theentire riparian zone is likely to be representative.



CONCLUSIONS

Riparian buffers can separate agriculture from streams, and buffer streams from sediment and nutrients inputs.However, several riparian buffer water quality functions rely on surface runoff being the dominant pollutantpathway. At this site, and under the conditions monitored, nutrient and sediment transport is dominated byB-horizon subsurface flow, which carries contaminant loads at least three times greater than surface runoff.

In this pasture catchment, where surface cover is good, sediment and nutrient trapping by riparian bufferswas extremely variable and linked to riparian hydrology. During the monitoring period the grass buffer trappedbetween 50 and 60% of the incoming SS, TN, TP and FRP loads in surface runoff. Surface runoff trappingefficiencies in the E. globulus buffer were lower, and between 10 and 50% of the SS, TN, TP and FRP loadswere retained in the buffer.

A key difference between the grass and E. globulus riparian buffers was their hydrologic response to intensesummer storms. Surface runoff was measured in the E. globulus riparian buffer during several summer stormsand evidence suggests that surface crusting and water repellence reduced the soil’s infiltration capacity. Inthe grass buffer all rain infiltrated.

Nutrient and sediment loads were similar in the E. globulus buffer during the summer and winter, despitemore events and larger flow volumes in the winter. During summer the nutrient concentrations are high andpasture cover is minimal, so the risk of sediment and nutrient transport by surface runoff in E. globulusriparian buffers is high.

This study has demonstrated that riparian buffers receiving surface runoff from pasture under natural rainfallcan trap nutrients and sediment. However, it also suggests that there are limitations placed on the surfacewater quality functions of buffers in environments where subsurface flow dominates, and additional waterquality measures are required to reduce nutrient and sediment loads.

ACKNOWLEDGEMENTS

This research was funded by the Western Australian Department of Agriculture and Land and Water Australia’sNational Riparian Lands Program. The cooperation of the land holders, Alex Campbell and Mike Cuss, isappreciated. Technical assistance was ably provided by Jamal Haragli, Greg Olma, Geraldine Janicke andJohn Grant. The constructive comments of two anonymous reviewers are gratefully acknowledged.

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performance of grass and eucalyptus riparian buffers in a pasture catchment, western australia, part...

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