Temporal and spatial patterns of salinity in a catchmentof the central wheatbelt of Western Australia

M. J. RobertsonA,D, R. J. GeorgeB, M. H. O’ConnorA, W. DawesC, Y. M. OliverA,and G. P. RaperB

ACSIRO Sustainable Ecosystems, Private Bag 5, PO Wembley, WA 6913, Australia.BDepartment of Agriculture and Food WA, PO Box 1231, Bunbury, WA 6231, Australia.CCSIRO Land and Water, Floreat, WA, Australia.DCorresponding author. Email: Michael.Robertson@csiro.au

Abstract. Many estimates have been made of the future likely extent of salinity at regional and national scales inAustralia; however, there are few detailed studies of changes in temporal and spatial patterns at catchment scale. This studywas conducted in theWallatin and O’Brien catchments in the low–medium rainfall zone of the central wheatbelt ofWesternAustralia, where we examined the spatial trends in saline land over the last 18 years and related these to the likely rate andextent of future salinisation.

The analysis showed that: (1) salinity has continued to expand post-1999 in landscape positions where there has beenwatertable rise and also in areas now at equilibrium even though rainfall has been below average; (2) increases in the area ofsalinity are still dominated by increases in the valley floor but there is now the emergence of many small, isolated outbreakson the adjacent slopes; (3) widely available satellite-derived salinity maps (LandMonitor) derived in 1998 provide a reliablebase-line for saline mapping but now underestimate the area of salt-affected land by 60%; (4) the trend in watertable levelsand time since clearing and interactions with proximity to uncleared native vegetation provide reliable predictors of salinityrisk; (5) episodic rainfall in areas of shallow watertables is proposed as a significant cause of the expansion in observedsalinisation, even though some of this may be transient.

These results are discussed in terms of management options for farmers and the likely long-term outlook for expansionof salinity in the catchment.

Introduction

In Australia, dryland salinity is a major land degradationproblem with over 2Mha of broadacre farm land currentlyaffected and scientists predicting up to 6Mha to be at risk(PMSEIC 1999; NLWRA 2001; ABS 2002). The clearingof deep-rooted native vegetation and its replacement withshallow-rooted annual agricultural plants is the principalcause of dryland salinity.

Various estimates have been made of the current andfuture likely extent of salinity within Western Australia(e.g. George 1990; George et al. 2005); however, there hasbeen little temporal analysis on individual catchments inWestern Australia, with the notable exception of thatconducted in the Upper Kent River catchment (Evans et al.1996). Monitoring changes to the patterns of salt-affected landat these finer scales inform larger scale estimates of the rates ofexpansion and where in the landscape expansion is occurring(Summerell et al. 2009). Both spatial and temporal patterns,together with local factors such land-use, groundwater trends,variations in micro-relief, and local hydrological processes, willdictate the choice of the most appropriate salinity managementresponse (Pannell 2001).

In the agricultural zone of Western Australia (WA), the statemost affected by dryland salinity, information on the spatial andtemporal patterns of salinity and landscape condition has been

provided by the Land Monitor project (Allen et al. 1999). LandMonitor provides land managers and administrators withbaseline salinity data for monitoring changes over time andestimates of areas at risk from secondary or future salinisation.It uses sequences of calibrated Landsat Thematic Mappersatellite images integrated with landform information derivedfrom height data, ground truthing, and other existing mappeddatasets to monitor changes in salinity and woody vegetation.The most recent estimates of LandMonitor were made in 1998.In this study we verify the accuracy of the 1998 estimatesin one catchment and update it with estimates made byfarmers in 2006–07 to gauge the recent rate of spread anddetermine associations between landscape position, trends ingroundwater rise, and history and the appearance of newoutbreaks of salinity.

Groundwater levels and their rates of change over timeinform estimates of the future area of land at risk fromsalinisation (George et al. 2008) and were adopted as thebasis for a consistent national analysis and reporting ofsalinity risk (NLWRA 2001). In addition, the drying trend inrainfall for much of the central and northern agricultural zone ofWA during the last 30 years (Smith et al. 2000) has been relatedto the stable or falling trends in groundwater levels since 2000 inthe some parts of the northern wheatbelt (George et al. 2008).Linking climate and salinity development has implications for

� CSIRO 2010 10.1071/SR09126 0004-9573/10/040326

CSIRO PUBLISHING

www.publish.csiro.au/journals/ajsr Australian Journal of Soil Research, 2010, 48, 326–336

the forecasts for the spread of salinity under an ongoing trend ofa drying climate in the north, and more variable climate in thesouth-east.

This study is conducted in the Wallatin and O’Briencatchments in the low–medium rainfall zone of the centralwheatbelt of WA, where mixed cereal and livestockfarms predominate. The study catchments occur in themiddle of the WA wheatbelt, and while no catchmentcould be considered typical of the whole wheatbelt region,these catchments may be regarded in terms of landscape,community, and environment as representative of much of it.The catchments are adjacent to the Woolundra Lakes catchment,a palaeodrainage which exhibits landforms associated withsalinity (e.g. playas and lunettes), many of which have beenreactivated since clearing.

The catchments have comprehensive soil–landscapemapping, history of vegetation clearing, groundwater trends,satellite salinity maps, and recent farmer records of new salinity(Dawes et al. 2007). This enables the project to addressthe following aims. (1) What have been the spatial andtemporal trends in area and location of salt-affected land for1998–2006 and how does this relate to ‘at risk’ estimates,soil–landscape patterns, groundwater trends, and underlyinghydrogeology? (2) What is the among-farm variation in suchtrends and what are the implications for the range of salinitymanagement responses required within a catchment? (3) Whatis the relationship between groundwater rise, time sinceclearing, and landscape position? (4) Has reduced rainfallchanged the patterns in salinity development between 1998and 2007?

Method

The catchments

The Wallatin and O’Brien Creek catchments are in the Shireof Kellerberrin 240 km east of Perth (Fig. 1). The locationrepresents the transition from the central to the easternwheatbelt of Western Australia. The total area is 44 457 ha(including the Woolundra Lakes, a largely salt-affected area);elevation ranges from 380 to 240m AHD in both catchments;catchment dimensions are 32 by 8–14 km for Wallatin and 18 by9–16 km for O’Brien; and catchment gradient is 0.43 and 0.78%in Wallatin and O’Brien, respectively.

The long-term (1900–2006) average rainfall is 330 mm/year(recorded at Kellerberrin Post Office) and annual evaporation is~2100mm/year. Evaporation exceeds rainfall in all months ofthe year except June and July. On average, ~75% of the rain isreceived between May and September. Compared to other areasin Australia, variability in winter rain is relatively low butsummer rainfall is variable and sporadic. Rainfall during thelast 30 years (1975–2006, av. 304mm) was 10% lower than the1900–2006 long-term average with rainfall in June and Julyhaving dropped by >25% (Ludwig and Asseng 2006). Duringthe same period there has been a small and not significantincrease in summer rainfall. These changes are consistentwith broader-scale changes in rainfall over the medium andhigh rainfall zones of the south-west of WA (Smith et al. 2000)and have related to changes in the rate of rise of watertables(George et al. 2008).

The broad landscape is described as the Zone of AncientDrainage with typical landforms described in Table 1. The soillandscape units have been mapped at a scale of 1 : 50 000 andclassified using a soil and topologically based classification(Bettenay and Hingston 1964). The Ulva unit, especially the‘upland sandplain’, is highly permeable to rainfall andgroundwater. Where this occurs alone or adjacent to the largerock monadnocks (Danberrin units), the potential forgroundwater recharge is high. The alluvium and colluvium ofthe major and minor valley floors are most affected by salinity.The gradational soils adjacent to the lake systems have acomplex mosaic of salinity that changes seasonally withfluctuating watertables and flooding.

Regolith thickness in the upper Wallatin Creek catchment(<15m) is less than that in comparable wheatbelt catchments(20–30m), as indicated by drilling and geomorphologicalanalysis, due to the abundance of Danberrin and Booraan soilsystems (George 1992; McFarlane and George 1992). Muchdeeper regolith (40–50m) occurs in the lower Wallatin andO’Brien catchments due to the prevalence of deep alluvialsediments. However, the general characteristics of the terrainare typical of the region (George 1992).

Aquifers in the catchment can be classified as localgroundwater flow systems (George et al. 1997), where most

Fig. 1. Map of Wallatin and O’Brien catchments and their location inWestern Australia.

Patterns of salinity in a wheatbelt catchment Australian Journal of Soil Research 327

of the recharge within one sub-catchment becomes dischargewithin the same sub-catchment. Groundwater catchment size isa function of terrain, geology, and weathering history. In thestudy area, most local flow systems are of the scale of <100 hain the uplands and 1000 ha in valleys. Catchment salinitymechanisms are dominated by 4 causal processes (afterNulsen and Henschke 1981; George et al. 1997; Coram et al.2000; Clarke et al. 2002): Type 1, bedrock highs; Type 2, localbreak of slope; Type 6, perched aquifers; Type 7, geologicalstructures. Salinity is generally expressed in the catchmentwhere there is a reduction in flow that results from shallowbedrock, convergent topography, changes in surface slope, orthe presence of faults or dykes (McFarlane and George 1992;George et al. 1997). In the medium rainfall zone of thewheatbelt, recharge to groundwater aquifers is estimated tohave increased from 0–1 to 10–30mm/year after clearing(George 1992; George and Coleman 2001).

Farming supports 27 families within the 2 catchments. Thedryland farming systems are based on grain production,predominately wheat and barley with lesser amounts of lupinsand canola (Robertson et al. 2009). Livestock systems varybetween properties but are not dominant in this area. Over 70%of natural vegetation was cleared for agriculture by the 1940sand now ~11% remains in scattered and generally small patches.There are 2 large nature reserves, both within the Wallatin Creekcatchment: Durokoppin Reserve (1030 ha) and Kodj KodjinReserve (204 ha) (Fig. 1). The catchments are atypical of thewheatbelt in that, to 2006, landholders have planted 1750 ha ofwoody perennial vegetation adding 4.9% further cover to the9.4% cover of uncleared remnant native vegetation (Smith2008). However, the number and area of new tree plantingshave reduced during the last 5 years (Smith 2008). The areamapped as saline in the shire in 1998 is 6% (Caccetta andBeetson 2000); however, individual farms especially in thevalley floor can have a large proportion of area affected.

Area and spatial pattern of salt-affected land

The LandMonitor dataset for area of consistently lowproductivity (AOCLP) in the catchments was available for1989 and 1998 (Caccetta and Beetson 2000). AOCLP isdefined as areas of consistently low normalised differencevegetation index (NDVI) across at least 3 consecutive springimages, ground-truthed by field inspection. Estimates ofAOCLP were verified in the field and errors of commissionand omission registered against mapped areas of drylandsalinity. The Wallatin Creek catchment was selected as one

of several state-wide to assess the methodology. Caccetta andBeetson (2000) define the methodology and results in detail.

An aerial photo of each farm, flown in November 2004, wassuperimposed with LandMonitor 1998 AOCLP and presented toeach farmer. Farmers were first asked to verify the accuracy ofthe 1998 LandMonitor estimates against what land on their farmthey remember as being saline/not saline in 1998. Farmers werethen asked to state whether land that had been classified assaline by LandMonitor in 1998 was in fact not saline (errors ofcommission). Farmers were also asked to identify land that wassaline in 1998 (according to their memory) but was not classifiedas such by LandMonitor (errors of omission). Next, farmers wereasked to add to the current aerial photograph any new salinitythat had appeared since the 1998 estimates, with their definitionof saline-affected land being guided by the LandMonitorclassification in 1998. This ensured that definitions of salinitywere consistent between LandMonitor and the farmers. Theywere also asked to indicate if any saline areas present in 1998had since disappeared; however, there were no recordings ofthis kind. Farmer hand-drawn estimates of new salinity weredigitised and added to the 1998 estimates.

The updated map of salt-affected land was then analysed asfollows. Each discrete polygon of salt-affected land, deemed tohave consistently low production, was attributed an area andthen classified as being a new outbreak (>100m from an existingsaline patch) or an outgrowth from an existing patch previouslyrecorded by LandMonitor. Polygons were also classified as waswhether or not they were located within the valley floor zone(below the break of slope). Polygons that straddled the valleyfloor boundary were classified according to where the dominantproportion fell. Polygons were classified for location on thisbasis because, as a generalisation, topographically low areasare the first where watertables come close to the soil surface,with associated water logging and salinity problems. This is therationale behind using area of the valley floor zone as a measureof potential salinity hazard (George et al. 2005). The valley floorzone is defined using height data and is described in more detailby Caccetta et al. (2010). Using this definition, the valley floorarea occupies 25 and 28% of Wallatin and O’Brien catchments,respectively.

Soil survey of salt-affected land

An additional estimate of the area of salt-affected land,independent to that of the farmers and LandMonitor, wasconducted via a catchment-wide soil survey. An on-groundsurvey of surface salinity was conducted, where soil sampling

Table 1. Soil landforms of the Wallatin–O’Brien catchment (based on Bettenay and Hingston 1964)

Soil landform Soil landform description Dominant nativevegetation

% of soil landformin catchment

Baandee Ancient drainage zone Halophytes 3.5Belka Broad flat alluvial (upper) valley floor (mainly ‘sandier’ soils) Salmon gum 1.9Booraan Upper dissected valley slopes White gum 21.8Collgar Lower valley slopes Mallee 13.0Danberrin Irregular low hills and gentle slopes with granite outcrops York gum 22.1Merredin Broad flat alluvial (lower) valley floor (mainly ‘heavier’ soils) Salmon gum 16.4Rocky hills Steeper hills with large areas of granite outcrop Tamma 4.1Ulva Remnants of lateritic sandplain Wodjil, grevillea 17.2

328 Australian Journal of Soil Research M. J. Robertson et al.

and analysis of soil for electrical conductivity was used toindicate the location of current salinity. This method requiresan approach to extrapolate from point samples to an arealestimate, by assigning an average value of salinity to anentire soil landscape unit. A detailed soil survey wascommissioned in the catchment (Wells 2004) and soils weremapped at 1 : 50 000 scale. The soil landscape map had 67 soillandscape units mapped, indicating both a landscape positionand a probable WA soil group (not differentiated by soil colour)(Schoknecht 2002). Soil profiles, which were sampled at 197points in the catchment, based on representation of the majorlandforms (Table 1), were classed into soil landscape units(Schoknecht et al. 2004) and WA soil group (Schoknecht2002) and measured for soil salinity. Soil salinity wasmeasured in 1 : 5 soil water mixture (EC) then converted intosaturated extract values (ECe) using the EC and a soil conversionfactor (based on soil type) from DAFWA (2004). This waschecked against expected soil properties based on their WA soilgroup type. The topsoil and subsoil ECe were broken into thefollowing salinity classes (mS/m): fresh <200, slightly saline200–400, brackish/moderate 400–800, highly saline 800–1600,extremely saline >1600. Land was classed as saline if it waseither extremely saline or highly saline.

Each soil-landscape mapping unit was assigned a subsoil andoverall salinity based on the most common properties fromthe point data. Hence, maps of soil salinity are for mostprobable property for that soil landscape unit.

Time trends and spatial patterns in the area of salt-affectedland were analysed by comparing estimates made fromLandMonitor, farmer estimates, and hydrograph analysis.

Groundwater catchment areas

Estimates were made of the likely size of the groundwatercatchment area underlying saline seeps on each farm in thecatchments. A groundwater catchment area is defined as thatarea where recharge is contributing to discharge at a saline seep,and hence can be thought of as the area requiring managementintervention to reduce recharge contributing to the saline area.

A comprehensive survey of groundwater catchments was notconducted; however, a subsample was made, being thosegroundwater catchments associated with saline seeps thatwere of current concern to farmers. Of the 494 discrete salinepatches identified in 2006, we analysed 17% on 22 farms thatwere visited over the period November 2006 to January 2007.A discussion was held on the current salinity risk situationbeing faced by the farmer, and the possible location ofoptions to manage current saline patches or seeps identifiedby the farmer. The discussion was followed by a field visit tothese seeps to evaluate the possible causes of current and futuresalinity, and hence possible options to manage salinity at thesite. An assessment was made of the boundary and area of thegroundwater catchment area contributing to the identified seep,using the approach described by Lewis (1991) and informed byinformation on any nearby bedrock or regolith exposures, theshape and characteristics of the surface of the landscape, andthe shapes and locations of any groundwater dischargefeatures. On-site visual inspections were supplemented withair photos flown in 2000 and November 2004, LandMonitor

1998, estimates of salt-affected land, groundwater records, andmaps of the main soil types, airborne gamma-K radiometrics,and airborne magnetic intensity. It should be noted that insuch landscapes, the groundwater catchment boundariesmay not coincide with the boundaries of the surface watercatchment (Clarke et al. 2002), although it is the exceptionthat groundwater would flow across surface water catchmentboundaries; rather, multiple groundwater catchments couldbe found within a single surface water catchment. Once theboundary of the contributing groundwater catchment wasidentified, feasible options to respond to the issues at eachsite were discussed with the farmer.

Date of clearing

Remnant native vegetation had previously been mapped for thearea in detail through farmer interviews, original survey maps,and aerial photography (Arnold and Weeldenburg 1991;G. W. Arnold and J. R. Weeldenburg, unpubl. data), whichenabled a date of vegetation clearing to be assigned to individualparcels of land, with an accuracy of �5 years. Clearing dateswere aggregated into the following classes for mapping andanalysis: 1890–1920, 1921–40, 1941–60, 1961–72, 1973–74,1975–85, and 2 further classes where the land was uncleared orthe date of clearing was unknown. Where possible these dateswere checked against the dates of alienation of these parcelsfrom State Lands Department records.

Analysis of groundwater levels

Although Wallatin and O’Brien catchments have been thesubject of much hydrological investigation over many years,the hydrograph record was patchy. Data are available from 1985to 1989, when Department of Agriculture and Food installed thepiezometers. However, there is a gap for most years to 2005.Unfortunately, as many of the original piezometers were drilledto investigate the causes of the saline seeps (McFarlane andGeorge 1992), many (68%) of the piezometers are located inlower landscape positions and valley floors, which have beencleared of native vegetation for longest. As a result, this biasesthe total set of hydrographs towards those that are likely to havestabilised, many of which have watertables near the groundsurface.

Hydrographs were inspected and minimum rates of changecalculated for the period between the late 1980s, when a largenumber of observations were made, and 2005, when piezometerreadings were resumed. The minimum rate of change wasestimated from the average measured groundwater levelbefore 1990, measurements in 2005, and the length of record;there were 12–38 measurements before 1990. To determinewhether the level had increased, stabilised, or decreased overtime, the average 2005 depth was compared to the average pre-1990 depth in a standard 2-tailed significance test at the 95%confidence level.

The year of clearing at the location of the piezometers,inferred from land alienation records and farmer interviews,was subtracted from 2000 to get an approximate time sinceclearing. There were 48 bores, 34 of which were from clearedagricultural land, 13 from areas near the border betweenuncleared and cleared land, and 1 from the interior of a

Patterns of salinity in a wheatbelt catchment Australian Journal of Soil Research 329

nature reserve and hence classified as ‘uncleared’. Correlationswere conducted between the minimum rate of change and timesince clearing. Bores were also defined by landscape positionranging from the upper slope to the valley floor.

To assist in interpretation of temporal patterns of salinityduring the period when there were no groundwater readings(1989–2005), HARTT (Ferdowsian et al. 2001) was used tointerpolate the groundwater levels for several bores on the valleyfloor at Wallatin Creek for the period between September 1989and February 2005, using data for 1985–89 and 2005–2009. Thealgorithm separates the influence of rainfall variability fromany underlying trend in groundwater levels by using either theaccumulated monthly or accumulated annual rainfall residualsas a surrogate for the influence of rainfall variability ongroundwater levels.

Results

Time trends and spatial patterns in the areaof salt-affected land

The accuracy of the 1998 LandMonitor estimates was assessedby farmers for their own farms in 2006. The area of land that hadbeen classified as saline by LandMonitor in 1998 and was infact not saline (i.e. errors of commission) amounted to a total of10 ha over 28 patches for the combined area of Wallatin andO’Brien, which is a small error (~1%) compared to the total areaof 900 ha classified as saline. Farmers were also asked to identifyland that was saline in 1998 (according to their memory) but wasnot classified as such by LandMonitor (errors of omission). Thisarea was 142 ha covering 31 patches, the majority (90 ha) beingin the valley floor (data not shown). When these correctionsare applied to the 1998 LandMonitor AOCLP, then 87% ofthe salinity in Wallatin and O’Brien occurs within the valleyfloor area.

The combined area of existing salinity identified byLandMonitor in 1998 for Wallatin and O’Brien Creekcatchments is 1045 ha (Table 2). The total for Wallatin of550 ha is close to the 496 ha reported by McFarlane andGeorge (1992) in their ground survey of salinity, even thoughin their survey they only mapped the larger patches of salt-affected land. Net increase in salinity between 1989 and 1998has been close to zero in the O’Brien catchment and within theerror of estimation from LandMonitor (Allen et al. 1999) and anincrease of 130 ha in the Wallatin catchment.

The estimates of surface soil salinity derived from soilsurvey conducted in 2003 were close to those estimated fromLandMonitor in 1998 (Table 2). Most of the surface soil salinitywas located on the Baandee and Collgar soil–landscape units(Table 1).

Table 2 includes an update of the area of saline land from1998 to the present as provided by farmer estimates made in2006. According to these farmer estimates, between 1998 and2006, the area of salt-affected land doubled (an increase of537 ha, 49%) in the Wallatin catchment and increased by~119 ha or 20% in O’Brien. An analysis of farm-by-farm areaof salt-affected land shows that between 1998 and 2006 themean increase in the area of salt-affected land per farm was 28 haand ranged from <10 ha on 14 farms up to 133 ha on the mostaffected farm (Fig. 2b). All but 3 farms conformed to the trendthat the higher the percent of the farm affected by salinity in1998, the greater the increase in saline area between 1998 and2006. There were 3 farms in the valley floor that were notableexceptions to this trend. On 2 of these farms, <60 ha of the farmwas salt-affected in 1998, but the increase in saline land between1998 and 2006 was >120 ha. On the third farm, 24% was salt-affected in 1998 but the increase in saline land between 1998 and2006 has been <5 ha. In 2006, the percentage of each farm withsalt-affected land ranged from 1 to 30% with a mean of 5.4%,with two-thirds of farms having <5% salt-affected land (Fig. 2a).

In 1998 there were 131 identifiable discrete patches of salt-affected land on the hill slopes of the Wallatin and O’Briencatchments, with a mean size of 0.8 ha and a median of 0.3 ha. Inthe valley floor there were 274 patches with a mean area of 3.2 haand a median of 0.4 ha, indicating that a few large patches weredominating the mean patch size. Since 1998, farmers indicatedthat there have been 52 new outbreaks of salt-affected land in thevalley floor of Wallatin and O’Brien. The mean size of theseoutbreaks was 10.7 ha, but the median was 3.6 ha indicating askewed distribution (Fig. 3). Outside the valley floor, on theadjacent hill slopes, there were 37 new outbreaks, with a meanand median of 2.8 ha and 2.3 ha, respectively, indicating lessskewness in the distribution of salt patches compared to thevalley floor. Approximately 80% (552 ha) of the new salinitywas expansion of already existing patches, while 20% (107 ha)was in new isolated patches.

Groundwater flow compartments

Analysis of saline seeps on 22 farms in the catchment involvedidentifying 85 associated groundwater catchment areas. Thefrequency distribution of the area of these contributinggroundwater catchments shows that most are <150 ha, many<80 ha, and none >500 ha (Fig. 4). In addition we did notfind any cases where the groundwater catchment on a farm,associated with a saline outbreak, straddled property boundaries.

Groundwater levels

A plot of the 62 groundwater bores (excluding 2 that had staticwater levels greater than 15m before 1989 and also in 2005)

Table 2. Estimate of saline land (ha) for the Wallatin and O’Brien catchments

Catchment Valley Saline areaA Farmer estimates of Saline areaB

floor area 1989 1998 Change saline area increase Surface Subsurface1989–98 (1998–06) soil soil

Wallatin Creek 6237 (25%) 421 (1.7%) 550 (2.2%) 129 537 550 (2.2%) 4648 (19%)O’Brien Creek 3407 (30%) 503 (4.4%) 495 (4.3%) –8 119 736 (6.4%) 2815 (25%)

AAs assessed by LandMonitor area of consistently low productivity.BAs assessed by catchment-wide soil survey.

330 Australian Journal of Soil Research M. J. Robertson et al.

shows that there were 36 bores with water levels closer than 2mof the surface in 1989 and these showed little change in 2005(Fig. 5a). Bores that had deeper water levels in 1989 had thegreatest minimum rate of rise between 1989 and 2005 (Fig. 5b).Of the 62 groundwater bores examined for trends between 1989and 2005: (i) 16 (26%) had declining trends from –0.005 to

–0.15 m/year, (ii) 17 (27%) were stable with trends from –0.005to +0.005 m/year, and (iii) 29 (47%) were rising at rates from+0.005 to +0.4 m/year (Table 3). Of the 62 bores, 49 (80%) werelocated in the mid/lower or lower positions of the landscape andaround half of these were either stable or declining in trend andthe other half were rising.

For all hydrographs analysed using HARTT (Ferdowsianet al. 2001), the p statistic for the accumulated annual residualrainfall was highly significant as a determinant for groundwaterlevel, giving confidence that the HARTT analysis could be usedto interpolate groundwater levels for the period when there wereno readings. Two examples of the interpolated groundwater

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Patterns of salinity in a wheatbelt catchment Australian Journal of Soil Research 331

levels are shown in Fig. 6. In both cases the strong relationshipbetween accumulated annual residual rainfall and the predictedgroundwater level is obvious; the p statistics for the residualrainfall were 2.2� 10–6 and 4.1� 10–5, respectively. Thefigures also show that when the accumulated residual rainfallwas consistently positive between mid-1999 and the end of2001, groundwater was predicted to be at its shallowest underthe valley floor. Furthermore, accumulated annual residualrainfall peaked in January 2000 and April 2000 in responseto 95mm of rain in January 2000 and 55mm in March 2000.

Clearing and salinity

In the Wallatin and O’Brien Creek catchments, land clearingstarted in the lower catchment adjacent to the Great EasternHighway, before the First World War, and was largelycompleted by the start of the Second World War (Fig. 7). Bycontrast, large areas of the upper catchment remained undernative woodlands and heath until the 1970s. McFarlane andGeorge (1992) linked the delayed response of salinity in theupper Wallatin catchment to the existence of remnant nature

reserves (Durokoppin and Kodj Kodjin Reserves, Fig. 1) andthis later clearing.

Land clearing information was used to explicitly examine thelink between clearing and trends in groundwater level. Figure 8shows the simple regression of rate of change in groundwaterlevels against years since clearing and shows an expectednegative trend – the longer the time since clearing the lowerthe minimum rate of rise as groundwater levels come toequilibrium. For land cleared in the last 70 years, all waterlevels are rising, whereas for land cleared earlier than this, allwater levels are stable or falling. Bores that were located near theborder between cleared and uncleared land have a rate ofgroundwater rise that is above the trend line for bores fromcleared land, indicating that such bores are behaving as if theyhad been cleared more recently than the date of clearing wouldindicate. This suggests that the adjacent native vegetation isaffording some protection, thus slowing rates of watertable rise.

Discussion

Assessment of salt-affected area

This study verified the overall accuracy of the LandMonitorestimates of salt-affected land (areas of consistently lowproductivity) through checking against farmer assessmentsand the soil survey. It gives confidence that the method isreliable, at least in this landscape of the WA wheatbelt, andcan be expected to give reasonable updated estimates of saline-affected land if a new survey were conducted. Errors ofcommission were small and within the expected error of themethodology of 3–13% (Allen et al. 1999). Errors of omission(e.g. the 90 ha in the valley floor of the Wallatin catchment thatwas saline in 1998 according to farmer memory, but was notclassified as such by LandMonitor) were more significant.

The area of salt-affected land as at 2006, with updates fromfarmers, was assessed to be 4.4% and 5.3% of the Wallatin andO’Brien catchments (Table 2), far less than the 25% and 30% ofthese catchments that occurs within the valley and indicatesstrongly that valley area is not equivalent to the area at risk. Thisalso substantiates the use of the term ‘Valley Hazard’ by Georgeet al. (2005) to denote where shallow watertables (and highsubsoil salinity) are likely; they note that the actual area at riskdepends on local factors such as land-use, groundwater trends,variations in micro-relief, and local hydrological processes thatare not well represented by current mapping and modellingtechnology.

Temporal trends

The increase in saline land since 1998, both in the valley floorand on adjacent slopes, indicates that salinity is an ongoing landdegradation problem even in a catchment that had been largelycleared for agriculture by the 1940s and where many of the boresin the valley floor show that groundwater levels have stabilised.Most (80%) of the new salinity is expansion of existing areas inthe valley floor, although a significant number of outbreaks havealso occurred on the adjacent slopes. As new outbreaks are largerin the valley floor, this confirms that the on-going threat is stillgreatest in this landscape position. The disproportionate increasein the number of saline patches (associated with later clearing)

Table 3. Number of bores with declining, stable, or rising trendsbetween 1989 and 2005 by landscape position

Groundwater Landscape position Totaltrend Upper Upper/mid Middle Mid/lower Lower

Declining 3 0 0 2 11 16Stable 2 0 1 3 11 17Rising 0 2 5 2 20 29

–2.00

–1.75

–1.50

–1.25

–1.00

–0.75

–0.50

–0.25

0.00

–500

–250

0

250

500

750

1000

1250

1500

–2.00

–1.75

–1.50

–1.25

–1.00

–0.75

–0.50

–0.25

0.00

Jan.’85

Jan.’87

Jan.’89

Jan.’91

Jan.’93

Jan.’95

Jan.’97

Jan.’99

Jan.’01

Jan.’03

Jan.’05

Jan.’07

Jan.’09

Gro

undw

ater

dep

th (

m)

–500

–250

0

250

500

750

1000

1250

1500

Acc

umul

ated

ann

ual r

esid

ual r

ainf

all (

mm

)

(a)

(b)

Fig. 6. Hydrographs for bores (a) 85WC05S and (b) 85WC05D in thevalley floor of theWallatin catchment, showing observed groundwater depth(*), predicted groundwater depth (–), and accumulated annual residualrainfall (- - -).

332 Australian Journal of Soil Research M. J. Robertson et al.

on the slopes suggests that salinity will continue to spread in thispart of the landscape.

Clearing and salinity

In other areas of the South-West of WA, the dominant driverbehind the observed trends in salt-affected area has been shownto be strongly related to time since clearing and groundwaterdepth (Peck and Williamson 1987). Similarly George et al.(1997) indicates this is a process that underpins thesalinisation of the wheatbelt, and as outlined in Fig. 8, isimplicated as a major driver in continued salinisation in thestudy catchments. However, as the catchments approachequilibrium, a process that may take 30–200 years (Georgeet al. 1997), climate begins to dominate changes in water level.

Climate and salinity

A significant finding from this study has been the documentedincrease in salt-affected land between 1998 and 2006 in both theWallatin and O’Brien catchments. In particular, during thisperiod, 2 valley floor farms experienced an increase in salt-affected land of >120 ha (to >25% of farm area) where only5% (<60 ha) of their farms were saline-affected in 1998. Thisalso took place in a period when the valley groundwaterbores displayed either a stable or falling trend (Table 3). Italso occurred within an area of the landscape that had beenlargely cleared for >60 years (Fig. 7).

We attribute this response to several factors, in particular the12 months between March 1999 and April 2000 when 500mmof rain fell, and to a lesser degree, a similarly wet period fromMarch 2005 to February 2006 when 450mm fell. Both periods

GDA94 datum MGA94 Zone 50 projection

N

E

S

W

Land clearing period to 1986YrClear86

Unknown year

Outside study area

Uncleared at 1986

Monitoring bores

Legend

Sub-catchments

Boundary

1920

1940

1984

Fig. 7. Land clearing history in Wallatin and O’Brien catchments. Data derived from land alienation records andfarmer interviews. Locations of groundwater bores are indicated.

Patterns of salinity in a wheatbelt catchment Australian Journal of Soil Research 333

included January rainfalls of 100mm. Figure 5 shows this trendas a plot of accumulated deviations of monthly rainfall from thelong-term mean and resulting interpolated groundwater levelsfor 2 valley floor bores. The figure shows the relatively lowrainfall period (negative slope) from 1975 to 1988 whenMcFarlane and George (1992) undertook baseline analysis ofthe area of salinity, and the period of positive slope (rainfallaccumulation) in the early 1990s and that identified above. Theclimate data identify that salinity development is underpinned bygroundwater trends, and depth to the watertable, but subject toexpansion after wet phases, such as between 1999 and 2000when the interpolated groundwater levels approached the soilsurface. This pattern of wet winters and peak summer rainfallsspikes watertables, as also observed by Speed and Kendle (2008)in WA, and thus exposes the capillary fringe to high evaporativeconditions (Nulsen 1981) and results in active salinisation.

In the case of the 2 farms that experienced a rapid expansionin salinity, both noted a difficulty in crop germination comingout of pastures that they attributed to high soil salinity levels.This suggests that salt was not leached from the profile by winterrains as the watertable dropped. With pressure on gross marginsdue to higher input costs, farmers are reconsidering croppingrisky areas. If valley soils with higher watertables are proneto such episodic events, their place as the most reliable andproductive parts of the farm business becomes questionable.

Spatial trends

The mapping of the size and location of salinity outbreaksand the inferred groundwater catchment areas revealed thehighly localised nature of the geo-hydrology of the region.Groundwater catchments contributing recharge to salineoutbreaks of concern to farmers were mostly <150 ha andwithout exception contained within the farm boundaries.

Associated with this is the spatial pattern of saline outbreaks.After the 2006 update of salt-affected land we documented494 discrete patches of saline land in the 35 000 ha ofWallatin–O’Brien, and 89 of these had emerged in the1998–06 period. Although 85% of the new saline area isfound in the valley floor (similar to the distribution of pre-existing salinity), only 58% of the new patches occur there.Larger patches of new salinity are occurring in the valley floorthan on the adjacent slopes, but there are ongoing smalleroutbreaks on the adjacent slopes.

Implications for farm management

The spatial pattern of salinity in a farming landscape affects thechoice of hydrologically effective and feasible managementoptions that are compatible with farm layout and operations,and the degree to which the benefits of intervention will becaptured by the farmer concerned (Pannell et al. 2001). Smalland isolated patches, while potentially easy to treat or containwithin farm boundaries, may not be compatible with farm layoutand operations, particularly in crop-dominant farming systems.On the other hand, spread of salinity adjacent to large outbreaksin the valley floor may require little modification to farm plansbecause of the ability to manage large contiguous patches ofsalt-affected land.

The localised nature of groundwater catchments and salinityoutbreaks found in these catchments has several implicationsfor managing salinity. Small areas requiring treatment meansthat options can be deployed at manageable scales and do notrequire large capital outlay or disruptive changes to farm layout.Benefits will accrue largely within the farm boundaries of wherethey are treated and hence will not be diluted by benefitsaccruing to neighbours. This acts as an incentive for farmersto invest their own funds combating problems on their ownfarms (Pannell et al. 2001). A further implication is that therewill be limits to which coordinated action by groups of farmerswill have an aggregate impact at the catchment scale that isgreater than the sum of its parts. Farmers in these catchmentsnow perceive salinity as a manageable problem and recognisethe options that they can adopt can be implemented within theboundaries of their farms.

The negativities associated with the widespread occurrenceof small treatment areas are that such areas may be difficult todeal with within the constraints of farm operations, layout,and management. For instance, small areas of lucerne will bedifficult to manage with large cropping paddocks and smallmobs of sheep. Engineering options such as deep opendrains may not be effective along their full length if thecharacteristics of groundwater flow systems change over shortdistances. Options that deal with discrete seeps on slopes, suchas siphons, may be a better choice.

One apparent precaution to protect against the effects ofthe wetter years such as 2000 and 2006 would be to bothmaximise cropping and/or increase the area of perennialpastures (e.g. lucerne or salt-tolerant systems) on the valleyfloor to maintain depleted soil water conditions to act as abuffer against spiked watertables that are associated withabove-average winter and summer rains. This is particularlyimportant in catchments that are likely to have of the order of

y = –0.0032x + 0.2656

R2 = 0.28

–0.050

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0 20 40 60 80 100

Time since clearing (years)

Min

imum

rat

e of

ris

e (m

m/y

ear)

ClearedAdjacent native vegUncleared

Fig. 8. Scatter plot of minimum annual rate of groundwater rise betweenlate 1980s and 2005 against years since clearing of the native vegetation foragriculture. Points are indicated as to whether they come from land withuncleared vegetation, land that has been cleared, and land that is near theborder between cleared and uncleared land. The regression line is only forbores from cleared land.

334 Australian Journal of Soil Research M. J. Robertson et al.

25% of the valley areas with shallow watertables and for whichprojected climate change may deliver more unusual rainfallevents (Hatton et al. 2003).

Conclusions

Our results have confirmed that satellite-derived salinity mapsprovide a reliable base-line for saline mapping, and thatgroundwater monitoring and time since clearing provide thereliable predictors of salinity risk. This study has highlighted theongoing threat of salinity in the agricultural areas of WesternAustralia against a background where much of the landscape hasbeen cleared for 60 years or more, groundwater levels haveequilibrated in the valley floor, and rainfall has been belowaverage for the past 30 years. Increases in the area of salinity arechanging from that dominated by increases in the valley floor tonow being associated with the development of small, isolatedoutbreaks on the adjacent slopes. Episodic rainfall in areas ofshallow watertables is a significant cause of the expansion inobserved salinisation. The widespread occurrence of smalltreatment areas means that such areas may be difficult to dealwith within the constraints of farm operations, layout, andmanagement.

Acknowledgments

This research was supported by the Grains Research and DevelopmentCorporation, CSIRO’s Water for Healthy Country Flagship, and theCatchment Demonstration Initiative. Thanks to Fay Lewis for data fromhydro-geological analysis and many stimulating discussions that influencedthis paper. Martin Wells of Land Assessment Pty Ltd provided the soil mapfor Wallatin–O’Brien. Input from the landholders of Wallatin–O’Brien inmapping salinity is gratefully acknowledged. Hamish Cresswell providedhelpful comments on an early draft of the paper.

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