Deep drainage and soil salt loads in the Queensland Murray-Darling Basin using soil chloride: comparison of land uses.
There is uncertainty about the risk and possible extent of secondary salinity in the Queensland Murray-Darling Basin (QMDB). The main contributor to this risk that can be managed is deep drainage. Deep drainage is an important component of the soil water balance because of its potential contribution to groundwater recharge and salinity risk. Deep drainage is defined as water that moves below the root-zone, whereas recharge refers to water that arrives at the groundwater surface (Walker et al. 2002). Soil water balance models are used to estimate the likely impacts of land use on deep drainage (Yee Yet and Silburn 2003; Owens et al. 2007; Robinson et al. 2010), salinity risk (Biggs et al. 2003), and stream salinity (Silburn and Owens 2005). Modelling provides an in-principle understanding of the relative deep drainage losses of a range of land uses and management systems (Keating et al. 2002; Yee Yet and Silburn 2003). However, there were few data for deep drainage under dryland farming in the QMDB before this study (Tolmie and Silburn 2003). Petheram et al. (2002) reviewed the data available Australia-wide and noted the need for more data in areas of summer-dominant rainfall. Without independent data, the models cannot be validated and there is greater uncertainty in assessment of salinity risk.
Summer-dominant rainfall areas such as the QMDB have been considered to have a lower risk of deep drainage than the winter-dominant rainfall zones of southern Australia, because a greater proportion of annual rainfall coincides with high potential evaporation (SalCon 1997; Walker et al. 2002). However, if cumulative infiltration (rainfall minus runoff and evapotranspiration) over any period exceeds the soil water deficit in the root-zone, drainage will occur. Rainfall increases in variability and intensity (e.g. daily totals) with decreasing latitude in eastern Australia. Large amounts of rainfall (>100mm) occur over short periods (days, months) and can exceed the soil water storage capacity. High potential evaporation is not necessarily converted to actual evapotranspiration, particularly during fallows. Thus, drainage occurs in summer-dominant rainfall areas but it is strongly episodic, and is dependent on the rainfall sequence (Walker et al. 1999; Tolmie et al. 2003) and the sequence of crops and fallows (Abbs and Littleboy 1998; Yee Yet and Silburn 2003).
Yee Yet and Silburn (2003) found that annual drainage in the QMDB is 'poorly related to annual rainfall, as it relates more to coincidences of larger rainfall events on near-full soil profiles'. Thus measurement of deep drainage over short periods will not be reliable in defining long-term average rate of deep drainage. However, over the long term, for any given land use, deep drainage is generally expected (from modelling) to increase with increasing annual average rainfall (Zhang et al. 2001; Petheram et al. 2002; Yee Yet and Silburn 2003), the proportion of winter-dominant rainfall (Keating et al. 2002), and with decreasing soil water-holding capacity (Yee Yet and Silburn 2003). The effect of soil water-holding capacity on average deep drainage was also demonstrated using chloride (Cl) mass-balance studies in Central Queensland (Radford et al. 2009).
The objective of this study was to measure deep drainage for a range of rainfall levels, soils, and land uses (native vegetation, pastures, and dryland cropping) in the eastern QMDB. Existing data on deep drainage in the region and methods of measuring deep drainage were reviewed by Tolmie and Silburn (2003). The Cl mass-balance method, based on soil Cl profiles, was selected as it is cost-effective for examining a large number of sites using standard soil sampling equipment. The mass-balance method has the advantage of being 'backward-looking' (i.e. no need to install equipment and wait for drainage to occur), is able to assess the average drainage over long periods, can detect small rates of drainage (e.g. a few millimetres per year), and can be applied reasonably rapidly to a number of sites.
The Cl ion is a naturally occurring anion in soils, mainly sourced from rainfall and dry-fall (although some may be derived from weathering of minerals). It is mobile, soluble, non-reactive, and not significantly adsorbed onto mineral surfaces, and is therefore often used as a tracer of water movement in soil (Thorburn et al. 1990, 1991; SalCon 1997; Walker 1998). Plants take up some Cl, but this is only significant where considerable plant biomass is removed from the site. The net loss via grain harvest is reportedly small (Xu et al. 1999), although it is surprisingly hard to find data to support this generally held view. We therefore analysed some grain samples for CI to demonstrate that this was indeed the case. Chloride can also be added in fertiliser, although such fertilisers were not used at the study sites to the best of our knowledge. In some cases, Cl can enter the soil profile from below via shallow groundwater, in which case the site is subject to groundwater discharge, and deep drainage cannot be calculated. This research also provides much-needed data on salt loads in the soils and upper regolith, which are used in salinity risk assessments (Chamberlain et al. 2007; Searle et al. 2007).
Our study employed both the steady-state mass-balance method (USSL 1954) and the transient mass-balance method (Rose et al. 1979) across a range of sites, soil types, and land uses in the QMDB. Effects of general land use, soil type, and plant-available water capacity (PAWC) on longer term deep drainage are presented in this paper. Effects of management practices in a number of farming system/tillage trials will be considered in more detail in a subsequent paper.
Materials and methods
The QMDB covers an area of ~261 000 [km.sup.2], about 25% of the Murray-Darling Basin, Australia's largest drainage system. The physiographic attributes of the basin (geology, topography, groundwater flow systems, soils, surface water, land use) are summarised by QMDCC (1998) and Biggs et al. (2003).
Sites where soil Cl data were obtained are shown in Fig. 1. All sites are in the eastern QMDB due to an emphasis on cropping areas, the location of long-term trial sites and catchment studies, and thus the availability of historical data. Details of sites are presented in Table 1.
The sites reported in this study are described fully in Tolmie et al. (2003), except for Brymaroo and Muckadilla. In all, 48 composited or averaged Cl profiles were collected across the 14 sites, including the various treatments (land uses, tillage treatments, or contour bays). In our search for data or sites, we specifically targeted locations where:
(i) Previous soil Cl sampling had occurred, with adequate sampling protocols (especially number of cores and depth greater than root-zone) and location information, and knowledge of the time since clearing, age of cultivation, cropping history, and soil type. A number of these sites were then re-sampled (Table 1) to give data at two or more times, so that the transient Cl mass-balance could be used.
(ii) Paired sites (e.g. uncleared v. cropping or pasture) with similar soils were available.
(iii) Other water balance data were available, e.g. from past runoff studies (Greenmount, Wallumbilla) or croppingtillage studies where a number of farming systems could be compared (Hermitage, Billa Billa 1, Nindigully), so that water balance models could be used with reasonable confidence and drainage compared with the Cl-based drainage estimates (Owens et al. 2004).
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Three sites were re-sampled (Table 1): the two runoff study sites, where six contour bays and a native vegetation reference site were sampled at each; and the Hermitage fallow management trial. Three new paired sites (Brigalow floodplain, Brymaroo, Muckadilla) were chosen based on similarity of soil morphology and topographic position of paired sites, and avoiding artefacts (e.g. edge effects, run-on). Brymaroo and Muckadilla were chosen as they had low PAWC. The Brymaroo paired site is the only site on Walloon Coal Measures, in the Eastern Darling Downs; the soil is a Red Chromosol with a PAWC of 133 mm (to 1.4 m, for cropping) and average annual rainfall is 600mm. The native vegetation was poplar box (Eucalyptus populnea). The cropped site was cropped for 52 years before sampling. The Muckadilla paired site was a subsoil constraints trial site of Dang et al. (2006), where native vegetation was brigalow (Acacia harpophylla); the soil is a sodic Grey Vertosol with a PAWC of 84 mm (to 0.9 m, for cropping) and average annual rainfall is 534mm. The site had been cropped for 20 years. The Brigalow floodplain site had strong gilgai in the native vegetation; mounds and depressions were sampled separately, as Silburn et al. (2011) found systematic variation in CI profiles in gilgai mounds and depressions. Gilgai could not be clearly identified in the cropped site after 62 years of cropping and were ignored.
Sites were geolocated and soil morphology described according to the standards of McDonald et al. (1990). A hydraulic soil coring rig was used to take soil samples for CI (and other chemical) analysis and measurement of field and air-dry soil moisture contents. Six to 10 cores were taken from each treatment. Soil was sampled as deep as possible, generally to 3-5 m or to shallower bedrock. Depth increments taken were the same as those used for previous sampling, if applicable, and ranged between 0.1 and 0.3m. In most cases, samples for each depth increment were bulked across cores within a treatment. We therefore cannot define the variance but have high confidence that we have defined the mean. Obtaining a good estimate of the mean, and sampling more sites and greater depths, was considered more important. Variability was explored at two contrasting sites (see Statistical analyses) and in a separate soil Cl study at Moonie (Silburn et al. 2011).
The pH, electrical conductivity ([EC.sub.1:5], 1 : 5 soil:water), and Cl were measured for each depth increment, using the methods of Rayment and Higginson (1992) (Cl method 5A2). Additional analyses were performed on samples for sites for which there were no existing chemistry data, particularly particle size analysis (PSA), exchangeable cations, and cation exchange capacity (CEC) (Rayment and Higginson 1992). Because most sites were long-term trial or catchment studies, field-measured bulk density and PAWC (Dalgliesh and Foale 1998) were often available. All Cl concentrations (mg/kg) were calculated on an oven-dry (OD) basis before drainage (mm/year) and mass of Cl (t/ha) were determined. All relevant data are stored in the Department of Environment and Resource Management Soil and Landscape Information database (SALI).
Chloride mass-balance is based on conservation of mass of Cl within or below the root-zone. We used steady-state mass-balance (USSL 1954) to calculate deep drainage under native vegetation, and transient mass-balance (Rose et al. 1979; Thorburn et al. 1990) for cleared sites (cropping or pasture). Equations for these methods are widely reported (Slavich and Yang 1990; Thorburn et al. 1991; Walker 1998; Tolmie et al. 2003; Silburn et al. 2009) and are therefore not reproduced here.
The steady-state analysis calculates the long-term average annual drainage rate as the Cl concentration in rainfall (mg/L) divided by the concentration in the soil (mg/L), multiplied by average annual rainfall (mm/year). The Cl concentrations in the soil (mg/kg) were converted to solute concentration (mg/L) by dividing by the moisture content at drained upper limit (DUL, g/g), following Thorburn et al. (1990, 1991). Calculated drainage is directly proportional to changes in rainfall, and Cl concentration in soil and in rainfall.
SODICS transient solute mass-balance
Thorburn et al. (1987) developed a computer program (SODICS) to solve the transient mass-balance equation of Rose et al. (1979) and determine the drainage rate from soil Cl data. In our case, a Microsoft Excel spreadsheet was used with Visual Basic code to iteratively solve the equation using a Solver function. The inputs are: rainfall (mm/year); time between sampling (years); solute concentration of rainfall (mg/L); Cl concentration for each soil layer (mg/kg) at two or more times; DUL for each soil layer (g/g); bulk density for each soil layer (kg/[m.sup.3]).
Bulk density is used to convert soil Cl concentrations to mass per unit area (kg/ha). Drainage calculation (steady-state or transient) is not affected by bulk density. Drainage estimated from transient mass-balance is only slightly sensitive to the amount and Cl concentration of rainfall (Tolmie et al. 2003). DUL is used to calculate the average Cl concentration in the leachate at a specified depth. Site notes, such as cropping history, fallow practice, and length of time under cultivation, are also required so that the drainage estimates can be attributed to a particular land use. Drainage rates obtained using SODICS are an average over the time period between soil samplings. In reality, drainage only occurs during a small proportion of this time, given the episodic nature of drainage events in the region (Yee Yet and Silburn 2003).
SODICS uses soil Cl profiles at two or more times. The first sampling is designated time zero (TO) and subsequent sampling times T1, T2, etc. In the case of paired sites, the native vegetation site is TO, and the cleared site (e.g. crop) with known time since clearing is T1. The equation is then solved for vertical flux of water (deep drainage) for each depth increment below the rootzone (because the root-zone is imprecise, as explained by Willis and Black 1996). The resulting drainage rates will tend to decrease with depth and then become similar below the rootzone.
For our purposes, two sampling and analysis approaches were used depending on the data available (Table 1):
(i) Time series analysis--comparison of CI profiles from the same site over time;
(ii) Paired site analysis--sampling two nearby sites at the same time, e.g. forest v. pasture, native grass v. cropping, so that the difference in drainage is determined. If drainage and change in Cl are small under the more native vegetation site, it can be assumed to represent the Cl status under the disturbed site/s at the time of the change in land use (T0). The low drainage rates and lack of change in soil Cl found for native vegetation over 25 years at the Brigalow catchment study (Thorburn et al. 1991; Silburn et al. 2009) indicate that this assumption is reasonable for soils of low permeability.
Leachate Cl concentrations and EC
The mean Cl concentration (mg/L) of leachate below 1.5 m was calculated by dividing the total Cl loss by the total deep drainage below 1.5m. Total dissolved ion (TDI) concentration was calculated by multiplying the Cl concentration by 1.648 (assuming all salt is NaCl). The leachate EC was calculated from the TDI by dividing by 0.7, an approximate ratio of TDI to EC (SalCon 1997).
Chloride in rainfall
The Cl concentration in rainfall was determined as a function of distance from the coast using equation 3 in Biggs (2006) (Table 2). This supersedes the value of 1.5 mg/L that was used for the sites in Tolmie et al. (2003) based on the data available at that time. The new Cl concentrations in rain are reasonably similar to 1.5 mg/L at western sites but are double this amount at eastern sites. Drainage rates for steady-state mass-balance consequently doubled in the east (c.f. Tolmie et al. 2003). Drainage rates calculated using transient mass-balance increased by 10% in the east (e.g. Greenmount and Hermitage) and did not change in the west. To give some perspective, a Cl concentration in rain of 1.5 mg/L is the equivalent of an annual Cl input of 6 kg/ha for 400 mm of rainfall, 9 t/ha for 600 mm of rainfall, and 11.3t/ha for 750mm rainfall, or about one-twentieth of the rate of reduction in soil Cl observed in the QMDB cropped sites. The Cl input with 650-700 mm of rain
requires ~1000 years to accumulate 10 t/ha of soil Cl when deep drainage rates are low, not a long time in soil formation or geological time scales. Many soils in the eastern half of the QMDB typically have 10-30 t/ha of Cl in the upper 1.5 m.
Chloride in grain
The CI removal via grain was investigated in south-western Queensland, at the Roma site of Dang et al. (2009). Ten soil cores were taken, and the CI mass was determined for each soil core to 1.5 m. Mature plants were sampled at each core site, grain was removed to determine yield per ha (corrected for moisture), and a subsample taken for CI analysis. Yield (3.72 t/ha, s.d. 0.46) was well above average (1.41 t/ha) due to favourable growing-season conditions. The Cl concentration in grain was 0.04% (s.d. 0.003). Average Cl removed in grain was 0.58 kg/ha when adjusted to the long-term average yield, compared with an average CI mass of 4311 kg/ha in 0-1.5 m soil and annual rainfall inputs of 22kg/ha at an eastern site (Greenmount) and 7 kg/ha at a western site (Nindigully). When this annual rate of Cl removal in grain was used in the transient Cl mass-balance in SODICS, estimated deep drainage decreased by 0.46% at the eastern site and by 0.1% at the western site. This is considered negligible compared with other sources of error.
For many of the sites, only averaged CI profile data were available or samples had been bulked across cores. Thus, statistical analysis was not possible in general. However, it is of value to perform statistical analysis for some sites where data are available, to illustrate the level of certainty that profiles under different land uses or sampling times are different. Statistical analysis was used to determine whether soil Cl profiles were significantly different for two contrasting cases: (i) the Warra site, where CI was measured in five profiles for each land use (native vegetation, 70 years of pasture or crop), which is a reasonably long time for differences to develop; (ii) the Billa Billa 1 site where CI was measured in three profiles after 10 years of zero tillage with stubble retained or conventional tillage with stubble removed. Three profiles is a small sample size and 10 years is a reasonably short time for differences to develop under a dryland situation.
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Two analyses were performed, taking into account that Cl data for different depths are not independent. Firstly, analysis of variance (ANOVA) was used to test for differences in mean Cl mass (t/ha) accumulated to 0.6, 1.2, 1.8, and 2.4m depths. Secondly, the Warra Cl concentration data were analysed as a repeated measures analysis using the method of residual maximum likelihood (REML). Cubic splines were fitted across depths for different land uses and compared with a common spline to assess whether different land uses had different shapes of CI patterns across depths. The correlation across depths was modelled trialling uniform, autoregressive order 1 and autoregressive order 2. Improvement in model fit was also tested by fitting heterogeneity across depths. Models were tested for improvement by comparing the change in deviance to a chi-square distribution. An autoregressive model of order 1 was found to model the (Cl) data better than a uniform model, and an autoregressive model of order 2 did not provide any additional improvement, so the autoregressive model of order 1 was used (estimate of autoregressive correlation was 0.788). There was significant improvement in the model by allowing heterogeneity of variances across depths; hence, this was incorporated. The analysis was performed using GENSTAT 5 release 11.1, second edition (Copyright 2008, VSN International Ltd).
Results and discussion
The Cl profiles from all sites are too numerous to present, but detailed data have been previously published (Tolmie et al. 2003) or are available from the authors. Example results of selected sites are presented below.
Example CI and EC profiles
The Cl profiles (mg/kg of soil) from a paired site (Brigalow floodplain; brigalow native vegetation and crop for 67 years on a gilgaied Grey Vertosol) (Fig. 2a) show the shallow depth to the Cl peak (about 0.5 m) common under native vegetation and the downward movement of Cl under cropping. The profile shapes (increasing then constant with depth) indicate 'normal' recharge for soils of moderate to low hydraulic conductivity, as defined by SalCon (1997). This is the theoretical shape expected for water and salt drainage by matrix flow in a uniform soil subject to evapotranspiration as shown by Raats (1974). There are no signs of palaeoclimate, diffusion to a watertable, or bypass flow, as described by Allison et al. (1994).
The Cl lost under cropping has not been stored in the lower profile, indicating that the Cl and drainage water has moved below 3 m. Soil water contents in the subsoil were greater under cropping than native vegetation (data not shown). The remnant soil water deficit that would have existed in the subsoil at the time of clearing (Radford et al. 2009; Silburn et al. 2011), due to deeper and potentially year-round water use by the native vegetation, has been filled to at least 3 m. This is consistent with the long time period since clearing of the cropped site. The cropped site also lost a substantial amount of salts other than Cl, as indicated by EC profiles (Fig. 2b) and the observation of gypsum and carbonate in the 0.4-1.0 m soil in gilgai mounds under native vegetation, but not in the cropped soil.
In contrast to all other sites (Table 1), Cl profiles (Fig. 3) from the Hermitage fallow management trial (Loch and Coughlan 1984; Dalal 1989; Turpin et al. 1998) do not have a 'normal' shape. The Cl peaked at about 2 m depth under native vegetation (grassland), and decreased with depth thereafter. This profile shape can indicate bypass flow through macropores, or intermittent capillary rise from, and diffusion of salts to, a watertable (Allison et al. 1994; SalCon 1997) or changes in texture. For this site, a watertable was likely to have influenced the shape of the CI profiles. Alluvial sites on the eastern Darling Downs, such as the Hermitage trial site, generally have shallow, fluctuating watertables (Silburn et al. 2006). Water levels in monitoring bores within 400m of the Hermitage trial have fluctuated between -5 and -10m since 1948. There are no significant changes in texture to 6 m at the site.
The Cl profiles under native vegetation at all other sites were near 'normal' (Tolmie et al. 2003), sometimes with a reduction in CI in deeper layers, associated with coarser texture or parent material that is less decomposed, or a continuous rise in CI concentrations in the deeper subsoil (e.g. Fig. 4). The profiles for all sites after cropping or pasture had total masses of Cl less than the corresponding sites under native vegetation (i.e. had lost Cl), and all profiles had moved downwards relative to those under native vegetation. Thus, some increase in deep drainage occurred for all cleared sites and the data are suitable for analysis with transient Cl mass-balance.
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Statistical analyses of CI differences due to land use
Both the spline fitting to concentration profiles and ANOVA for cumulative CI mass profiles indicated that CI profiles were significantly different for native vegetation, pasture, and cropping at the Warra site (Fig. 4). Fitting separate splines across land uses for the Cl concentration data provided a significant change in deviance from fitting a common spline across land uses, indicating that there was a significant difference in the shape of the trend of Cl across depths for the different land uses. Sub-setting the data into each pair of land uses and re-analysing using the same model showed that each land use had a significantly different-shaped response to CI across depth to each other land use. ANOVA indicated that cumulative Cl masses were significantly different for the three land uses to depths of 0.6, 1.2, 1.8, and 2.4m (P<0.001), and 5.1 m (P< 0.01).
Analysis of cumulative Cl mass profiles at the Billa Billa 1 site by Tolmie et al. (2003), where only three cores were used per treatment and with only 10 years between Cl sampling, also found significant differences in Cl to 1.8 and 2.4m (P<0.05) and 3.3 m (P<0.01) between tillage treatments. Turpin et al. (1998) found that soil Cl profiles were significantly different for the tillage treatments in the Hermitage trial. We are therefore reasonably confident that CI profiles are different between land uses at the long-term sites, where differences in Cl masses are greater than for Billa Billa 1, and six to nine cores were bulked.
Chloride under native vegetation
On average, 25 t/ha of soil CI was stored under native vegetation to 2.4 m depth, ranging from 6 to 54 t Cl/ha (Table 3). This range is consistent with storage of 600-5400 years of rainfall Cl input (assuming 10 kg/ha.year) and equates to total NaCl salt (ignoring solid-phase salts such as carbonate or gypsum) of 41.9t/ha on average, ranging from 9.6 to 89 t/ha. An elevated level of CI occurs at depths of less than 1 m in all soils under native vegetation. This large mass of salt stored in Vertosols and Sodosols, which occurs in 90% of cropped land in the QMDB, indicates the large salinity hazard inherent to the region.
Deep drainage under native vegetation
Deep drainage rates (from steady-state analysis) were low under native vegetation (Table 3), averaging 0.3 mm/year for Sodosols and Grey Vertosols. These drainage rates under native vegetation are consistent with those of Jolly (1989), Thorburn et al. (1991), and Cook et al. (2002) for similar soils, as summarised by Tolmie and Silburn (2003) and in Table 4. Drainage was somewhat higher for the Red Chromosol at Brymaroo than for Sodosols and Grey Vertosols. Drainage rates were higher, about 1-2mm/year, and Cl levels commensurately lower, for the basaltic upland Black Vertosols at the wetter, eastern Darling Downs sites, similar to results reported by Young and MacLeod (2002) for Black Vertosols on the Liverpool Plains (Table 4).
Some soils in the QMDB and surrounding areas have naturally low levels of Cl under native vegetation (e.g. <20mg/kg). Examples include Dermosols (on upper slopes), Red Kandosols, and Red Chromosols (Table 4), due to their shallowness and low PAWC. The low Cl indicates that drainage rates are high enough to prevent accumulation of Cl. The transient mass-balance cannot be used in these cases; the soils contain too little Cl to apply the method after a change in land use. Steady-state analysis gives a lower limit to drainage which will maintain the low soil Cl. Drainage rates for these soils are considerable greater (Table 4) than for the Vertosols and Sodosols (Tables 3 and 4). Red Ferrosols also have higher deep drainage (Bell et al. 2005), due to their higher permeability and low water-holding capacity.
Long-term cropping sites
The soil Cl changes and drainage estimates are affected by many factors, including rainfall, soil type, time period considered, and the cropping and tillage system. To allow comparison, we selected cropping sites with longer periods of cropping (~20-70 years) that were predominantly under winter cropping-summer fallow and conventional tillage (Table 5) and should give a reasonable estimate of the long-term average. In summary, these sites indicate the following:
(i) Large changes in soil Cl occurred since clearing of agricultural land in the eastern QMDB. On average, these sites lost 48% (0.2 t/ha. year) of Cl from 0-1.5 m depth and 43% (0.3 t/ha. year) from 0-3m depth. In contrast, only about 0.01 t/ha.year of Cl is input via rainfall.
(ii) Deep drainage averaged about 8 mm/year and ranged from 2 to 18 mm/year. Drainage rates are a reasonably low proportion of rainfall (average 1.25%, range 0.4-2.5%) but are considerably higher than under native vegetation (0.05-0.2%).
Loss of soil Cl
Compared with paired native vegetation sites, the long-term cropped sites had lost an average of 7 t/ha (50%) from 0-1.5 m soil depth, and ranged from 1.4 t/ha (13% at Billa Billa 2, Grey Vertosol) to 24t/ha (84% at Warra, Grey Vertosol) (Table 5). The two Sodosols (Billa Billa 1 and Fairlands North) lost 5.3 t/ha (34%) and 8.0 t/ha (49%), reasonably similar to the average. Interestingly, a Sodosol had greater deep drainage than two Vertosols at the Brigalow catchment study, central Queensland (Silburn et al. 2009). Sodosols would generally be viewed as having poorer internal drainage than cracking clays. However, they have lower PAWC (<140 mm) under cropping than the Vertosols, which enhances deep drainage and Cl leaching (Radford et al. 2009). The Red Chromosol, which also has a low PAWC (133mm), lost 3.9 t/ha or 85% of original Cl in 52 years of cropping.
The large losses of Cl (and other salts/ions) from soils are of concern because of the leachate salinity (discussed later) and the mass of salt moving towards or entering groundwater. On the other hand, the data show that primary (or inherent) soil salinity has decreased and potential root depth may have increased in cultivated soils in the QMDB, as found by Dang et al. (2006), although other soil properties (such as sodicity or pH) may still limit rooting depth.
For these selected sites (Table 5), drainage increased with average annual rainfall (Fig. 5), as expected from SaLF (Shaw and Thorburn 1985), catchment data (Zhang et al. 2001; Petheram et al. 2002), and soil water balance modelling (Keating et al. 2002; Yee Yet and Silburn 2003). The Brigalow floodplain site is the most significant outlier, with lower drainage than other sites with similar rainfall. This site may have had more summer crop or pasture leys coinciding with wet periods than other sites. Native vegetation at the Brigalow floodplain site included black tea tree (Melaleuca bracteata), which is a local indicator of poorly drained soils.
For soils other than Black Vertosols (which were also two of the wettest sites), deep drainage is also related to PAWC (for those sites where PAWC data were available, Fig. 6). Long-term average deep drainage was lower for soils with greater PAWC, as expected from soil water balance modelling (Abbs and Littleboy 1998; Keating et al. 2002; Ringrose-Voase et al. 2003; Yee Yet and Silburn 2003; Silburn et al. 2007a) and measurements in the Fitzroy Basin, central Queensland (Radford et al. 2009). Keating et al. (2002) concluded that soils with higher PAWC could reduce drainage losses because they are able to generate a larger dry soil buffer between rainfall inputs and evapotranspiration outputs.
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There is still considerable variation in drainage in both relationships (Figs 5, 6), presumably due to differences in other soil properties, farming practices, and time periods sampled. The relationship between deep drainage and PAWC was not improved by plotting PAWC against deep drainage as a percentage of rainfall (Y = -0.0060X+ 2.522, [R.sup.2] = 0.78) (data not shown). This improved the fit of the Black Vertosols with the other soils only slightly. There appears to be a systematic difference between groups of soils, which needs to be confirmed with more data for Black Vertosols of basaltic origin. The most obvious difference is subsoil sodicity. In southern Queensland, most Vertosols are sodic to extremely sodic in the subsoil, with the exception of Black Vertosols of purely basaltic origin. Even within a soil class, drainage is variable (e.g. the many Grey Vertosols in Fig. 5). Relationships with site-specific soil properties (e.g. PAWC, subsoil exchangeable sodium percentage, clay content, and CEC) need to be investigated further, preferably by pooling these data with data from across Queensland (e.g. Radford et al. 2009; Silburn et al. 2009, 2011).
To provide some perspective, drainage of 10mm/year translates to a groundwater rise of 0.2 m/year, or 10m in 50 years (assuming no loss of groundwater occurs and a storativity or available porosity of 0.05 v/v). The consequences of this increase in drainage depend on local conditions (depth to watertable and groundwater discharge capacity) and the proportion of the (groundwater) catchment experiencing this level of increased drainage.
Leachate Cl concentrations and EC
The concentrations of salts in the leachate are of interest as they will potentially affect groundwater and will determine whether the recharge contributes to groundwater resources (increased supply of fresh water) or to its degradation (e.g. Leaney et al. 2003). The Cl concentrations in leachate averaged 1290 mg/L (range 630-2230mg/L) for eastern Darling Downs and 3190mg/L (1180-5000mg/L) for other sites. Leachate Cl concentrations at nine of the 11 sites not on the eastern Darling Downs would be rated as 'extreme' and 'generally too saline' for use for irrigation (table 40 in SalCon 1997). The other two non-eastern sites, and one Darling Downs site, would be rated as 'very high' and suitable for irrigation of very tolerant crops. The other two Darling Downs sites rate as borderline between 'medium' and 'high', suitable for moderately tolerant to tolerant crops.
These estimates do not include other ions that might be leached from the unsaturated zone. Equivalent average TDI concentrations were 2130mg/L for eastern Darling Downs and 5250 mg/L for other sites, if all salts are NaCl. Average leachate EC was ~3040 and 7500 [micro]S/cm, respectively; 80% of sites had leachate EC >2250 [micro]S/cm and 6720 [micro]S/cm, respectively. Thus, the leachate water is generally not a source of good quality recharge. In areas of good quality groundwater (e.g. Cl <400mg/L), the leachate will cause an increase in groundwater salinity, although the magnitude of the increase will depend on the volume of groundwater and the volume of recharge.
Four sites studied included pasture as well as cropping and native vegetation treatments (Table 6). These are discussed in order of decreasing rainfall.
(1) Drainage under sown pasture (9.8 mm/year, 1986-2002) on ex-cultivated land at Greenmount was about half that on adjacent, cultivated land, but was still five times greater than that under native vegetation. It is surprising that drainage occurs in this case, given that the pasture uses water from below 2 m of soil depth with a large PAWC (>260 mm). However, water balance modelling confirms that even under these conditions, the soil is occasionally filled and long-term average drainage occurs at rates close to that from the Cl study (Owens et al. 2004). Thirty percent of the soil Cl present in 1986 was lost from the 1.6 m soil depth when the site was re-sampled 16 years latter.
(2) At the Billa Billa 2 Grey Vertosol site of Dalal and Mayer (1986) and Page et al. (2003) (rainfall 613mm/year), drainage below 1.5 m under pasture (1.4 mm/year) during the 23 years since cleating was about one-third of that under cropping (3.7 mm/year) and about three times that under native vegetation (0.5 mm/year). The Cl lost from 0 to 1.0 m under pasture was stored between 1-2.5 m and no Cl loss occurred below 2.4 m depth. Thus, little or no drainage occurred below 2.4 m, because either the drainage water and Cl were being stored between 1 and 2.5 m in dry soil remaining from the previous native vegetation, or the pasture root depth was below 1.5 m.
(3) At the Wallumbilla runoff study (Grey Vertosol, rainfall 587 mm), drainage below 1.5 m under pasture at the top of the slope (in 36 years since clearing) was 1.0mm/year, compared with 0.2 mm/year under adjacent native vegetation. In comparison, drainage under cropping varied from 4 to 11 mm/year. However, some of this difference may be because of the different hillslope positions of pasture and cropping; deep drainage was greater in lower-slope cropped bays than in upper-slope cropped bays (Table 5). This pasture was also one of the few sites where there were indications by the shape of the Cl profile that bypass flow may have occurred in the root-zone. Most Cl loss occurred below 1.5 m; drainage at 1.5 m was 1.0 mm/year, while drainage at 2.1 m and 2.4 m was 3 mm/ year. The Cl loss was 0.7 t/ha below 1.5 m (5%) and 3.7 t/ha below 2.4 m (13%).
(4) At the Nindigully fanning systems trial (Grey Vertosol, rainfall 497 mm), the grass/legume pasture treatment ran for 6 years, following cropping. Drainage below 1.5 m (1.7 mm/year) was 4.7 times less than under conventional cropping and 7.6 times greater than under native vegetation. The Cl loss was 0.2 t/ha below 1.5 m (5%) (Table 6). As the data covered only a period of 6 years at this site, the drainage rate may not represent the long-term average.
The data give some indication that deep drainage under pasture decreases with decreasing rainfall and is lower than for cropping but greater than that under native vegetation. However, this represents a small sample of drainage rates under pasture. All soils had moderate to high PAWC (>198mm). Drainage under pasture is expected to be greater on soils of lower PAWC and with increasing rainfall, based on water balance modelling (Yee Yet and Silburn 2003; Silburn et al. 2007a; Robinson et al. 2010) and the trends found here for cropping. The Cl method provided useful drainage rates under pasture, as long as the time since clearing, and therefore the change in Cl profiles, is reasonably large (e.g. several decades). The inefficiency of pasture at reducing drainage in eastern, wetter areas, under good conditions for pasture growth (e.g. Greenmount), requires further investigation. Sampling to greater soil depth (e.g. Silburn et al. 2011) would help confirm the time lag for deep drainage to begin contributing to groundwater research.
Transient Cl behaviour
After a change in average drainage rate, for instance due to change in land use, the soil Cl profile will change, eventually approaching a new equilibrium, increasing or decreasing to adjust to the new drainage rate. These processes are represented numerically by the SODICS equations (Thorburn et al. 1990). For the increases in drainage typically found for cultivated soils in the QMDB, this new equilibrium will be established over a time scale of 30 to >200 years (depending on drainage rate), with a new soil concentration only a fraction of the original (Fig. 7; Silburn et al. 2007b, 2009). The change is rapid initially and decays exponentially, so a large proportion of the change will probably occur in the first third of the time to equilibrium--in reality it will also be influenced by runs of wet and dry periods.
For example, for a change in average drainage from 0.2 to 14 mm/year on a Grey Vertosol, about half of the change in soil Cl occurs within 10 years and 95% of the new equilibrium is established in 46 years. The resultant average soil Cl concentration is 6% of the original and soil Cl to 1.5 m depth decreases from 17 to 1 t/ha. For a new drainage rate of 5mm/year, 95% of the final equilibrium is established in 130 years with a final soil Cl mass of 1.2 t/ha, i.e. most Cl is still leached from the soil, it just takes longer. Thus, even though the drainage rate is a small proportion of rainfall, it is large enough to flush much of the Cl and other salts from the soil over a period of several decades.
Changes in soil Cl over time since clearing for two contour bays at Wallumbilla are shown in Fig. 7. Half the original soil Cl (0-1.5m soil) was lost in bay 3 by 1996 (30 years) and in bay 2 by 2002 (36 years). The time to approach a new steady-state (95% of final soil concentration) in equilibrium with the increase drainage rates after clearing is 120 years for bay 2 (mean deep drainage 6.4 mm/year) and 78 years for bay 3 (12 mm/year). The theoretical change in Cl over time (for a constant drainage rate) is an exponential decline (Thorburn et al. 1990), as shown in Fig. 7. However, drainage in the field occurred during episodic wet periods and changes in soil Cl were somewhat irregular.
Changes in soil Cl over time since clearing (and various average drainage rates) are illustrated in Fig. 8 with data from Nindigully 1 and 2 sites cleared in two different years. Before the start of the Nindigully 1 trial, almost half the soil Cl was lost from 0 to 1.5 m depth in 40 years. Soil Cl during the trial (established in 1996) varied from year to year due to wet and dry periods and experimental error; however, the overall downward trend is clear. These data and the numerical analysis with SODICS indicate the inevitability of salt leaching after hydrologic change induced by land clearing.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
Management of deep drainage and salinity risk
The study confirmed several conclusions from soil water balance modelling for cropping in the QMDB (Yee Yet and Silburn 2003; Silburn et al. 2007a): deep drainage is considerably greater under cropping than native vegetation; deep drainage increases with increasing average annual rainfall, all else being equal; deep drainage increases with decreasing soil PAWC, all else being equal; deep drainage is greater under cropping than pasture, which has greater deep drainage than native vegetation.
The increase in drainage for soils with lower PAWC indicates that to manage salinity, there is more potential to control recharge by changing management on soils of low PAWC. Soil water balance modelling showed that the reduction in drainage with pasture replacing cropping was several times greater on soils with low PAWC than on soils with high PAWC such as Vertosols (Silburn et al. 2007a; Robinson et al. 2010). Abbs and Littleboy (1998) and Ringrose-Voase et al. (2003), using water balance simulations for the Liverpool Plains (NSW), found opportunity cropping also gave less control of deep drainage on poorer (low PAWC) soils, and that pasture was a preferred land use for recharge control. Soils with low PAWC often occur in higher landscape positions than soils with high PAWC (e.g. Vertosols). Controlling deep drainage in upper landscape positions (e.g. by growing pasture rather than crops) will reduce groundwater flow into lower areas; if soils in the lower areas have greater PAWC then there are viable options (e.g. opportunity cropping) for continuing cropping with low deep drainage. However, if cropping is continued in the upper areas, salinity may affect the lower areas irrespective of the land use.
To determine the salinity risk associate with the deep drainage observed here, measurements of the moisture storage capacity and moisture status (water content and matrix suction; Jolly et al. 1989; Allison et al. 1994) of the unsaturated zone below the root-zone are needed. Storage of deep drainage water in the unsaturated zone leads to a time-lag before groundwater begins to rise. These data would also give an alternative estimate of extent of deep drainage (Silburn et al. 2011), although some additional water may have moved below the depth sampled. We have begun using a combination of coting and geophysical imaging to characterise the unsaturated zone in the QMDB (Foley et al. 2010).
The transient Cl method
Transient Cl mass-balance was used to provide a reasonably rapid appraisal of deep drainage across the cropping lands in the QMDB, and in the Fitzroy Basin by Radford et al. (2009). This method, particularly the paired site approach, was particularly useful because it is backward-looking and effectively 'reconstructs' an average drainage history since clearing. In contrast, other methods, such as soil physics (Darcy's Law), lysimetry, and measured water balance methods (Walker et al. 2002), measure drainage as it occurs. Therefore, a considerable period of monitoring (e.g. decades) is required to estimate the long-term average drainage, as drainage under dryland agriculture in the QMDB is highly episodic (Yee Yet and Silburn 2003). For example, Cl data for the Hermitage trial indicate little or no drainage over periods as long as 10 years (i.e. the recent drought-affected decade).
The Cl method can provide drainage estimates for long periods of time (e.g. decades) and can detect low rates of drainage. However, an adequate number of soil cores must be taken, obvious features such as gilgai mounds and depression should be sampled separately (Silburn et al. 2011), and soil should be sampled to below the 'conventional' depth of 0.9-1.5 m (Tolmie et al. 2003). As a general rule, sampling to greater soil depths is advised: (a) to ensure sampling below the root-zone, (b) because each layer sampled below the root-zone gives an estimate of drainage for the profile, (c) to capture the Cl profile as it moves deeper at greater times after clearing, and (d) to measure the full extent of salt load and loss. Silburn et al. (2011) found surprisingly small variability in soil Cl within carefully selected mounds and depressions of gilgaied Grey Vertosols, but large differences between mounds and depressions. Thus, variability appears to be related to fairly obvious systematic microrelief and morphologic differences, which must be taken into account during sampling.
The SODICS method used to calculate drainage from the change in soil Cl is more likely to under-estimate, rather than overestimate, drainage. If incomplete mixing and/or bypass flow (in macropores) occurs, drainage will be greater than calculated by SODICS. Thus, these results represent a lower limit to drainage.
Range of soils sampled
In this study, we only sampled soils with sufficient Cl to allow the transient Cl balance to be used, mainly Vertosols and Sodosols. Soils other than Vertosols and Sodosols occur in only 10% of the cropped land in the QMDB; however, they may be locally important recharge areas (e.g. Doherty and Stallman 1992). Some of these soils have naturally low Cl under native vegetation. For example, Red Kandosols under native vegetation (mulga or mulga/cypress) at five sites sampled by Harms and Dalal (2003) contained little Cl in soil profiles to 1.5 m. Drainage rates for soils with low Cl levels under native vegetation (using steady-state Cl balance) (Table 4; Tolmie and Silburn 2003) indicate that these soils potentially contribute large rates of deep drainage. Further work is needed to define drainage on such soils under various land uses, their distribution, and their contribution to salinity risk. Methods other than Cl mass-balance will need to be used. These soils often occur in upper landscape positions and may make an important contribution to total groundwater catchment recharge (Thorburn et al. 1986; Doherty and Stallman 1992; Silburn et al. 2007a). They probably contribute drainage of low salinity (i.e. a potential source of good quality groundwater and of salinity dilution) but may put hydraulic pressure on more saline groundwater.
We report results for soil Cl changes and deep drainage (below the crop root-zone) determined by transient soil Cl massbalance, on heavier textured soils (mainly Vertosols and Sodosols) under dryland cropping and pasture in the eastern QMDB. Large masses of soil Cl were stored under native vegetation and deep drainage was low. Large losses of soil Cl occurred where native vegetation was cleared and cropping was practised, and smaller losses occurred under cleared land planted to pastures. Deep drainage under cropping increased with greater long-term average rainfall and with lower soil PAWC, confirming results from soil water balance modelling. Deep drainage increased in the order: native vegetation < pasture < annual winter cropping. The data and the numerical analysis with SOD1CS indicate the inevitability of salt leaching after an increase in deep drainage induced by land clearing, resulting in movement of a large mass of Cl (and other salts) downwards towards groundwater. The leachate is of poor quality and will increase salinity if added to groundwater of good quality. Transient soil Cl mass-balance is probably the only affordable method available for measuring long-term deep drainage in these conditions.
Cropping areas in the QMDB have the precursors for salinity high salt loads and an increase in drainage after clearing. Salinity outcomes will depend on the properties of the underlying regolith and groundwater systems. However, we believe there are viable options for managing deep drainage in the QMDB, through the use of summer crops, and particularly opportunity cropping and pastures.
Funding from NHT project 'Managing dryland salinity in the Queensland Murray-Darling Basin', National Action Plan for Salinity and Water Quality project SIP-AG07-NRW and GRDC project DNR15 'How much water is leaking from dryland agriculture measurement and solutions' is gratefully acknowledged. Our sincere thanks to: Alison Vieritz whose Microsoft Excel version of SODICS was adapted to calculate drainage; Don Pegler and Graeme Wockner for assistance in the field; Dr Greg Thomas, Dr Kathryn Page, Dr Jill Turpin, Geoff Titmarsh, Ben Harms, Dr Ram Dalal, and Neal Dalgliesh for sharing data and providing site information; Bob Amos, Viv and Barbara Taylor, Ron Muller, Ches and Del Priebbenow, Warren Wilson and Noel Perrina for access to field sites and site information; Denis Orange and Brett Robinson for sampling the Muckadilla site; David Burton for preparing maps; Dr John Standley and the teams at Leslie Research Centre and Indooroopilly Resource Sciences analytical laboratories for soil analyses; Ian Jolly and Rick Young for critical review of the manuscript.
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Manuscript received 17 August 2010, accepted 21 March 2011
P. E. Tolmie (A,E), D. M. Silburn (A,B), and A. J. W. Biggs (C,D)
(A) Agricultural Production Systems Research Unit, Department of Environment and Resource Management, PO Box 318, Toowoomba, Qld 4350, Australia.
(B) eWater Cooperative Research Centre.
(C) Department of Environment and Resource Management, PO Box 318, Toowoomba, Qld 4350, Australia.
(D) The University of Queensland, School of Land, Crop and Food Sciences, St Lucia, Qld 4072, Australia.
(E) Corresponding author. Email: email@example.com
Table 1. Summary of sampling sites ASC, Australian Soil Classification (Isbell 1996); TS, time series sampling; PS, paired site sampling; n.a., not available Site Dates sampled Location (A) (lat. S, long. E) 1 Billa Billa 1 fallow Apr. 1985-93 -28.167 150.250 management trial 2 Billa Billa 2 soil Dec-1999 -28.433 150.300 fertility study 3 Brigalow floodplain Sep. 2002 (D) -26.814 150.796 4 Brymaroo runoff study July 2004 (D) -27.252 151.605 5 Calingunee infiltration June-1991 -28.083 150.433 study 6 Condamine Dec-1999 -27.000 149.933 7 Fairlands North 1991 -26.471 149.103 infiltration study 8 Greenmount 1977, 1986, -27.767 151.917 runoff study 1996, 2002 (D) 9 Hermitage fallow Dec. 1989, -28.210 152.100 management trial June 2002 (D) 10 Muckadilla subsoil June 2006 (D) -26.708 148.860 constraint study 11 Nindigully 1 farming Apr. 1996-2001, -28.501 148.733 systems trial 2002 12 Nindigully 2 carbon 2001 -28.464 148.718 study (sites 38, 39) (E) 13 Wallumbilla runoff May 1996, -26.467 149.100 study May 2002 (D) 14 Warra soil Dec-1999 -26.783 150.883 fertility study Site Soil type Geomorphic (ASC) unit (B) 1 Billa Billa 1 fallow Red Sodosol Brigalow uplands management trial 2 Billa Billa 2 soil Grey Vertosol Brigalow uplands fertility study 3 Brigalow floodplain Grey Vertosol alluvia 4 Brymaroo runoff study Red Chromosol Walloon coal measures 5 Calingunee infiltration Grey Vertosol Brigalow uplands study 6 Condamine Brown Vertosol Alluvia 7 Fairlands North Brown Sodosol Brigalow uplands infiltration study 8 Greenmount Black Vertosol Basaltic uplands runoff study 9 Hermitage fallow Black Vertosol Alluvia management trial 10 Muckadilla subsoil Grey Vertosol Unweathered constraint study sediments 11 Nindigully 1 farming Grey Vertosol Alluvia systems trial 12 Nindigully 2 carbon Grey Vertosol Alluvia study (sites 38, 39) (E) 13 Wallumbilla runoff Grey Vertosol Brigalow uplands study 14 Warra soil Grey Vertosol Alluvia fertility study Site Rainfall (C) Depth (mm/year) (m) 1 Billa Billa 1 fallow 613 0-3.0 management trial 2 Billa Billa 2 soil 613 0-3.0 fertility study 3 Brigalow floodplain 612 0-3.0 4 Brymaroo runoff study 600 0-1.8 5 Calingunee infiltration 617 0-1.6 study 6 Condamine 648 0-3.0 7 Fairlands North 587 0-1.6 infiltration study 8 Greenmount 731 0-3.0, 0-3.0 runoff study 9 Hermitage fallow 672 0-5.0+ management trial 10 Muckadilla subsoil 543 0-3.1 constraint study 11 Nindigully 1 farming 497 0-1.2, 0-3.0 systems trial 12 Nindigully 2 carbon 497 0-1.2 study (sites 38, 39) (E) 13 Wallumbilla runoff 587 0-1.5, 0-2.4 study 14 Warra soil 657 0-5.1 fertility study Site Land uses Sampling method 1 Billa Billa 1 fallow Crop, native veg. TS, PS management trial 2 Billa Billa 2 soil Crop, pasture, native veg. PS fertility study 3 Brigalow floodplain Crop, native veg. PS 4 Brymaroo runoff study Crop, native veg. PS 5 Calingunee infiltration Crop, native veg. PS study 6 Condamine Crop, pasture, native veg. PS 7 Fairlands North Crop, native veg. PS infiltration study 8 Greenmount Crop (bays 1-5), TS runoff study pasture (bay 0) 9 Hermitage fallow Crop, native veg TS, PS management trial 10 Muckadilla subsoil Crop, native veg. PS constraint study 11 Nindigully 1 farming Crop, pasture, fallow, TS, PS systems trial native veg. 12 Nindigully 2 carbon 2 x crop, native veg. PS study (sites 38, 39) (E) 13 Wallumbilla runoff Crop (bays 0-1) pasture, TS, PS study native veg. 14 Warra soil Crop, pasture, native veg. PS fertility study Site Reference describing site and previous research 1 Billa Billa 1 fallow Thomas et al. (1995) management trial 2 Billa Billa 2 soil Dalal and Mayer (1986), fertility study Page et al. (2003) 3 Brigalow floodplain n.a. 4 Brymaroo runoff study Dan Rattray, pers. comm. 5 Calingunee infiltration Connolly et al. (1992), study Connolly (2000) 6 Condamine Page et al. (2003) 7 Fairlands North Connolly et al. (1992), infiltration study Connolly (2000) 8 Greenmount Freebairn and runoff study Wockner (1986) 9 Hermitage fallow Turpin (1995), management trial Turpin et al. (1998) 10 Muckadilla subsoil Dang et al. (2006) constraint study 11 Nindigully 1 farming Thomas et al. (1998) systems trial 12 Nindigully 2 carbon Harms and Dalal (2003) study (sites 38, 39) (E) 13 Wallumbilla runoff Wockner et al. (1996), study Freebairn et al. (2009) 14 Warra soil Dalal and Mayer (1986), fertility study Page et al. (2003) (A) Coordinates represent approximate site location only, using Geocentric Datum of Australia (GDA 94). (B) Biggs et al. (2005). (C) Rainfall is long-term (>100 years) average, except Muckadilla (1986-2006). (D) Sampled by the authors. (E) Same property as Nindigully 1, different locations. Table 2. Chloride concentration in rain determined using the equation of Biggs (2006) Site Distance Mean annua CI in rain to coast rainfall (mg/L) (km) (mm/year) Greenmount 142 730 3.023 Hermitage 142 672 3.328 Warra 239 657 2.102 Brymaroo 166 630 2.920 Brigalow floodplain 249 612 2.172 Calingunee 307 617 1.774 Billa Billa 1 330 613 1.668 Billa Billa 2 334 613 1.652 Fairlands North 339 587 1.669 Condamine 346 648 1.512 Wallumbilla 377 587 1.542 Muckadilla 433 543 1.420 Nindigully 502 497 1.396 Table 3. Summary of chloride and drainage (steady-state analysis) under native vegetation ASC, Australian Soil Classification (Isbell 1996); n.d., no data available. Brigalow, Acacia harpophylla; belah, Casuarina cristata; Mitchell grass, Astrebla spp.; poplar box, Eucalyptus populnea Site Soil type Mean (ASC) rainfall (mm/year) Warra Grey Vertosol 657 Condamine Brown Vertosol 648 Calingunee Grey Vertosol 617 Billa Billa 1 Red Sodosol 613 Billa Billa 2 Grey Vertosol 613 Brigalow floodplain (A) Grey Vertosol 612 Fairlands North Brown Sodosol 587 Wallumbilla Grey Vertosol 587 Muckadilla Grey Vertosol 543 Nindigully 1 Grey Vertosol 497 Nindigully 2 Grey Vertosol 497 Mean (western) 587 Greenmount Black Vertosol 730 Hermitage Black Vertosol 672 Brymaroo Red Chromosol 630 Mean (eastern Darling 667 Downs) Mean (all sites) 607 1.5 m depth Site Vegetation Cl Drainage (t/ha) (mm/year) Warra Brigalow 28.3 0.20 Condamine Brigalow 14.0 0.40 Calingunee Brigalow, belah 38.1 0.15 Billa Billa 1 Brigalow 15.5 0.35 Billa Billa 2 Belah 11.0 0.46 Brigalow floodplain (A) Brigalow 17.5 0.37 Fairlands North Belah, brigalow 16.4 0.28 Wallumbilla Brigalow, belah 15.1 0.24 Muckadilla Mitchell grass 24.9 0.23 Nindigully 1 Coolibah 8.04 0.21 Nindigully 2 Coolibah 7.94 (B) 0.23 Mean (western) 17.9 0.28 Greenmount Pasture, woodland 5.82 (C) 1.92 Hermitage Grassland 12.2 0.80 Brymaroo Poplar box 4.52 0.88 Mean (eastern Darling 7.52 1.20 Downs) Mean (all sites) 15.7 0.48 2.4 m depth Site Cl Drainage (t/ha) (mm/year) Warra 54.1 0.17 Condamine 25.4 0.40 Calingunee n.d n.d. Billa Billa 1 n.d n.d. Billa Billa 2 19.9 0.59 Brigalow floodplain (A) 31.0 0.40 Fairlands North n.d n.d. Wallumbilla 28.2 0.22 Muckadilla 37.9 0.20 Nindigully 1 15.8 0.26 Nindigully 2 n.d n.d. Mean (western) 30.3 0.32 Greenmount 9.0 2.31 Hermitage 27.0 0.62 Brymaroo 5.82 (D) 0.95 Mean (eastern Darling 14.0 1.28 Downs) Mean (all sites) 25.4 0.61 (A) Mean of mound and depression. (B) 1.2 m depth. (C) 1.6 m depth. (D) 1.8 m depth. Table 4. Summary of published drainage (steady-state analysis) under native vegetation ASC, Australian Soil Classification (Isbell 1996) Site Soil type Mean rainfall (ASC) (mm/year) Basaltic uplands, Black Vertosols Eastern Darling Black Vertosol 730 Mountain Downs, Qld coolibah (A) Liverpool Plains NSW Black Vertosol 680 Undulating Cretaceous sediments or associated alluvia Theodore Qld Grey Vertosol 700 Brigalow Warra Qld Grey Vertosol 657 Billa Billa Qld Grey Vertosol 613 Talwood (Wycanna) Brown Sodosol Grey 557 Qld Vertosol Grey Vertosol Wallumbilla Qld 'Red brown earth' (D) 587 Soils with naturally low Cl Eastern Darling Black Dermosol, 730 Downs, Qld upper hill slope Manilla, NSW Red Chromosol 660 Warrego (Charleville), Red Kandosol 480 Qld Site Vegetation Drainage (mm/year) Basaltic uplands, Black Vertosols Eastern Darling 1-2 Downs, Qld Liverpool Plains NSW Pasture, woodland 1-2 Undulating Cretaceous sediments or associated alluvia Theodore Qld <0.3 Warra Qld Brigalow 0.15 Billa Billa Qld Belah 0.1 Talwood (Wycanna) Poplar boxBrigalow <0.1 <0.1<0.1 Qld Belah Wallumbilla Qld Forest (unspecified) <1 Soils with naturally low Cl Eastern Darling Mountain coolibah (A) >30 Downs, Qld Manilla, NSW Not specified >10-20 Warrego (Charleville), Mulgan (B) >5-10 Qld Site Source Basaltic uplands, Black Vertosols Eastern Darling Tolmic and Silburn (2003) (C) Downs, Qld Liverpool Plains NSW Young and MacLeod (2002) Undulating Cretaceous sediments or associated alluvia Theodore Qld Thorburn et al. (1991) Warra Qld Jolly (1989) Billa Billa Qld Jolly (1989) Talwood (Wycanna) Cook et al. (2002) Qld Wallumbilla Qld Tolmie and Silburn (2003) (C) Soils with naturally low Cl Eastern Darling Tolmie and Silburn (2003) (C) Downs, Qld Manilla, NSW Tolmie and Silburn (2003) (C) interpreted from Young and MacLeod (2002) Warrego (Charleville), Tolmie and Silbum (2003) Qld (A) Eucalyptus orgadophila. (B) Acacia aneura. (C) Interpreted from the Cl data of Titmarsh et al. (2002). (D) Titmarsh et al. (2002). Table 5. Summary of chloride loss since clearing and drainage (transient analysis) under long-term cropping with mostly winter cereals-summer fallow, conventional tillage ASC, Australian Soil Classification (Isbell 1996); n.d., data not available Site Soil type (ASC) Warra Grey Vertosol Condamine Brown Vertosol Billa Billa 1 Red Sodosol Billa Billa 2 Grey Vertosol Brigalow (A) Grey Vertosol Fairlands North Brown Sodosol Wallumbilla Grey Vertosol Bay 1 (site included 6-year pasture) Bay 2 Mean bays 1-2 (upslope) Bay 3 Bay 4 Mean bays 3-4 (downslope) Muckadilla Grey Vertosol Nindigully 1 Grey Vertosol 1971-1996 1971-2002 Nindigully 2 Grey Vertosol Mean (western) Greenmount Black Vertosol Bay 1 Bay 2 Bay 3 Bay 4 Mean Hermitage Black Vertosol Brymaroo Red Chromosol Mean (eastern Darling Downs) Mean (all sites) Site Mean rainfall Crop (mm/year) (years) Warra 657 70 Condamine 648 42 Billa Billa 1 658 18 Billa Billa 2 654 23 Brigalow (A) 612 67 Fairlands North 587 23 Wallumbilla 587 36 Bay 1 Bay 2 Mean bays 1-2 Bay 3 Bay 4 Mean bays 3-4 Muckadilla 534 20 Nindigully 1 497 1971-1996 40 1971-2002 46 Nindigully 2 497 34 Mean (western) 589 37 Greenmount 730 19 Bay 1 Bay 2 Bay 3 Bay 4 Mean Hermitage 672 42 Brymaroo 630 52 Mean (eastern Darling Downs) 677 38 Mean (all sites) 608 37 1.5m depth Site Cl loss Drainage (t/ha) (%) (mm/year) Warra 23.9 (84) 6.8 Condamine 8.1 (58) 12.4 Billa Billa 1 5.3 (34) 10.4 Billa Billa 2 1.4 (13) 5.2 Brigalow (A) 6.6 (38) 2.5 Fairlands North 8.0 (49) 8.1 Wallumbilla Bay 1 4.3 (29) 4.1 Bay 2 7.2 (48) 6.4 Mean bays 1-2 5.8 (38) 5.3 Bay 3 11.1 (74) 12.1 Bay 4 9.7 (64) 8.1 Mean bays 3-4 10.4 (69) 10.1 Muckadilla 7.4 (30) 7.5 Nindigully 1 1971-1996 2.8 (35) 2.2 1971-2002 4.3 (53) 3.1 Nindigully 2 3.8 (C) (48) 3.9 Mean (western) 7.6 (45) 6.7 Greenmount Bay 1 2.3 (C) (57) 18.2 Bay 2 1.4 (D) (35) 11.7 Bay 3 2.7 (D) (67) 16.2 Bay 4 2.8 (D) (70) 12.5 Mean 2.3 (D) (57) 14.7 Hermitage 8.1 (66) 8.7 Brymaroo 3.9 (85) 9.2 Mean (eastern Darling Downs) 4.75 (70) 10.8 Mean (all sites) 7.0 (50) 7.62 2.4m depth Site C1 loss Drainage (B) (t/ha) (mm/year) Warra 35.6 6.6 Condamine 13.2 16.0 Billa Billa 1 n.d n.d Billa Billa 2 1.68 5.2 Brigalow (A) 6.9 2.8 Fairlands North n.d n.d Wallumbilla Bay 1 6.8 4.7 Bay 2 12.7 9.6 Mean bays 1-2 9.7 7.1 Bay 3 16.7 11.6 Bay 4 14.0 9.0 Mean bays 3-4 15.3 10.3 Muckadilla 9.5 12.2 Nindigully 1 1971-1996 n.d n.d 1971-2002 n.d n.d Nindigully 2 n.d n.d Mean (western) 13.1 8.8 Greenmount Bay 1 n.d n.d Bay 2 n.d n.d Bay 3 n.d n.d Bay 4 n.d n.d Mean n.d n.d Hermitage 11.2 7.2 Brymaroo 4.9 (E) 11.1 (E) Mean (eastern Darling Downs) Mean (all sites) 12.0 8.72 (A) Brigalow floodplain. (B) Mean of all depths available 1.5-3.0 m. (C) 1.2 m depth. (D) 1.6 m depth. (E) 1.8 m depth. Table 6. Chloride and drainage at sites in pastures Land use Time Cl lost from (years) 0-1.5 m (t/ha) (%) Greenmount (Black Vertosol, rainfall 730mm/year) Sown purple pigeon grass (A) 16 (1986-2002) 1.04 29 Billa Billa 2 (Grey Vertosol, rainfall 613 mm/year) Mix of buffel (B) and native 18 (1971-2000) 1.76 11 grasses Wallumbilla (Grey Vertosol, rainfall 587 mm/year) Buffel grass (B) 36 (1966-2002) 0.69 5 Nindigully 1 (Grey Vertosol, rainfall 497mm/year) Grass-legume pasture (bambatsic, 6 (1996-2002) 0.19 4 lucerne (D) and annual medics E) Land use Av. drainage Leachate Leachate (at 1.5 m) Cl conc. EC (mm/year) (mg/L) (AS/Cm) Sown purple pigeon grasS (A) 9.8 660 1560 Mix of buffel (B) and native 3.3 1990 4680 grasses Buffel grass (B) 1.0 1920 4510 Grass-legume pasture (bambatsi, 1.7 1900 4460 (C) lucerne (D) and annual medics (E)) (A) Setaria incrassate. (B) Cenchrus ciliaris. (C) Panicum coloratum, (E) Medicago saliva. Mixture of equal amounts of cvv. Sava and Kelson (Medicago scutellata L. Mill.), Caliph, Sephi, and Keg (M. truncatula Gaertn.).
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|Author:||Tolmie, P.E.; Silburn, D.M.; Biggs, A.J.W.|
|Date:||Aug 1, 2011|
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