The Cotton Research and Development Corporation have demonstrated a strong commitment to supporting research in water use efficiency across the industry. One component of water use in irrigation is the identification of the risk of water being wasted through deep drainage, which is defined as the rate of water that is lost below the depth of plant roots. The cotton industry also recognises that excessive deep drainage can also potentially lead to the development of shallow water tables often blamed for root zone salinisation affecting crop performance. There are many methods that exist to estimate the deep drainage however they need specialised and expensive instrumentation and, the measurement is often tedious requiring highly specialised skills. With this in mind the CRDC supported this project with the charge to develop a cheap and easy protocol to estimate the risk of deep drainage. The method should not require any specialised equipment, should not be time consuming, and can be operated by growers and IDO’s within the industry.

The initial work involved the successful completion of experiments that determined the minimum number of observations that need to be taken to in a cotton field to give a good estimate the potential deep drainage and the implementation of a modified sub-soil hydraulic conductivity using the falling head lined-borehole technique (FHLBT) appropriate for a cotton growing system. The readily available Microsoft EXCEL was chosen as the platform to develop the user-friendly interface for managing the data to estimate the potential and required deep drainage. Testing of the method was conducted on Field 11 at Auscott Moree, which was chosen as this site is representative of the soil in the cotton growing area, having heavy clays derived from alluvial material and less clayey and/or leaky soil traversing the field. Results showed that the method could successfully identify the significant difference in the potential deep drainage occurring within Field 11, corresponding to soil with different soil clay contents and leaky areas. To make the estimate of potential deep drainage more meaningful the Potential and Required Deep Drainage Interface incorporates a leaching requirement that is needed to prevent excess salts build up in the sub-soil that may affect crop.

The main outcomes to benefit the cotton industry are:

• the Development of a protocol that can be used to assess the potential deep drainage on a cotton farm

• demonstrating the importance of strategically placing sampling sites to capture the within field soil variability, which will increase the opportunity for a better representation of the potential deep drainage.

• the development of a user-friendly interface in commonly available software to calculate the potential deep drainage.

• and the incorporation a module into the Potential and Required Deep Drainage software to allow growers to determine the drainage required to avoid subsoil salinisation that could potentially reduce crop performance.