Understanding and managing groundwater and salinity in a tropical conjunctive water use irrigation district

agricultural water management 95 (2008) 1167–1179

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Understanding and managing groundwater and salinity in a tropical conjunctive water use irrigation district Cuan Petheram a,*, Keith L. Bristow a,b, Paul N. Nelson c a

CSIRO Land and Water, PMB Aitkenvale, Townsville, QLD 4814, Australia CRC for Irrigation Futures, PMB Aitkenvale, Townsville, QLD 4814, Australia c James Cook University and the Department of Natural Resources and Water, PO Box 6811, Cairns, QLD 4870, Australia b

article info

abstract

Article history:

Agricultural production around the world is increasingly being constrained by hydrological

Received 7 August 2007

factors—such as over-extraction of groundwater in some locations, rising water tables in

Accepted 30 April 2008

others, and worsening groundwater quality in general. One such area is the Lower Burdekin

Published on line 20 June 2008

irrigation area in northern tropical Australia, where rising watertable levels and increasing salinity concentrations within alluvial deposits are causing concern. The aim of this study

Keywords:

was to improve understanding of the processes driving trends in groundwater quantity and

Irrigation

quality in Mona Park, a conjunctive water use irrigation district in the Lower Burdekin. The

Groundwater

analysis is intended to enable land and water managers to explore alternative policy and

Salinity

management practices to help support the reversal in current trends, and to improve water

Conjunctive water use

table conditions in terms of both water quantity and quality. Key lessons that are applicable

Gypsum

to the development of new irrigation schemes in wet–dry tropical regions elsewhere in the

Lower Burdekin

world are emphasised. This study demonstrated that simple qualitative methods that link historical developments and observed climatic and hydrological trends can support development of a robust understanding of groundwater behaviour. The results showed that to minimise groundwater accessions in wet–dry tropical regions, a large soil water deficit should be maintained in the unsaturated zone prior to the onset of the wet season to buffer against potentially large wet season recharge events, and that this strategy should be implemented from when irrigation is first commenced. It is very clear that groundwater systems under or down gradient from irrigated areas need to be managed adaptively, such that: (1) timely decisions are made in response to changes in watertable level and groundwater quality; and (2) suitable mechanisms are in place to ensure farmers have the financial incentives and flexibility to respond in the short-term. The work also demonstrated that the establishment of good baseline data prior to irrigation development, and long-term analysis (>30 years) involving various combinations of wet and dry periods, are required in order to build a comprehensive understanding of potential groundwater behaviour and adaptive management needs. Crown Copyright # 2008 Published by Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +61 7 4753 8500; fax: +61 7 4753 8600. E-mail address: [email protected] (C. Petheram). 0378-3774/$ – see front matter . Crown Copyright # 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2008.04.016

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agricultural water management 95 (2008) 1167–1179

Introduction

Rising groundwater levels and increasing salinity concentrations within alluvial deposits adjacent to the Burdekin River in northern tropical Australia are causing concern for farmers and regional water managers. A particular region in question is Mona Park, a conjunctive water use district within the Burdekin Haughton Water Supply Scheme (BHWSS), which lies between the Burdekin and Haughton Rivers in the Lower Burdekin (Fig. 1). Prior to development of the surface water supply scheme in the BHWSS in the late 1980s, over-extraction of groundwater in the Delta, one of northern Australia’s oldest irrigation schemes incorporating the North and South Burdekin Water Boards (Fig. 1), was a primary concern. Numerous instances of seawater intrusion in this region have been reported over the last 60 plus years (Credlin, 1979; Narayan et al., 2007). In contrast, wide spread use and application of ‘imported’ surface water in the BHWSS has resulted in rising watertable levels to varying degrees across the scheme. Particular concern arose for the Mona Park district (Fig. 1) after a large groundwater mound formed during the wet season of 2000.

Although watertable levels in the Mona Park district have subsequently declined somewhat, concern remains about the sustainability of irrigation in the region and how best to manage the groundwater and associated salinity. Shaw (1988) warned of the potential for rising watertable levels and salinisation prior to the BHWSS’s inception in 1987, but there have been few documented investigations of groundwater in the BHWSS, no formal drainage management strategies implemented, and little response to bore monitoring data, which have in general shown steadily rising watertable levels since the BHWSS was established. Most groundwater investigations in the Lower Burdekin have centred on the Delta (e.g. Wiebenga et al., 1975; O’Shea, 1967; McMahon et al., 2000; Arunakumaren et al., 2000; Lawrie et al., 2004; Narayan et al., 2007). Notable exceptions were studies by Shaw et al. (1982, 1984) and Doherty (1990) who investigated local scale groundwater movement on the dissected granite and granodiorite uplands of the right bank of the Burdekin River. The hydrogeology of this district is unique in the Lower Burdekin with groundwater movement occurring primarily within the fractured granodiorite and controlled by geological features like dykes. Only one ground-

Fig. 1 – The Lower Burdekin showing the Burdekin Haughton Water Supply Scheme (BHWSS); conjunctive water use scheme, and the North and South Burdekin Water Board areas where irrigation is largely groundwater based. Mona Park is identified by the polygon in the centre of the image.

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water investigation has been documented since the inception of the BHWSS on the left bank of the Burdekin River. In that investigation at the outer extremity of the BHWSS, Narayan et al. (2004) examined the influence of an on-stream storage weir on the Haughton River on local watertable levels. Several review and discussion papers provide cursory attention to groundwater issues in the BHWSS (e.g. SKM, 1993; PPK, 2002), but no readily available studies document detailed investigations into the hydrogeology or groundwater trends in the greater BHWSS region. In addressing this issue, this paper is the first to document and discuss development of the BHWSS and the associated spatial and temporal variation in water use and groundwater behaviour, with a focus on understanding the causes of the groundwater mound in the Mona Park region. By doing so, this paper also aims to demonstrate that simple qualitative methods can generate a robust understanding of groundwater behaviour. The lessons learnt in the BHWSS will be useful to future irrigation developments in northern Australia and elsewhere in the world.

2.

Site description

Mona Park is situated on the coastal alluvial floodplain of the Burdekin River. Surface elevation ranges from 18 m at the northern boundary to 26 m on the bank of the Burdekin River. Mona Park is bounded to the north-east by Mount Kelly (80 m AHD), a granitic bedrock intrusion, and to the east by the deeply incised Burdekin River.

2.1.

Climate

The average annual rainfall and evaporation in the Lower Burdekin is about 1000 mm/year and 2080 mm/year, respectively. However, rainfall is highly variable with annual totals

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ranging from <200 mm/year to >1800 mm/year. In addition to having a highly variable inter-annual rainfall, the Lower Burdekin also exhibits a strongly seasonal trend with two thirds of the rain falling in summer between January and March (Fig. 2).

2.2.

Geology

The Lower Burdekin delta and alluvial floodplain consist of a complex layering of unconsolidated Quaternary sediments of limited lateral continuity overlying a basement of igneous origin (Hopley, 1970; Fielding et al., 2005). The deltaic deposits tend to be comprised of coarser material and have fewer clay and fine sediments than the upper alluvial deposits on the left and right banks of the Burdekin River upstream of Mount Kelly. The finer material in the BHWSS is likely to have resulted from the deposition of clays and fine silts during overbank flow events. The upper clay layer in the BHWSS has a thickness of between 2.5 and 20 m, and directly overlies either bedrock or more coarse sand and gravel deposits. The Mona Park region has some of the thickest deposits of coarse sediment in the BHWSS. Along the western boundary of Mona Park bore logs indicate very little clay in the upper alluvium. However, the depth to which clay material extends increases in an easterly direction, i.e. towards the Burdekin River.

2.3.

Soils

Soils of the BHWSS have been classified into four broad management categories: cracking clays or Vertosols (43% of the area), sodic duplex soils or Sodosols (35%), non-sodic duplex soils (12%) and gradational and uniform non-cracking soils (10%) (Donnollan, 1991; Isbell, 1996). Of the surveyed area, 72% (all of the Sodosols and most of the Vertosols) is comprised of soils with a sodic B horizon (exchangeable

Fig. 2 – Average monthly rainfall (black) and average monthly evaporation (grey) recorded at the Ayr DPI station (55 years of data) (Source: SILO climate dataset http://www.nrw.qld.gov.au/silo/ppd/index.html).

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sodium percentage or ESP >6). The non-sodic soils generally have low electrical conductivity (EC) and ESP and high permeability. The sodic Vertosols are generally strongly sodic (ESP >15) and moderately saline by 0.9-m depth. These properties restrict plant available water holding capacity to <100–130 mm and plant rooting depth to 0.6–0.9 m. The Sodosols are usually located on slightly sloping land, just upslope of the Vertosols. The Sodosols of the BHWSS have been subdivided into three groups, based on ESP levels at a depth of 0.3 m. This depth was chosen as it corresponds with the top part of the B horizon in this area. The groups are subnatric (ESP 6–14), mesonatric (ESP 15–25) and hypernatric (ESP >25). ESP, pH and EC rise rapidly with depth in these soils, with most Sodosols having an ESP >25 by 0.6-m depth. These characteristics of the Sodosols limit plant-available water holding capacity to 70–95 mm and plant rooting depth to 0.4– 0.6 m. Soil sodicity is considered the most restrictive limitation to plant growth in the area (Day et al., 1995; Nelson and Ham, 2000) due mainly to low rates of water penetration and low plant available water holding content (Shaw et al., 1988). Soils of the area have been extensively modified by agricultural development and management. Many areas have been levelled for irrigation, changing the depths and composition of horizons. Elsewhere, it has been found that long-term irrigation with sodic and saline-sodic water increases soil EC and ESP (Slavich et al., 2002; O’Choudhary et al., 2004). Soil EC and ESP have decreased where low salinity irrigation water has been used (the predominant case) in combination with gypsum (CaSO42H2O) and adequate leaching (Nelson et al., 2001).

2.4.

Hydrology

The gradient of the watertable in the BHWSS indicates a northward flow of groundwater towards the coast (Fig. 3). With the introduction of surface water for irrigation following completion of the Burdekin Falls dam in 1987, watertable levels across the BHWSS rose dramatically. While at the regional scale the groundwater flow direction remained northward some areas appeared to experience greater rises in water level than others. One such area was Mona Park. The variability in water level rise is likely to be a function of many different factors, including differing development histories, soil types, vegetation, irrigation efficiencies, channel leakages, groundwater extraction and aquifer heterogeneity. Groundwater pumping in the BHWSS is limited to the Mona Park, old Clare, Jardine, and a small numbers of bores located in the Northcote and Mulgrave districts (districts are shown in Fig. 4). Away from the Burdekin River, the density of groundwater pumping/production bores in the BHWSS is lower (Fig. 4), which largely reflects that these regions have a lower hydraulic conductivity and hence lower bore yield. Since the inception of the BHWSS the use of surface water has been metered, but these data are not readily accessible. Metering of groundwater usage only commenced in the late 1990s. Groundwater hydrographs from the Department of Natural Resources and Water (NRW) observation bores near the Burdekin River (not shown) provide evidence that in some areas there is a direct connection between the groundwater system and the Burdekin River.

Fig. 3 – Groundwater contour map of the area surrounding Mona Park for April 2003. White circles indicate location of observation bores and the arrows show the direction of groundwater flow. The Mona Park boundary is illustrated by the polygon. The thick wavy line indicates the location of the Burdekin River. The Clare Weir on the Burdekin River is shown by the dark triangle. The letters J, M, C and P show the location of observation bores 11910113 (Jardine), 12000128 (Mona Park), 12000258 (Clare) and nested piezometer 11910980 (Jardine), respectively.

3.

Method

The general approach used was to analyse the spatial groundwater trends, climate data and land use history to provide an explanation of groundwater behaviour in the region. There is fortunately, within the BHWSS, a welldistributed network of groundwater observation bores that are monitored on a bi-monthly basis and these provided key data for this analysis (Fig. 3). Within the Mona Park district many of these observation bores have been monitored since 1964. The documentation on the development history of the BHWSS prior to this study was ad hoc and piecemeal. A more

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Fig. 4 – Key districts surrounding Mona Park: (1) Jardine; (2) Northcote; (3) original Mona Park—groundwater only prior to 1989; (4) Mona Park—river water only; (5) Mona Park— surface water only (was originally also known as New Clare); (6) Burdekin Agricultural College; (7) Old Clare; (8) Mulgrave. Today Mona Park includes the areas bounded by Sections 3–5. Dark circles indicate the location of groundwater production bores.

accurate and coherent description of the development history of the region than has previously been available is therefore provided in the next section.

3.1. Development history of the Burdekin Haughton Water Supply Scheme In this paper the name BHWSS is used to describe that region of the Lower Burdekin that is supplied bulk surface water by SunWater. Prior to 2000, the BHWSS was referred to as the Burdekin River Irrigation Area (BRIA). Irrigated agriculture commenced in the BHWSS in July 1949 with the opening of the first 10 of 40 irrigated tobacco farms (a farm being comprised of 1 or more Lots) at Clare (i.e. Old Clare, Fig. 4). The tobacco farms were irrigated using river water, which was supplied by a network of channels and two river pumping stations (Credlin, 1979).

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In 1965 irrigated agriculture commenced in Mona Park using groundwater (Section 3 in Fig. 4), where each 26.3-ha Lot was allocated 95 ML/year (i.e. 3.6 ML/ha per year), as well as water sourced from the Burdekin River (Section 4 in Fig. 4). The groundwater use section of Mona Park was the first district to extract ‘substantial’ quantities of groundwater for agriculture in the BHWSS. Today Mona Park still has the greatest density of production bores in the BHWSS. During a run of dry years in the early to mid-1980s, farmers in the ‘river water only’ section of Mona Park (Section 4 in Fig. 4) were unable to extract water for irrigation from the Burdekin River after July. In 1979 the first phase of Clare Weir (8000 ML, location shown in Fig. 3) was completed (storage commenced in 1978). In 1985 flap gates were installed which increased the capacity of Clare Weir to approximately 15,500 ML. The Clare Weir was to provide a pumping pool for irrigation water for the proposed BHWSS. The BHWSS was realised in 1987 with the completion of the Burdekin Falls dam (1860 GL) and the Haughton and Barratta main channels. Until this time much of the BHWSS had been devoted to beef production on rainfed pastures. Farms in the Jardine and Northcote regions did not receive water from the BHWSS for several years after Mona Park was connected (Mona Park was connected in 1989). A few years after the introduction of surface water to the BHWSS (i.e. 1990– 1991) many farmers started applying large quantities of gypsum to their land in an effort to improve infiltration and maintain the structure of their soils (Gary Ham, personal communication, 2008) (see Section 2.3). At the time of development it was estimated that across the Scheme 12.5% of the irrigation water applied for by irrigators would return to the groundwater system. Hence, in an attempt to pre-empt a potential rise in the watertable, a conjunctive water use policy was introduced in 1989 where most farmers were required to extract one part groundwater for every eight parts of surface water applied, referred to as their ‘nominal allocation’. However, in practice, only about half the irrigated farms in the BHWSS are able to exploit commercial quantities of groundwater. NRW have tried to adaptively manage watertable levels in the BHWSS by allowing farmers to pump a greater or lesser percentage of their nominal allocation, referred to as ‘announced allocation’. Announced allocations vary on a monthly basis and between districts. During the 2000 wet season, in an attempt to address the ongoing issue of rising watertables, unlimited groundwater use allocations were announced. However, in reality only a limited number of farmers were able to capitalise on the increase in groundwater allocation because of poor yielding bores and/or high salinity groundwater. In some circumstances, production bores were no longer in place.

3.2.

Watertable level

To assist interpretation and explanation of the groundwater trends in the Mona Park district, hydrographs from three observation bores were plotted and these are shown in Fig. 5. One observation bore is located in Mona Park (Rn 12000128), one up-gradient in Clare (Rn 12000254) and one down-gradient in Jardine (Rn 11910113). These bores were selected because: (1) their groundwater hydrographs were deemed to be representative of their respective areas; (2) each hydrograph

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Fig. 5 – Selected hydrographs from observation bores at Jardine (top), Mona Park (middle) and Clare (bottom). The location of these observation bores is shown in Fig. 3 by the letters J, M and C, respectively. Solid dots represent watertable level observations and the thin jagged line is the rainfall mass residual curve. On the top two charts the ground surface elevation is shown by a solid horizontal line (the ground surface elevation is 24.7 mAHD at the Clare observation bore). The horizontal dashed line in the middle chart indicates the approximate elevation of the Burdekin River bed. The three vertical dashed lines A–A0 , B–B0 and C–C0 indicate the completion of the Clare Weir, the completion of construction of the surface water scheme in the BHWSS, and a change in groundwater pumping allocations, respectively. The vertical shaded rectangles to the right of the hydrographs are a qualitative representation of the likely effective porosity in each observation bore. These were derived from the stratigraphy logs, with clay (i.e. low effective porosity) being dark and stones/boulders being the lightest shading. Observation bores 11910113, 12000128 and 12000254 are screened at approximately S6, S3.5 and S1.5 m AHD, respectively.

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has more than 40 years of record; (3) the three bores are aligned along the regional groundwater flow path; and (4) prior to development of the BHWSS each region had a very different land-use history. Mona Park has a long history of groundwater pumping, Jardine was largely undeveloped and used only for extensive grazing, and the Lots in Clare adjacent to the Burdekin River have a long history of riparian pumping. Because groundwater is strongly influenced by the material through which it moves, groundwater hydrographs were analysed in conjunction with their bore logs (qualitative illustrations of bore log material are plotted adjacent to their bore hydrographs in Fig. 5). To further assist interpretation, a monthly rainfall mass residual curve (RMRC) was plotted on each chart. A monthly RMRC is the cumulative difference between the actual rainfall for a particular month and the long-term mean monthly rainfall. A falling limb on the RMRC indicates a drier than average period and a rising limb indicates a wetter than average period. In the absence of water application and deep drainage measurements, the RMRC can be used as a proximal indicator for crop water irrigation requirement and recharge from rainfall.

3.3.

Groundwater quality

Most of the bores in Mona Park are screened near the basement (i.e. about 20–25 m below surface) so concentration measurements are not necessarily representative of the groundwater concentrations at the top of the watertable. Understanding how salts move in the saturated zone requires nested piezometers. However, few nested piezometer sites with a long period of record (>30 years) were identified within the vicinity of Mona Park. To interpret trends in groundwater quality, data from four additional observation bores with a long period of record and one nested piezometer site with 10 years of record are presented and discussed in Section 4.2.

4.

Results and discussion

Trends in watertable level and groundwater salinity in the BHWSS are discussed in this section, followed by future trends and management options for the Mona Park region.

4.1.

Trends in watertable level

4.1.1.

1964–1988 (i.e. 1964 to B–B0 in Fig. 5)

Prior to the development of the BHWSS (i.e. before 1988) watertable levels beneath Mona Park and down-gradient of Mona Park (i.e. Northcote and Jardine) mirrored rainfall trends. This is evident in the top two charts in Fig. 5 where the groundwater hydrographs mirror the RMRC. The watertable level in the Mona Park observation bore (Rn 12000128) is more responsive than that in the observation bore at Jardine (Rn 11910113). Fluctuations in the former were between about 4 and 8 m, compared with 4 and 6 m in the latter. Furthermore, the Mona Park hydrograph is not as ‘smooth’ as that in the Jardine hydrograph. These observations are deemed to reflect the larger deep drainage rates beneath irrigation in Mona Park as well as the larger volumes of groundwater being pumped for irrigation in that area.

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During the dry years of the mid to late 1960s groundwater pumping and evaporation of irrigation water in Mona Park kept the watertable levels lower than they would have been if there had been no pumping. It is likely that there was a reduction in groundwater pumping during the wet years of the 1970s, which served to exacerbate the rise in watertable level. The inter-annual variability in the Mona Park hydrograph (i.e. jagged shape) is due to seasonal demand for groundwater. In the Clare area prior to the BHWSS, watertable records show a more subtle response to rainfall. This is probably because Lots adjacent to the Burdekin River were already applying water sourced from the river. That none of the hydrographs display any obvious response to the construction of the first phase of Clare Weir (i.e. vertical line A–A0 in Fig. 5) is indication that the storage behind the weir is not a key contributor to the elevated watertable levels in the Mona Park district.

4.1.2.

1988–2000 (i.e. B–B0 to C–C0 in Fig. 5)

The introduction of surface water into the BHWSS in the late 1980s coincided with a wetter than average period, due to tropical cyclones over three consecutive years. In response to these wet years and in conjunction with the importation of surface water into the region a step change in watertable level is observed in all three hydrographs during this period (note that surface water was introduced to the Jardine and Northcote regions several years after Mona Park). However, the step change is most pronounced in the Mona Park observation bore. This is because prior to 1989 groundwater pumping in Mona Park had kept the watertable levels ‘artificially’ low, and hence they were more responsive to a decrease in discharge (i.e. groundwater pumping) and an increase in recharge. During the dry period in the early 1990s there was a plateau in the watertable levels in Clare and Jardine. This is in contrast to observations made before the development of the BHWSS, where watertable levels in these districts declined during dry periods. In a number of observation bores in Mona Park a slight decline in watertable level is observed at this time, a function of groundwater pumping and groundwater discharge to the Burdekin River. These observations suggest that relatively little deep drainage is occurring in response to irrigation events during the dry season, and where it is occurring, it is less than or equal to the discharge capacity of the groundwater system. That (1) the watertable levels in the BHWSS show little to no change during dry years; and (2) watertable levels underwent unprecedented rates of rise during many of the wet seasons after 1987, suggest that the main effect of irrigation on the subsurface water balance has been to maintain a high soil water content in the unsaturated zone prior to and throughout the wet season. A consequence of having a relatively small soil water deficit in this zone is that there is little to no buffer to mitigate groundwater accessions during large tropical rainfall events. Contour maps of the groundwater surface after 1990 show a persistent groundwater mound/ridge along the western boundary of Mona Park (Fig. 6). This boundary coincides with a surface water supply channel. Observations made at observation bore 12000127 (Fig. 7) show that watertable levels rose even during dry periods, which suggests that channel

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Causal factors for the sudden and large rise in watertable levels in Mona Park are: (1) over 500 mm of rainfall was received in one month; (2) the watertable was shallower than it had ever been previously; (3) irrigation during the dry season, which maintained a high level of water in the soil profile; and (4) the low specific yield in the zone of watertable rise. The central portion of the Mona Park aquifer stratigraphy changes from a sandy clay to a heavy clay at an elevation of about 17 m AHD (Fig. 5). Clay has a lower specific yield (2%; Johnson, 1967) than sandy clay (7%; Johnson, 1967). Prior to the large rainfall events during February 2000, watertable levels were immediately below the clay layer. The effect of the large volume of recharge (due to points 1–3) on watertable level would have been amplified by having a low specific yield. West of line Z–Z0 in Fig. 7, observation bores in Mona Park exhibit a smaller rise in watertable levels in early 2000 than those east of line Z–Z0 (2 m, cf. 4 m). Lithology logs for these bores indicate the presence of sand throughout the profile, and without the upper clay layer evident in bore 12000128. The reason the watertable level rise was smaller at bore 11910113 than 12000128 is partly because the specific yield of the material immediately above the saturated zone was greater at bore 11910113 than at bore 12000128 (sandy clay cf. clay). Despite much of the Mona Park district having fine textured soil in its upper profile, deep drainage rates are likely to be large, due to the application of gypsum by irrigators and the long history of saline groundwater use (EC >1 dS/m). Fig. 6 – Three-dimensional representation of Mona Park groundwater mound in April 2000 (A—atypical) and in April 2003 (B—typical).

leakage may be contributing to the groundwater mound/ridge and the decrease in groundwater salinity. Channel flow information would provide confirmation of whether channel leakage is occurring or not and if so enable the volume of water leakage to be quantified. With the onset of a series of wet years in the late 1990s watertable levels in all three regions steadily increased culminating with a very rapid rise in response to a three month wet period in early 2000 (C–C0 in Fig. 5). The response to the rainfall event in early 2000 in the eastern half of Mona Park (i.e. east of the line Z–Z0 in Fig. 7) was especially large and rapid, with watertable levels increasing by as much as 4 m. This same response was not observed to the same degree in observation bores outside of Mona Park (where the change was <2 m), suggesting a localised and prominent groundwater mound had formed in the Mona Park area (Fig. 6A). Key points to note about the watertable response in Mona Park with respect to this event are: (1) the groundwater mound dissipated very quickly (see Fig. 5 vertical line C–C0 and Fig. 7 vertical line B–B0 ); (2) the centre of the mound had shifted east and was centred in the middle of Mona Park (Fig. 6A) rather than along the western boundary, which is usually the case (Fig. 6B); (3) the mound was very prominent; (4) observation bores along the western boundary of Mona Park (Fig. 7) did not exhibit the same rapid response; and (5) never had such a rapid rise been observed in Mona Park or anywhere else in the BHWSS.

4.1.3.

2000 to present (C–C0 to present in Fig. 5)

Groundwater allocations were changed after 2000 in response to concerns about the rising water tables, and farmers were allowed to pump unlimited quantities of groundwater. In reality, only a limited number of farmers in the BHWSS could take advantage of this change in allocation. This is because many were already constrained by the capabilities of their production bores and in some cases production bores had been dismantled or allowed to fall into disrepair through lack of maintenance. Of those that had the capacity to increase their groundwater pumping many were limited by the quality of their groundwater, with many farmers already utilising their optimum surface water–groundwater mix (Ray McGowan, personal communication, 2005). This optimum mix varies from farmer to farmer. The general reduction in watertable levels after 2000 is most likely a function of an increase in groundwater pumping in the Mona Park district and lower than average rainfall in the succeeding years. The large seasonal differences in the watertable levels post 2000 is thought to be a function of the increased groundwater pumping and the fact that the watertable level is fluctuating within a zone of relatively low specific yield (Fig. 5). With fewer production bores in the Clare and Jardine districts, watertable levels stabilised rather than fell during the lower than average rainfall years after 2000.

4.2.

Trends in groundwater salinity

Trends in groundwater salinity were related to the amount of recharge, the source of water entering the soil (rainfall, groundwater or surface water), the addition of salt mobilised from the unsaturated zone and lateral groundwater flow, as

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Fig. 7 – Groundwater EC contour maps of the Mona Park district at June 1976 (left) and July 1994 (right). Arrows indicate direction of groundwater flow, where the larger the arrow the steeper the gradient of the watertable. Line Z–Z0 illustrates an imaginary boundary between observation bores (circles) in which a rapid response was observed in early 2000 (i.e. east of Z–Z0 ) and those in which a rapid response was not observed (i.e. west of Z–Z0 ). Mona Park is illustrated by the black polygon. Charts illustrate trends in watertable levels (dark diamonds) and salinity concentrations (light squares). Numbers on the contour maps correspond to the chart numbers. The two vertical dashed lines A–A0 and B–B0 indicate the completion of construction of the surface water scheme and a change in groundwater pumping allocations, respectively. Nested piezometer 11910980 is located approximately 5 km north of Mona Park and is shown in Fig. 3 by the letter P.

described further below. When examining the trends it should be kept in mind that most of the EC data shown are from the bottom of the groundwater column, and trends were probably more pronounced at the top of the groundwater column (see bore 11910980 in Fig. 7). In the Burdekin region we can surmise that the unsaturated zone (>10-m thick) contains considerable quantities of salt. Although we have not been able to find any data on salt

content below 1.5-m depth, salt contents generally increase to that depth (Donnollan, 1991). Under natural conditions, salts have steadily accumulated over time from rainfall and possibly also from weathering (Shaw et al., 1988). Added salts have generally moved down the profile to form concentration bulges at depths relating to salt solubility, soil permeability and the water balance. Within the unsaturated zone, the maximum concentration of calcium carbonate is generally

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reached just below the root zone and sodium chloride below that. In a given area, clayey profiles contain more salt than sandy profiles as there is less deep drainage through clayey soils. Shaw et al. (1988) showed that subsoil salt contents are greatest in soils with 40–60% clay and mixed mineralogy. Prior to irrigation starting, groundwater EC was fairly low (<0.5 dS/ m, Fig. 7) and had probably been relatively stable. After irrigation started, groundwater EC increased until it reached a new equilibrium, due to movement of salt from the unsaturated zone into the groundwater (Fig. 7). In a sandy area with high permeability (bore 12000127), the increase in groundwater EC upon commencement of irrigation was rapid, with a new equilibrium EC (1 dS/m) being attained in about 5 years. In a clayey area with lower permeability (bore 12000100) the new equilibrium EC (2.5 dS/m) was reached more slowly, after about 10 years. The marked difference in the new equilibrium EC between the two sites was presumably due to the larger amount of salt being stored in the unsaturated profile with higher clay content. At the two sites where river water was used for irrigation rather than groundwater, the groundwater EC reached 1–1.5 dS/m (late 1970s, bores 12000187 and 12000129). There was a slight decline in groundwater EC in most of the bores in the mid-1970s, which may have been related to dilution due to deep drainage following the high rainfall events and river recharge. The only major deviation from equilibrium EC between the mid-1970s and late 1980s was in bore 12000187, which experienced a spike in EC between 1982 and 1986. In the 1980s the direction of groundwater flow in Mona Park was approximately northwest (i.e. similar to that for 1976 as illustrated in Fig. 7), and hence the spike observed in bore 12000187 was very unlikely to be due to lateral groundwater flow, because the groundwater up-gradient was relatively fresh (i.e. <1 dS/m). The early mid1980s had a run of dry years (see RMRC in Fig. 5) and the Lot adjacent to bore 12000187 was one of the few Lots in the river water only section of Mona Park to have access to a groundwater production bore (Section 4 in Fig. 4). Therefore, the spike in EC in bore 12000129 in the mid-1980s can be attributed to use of groundwater with higher EC (0.8 dS/m) than the river water (<0.3 dS/m), which would have enhanced soil structure and resulted in an increase in leaching of salts to the underlying groundwater system. Data from nested piezometers in the districts neighbouring Mona Park support the notion that the increase in groundwater EC after development of irrigation was due to the salts in deep drainage water. For example, the Lots surrounding bore 11910980 were developed in the early mid-1990s. Here it can be seen that groundwater EC in the upper part of the aquifer increased over a period of about 6 years after development, while the groundwater EC in the lower part of the aquifer remained largely unchanged. In 1989 irrigation in most of Mona Park had switched from groundwater to surface water, which should have led to a stabilisation or decrease in groundwater EC. With the introduction of surface water, water allocations were doubled and rainfall was high, which led to increased deep drainage and a rise in watertables. Those processes alone would lead to a dilution of groundwater. However, in the area that switched to surface water irrigation, there was a spike in groundwater EC during the 1990s (bores 12000100 and 12000127, Fig. 7).

While reasons are unclear at this stage, the additional salt in the groundwater could have originated from lateral influx of more saline groundwater, and/or a second flushing of salt from the unsaturated zone. That the spike in groundwater EC was larger and broader in the clayey profile (12000100) than the sandy profile (12000127) and that time series plots of groundwater EC isohytes (not shown) do not indicate any evidence for the lateral influx of more saline groundwater into Mona Park, suggests the latter. The addition of gypsum to the soil surface could have been the primary source for the salts in the unsaturated zone. When groundwater was used for irrigation, soil permeability was maintained, but after the switch to surface water, soil permeability decreased, so farmers started to apply gypsum to bring it back to desirable levels. The groundwater used for irrigation had sufficiently high EC (1–2 dS/m) and low SAR (3–4) for clay dispersion to be minimised and adequate infiltration rates to be maintained (Rengasamy and Olsson, 1991; Nelson et al., 2001; Bethune and Batey, 2002). Irrigation with saline water helps to maintain permeability by keeping EC of the soil solution above the threshold electrolyte concentration, thereby preventing clay dispersion. After the shift from irrigation with groundwater to surface water, slow infiltration rates became problematic and farmers in the BHWSS started to apply gypsum. Gypsum stabilises pores and increases infiltration, by decreasing clay dispersion through increased EC and lowered ESP. Sodic soils are very common in the area; except along the levee soils of the Burdekin River (BPC, 1976). Since 1990 gypsum has commonly been applied to crops in Mona Park at rates of about 2–5 t/ha every few years (Evan Shannon, personal communication, 2008). Concerns about possible effects of gypsum applications on groundwater quality and levels have been raised previously (Slavich et al., 1995; Surapaneni et al., 2002), but we have not found any studies where direct effects on groundwater have been documented. Gypsum application appears to have influenced watertable levels and groundwater chemistry by increasing soil permeability and therefore deep drainage, and by displacing salts from the unsaturated zone into the groundwater. Gypsum has a saturation concentration of 0.2 g/l in pure water and higher in the presence of NaCl. The saturation concentration of gypsum in pure water corresponds to an EC of 2 dS/m. The addition rates of water and gypsum result in deep drainage water that is initially saturated in gypsum. However, as the water moves through the unsaturated profile, dissolved Ca2+ and SO42 are replaced by Na+ and Cl, which are retained less strongly by the soil. Salt stores in the unsaturated zone of the original Mona Park district would have been large due to more than 20 years of irrigation using saline groundwater (EC between 1 and 2.5 dS/m). After this spike in groundwater EC, EC returned to fairly low levels, indicating that continued leaching depleted the store of soluble salts in the unsaturated zone. Changes in the direction of groundwater flow also appear to have influenced groundwater EC and in some observation bores the influence of lateral groundwater flow on groundwater EC confounds interpretation. The groundwater system in the south-eastern corner of Mona Park (e.g. bore 12000169) appears to have a strong connection with the Burdekin River

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(e.g. Fig. 6 and large intra-annual variability in watertable level in bore 12000169). Prior to the 1990s groundwater EC in this area was low (0.5 dS/m) as a result of river recharge (in Fig. 7 note the EC gradient and the direction of groundwater flow away from the river). However, with the introduction of surface water and a change in the direction of groundwater flow in the 90s groundwater EC steadily increased (in Fig. 7 note the EC gradient and direction of groundwater flow towards the river in 1994).

4.3.

Future trends and management options in mona park

Based upon historical trends analysed in this study, watertable levels are expected to continue to rise in most districts of the BHWSS under future wet periods. This is because the unsaturated zone underlying much of the BHWSS has decreased in depth and consequently its capacity to buffer against recharge events is considerably reduced. Because the specific yield of the heavy textured upper soil layers is smaller than the coarse, deep soil layers, rises in watertable level are likely to be of greater magnitude than in the past. To mitigate against future recharge events will require a combination of strategies, i.e. minimising diffuse deep drainage, enhancing groundwater discharge; and greater scrutiny of supply channels and tail water drains for leakage. It appears that the groundwater EC in the Mona Park district is decreasing and in some areas has attained a new, lower equilibrium. However, there is no room for complacency, as the recommended maximum salinity value for sugarcane is <2 dS/m (Landon, 1984) and for some horticulture is <1.5 dS/m (ANZECC, 2000). Hence, for productivity and economic viability it is imperative that groundwater EC continues to decrease and that deep drainage is reduced. It is apparent from this study that to minimise diffuse deep drainage in wet–dry tropical areas it is necessary to maintain a low soil water content in the unsaturated zone prior to the onset of the wet season, so as to provide a buffer that could mitigate recharge following large rainfall events. One option for achieving this would be to improve irrigation efficiency on farms by considerably reducing furrow lengths. Furrows are about 600 m in length in Mona Park, but considerably longer in other parts of the BHWSS, with some furrows being longer than 2 km. In southern Queensland Smith et al. (2005) demonstrated that on a group of permeable soils and with furrow length similar to those found in Mona Park, substantial improvements in irrigation efficiency under furrow irrigation could only be achieved by decreasing furrow lengths by at least half. Many farmers are, however, hesitant to reduce furrow length because of the costs involved in reconfiguring the fields and the associated increased costs of harvesting. If this is not an option then other obvious alternatives to furrow irrigation are overhead and/or trickle irrigation, although these also require fields to be reconfigured and have substantial economic implications, at least in the short term. To further reduce diffuse deep drainage farmers may need to reconsider the way in which they use soil amelioration agents like gypsum. In those areas of the BHWSS where ESP is very high, the limitations to agricultural productivity may be such that it is not viable to discontinue their application. In areas such as this, extreme caution should be exercised in the

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future before land is released for irrigation. The impact of repeated application of soil amelioration agents like gypsum on deep drainage and groundwater quality is an area of study that has been identified as requiring further investigation (Surapaneni et al., 2002). Because the depth to groundwater in many parts of the BHWSS is now relatively shallow, to manage groundwater tables during wetter regimes may require the discharge capacity of the groundwater system to be increased. One way of doing this is to extract greater volumes of groundwater. While attempts have been made to adaptively manage groundwater allocations, these have been largely ineffective because: (1) the announced allocations have not been sufficiently linked to watertable levels and trends; and (2) mechanisms to encourage farmers to use more groundwater have not been investigated. In the Mona Park region for example, many farmers have already optimised their conjunctive use of surface water and groundwater with respect to crop yield and in some other districts of the BHWSS the groundwater quality is poor and further exploitation of ‘saline’ groundwater may accelerate the rate at which salts are concentrated in the near surface zone. An alternative to utilising the groundwater on-farm would be to dispose of the extracted groundwater to the Burdekin River. This will have issues for downstream users and aquatic ecosystems, including the North and South Burdekin Water Boards who use water from the Burdekin River to recharge the groundwater system in the Delta via artificial recharge pits (Bristow, 2004). Another option would be to extract groundwater, store it and then dispose of it during periods of high flow in the Burdekin River. Although the Water Boards do not extract much water during high flow periods because the high suspended sediment loads associated with these events reduces the infiltration of their artificial recharge pits, adopting this strategy would be expensive and would require large areas of land to be acquired for storage.

5.

Conclusions

Analysis of historical information and groundwater and rainfall trends at Mona Park enabled a sound understanding of system behaviour to be developed for this tropical conjunctive water use irrigation district. The results from this study indicate that diffuse deep drainage and channel leakage are key factors contributing to rising watertables and that farm management practices, such as the application of gypsum or changing from river water to groundwater, may have a large effect on the underlying groundwater EC. Hence, to understand and model groundwater quality at the irrigation district scale, a deliberate effort is required to document the history of farm practices. Our analysis indicates that under current management practices, the watertable levels will rise further during future wet periods, with responses to increased recharge likely to be rapid and large. This is mainly because the low specific yield in the upper parts of the aquifer will amplify the affect of recharge on watertable level. Reduction in groundwater accessions could be achieved by irrigators and/or irrigation districts, by adopting one or more

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changes in practice: reduction in field row lengths; use of more efficient irrigation delivery systems, such as overhead or trickle irrigation (instead of furrow irrigation); improved irrigation scheduling to meet crop water requirements; improved conjunctive water use rules that accommodate irrigation water quality requirements; improved management of water delivery systems that minimise channel leakage; regional coordination and management of production bores; and where necessary, implementation of dewatering arrays and appropriate disposal of the extracted water. However, it needs to be kept in mind that watertable rise is inevitable, even in a well-managed irrigation system, because a leaching fraction is necessary to prevent salt accumulation in the root zone. Therefore, discharge into natural channels or constructed drains must be managed so that the watertable does not rise to within 2 m of the soil surface. A key point arising from this study was that minimisation of groundwater accessions in wet–dry tropical regions requires a large soil water deficit be maintained in the unsaturated zone prior to the onset of the wet season, to provide a buffer against potentially large wet season recharge events. Strategies to achieve this need to be implemented from the initial commencement of irrigation; otherwise the task becomes increasingly difficult as the unsaturated zone decreases in depth with rising watertable. It is also clear that groundwater systems under or downgradient of irrigated areas need to be managed adaptively, and require appropriate incentive mechanisms to be in place to ensure farmers can make timely decisions and take onfarm action in response to changes in watertable level and groundwater quality. The study showed that long-term (>30 years) analysis involving various combinations of wet and dry periods is required to build a comprehensive understanding of the likely range in potential groundwater behaviour and that establishing good baseline data prior to irrigation development is vital in providing the basic level of understanding of hydrology required for irrigation management.

Acknowledgements The authors acknowledge the assistance provided by Graham Dumaresq from SunWater in providing information to help construct the development history of Mona Park. Ian Duncan, Gary Jensen, Ray McGowan from NRW, Gary Ham from HamAg consulting, Tom McShane from BBIFMAC and Carl List from Carl List Drilling are acknowledged for their input regarding conceptualisation of the Mona Park groundwater system and general information regarding development and management within the BHWSS. Tom McShane (BBIFMAC), Lucy Reading (University of Queensland) and Dr Richard Cresswell (CSIRO) and two anonymous reviewers are thanked for commenting on earlier versions of this paper. This study was supported in part by Queensland Department of Natural Resources and Water (NRW), CSIRO Land and Water (CLW), the CRC for Irrigation Futures (CRC IF), and the Northern Australia Irrigation Futures (NAIF) project.

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