Deep drainage below the root zone is still the least understood component of the water balance, especially in cracking clay soils. It represents a waste of a valuable resource and can leach nitrogen out of the root zone. It has the potential to cause watertables to rise, with the accompanying risk of salinity. Drainage can move contaminants, such as salt and agrochemicals, into the groundwater.

The lysimeter facility at the Australian Cotton Research Institute, near Narrabri NSW, was used to study drainage, its contaminants and its interaction with groundwater in a heavy clay soil under a furrow-irrigated cotton – wheat rotation from 2006 to 2011.

Drainage during the cotton seasons varied from 0 – 74 mm, under wheat it was negligible and under fallow it was 23 mm. Drainage occurred in two forms: matrix drainage and by-pass drainage. The former occurs when the water storage capacity of the soil is filled due to prolonged rainy periods with any extra water becoming drainage. Drainage rates are not high (<0.5 mm/day) but can continue for periods of a month.

By-pass drainage occurs after furrow irrigation when water flows rapidly down macropores and by-passes the matrix of the subsoil. Peak drainage rates are reached 25 hours after irrigation and can reach more than 3 mm/day. The rate then declines exponentially over a week to about 0.5 mm/day. The amount of by-pass drainage appears to be controlled by the soil water deficit in the upper metre of soil. Drainage increases as the 0 – 0.5 m layer becomes drier, possibly due to greater cracking. However, larger deficits in the 0.5 – 1.0 m layer decrease drainage and appear important in mitigating by-pass drainage.

The risk of by-pass drainage is greatest when irrigation is necessary early in the cotton season, when the crop is too small to create a subsoil deficit between irrigations, especially if the subsoil was already wet before sowing.

The risk of matrix drainage can be minimized by managing both the rotation and irrigation scheduling to ensure there is sufficient deficit to accommodate likely inputs of water and irrigation at any time of year. Nevertheless there will always be times of above average rainfall when the profile is filled to capacity and drainage occurs.

However, some drainage is necessary to leach salts from the irrigation water that accumulate in the root zone. The electrical conductivity (EC) of matrix drainage is greater than by-pass drainage, suggesting matrix drainage is more efficient at leaching salt.

In addition to salt, drainage leaches nitrogen from the topsoil. During the 2008/09 cotton season approximately 9.5 kg N/ha – equivalent to 6% of that applied as fertilizer – was lost in drainage.

Seasonal drainage from the root zone appears to recharge the watertable at 16 m depth within weeks, although this result is still tentative. There is continuous downwards leakage of salty water from the upper, watertable aquifer into the lower confined aquifer, from which water is extracted for a variety of uses. This leakage is exacerbated by pumping from the lower aquifer.

The lysimeter was also used to test less expensive methods of estimating drainage. A barrel lysimeter installed near the lysimeter facility overestimated drainage, whereas the chloride mass balance method underestimated drainage.

Water management in the cotton industry has continued to be of fundamental importance to sustainability and production. Dealing with climate variability that requires the tools of water management be more sensitive to atmospheric factors. The IrriSAT suite of tools integrate atmospheric measurements with plant vigour measurements, creating tools which reflect very well the current climate, including extremes and spatial variability in crops.

The IrriSAT tools capture a foundation dataset for the reporting and management of variability of plant water use. IrriSAT is a weather based irrigation scheduling service. It uses satellite imagery to better determine crop coefficients that are needed to calculate crop water use. The system uses on-ground weather stations to measure sunlight hours and intensity, cloud cover, rainfall and wind which are all used to calculate a potential water use in the past 24 hours. This information when combined with the satellite-determined crop coefficient for a particular crop allows an actual water use figure to be calculated on a site specific basis for low cost.

During the 2010/2011 irrigation season an initial trial was undertaken to assess the potential of the IrriSAT system for use in water management in cotton production systems. Ten consultants took part in the trial with 304 individual paddocks or blocks being monitored. Total area monitored was approximately 20 000 ha.

Based on the results of the trials and feedback received from the trial participants there were three potential applications of the IrriSAT technology identified and initially assessed for their useability and function which could benefit the cotton industry. These are:

1. Providing site specific irrigation scheduling information

2. Regional water use benchmarking

3. In season yield forecasting

Each one of these components has clear benefits for the cotton industry. From this initial trial it was clear that there is significant interest in the approach by water managers in the cotton industry and real benefits for water management.

The experiments reported here investigated the influence of high or low concentrations of nitrogen, phosphorus and potassium on the multiplication of X.campestris pv. malvacearum in cotton leaf tissue.

The Cotton Growing Practices 2016 survey was conducted in Spring 2016 by Roth Rural on behalf of CRDC. It gathered information on farming practices and views of cotton growers about the 2015-16 season on the issues of nutrition, plant growth regulation, farm hygiene, chemical application management, harvesting, solar energy, CottonInfo and participation in research trials. This information is collated and analysed as an industry data set to help inform cotton research and industry policy and provide information for cotton growers about management practices across their industry.

Survey responses were received from 176 growers, representing 35 per cent of the irrigated and 15 per cent of the dryland 2015-16 crop area.