Water is essential for producing high yielding and high quality cotton and cotton is demanding in its water consumption requiring, on average, 5.2 ML/ha for peak production in Australia (Cotton Australia 2015). With the majority of the Australian cotton industry being water limited and changing climatic conditions indicating continued water scarcity, increased research into producing more cotton per unit of water has resulted. This approach is termed Water Use Efficiency (WUE) and is described by Howell (2000) as crop yield per unit of water use. Cotton crop yields would benefit from evaluation on bales produced per megalitre of water used, rather than on a per hectare basis (Roth et al. 2013).

In Australia water resource availability is highly variable due to the range of climates found across the country. The majority of Australia’s runoff (65%) is positioned in three drainage partitions located towards the far-north of the country, associated with tropical/sub-tropical climates. However, the majority of irrigated cotton (96%) is situated within the Murray Darling Basin (southern Queensland and New South Wales) which accounts for a mere 6.1% of national runoff (Murray Darling Basin Authority 2015). Hence, the majority of Australia’s irrigated cotton is concentrated where water resources are the most restricted (Chartres and Williams 2006; Roth et al. 2013).

Currently 16,660 GL of the 70,000 GL extracted in Australia is used in agriculture (Chartres and Williams 2006). In the 2011-12 season 14% (2231 GL) of this 16,660 GL was required to produce 4,240,000 bales (227 kg of lint per bale) of Australian cotton (Cotton Australia 2015). In general, the trend for Australian irrigated agricultural industries as a whole is increased water demand, highlighting that water availability is now the most limiting factor for not just cotton production (Chartres and Williams 2006; Roth et al. 2013), but agriculture in general.

Furthermore, irrigated agriculture will be integral to meeting demand if food production is to be doubled by 2050 (FAO 2009), as the challenge is to essentially produce more with less (Fraiture et al. 2007). The opportunity to grow crops close to their yield potential in areas which would otherwise be unable to, under rain fed conditions, is allowed for via irrigation. Hence, these crops primarily rely on irrigation water supply for optimal production (Howell 2000; Roth et al. 2013), meaning that in uncertain, or marginal, climatic conditions, minimising water losses is inextricably linked to production and profitability.

WUE involves the management of inputs and losses water. Inputs impact the volume of soil water contained within the profile (Roth et al. 2013). Characteristics such as porosity, bulk density and hydraulic conductivity affect a soils ability to hold water and a plant’s ability to access it (Radford et al. 2000). Inputs consist of irrigation and rainfall. Utilising rainfall reduces the irrigation requirement and improves irrigation efficiency (Cull et al. 1981). The effectiveness of these events is a function of soil infiltration and evaporative demand. Each cotton plant in a linear row has access to a certain volume of soil water in the row and inter-row space. The volume available is dependent on the row-spacing configuration. Wider row-spacings allow for greater access of soil moisture per plant (Roche et al. 2006). However, plant available soil moisture is limited by destructive management practices such as uncontrolled traffic (Chan et al. 2006).

The Australian cotton industry is currently considering a transition to 1.5 m row-spacing. The 1.5 m row benefits WUE, soil health and enterprise integration. Plants have access to larger volumes of soil water which increases utilisation of rainfall while reducing irrigations. Low plant densities per hectare would assist in reducing water input requirements (Enciso-Medina et al. 2002; Brodrick et al. 2012b). Current practice involves uncontrolled traffic in fields. This increases compaction and negatively impacts bulk density, mechanical impedance, porosity and hydraulic conductivity (Radford et al. 2000; Chan et al. 2006). The 1.5 m row-spacing enables Controlled Traffic Farming (CTF) where all machinery is driven on the same 3 m wheel tracks. Compaction is minimised to 15 – 20% of the total land area and allows for the soil structure to recover. Over time, water infiltration and root penetration will expand, demonstrated by an increase in yield (Tullberg 2000; Tullberg et al. 2007; Hamza and Anderson 2005; Antille et al. in press). The 1.5 m row-spacing enables enterprise integration through crop rotations with grains. Currently, 3 m is the factory standard wheel track width of a combine harvester. It is simpler for other machinery (e.g. pickers and spray rigs) to be adjusted to this track width than to adjust the combine. The combine is required during wheat harvest, the other major crop in a cotton rotation (Chan et al. 2006). Hence compaction is minimised on the entire farm all year round. The benefits of CTF are then experienced by all crops.

Conventional row-spacing in Australia is on 1.0 m rows (CRDC 2013). The reason being, that this was the width required for a mule to pass between crop rows with minimal trampling. Since then, multi-row cotton pickers have been arranged to fit this 1.0 m standard. Progressively plant breeding and cotton genetics followed suit and were evaluated on their ability to yield successfully in this configuration. CTF cannot be implemented in these systems as machinery would have wheels on a hill and a furrow in a 1.0 m row-spacing configuration (Masek et al. 2010) (Figure 1). Machinery implements would have to be offset to compensate for the three rows that would need to be picked in one run as opposed to the normal two. The centre for gravity would be quite high creating an engineering issue. Logistically this would become unnecessarily difficult (Tullberg et al. 2007), especially since 1.5 m wide row spacing accommodates for 3 m tracks comfortably.

In recent times other configurations have become increasingly more common (Clark and Carpenter 1992). These include Ultra Narrow Row (UNR) (< 0.4 m) and Narrow Row (0.75 m) (Roche et al. 2006). Row configurations influence yield, plant vigour as well as WUE (Figure 2). To determine the most suitable row spacing farm managers must consider a variety of factors. These include water availability, local climate, soil type and machinery logistics (Roth et al. 2013). Increasing or decreasing row spacing from the conventional 1.0 m can provide various advantages and disadvantages (Clark and Carpenter 1992). UNR and narrow row spacing reduces time to crop maturity (i.e. when the plant stops producing new fruit) and increases in yield per hectare (Brodrick and Bange 2010). This is important in regions where cotton seasons are particularly short (e.g. Riverina in NSW) (Jost and Cothren 2001; Brodrick et al. 2013). CTF can be implemented into a narrow row system but WUE is found to decrease with decreasing row-spacing, therefore it is not a suitable configuration for the water limited regions of northern NSW (Stone and Nofziger 1993). A substantial amount of research has been conducted on UNR and narrow row spacing in cotton (Clark and Carpenter 1992; Jost and Cothren 2001; Brodrick and Bange 2010; Brodrick et al. 2013). However little is known about the effect of 1.5 m row configurations on WUE, yield and fibre quality compared with conventional row spacing