Sugarcane water use from shallow water tables

Agricultural Water Management 65 (2004) 1–19

Sugarcane water use from shallow water tables: implications for improving irrigation water use efficiency Caecelia A. Hurst a , Peter J. Thorburn a,∗ , David Lockington b , Keith L. Bristow c a

CRC for Sustainable Sugar Production and CSIRO Sustainable Ecosystems, 306 Carmody Road, St Lucia, Qld 4068, Australia b Environmental Engineering, University of Queensland, St Lucia, Qld 4072, Australia c CSIRO Land & Water and CRC for Sustainable Sugar Production, University Road, Townsville, Qld 4814, Australia Accepted 17 July 2003

Abstract The resource potential of shallow water tables for cropping systems has been investigated using the Australian sugar industry as a case study. Literature concerning shallow water table contributions to sugarcane crops has been summarised, and an assessment of required irrigation for water tables to depths of 2 m investigated using the SWIMv2.1 soil water balance model for three different soils. The study was undertaken because water availability is a major limitation for sugarcane and other crop production systems in Australia and knowledge on how best to incorporate upflow from water tables in irrigation scheduling is limited. Our results showed that for the three soils studied (representing a range of permeabilities as defined by near-saturated hydraulic conductivities), no irrigation would be required for static water tables within 1 m of the soil surface. Irrigation requirements when static water tables exceeded 1 m depth were dependent on the soil type and rooting characteristics (root depth and density). Our results also show that the near-saturated hydraulic conductivities are a better indicator of the ability of water tables below 1 m to supply sufficient upflow as opposed to soil textural classifications. We conclude that there is potential for reductions in irrigation and hence improvements in irrigation water use efficiency in areas where shallow water tables are a low salinity risk: either fresh, or the local hydrology results in net recharge. © 2003 Elsevier B.V. All rights reserved. Keywords: Capillary rise; Upflow; Evapotranspiration; Groundwater; Crop water use; Water balance

∗ Corresponding author. Present address: CSIRO Sustainable Ecosystems, Level 3, Queensland Bioscience Precinct, 306 Carmody Road, St Lucia, Qld 4067, Australia. Tel.: +61-7-3214-2316; fax: +61-7-3214-2206. E-mail address: [email protected] (P.J. Thorburn).

0378-3774/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-3774(03)00207-5

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C.A. Hurst et al. / Agricultural Water Management 65 (2004) 1–19

1. Introduction Upflow from shallow water tables can be a significant component in the root zone water balance of cropping systems (Benz et al., 1984; Meyer et al., 1989). Water tables should therefore be viewed as a potential water resource for crops provided upflow does not contribute to processes of soil degradation, such as salinisation or acidification, nor limit crop growth through waterlogging (Benz et al., 1984). Before reliance on water tables is promoted for crop water use, it will be necessary to assess the salinity hazard based on the ground water quality, the crop’s threshold salinity level, and the overall salt balance (Grimes et al., 1984). Shallow water tables referred to in this paper are assumed to be of such quality so as to not result in salinisation of the root zone. Where water tables are potential resources, irrigation can be reduced with no detriment to crop yield (Patel and Joshi, 1985; Hunsigi and Srivastava, 1977; Ayars and Hutmacher, 1994). Reduced irrigation above shallow water tables not only results in more efficient use of water resources, it also lowers the risk of waterlogging and nutrient losses below the root zone (Pitts et al., 1990; Shih and Gascho, 1980). The magnitude of upflow, and hence possible reduction in irrigation, depends on the water table depth, soil hydraulic properties, and on plant characteristics such as proximity of roots to the water table (Thorburn, 1997). Despite this realisation that fresh shallow water tables can help meet a crop’s water requirements, there is uncertainty as to how irrigation management should be adjusted to account for potential contributions from shallow water tables. Grimes et al. (1984) advocated the use of plant-based water measurements to adjust scheduling in areas where shallow water tables occur. The use of pan-based coefficients requires these coefficients to be modified to reflect groundwater contributions. The adjusted cotton coefficients developed by Ayars and Hutmacher (1994) are one of the few examples of such modifications. These modifications were based on measured upflow rates in lysimeters over 2 years for static water table depths of 1.2 and 2 m. Crop coefficients were reduced by the proportion of evapotranspiration supplied by upflow from the groundwater, for the two water table depths. This calculation assumes that evapotranspiration is the same for all water table depths. Kang et al. (2001) showed this was not the case for wheat and maize crops, with evapotranspiration reducing with increasing water table depths, despite all treatments being irrigated whenever the soil water tensions in the top 0.60 m fell below 0.1 MPa. When the modified coefficients of Ayars and Hutmacher (1994) were applied in a field experiment in the San Joaquin Valley, water savings were made without reducing cotton yields. However, it is not clear how applicable these modified coefficients will be to crops in other areas, because of possible differences in soil types, rooting depths, water demand, and water table depths. For the method to be applied elsewhere, a new set of experiments to determine site-specific coefficients may be required. It would be useful if more generic approaches to improving irrigation management in the presence of shallow water tables could be developed. Shallow water tables are a common feature in many sugarcane (Saccharum spp.) growing regions (e.g. Omary and Izuno, 1995). In Australia, sugarcane is mainly grown along the north-eastern tropical and sub-tropical coastline and shallow water tables, either permanent or temporary, are a common feature of this region (Sweeney et al., 2001). Water availability is a major limitation to crop production in many of these regions and profitable crop production depends on partial or full irrigation (Tilley and Chapman, 1999). The industry is coming


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under increasing pressure from government and community groups to demonstrate increased water use efficiency, and costs of irrigation (both in pumping and purchasing of water) are increasing (Kingston, 1994). The industry has, however, to date only focused on negative aspects of shallow water tables, such as waterlogging (Rudd and Chardon, 1977) and salinity (Kingston, 1993), and neglected their potential as a water resource. Sugarcane crops in other parts of the world have achieved commercial yields from sub-irrigation methods, indicating substantial use of water from shallow water tables (Escolar et al., 1971; Torres and Hanks, 1989; Juang and Uehara, 1971; Pitts et al., 1990). This would suggest that where irrigation scheduling does not involve water table monitoring, there is potential to increase water use efficiency and reduce costs associated with surface irrigation. Practical guidelines are required for modifying irrigation scheduling in the presence of shallow water tables. Because such recommendations have not yet been developed, this industry can provide a test for developing a generic framework for transforming knowledge on water use from water tables into irrigation management information. The aims of this paper are to review information on the water resource potential of shallow water tables for sugarcane, and especially for that grown in Australia, to demonstrate the likely impact of shallow water tables on irrigation management, and to suggest ways in which irrigation management may be improved in the presence of shallow water tables. The latter two aims are addressed through modelling and scenario analysis to provide insights on (a) the depth at which water tables can meet the crops’ water requirements, (b) the depth at which water table contributions become negligible, and (c) possible changes to irrigation management that would result in improved irrigation water use efficiency/crop response to irrigation when water tables lie within these extremes. Irrigation water use efficiency is defined as the difference in mean crop yield between irrigated and rainfed treatments, divided by the difference in irrigation water applied (Inman-Bamber et al., 1999).

2. Impact of shallow water tables on evapotranspiration, yield, and rooting characteristics of sugarcane Studies of water table contributions to sugarcane evapotranspiration indicate considerable irrigation water savings are possible. Water tables can contribute high proportions, and sometimes all of the evapotranspiration requirements for sugarcane crops with no detriment to yields. For a sugarcane crop in Columbia, Cenicana (1984, as cited in Torres and Hanks, 1989) found that yields were not improved by irrigation when a water table was maintained between 1.2 and 1.5 m. In India, a water table at 1 m provided 65% of sugarcane evapotranspiration in a sandy loam soil (Hunsigi and Srivastava, 1977). In another Indian study (Gupta and Yadav, 1993) groundwater contributions were 91, 86 and 55% of sugarcane potential evapotranspiration (PET) for water tables at 0.2, 0.4 and 0.6 m in a sandy loam soil during summer, respectively. Sugarcane yield was, however, significantly lower at the 0.2 m water table depth, possibly because of water logging. This result suggests that a significant component of the upflow was actually directed towards evaporation from the soil. Omary and Izuno (1995) found sugarcane evapotranspiration ranged from 20 to 106% of measured pan evaporation from subirrigation alone with water tables fluctuating between 0.4 and 1 m. Estimates of sugarcane evapotranspiration were calculated during periods where

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changes to water table heights could be equated to evapotranspiration. The highest rates were achieved when water tables were close to the soil surface during high growth periods. The above studies provide sufficient evidence that sugarcane is capable of exploiting shallow groundwater resources when the water table is within 1.5 m of the soil surface. Investigation of upflow rates from water tables deeper than 1.5 m and their contribution towards the crops water requirements still requires detailed investigation. The ability of sugarcane to extract shallow groundwater is further supported by experiments where sugarcane yields have shown a small or negligible response to reductions in irrigation in environments where responses to irrigation have been expected (Table 1). In particular, Hunsigi and Srivastava (1977) recorded small differences in cane yields (5 t/ha) between two irrigation treatments (irrigated at 50 and 75% of available soil moisture) in the presence of a shallow water table. However, when input from the water table was restricted, the cane yield difference between the irrigation treatments was much larger (18 t/ha).

Table 1 Sugarcane yields for various irrigation regimes over shallow water tables for plant crops (P) and ratoon crops (R) Study, location and soil type

Depth of water table

Irrigation treatment

Cane yield (t cane/ha)

Comments

Hunsigi and Srivastava (1977), India, red sandy loam

Fluctuating between 0.22 and 1.22 m

1.0 × Eo , 37 mm 0.75 × Eo , 37 mm 0.5 × Eo , 37 mm 1.0 × Eo , 19 mm 0.75 × Eo , 19 mm 0.5 × Eo , 19 mm

156ab

Eo —open pan evaporation Irrigation was applied whenever deficits of 37 or 19 mm (as per treatment) occurred Yields followed by the same letters are not significantly different ASM—available soil moisture

Fluctuating between 0.22 and 1.22 m

75% ASM 50% ASM

174 179

No water table (plastic sheeting at 0.9 m depth)

75% ASM 50% ASM

156 138

No rainfall or irrigation amounts given

Fluctuating between 0.4 and 1 m

P 0.5 × Eo

110a

0.7 × Eo 0.9 × Eo

111a 109a

R 0.3 × Eo 0.5 × Eo 0.7 × Eo 0.9 × Eo

91a 101a 105a 104a

Eo —open pan evaporation Irrigation was applied whenever a deficit of 80 mm occurred Yields followed by the same letters are not significantly different No rainfall given

Patel and Joshi (1985), India, heavy black soils

151ab 136a 164b 160ab 160ab


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There are other experiments relevant to shallow water table impacts where constant water tables have been maintained within lysimeters and sugarcane yields compared over a range of water table depths. These studies generally show that yields are insensitive to water table depth where depths are greater than approximately 0.5 m (Table 2). Where water tables are shallow than this yields may decrease, presumably due to water logging. However, this response is variety specific. Percent sucrose or alternative measures of sugar contents are generally unaffected by water table depths (Gupta and Yadav, 1993; Hunsigi and Srivastava, 1977; Escolar et al., 1971). This implies that water tables below 0.5 m can supplement crop water use without affecting crop yields. Very little research has focused on the effects of shallow water tables on growth and distribution of sugarcane roots and their ability to extract water at saturated or near saturated conditions. The main effect of static water tables on rooting distribution may be variety dependent. The majority of studies show that depth of rooting ceases within approximately 0.1 m of static water tables (Juang and Uehara, 1971; Pitts et al., 1990; Gosnell, 1971). Juang and Uehara (1971) showed no difference in dry root weight with water table depths between 0.3 and 0.8 m, although maximum depth of rooting was affected. Gosnell (1971) measured significantly lower root weights when the water table was at 0.25 m compared to depths between 0.5 and 1.25 m. Pitts et al. (1990) found that the depth of the water table influenced the root density and root distributions for water tables at 0.45 and 0.75 m. There were a greater number of roots in the deeper water table treatment with a lower proportion contained within the top 0.15 m of the root zone when compared to the 0.45 m water table. A recent study by Chabot et al. (2002) showed that when a water table was within 0.45 m of the surface the majority of sugarcane root water uptake occurred within the upper part of the water table, even where 70% of roots were within 0.2 m of the surface. While the root system was established with a greater water table depth that was subsequently reduced to 0.45 m in this study, Eavis (1971) also recorded very fine roots growing 0.75 m into a water table maintained at 0.15 m. This suggests that some varieties of sugarcane will actively extract from the saturated zone.

Table 2 Yields for various sugarcane varieties over a range of water table depths Study, location and soil type

Variety

Gupta and Yadav (1993), India, sandy loam

Co. 1148

Depth of water table (m)

0.2 0.4 0.6 Pitts et al. (1991), Florida, Malabar fine sand (98% sand)

CP 72-1210

0.35–0.5 0.75–0.9

Cane yield (t/ha)

Comments

P

Harvested at 12 months of age

78a 94b 99b

61.9 mm received via rainfall

P*

*Two season average

102a 93b

Harvested at 12 months Average rainfall of 1100 mm, subirrigated only

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Table 2 (Continued ) Study, location and soil type

Variety

Juang and Uehara (1971), Taiwan, clay loam and sandy loam

F 146

Escolar et al. (1971), Puerto Rico, Tao loam

Gosnell (1971), Rhodesia, loamy sand

Depth of water table (m)

Cane yield (t/ha)

Comments

0.3 0.5 0.8

P Clay loam 80a 82a 87a

Harvested at 8 months of age The yield measure is that of total above ground dry weight

Sandy loam 30a 39a 67b

Subirrigated from 2 months

0.3 0.5 0.8 PR 980

0.3 0.46 0.6 0.76 0.91 1.22

321 368 387 346 411 482*

*Cane yield for PR 980 is asterisked as highly significant, but no further details as to which other treatments it is significantly different from is given in the publication

PR 1028

0.3 0.46 0.6 0.76 0.91 1.22

401a 440a 412a 440a 429a 447a

Yields based on three successive plant cane crops No information on rainfall given

PR 1059

0.3 0.46 0.6 0.76 0.91 1.22

287a 272a 374a 343a 376a 405a

Is unclear as to whether subirrigation is the only means of irrigation Yields were converted from lb/lysimeter using lysimeter diameter of 0.61 m

0.25 0.5 0.75 1.0 1.25

P 529a 705b 893c 964c 952c

Surface irrigation was also applied, but the amount for each treatment was not given

0.25 0.5 0.75 1.0 1.25

IR 247a 580b 917c 972c 956c

0.25 0.5 0.75 1.0 1.25

2R 165a 423b 717c 764c 749c

NCo 310

Yields followed by the same letters are not significantly different.


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According to theory, capillary rise rates are affected by the presence of plants, and decrease as the distance between the maximum depth of active roots and the water table increases (Thorburn, 1997). In effect, extraction of water by roots decreases the distance upflow travels in the soil. Further work is required to quantify the distance water tables can exist below the active root zone but still impact positively on the crop water balance. Results of the field experiments referenced above show that considerable reductions in surface irrigation could occur for sugarcane grown above shallow water tables and that modifications to irrigation scheduling methods (e.g. crop or pan factors) will benefit from improved knowledge of the interactions between soil type, irrigation frequency, depth to water table, sugarcane variety and rooting characteristics. What these studies do not reveal is the optimal irrigation management that maximises groundwater contributions to sugarcane growth while maintaining high yields. The use of field experiments, while critical, will only supply information on the particular treatments under investigation. For example, reduced crop factors estimated from field experiments over a range of water table depths may not be applicable to water tables outside the particular range studied, or for different soil types. Modelling and scenario analysis can complement field work by exploring alternative irrigation management strategies under a much wider range of conditions.

3. Simulation study While numerous examples in the literature indicate that reduced surface irrigation above shallow water tables has no adverse effect on crop yield, some fundamental questions in terms of irrigation management remain unanswered. These questions include: (a) When does upflow meet all plant water requirements so that no irrigation is required? (b) When is upflow negligible, such that the presence of a shallow water table has no effect on irrigation scheduling? and (c) How can reduced irrigation be applied without having negative impacts on yield? This section illustrates how a modelling environment can provide insight into irrigation requirements by analysing various soil–crop–water table conditions. Upflow largely depends on soil properties, water table depths, and rooting characteristics (Thorburn, 1997). The impact of these factors on irrigation water requirements for a crop such as sugarcane was investigated by modelling upflow and evapotranspiration for a range of soils, water table depths and rooting depths. Potential water savings implied by the results of these simulations were assessed for the Australian sugar industry. Conditions in the simulations were chosen to represent a closed canopy sugarcane crop during conditions of high potential crop water use commonly found in irrigated sugarcane areas. 3.1. Model description Simulations of various soil–crop–water table conditions were run using SWIMv2.1 (Verburg et al., 1996). SWIMv2.1 employs Richards’ equation (Richards, 1931) to describe

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water flow in one-dimensional soil profile that may be vertically heterogeneous. It can be used to simulate runoff, infiltration, redistribution, solute transport and redistribution of solutes, plant uptake and transpiration, evaporation, deep drainage and leaching. The limitations of the model include an inability for any feedback between plant growth and soil moisture conditions. SWIMv2.1 has been successfully applied to both water and solute transport simulations (Verburg et al., 1996). Similar Richards’ equation-based numerical models have found close agreement between predicted and observed soil moisture profiles in cropping systems (Torres and Hanks, 1989; Prathapar et al., 1992). Required inputs to SWIMv2.1 include soil hydraulic properties throughout the soil profile, water inputs and evapotranspiration, root distribution, and vegetative growth. The smoothed Brooks-Corey form of both the moisture characteristic (taken from Hutson and Cass, 1987) and hydraulic conductivity (Brooks and Corey, 1964, 1966) functions were used in this study to describe the soil hydraulic properties. Soil evaporation and plant transpiration are incorporated as sink terms in the Richards’ equation. PET is required as input in the model. The method for determining PET is chosen by the user, although common approaches include pan evaporation (with or without a crop correction factor), or Penman-Monteith or Priestly-Taylor. Vegetation in the model is assigned a maximum fraction of PET, which essentially sets a maximum transpiration rate, with the remainder of PET being available for soil evaporation. SWIM approximates the impact of plant growth on soil water extraction by changing the fraction of maximum transpiration that can occur over time, allowing increasing or decreasing transpiration through time. These fractions describing transpiration are input as either parameters describing the shape of a sigmoidal curve representing the fraction of maximum potential transpiration with time, or as specific fractions for specific times. In the latter case, SWIM uses linear interpolation to determine fractions between the times specified. Root distributions are described as functions of depth and time relative to a maximum root length density. Rooting characteristics with depth can be input as an exponential function or as pairs of data specifying the fraction of maximum root length density at specified depths. For both of these cases, the changes with roots over time are assumed to follow the same sigmoidal distribution as the fraction of maximum potential transpiration. The other option for describing the spatial and temporal rooting characteristics is to input a matrix of relative root length density with depth and time. Water uptake by the roots is equated to transpiration, which may be limited by either the potential transpiration rate or soil water availability. The actual rate of water uptake from each soil layer is a function of root length density and soil–plant water potential gradients. Root radius and root conductance affect the supply rate of the soil. 3.2. Parameter values and scenario descriptions In this study PET was input as pan evaporation without a crop correction factor. The maximum transpiration rate was set at 0.85 × PET (Holden, 1998). To represent a mature crop with a closed canopy, the fraction of potential transpiration was set to 1 throughout the simulation, effectively starting and finishing the simulation with maximum growth. The relative fraction of maximum root length density followed a generic root distribution described by Danielson (1967). This distribution specifies that 40, 30, 20 and 10% of the


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roots be found in each quarter of the root zone. Within the SWIM modelling framework, this meant that the maximum root length density was assigned to the surface quarter, and was reduced accordingly for successive quarters. That is, fractions of maximum root length density of 1, 0.75, 0.5 and 0.25 were set for successive quarters of the root zone. Torres and Hanks (1989) found agreement between predicted and measured lysimeter upflow rates using this root distribution for spring wheat. Maximum root length densities measured for sugarcane have ranged between 0.01 and 0.0275 mm/mm3 for mature plants (Ball-Coelho et al., 1992), although Australian sugarcane roots are reported in the range of 0.01–0.015 mm/mm3 (M. Robertson, personal communication, 2001). Because of the uncertainty in this parameter, a range of maximum root length densities (0.01, 0.015 and 0.025 mm/mm3 ) was used in combination with other variables (soil type and maximum rooting depth). Root radius was set at 0.24 mm, the average found by Ball-Coelho et al. (1992), and root conductance at 1.4 day-6 mm2 /h/mm root (Bristow et al., 1984). Rooting depths have the potential to affect the rate of capillary rise by impacting on the distance water is required to move through the unsaturated zone from the water table to the roots (Thorburn, 1997). In the model application, rooting depths were increased in 0.1 m increments from a depth of 0.8 m to a distance 0.2 m above the water table. When the rooting depth was increased, the fraction of maximum root length density assigned to the bottom quarter of the root zone (0.25) was applied to the root zone below 0.8 m. The fractions of maximum root length density down to 0.6 m remained unchanged (1, 0.75 and 0.5 for depths of 0–0.2, 0.2–0.4, and 0.4–0.6 m, respectively). Initial simulations were carried out for the maximum root length density of 0.01 mm/mm3 and the 0.8 m rooting depth (the lowest maximum root length density and shallowest rooting depth). If this combination of maximum root length density and rooting depth could meet potential transpiration rates in a particular soil, then simulations with higher root length densities and deeper rooting depths would also meet these transpiration rates. If this combination failed to meet potential transpiration rates, then the effect of higher root length densities and deeper rooting depths was investigated. It was assumed that roots were able to function throughout the entire root zone, even when the soil was close to saturation, as discussed above. The minimum distance between the root zone and water table was 0.2 m. Water inputs (rainfall and irrigation) were set to zero to represent extreme dry conditions, with PET rates of 10 mm/day. Maximum potential crop transpiration was therefore 8.5 mm/day. If potential transpiration and evaporation can be supplied by the system, actual sugarcane evapotranspiration will be equivalent to PET. Simulations were run over a period of 100 days, both to overcome the effects of initial conditions, and to approximate the time period for which sugarcane has a closed canopy during periods of high potential evapotranspiration (January–March) in the Australian sugar industry. Initial soil water potentials (−10 kPa) for all simulations were set to approximate field capacity, such as may occur following a large rainfall event. Hydraulic properties were taken for three soil types were from a range of soils previously characterised in one of the main irrigation areas (Bundaberg) (Verburg et al., 2001) in the Australian sugar industry. This area is one of the few that has detailed measurements of soil hydraulic properties. Uniform soil profiles of differing permeability (see Fig. 1) were

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Fig. 1. Fitted soil hydraulic conductivity functions for the three soils used in this study.

simulated by taking data from an AP horizon of a Fairymead soil, and an Alloway soil, and a B22 horizon from an Oakwood soil (Table 3) (Verburg et al., 2001). The soil permeability (as defined by the near saturated hydraulic conductivity) of the Oakwood and Alloway soils are at the extremes of the range typically found in soils used for cropping (Marshall et al., 1996); the Fairymead soil is more conductive than this range. Water content profiles and transpiration rates were simulated for each soil type in the absence of a water table using a unit hydraulic gradient at the bottom boundary. These scenarios were modelled for comparative purposes to illustrate a “control” condition where water tables have no impact on soil water content profiles and plant water use. Constant water tables at depths of 1, 1.5 and 2 m were then simulated via a constant potential (ψ = 0 kPa) bottom boundary condition. These simulations were carried out to investigate the effect of Table 3 Soil hydraulic property data for the soils used in the simulations: θ s is the saturated volumetric water content; θ r is the volumetric residual water content; ψe is the air entry potential; b is the slope in the smoothed Brooks-Corey water retention function; Ksat is the saturated hydraulic conductivity; θ is the volumetric water content; ψ is the matric potential Properties

Fairymead soil

Alloway soil

Oakwood soil

Field texture θ s (mm/mm3 ) θ r (mm/mm3 ) ψe (mm) b Ksat (mm/h) θ (ψ = −10 kPa) θ (ψ = −100 kPa) θ (ψ = −1500 kPa)

Medium clay 0.456 0 41.7 12.75 1617.2 0.356 0.308 0.216

Sandy loam 0.475 0.040 116.3 2.30 394.7 0.201 0.104 0.049

Light medium clay 0.38 0.097 80.9 7.64 47.1 0.298 0.246 0.202

Source: Verburg et al. (2001).


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water tables on simulated soil water contents and transpiration rates, and hence potential irrigation requirements. Irrigation requirements were assessed from simulated transpiration rates once these transpiration rates approached steady state, which usually occurred around 30 days into the simulation. Combinations of soil type, water table depth, maximum root length density, and rooting depth were simulated to investigate their effects on water use and hence irrigation requirements. When evaporative demand is lower than that used in this study, irrigation requirements are likely to be even less than indicated. 3.3. Results of scenario analysis 3.3.1. Fairymead soil—“high permeability” When no water table was present, simulated transpiration rates (data not shown) fell below potential rates (8.5 mm/day) after 10 days. Water extraction via evaporation and transpiration continued over the 100 days but declined to values less than 0.2 mm/day. For the 1, 1.5 and 2 m water table simulations, potential transpiration rates of 8.5 mm/day were maintained over the 100 days, even with the lowest maximum root length density (0.01 mm/mm3 ) and shallowest rooting depth (0.8 m) (Fig. 2). The resulting water contents appeared steady and were considerably greater than the water content profile simulated in the absence of a water table (Fig. 3a). Water contents above field capacity occurred in the majority of the soil profile for the 1 m water table depth. The water content profiles are slightly lower when the water tables are deeper, but upflow rates are high enough to meet potential transpiration rates. On the basis of these simulations, no irrigation would be required for mature sugarcane crops above water tables as deep as 2 m in this soil, provided functioning roots extended to a depth of at least 0.8 m. 3.3.2. Alloway soil—“medium permeability” When no water table was present, simulated transpiration rates fell below potential rates after 14 days in the Alloway soil. Simulated transpiration rates over the 100-day period were equal to potential transpiration rates for the 1 and 1.5 m water table simulations with the lowest maximum root length density (0.01 mm/mm3 ) and shallowest rooting depth (0.8 m) (Fig. 2). The simulated soil water content profile at the end of 100 days was above field capacity for the 1 m water table. For the 1.5 m water table depth, the simulated water content profile contains water easily extractable by plants (i.e. ψ > −100 kPa; Inman-Bamber et al., 1998) at depths below 0.2 m (Fig. 3b). With a water table at 2 m, a root distribution of 0.01 mm/mm3 and a rooting depth of 0.8 m, simulated transpiration rates fell to 4.6 mm/day after 32 days and then remained steady (Fig. 2). The soil water content profile to a depth of 0.5 m was similar to that simulated in the absence of a water table (Fig. 3b). Increasing the maximum root length density to 0.025 mm/mm3 resulted in higher transpiration rates (5.6 mm/day) in the later part of the simulation. For these 2 m water table simulations, rooting depth had a much greater effect on whether potential transpiration rates were met, with simulated transpiration rates exhibiting more sensitivity to maximum root length densities as the distance between the water table and root zone decreased (Fig. 4a). Potential transpiration rates were maintained in the 2 m water table simulation regardless of root length

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Fig. 2. Simulated transpiration rates at the end of 100 days for a rooting depth of 0.8 m for the (a) 1 m water table (b) 1.5 m water table, and (c) 2 m water table. All transpiration rates had reached steady state by day 50.

densities, provided the distance between the water table and root zone did not exceed 0.7 m. The results of these scenarios suggest that all water requirements of a mature sugarcane crop would be supplied by upflow from a water table in this soil, provided the water table was no deeper than 1.5 m. For deeper water tables, irrigation may be required depending on the rooting distribution. Fifty-four percent of sugarcane transpiration was met by a water table at 2 m using the shallowest rooting depth (0.8 m) and lowest maximum root length density (0.01 mm/mm3 ). 3.3.3. Oakwood soil—“low permeability” Transpiration rates fell below potential rates after 10 days during simulations with no water tables. With a water table at 1 m and a maximum root length density of 0.01 mm/mm3 and rooting depth of 0.8 m, upflow was able to supply enough water to maintain potential transpiration rates over the 100 days. With a water table at 1.5 m and the same root length


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Fig. 3. Simulated water content profiles with a maximum root length density of 0.01 mm/mm3 and a rooting depth of 0.8 m for (a) Fairymead soil (b) Alloway soil and (c) Oakwood soil. Water content profiles are given for the 1, 1.5, and 2 m water table depths, and for no water table. Water contents corresponding to field capacity (−10 kPa) and the lower limit of readily available water (−100 kPa, Inman-Bamber et al., 1998) are shown for reference.

density (0.01 mm/mm3 ) and rooting depth (0.8 m), simulated transpiration rates decreased to 6.9 mm/day after 25 days and then remained steady (Fig. 2). When the maximum root length density was increased to 0.025 mm/mm3 with the same rooting depth (0.8 m) simulated transpiration rates equalled potential rates for the full 100-day period. In the 1.5 m water table simulations, root length density only had an effect on transpiration rates for rooting depths up to 1 m, after which, potential transpiration rates were maintained for the full 100 days for all simulations (Fig. 4b). In simulations with a 2 m water table, transpiration rates were dependent upon the combined effects of maximum root length density and rooting depth (Fig. 4c). Simulated

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Fig. 4. The effects of root length density and rooting depth on steady state transpiration rates for (a) Alloway soil with a 2-m water table, (b) Oakwood soil with a 1.5-m water table, and (c) Oakwood soil with a 2-m water table.

transpiration rates were equal to potential rates only when roots were within 0.4 m of the water table. At greater distances from the water table simulated transpiration rates were less sensitive to root length densities, suggesting the system becomes limited by the soil’s ability to transmit water over larger distances. When the water table was at 2 m with roots to 0.8 m and a maximum root length density of 0.01 mm/mm3 , simulated transpiration rates declined to 2.9 mm/day after 26 days and then remained steady. Soil water contents at the end of the 100-day simulations for the 1 m water table were less than the water content equivalent to ψ = −100 kPa in at least half of the root zone. Thus much of the soil water would be difficult for plants to extract when water tables are deeper (Fig. 3c). Results from the simulations indicate that while no irrigation would be required for mature sugarcane grown above water tables to depths of 1 m in this soil, irrigation may be required for deeper water tables depending on rooting characteristics. Conservative estimates from simulations with maximum root length densities of 0.01 mm/mm3 and rooting depths of 0.8 m indicate that at least 80 and 34% of potential transpiration could be met by upflow from water tables at 1.5 and 2 m, respectively.


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4. Discussion Experimental findings of sugarcane crops receiving all or large proportions of water requirements from water tables at 1 m depth (Cenicana, 1984, as cited in Torres and Hanks, 1989; Hunsigi and Srivastava, 1977) are supported by our simulations of potential upflow from water tables for a range of soils (Fig. 2). It appears that a general rule of not applying irrigation to mature sugarcane when water tables are shallower than 1 m may be applicable to a wide range of soil types. The influence of soil hydraulic characteristics on upflow becomes more evident when water tables are deeper than 1 m. Experimental results confirm that, for some soils, water tables as deep as 1.5 m can supply all the water requirements of sugarcane (Cenicana, 1984, as cited in Torres and Hanks, 1989). The ability of upflow to meet a sugarcane crop’s water requirements has not yet been quantified by experimental means where water tables are deeper than 1.5 m. The results from the simulations reported in this paper confirm that guidelines for modifying irrigation scheduling for the presence of shallow water tables will need to be soil specific. Our results, like those of Thorburn et al. (2003), also indicate that soil texture per se is not a useful identifier of potential upflow rates. The Fairymead soil has the heaviest texture of the three soils modelled (medium clay), yet is able to supply upflow at 8.5 mm/day from water tables at 2 m. The sandy loam (Alloway) and light medium clay (Oakwood) soils are only able to supply 4.6 and 2.9 mm/day, respectively, from the same water table depth. The depth of water table that can supply potential sugarcane evapotranspiration for the three soils is in the same order as the near saturated hydraulic conductivities (Fig. 1). This suggests that near saturated hydraulic conductivity is a better indicator of potential upflow rates, with distances between the root zone and water table that maintain potential transpiration rates increasing with higher near saturated hydraulic conductivities. These findings are in accordance with those of Talsma (1963) who found that hydraulic conductivities at low suctions are more important for determining limiting flux rates for water table depths, unless hydraulic conductivities decrease slowly with increasing suction. Guidelines for irrigation requirements in some situations will be dependent on the rooting depth and root length density found in the soil profile. These findings are in accordance with previous modelling studies (Thorburn and Meyer, 1997), and also with theoretical predictions that actively extracting roots can increase capillary rise fluxes (Thorburn, 1997). Prior investigation of the sensitivity of capillary upflow rates to a range of soil and crop input parameters in the SWIM model indicated that during full plant development rates of capillary upflow are sensitive to the root length density in close proximity to the water table (Thorburn and Meyer, 1997). Results presented in this paper suggest that an increase in root length density had a much greater effect on steady state transpiration rates when roots are closer to the water table. At larger distances, steady state transpiration rates were influenced more by soil characteristics, and especially the soil hydraulic characteristics. Despite the model sensitivity to root length density and depth of rooting, model output can be used to provide a range of transpiration rates that water tables will be able to meet. This information when combined with local knowledge, such as barriers to root proliferation in the subsoil, can be used to develop site specific solutions. The sugarcane crop has been represented in the simulations in this study through (1) the root length densities and depths that specify the demand for soil profile water, and

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(2) the potential evapotranspiration conditions which occur in sugarcane growing areas of Australia. The results of these simulations will be applicable to other crops with similar rooting characteristics and localities with similar potential evapotranspiration conditions. For example, similar root length densities and rooting depths have been measured for irrigated maize (Liedgens and Richner, 2001; King et al., 1995), and lucerne (Smith et al., 1996). As irrigation is commonly conducted in arid areas, the conditions of environmental demand used in our study may be reasonably common. At lower evaporative demands, the predictions of possible irrigation reductions will be conservative. Irrigation scheduling using concepts of soil water availability, such as readily available water or plant available water, is unlikely to be useful for systems with shallow water tables. The concept of replenishing a soil water reservoir as it becomes depleted is unable to be applied to root zones where upflow is a significant component in the water balance and depletion of stored soil water is not evident. For example, the upper half of the root zone of the Oakwood soil with a water table at 1.5 m (Fig. 2c) is extremely dry and water is unlikely to be available to plants. While the lower half of the root zone is close to saturation, the low root densities at these depths prevent extraction of enough water to meet potential transpiration rates. The Australian sugar industry has a current target of increasing water use efficiency by 6% (Anonymous, 1999). The scenarios modelled in this study suggest that in areas with fresh shallow water tables, increasing reliance on this potential resource could help meet this target. Given that shallow water tables are currently not incorporated in root zone water balances for sugarcane crops in Australia, there are major opportunities for increasing irrigation water use efficiency in several sugarcane growing areas. In areas where irrigation allocations depend on the reliability of irrigation supplies, allocations can range between 24 and 74% of the required irrigation (Tilley and Chapman, 1999; Holden, 1998). Improved ability to draw on shallow water tables as a source of crop water in these areas will enable more efficient use of restricted water, and potentially increase yields. The current recommended pan factor for irrigation scheduling in the sugar industry during full canopy development is 0.85 (Holden, 1998). Average measured daily evapotranspiration rates can be as high as 7.4 mm during the summer months for sugarcane growing areas in Australia (Australian Bureau of Meteorology, http://www.bom.gov.au/). Thus over 100 days the cumulative equivalent evapotranspiration could be as high as 740 mm. If all sugarcane irrigation water requirements could be supplied by shallow water tables, this would lead to substantial water savings from surface storages in major irrigation areas. The long-term benefits of reduced surface irrigation above water tables will be dependent on the source of water table recharge. If a shallow water table is as a result of over-irrigation, then reducing surface irrigation will result in the lowering and eventual disappearance of the water table. While this provides a short-term benefit in terms of water supply, a long-term benefit is also achieved in terms of reducing salinisation of the root zone. If the local discharge and recharge is small in comparison to the catchment recharge and discharge characteristics, then the shallow water table is a natural phenomenon that would occur regardless of our cropping system characteristics. This is the suspected case in many of the coastal growing areas, where sugarcane crops are grown on areas once vegetated by tea tree species. Under these conditions, there is the potential for long-term use of the water table. In either case, the irrigator would be required to monitor the depth of the water table to


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determine the appropriate level of irrigation reduction. If the water table is not taken into account, then the risk of recharging the shallow water table from irrigation water is high. Such an inefficient use of surface irrigation water will contribute to raising the water table and an increasing risk of associated problems such as waterlogging and salinity. Caution should be exercised in areas with potential acid sulphate soils, or where there is a danger of reducing the water table to below sea level height in areas where salt water intrusion may occur. Based on the findings of this study it is recommended that additional work be carried out to investigate a broader range of conditions, such as heterogeneous soils, different water qualities, and different potential evapotranspiration rates, and to assess predictions of upflow with field measurements. The soils chosen in this study, while representing a range of soils found in the Bundaberg sugar region, do not include layered soils or soils with extremely low permeabilities. A plant growth model coupled with the type of soil water balance modelling used in this study could also be used to explore plant physiological responses to the presence of shallow water tables, particularly with regards to sucrose accumulation. 5. Conclusions There is scope for increasing the irrigation water use efficiency in areas of irrigated sugarcane where fresh shallow water tables are present. Both previous experimental studies and our simulation study suggest that mature sugarcane can uptake a large proportion of their water requirement from shallow water tables. For the uniform soil types investigated in our simulations, sugarcane will not require surface irrigation when fresh water tables are within 1 m of the soil surface. This rule would be applicable to other crops with maximum root length densities greater than 0.01 mm/mm3 and rooting depths of at least 0.8 m. Our data also suggests that for uniform soil types: (a) if near-saturated hydraulic conductivities are high, then deeper water tables (>1 m) may also be capable of meeting the water requirements of mature sugarcane; (b) water tables as deep as 2 m are still likely to have a positive impact on the sugarcane water balance; and (c) where irrigation is required, the amount will be dependent on local rooting characteristics such as rooting depth and density. Further refinement of these findings will require additional work to address issues such as soil layering, and how sensitive predictions of required irrigation are to hysteresis and preferential flow. Investigation of plant responses to water tables of varying water quality is also required. As competition for water increases, along with escalating water consumption costs, practical management guidelines for irrigating in the presence of a shallow water table will be of major benefit to the Australian sugarcane and other agricultural industries. Modelling and scenario analysis can be used to help generate these guidelines, provided that estimates of irrigation requirements for water tables below 1 m take into account the potential influence of rooting distributions. Acknowledgements The senior author acknowledges the financial assistance of a CRC for Sustainable Sugar Production postgraduate award. This work was also supported in part by

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CSIRO and the Land and Water Australia National Program for Irrigation Research and Development.

References Anonymous, 1999. Rural Water Use Efficiency Initiative Milestone Report One for the Sugar Industry. Queensland Department of Natural Resources, Brisbane, Australia. Ayars, J.E., Hutmacher, R.B., 1994. Crop coefficients for irrigating cotton in the presence of groundwater. Irrigation Sci. 15, 45–52. Ball-Coelho, B., Sampaio, E.V.S.B., Tiessen, H., Stewart, J.B., 1992. Root dynamics in plant and ratoon crops of sugar cane. Plant Soil 142, 297–305. Benz, L.C., Doering, E.J., Recihman, G.A., 1984. Water-table contribution to alfalfa evapotranspiration and yields in sandy soils. Trans. ASAE 27, 1307–1312. Bristow, K.L., Campbell, G.S., Calissendorff, C., 1984. The effects of texture on the resistance to water movement within the rhizosphere. Soil Sci. Soc. Am. J. 48, 266–270. Brooks, R.H., Corey, A.T., 1964. Hydraulic Properties of Porous Media. Hydrological Paper No. 3. Civil Engineering Department, Colorado State University, Fort Collins, CO, USA. Brooks, R.H., Corey, A.T., 1966. Properties of porous media affecting fluid flow. J. Irrigation Drain. Div. Am. Soc. Civil Eng. 92 (IR2), 555–560. Chabot, R., Bouarfa, S., Zimmer, D., Chaumont, C., Duprez, C., 2002. Sugarcane transpiration with shallow water-table: sap flow measurements and modelling. Agric. Water Manage. 54, 17–36. Danielson, R.E., 1967. Root systems in relation to irrigation. In: Hagan, R.M., Haise, H.R., Edminster, T.W. (Eds.), Irrigation of Agricultural Lands. American Society of Agronomy, Wisconsin, pp. 390–413. Eavis, B.W., 1971. Effects of flooding on sugarcane growth. 2. Benefits during subsequent drought. In: Proceedings of the 14th ISSCT Congress, pp. 715–721. Escolar, R.P., Allison, W.F., Juan Jr., Juarez, 1971. The effect of water table depth on the yield of sugarcane. In: Proceedings of the 14th ISSCT Congress, pp. 722–726. Gosnell, J.M., 1971. Some effects of a water-table level on the growth of sugarcane. In: Proceedings of the 14th ISSCT Congress, pp. 841–849. Grimes, D.W., Sharma, R.L., Henderson, D.W., 1984. Developing the Resource Potential of a Shallow Water Table. Californian Water Resources Center Bulletin No. 188, August 1984. Gupta, R., Yadav, R.L., 1993. Ground water contribution to evapo-transpiration of sugarcane during summer. Cooperative Sugar 25, 113–115. Holden, J.R., 1998. Irrigation of Sugarcane. Bureau of Sugar Experiment Stations, Brisbane, Australia. Hunsigi, G., Srivastava, S.C., 1977. Modulation of ET (evapotranspiration) values of sugar cane because of high water table. In: Proceedings of the 16th ISSCT Congress, pp. 1557–1564. Hutson, J.L., Cass, A., 1987. A retentivity function for use in soil-water simulation models. J. Soil Sci. 38, 105–113. Inman-Bamber, N.G., Muchow, R.C., Holden, J.R., Robertson, M.J., Ham, G.J., 1998. Soil water extraction by sugar cane beyond the readily available limit. In: Proceedings of the Australian Society of Sugar Cane Technologists, vol. 20, pp. 112–117. Inman-Bamber, N.G., Robertson, M.J., Muchow, R.C., Wood, A.W., Pace, R., Spillman, A.M.F, 1999. Boosting yields with limited irrigation water. In: Proceedings of the Australian Society of Sugar Cane Technologists, vol. 21, pp. 203–211. Juang, T.C., Uehara, G., 1971. Effects of ground-water table and soil compaction on nutrient element uptake and growth of sugarcane. In: Proceedings of the 14th ISSCT Congress, pp. 679–687. Kang, S., Zhang, F., Hu, X., Jerie, P., Zhang, L., 2001. Effects of shallow water table on capillary contribution, evapotranspiration, and crop coefficient of maize and winter wheat in a semi-arid region. Aust. J. Agric. Res. 52, 317–327. King, C.A., Schwamberger, E.C., Wallace, M., Smith, D.J., Meyer, W.S., Thorburn, P.J., 1995. Water Use, Plant Development, and Capillary Upflow from Saline Water Tables in Four Soils for Irrigated Maize. Technical Report 95.8 CSIRO Division of Water Resources, Canberra, Australia.


19

Kingston, G., 1993. Geo-hydrology of Soil and Water Salinity in the Maryborough Basin. Unpublished Ph.D. Thesis, Griffith University, Brisbane, Australia. Kingston, G., 1994. Benchmarking yield of sugarcane from estimates of crop water use. In: Proceedings of the Australian Society of Sugar Cane Technologists, vol. 16, pp. 201–209. Liedgens, M., Richner, W., 2001. Minirhizotron observations of the spatial distribution of the maize root system. Agron. J. 93, 1097–1104. Marshall, T.J., Holmes, J.W., Rose, C.W., 1996. Soil Physics, 3rd ed. Cambridge University Press, New York. Meyer, W.S., Prathapar, S.A., Barrs, H.D., 1989. Water flux to and from shallow water tables on two irrigated soils. In: Humphreys, E., Muirhead, W.A., vanderLilij, A. (Eds.), Management of Soil Salinity in South East Australia. Australian Society of Soil Science, Inc., Riverina Branch, Conference Proceedings, Albury, Australia, 18–20 September. Omary, M., Izuno, F.T., 1995. Evaluation of sugarcane evapotranspiration from water table data in the Everglades agricultural area. Agric. Water Manage. 27, 309–319. Patel, K.R., Joshi, R.S., 1985. Response of sugarcane to different levels of irrigation under high water table conditions. Madras Agric. J. 72, 577–581. Pitts, D.J., Myhre, D.L., Shih, S.F., Grimm, J.M., 1990. The effect of two water-table depths on sugarcane grown on a sandy soil. Soil Crop Sci. Soc. Fl. 49, 54–59. Pitts, D.J., Myhre, D.L., Tsai, Y.J., Shih, S.F., 1991. Effects of water-table depth on water relations and yield for sugarcane grown in sand. Am. Soc. Sugar Cane Technol. 11, 29–37. Prathapar, S.A., Robbins, C.W., Meyer, W.S., Jayawardane, N.S., 1992. Models for estimating capillary rise in a heavy clay soil with a saline water table. Irrigation Sci. 13, 1–7. Richards, L.A., 1931. Capillary conduction of liquids through porous media. Physics 1, 318–333. Rudd, A.V., Chardon, C.W., 1977. The effects of drainage on cane yields as measured by water-table heights in the Macknade mill area. In: Proceedings of the Queensland Society of Sugar Cane Technologists, 44th Conference, pp. 111–117. Shih, S.F., Gascho, G.J., 1980. Water requirement for sugarcane production. Trans. ASAE 23, 934–937. Smith, D.J., Meyer, W.S., Barrs, H.D., Schwamberger, E.C., 1996. Lucerne Daily Water Use and Capillary Upflow Using Weighing Lysimeters: Effects of Soil Type and Groundwater Salinity. Technical Report 96.6 CSIRO Division of Water Resources, Canberra, Australia. Sweeney, C.A., Thorburn, P.J., Bristow, K.L., 2001. Water table contributions to sugarcane production: impacts on irrigation water use efficiency. In: Proceedings of the Australian Society of Sugar Cane Technologists, vol. 23, pp. 116–121. Talsma, T., 1963. The control of saline groundwater. Meded Landbouwhogeschool Wageningen 63, 1–68. Thorburn, P.J., 1997. Land management impacts on evaporation from shallow saline water tables. In: Taniguchi, M. (Ed.), Subsurface Hydrological Responses to Land Cover and Land Use Changes. Kluwer Academic Publishers, Boston, pp. 6–71. Thorburn, P.J., Meyer, W.S., 1997. Response of simulated upflow from shallow water tables to variations in model parameter values. In: Taniguchi, M. (Ed.), Subsurface Hydrological Responses to Land Cover and Land Use Changes. Kluwer Academic Publishers, Boston, pp. 61–71. Thorburn, P.J., Cook, F.J., Bristow, K.L., 2003. Soil-dependent wetting from trickle emitters: implications for system design and management. In: Thorburn, P.J., Bristow, K.L., Annandale, J. (Eds.), Micro-irrigation: Advances in System Design and Management. Irrigation Science (in press). Tilley, L., Chapman, L., 1999. Benchmarking Crop Water Index for the Queensland Sugar Industry: Desktop Audit Stage 1 Final Report. Queensland Department of Natural Resources, Brisbane. Torres, J.S., Hanks, R.J., 1989. Modeling water table contribution to crop evapotranspiration. Irrigation Sci. 10, 265–279. Verburg, K., Ross, P.J., Bristow, K.L., 1996. SWIMv2.1 User Manual. Divisional Report No. 130, CSIRO Division of Soils, Canberra, Australia. Verburg, K., Bridge, B.J., Bristow, K.L., Keating, B.A., 2001. Properties of Selected Soils in the Gooburrum— Moore Park Area of Bundaberg. Technical Report 09/01, CSIRO Land and Water, Canberra, Australia.