— 23 IREC Farmers' Newsletter No. 200 — Spring 2018 Trials across the cotton growing valleys in 2017–18 aimed to quantify the runoff component of the nitrogen loss pathway, and to highlight the relationship between irrigation and nitrogen management. BOOSTING the nitrogen use efficiency and water use efficiency of cotton farms are key objectives of the Australian cotton industry. Research by Jon Baird (2016) and Ben McDonald (2017) identified up to 10% fertiliser nitrogen losses in the first two irrigations, and these were influenced by pre-season fertiliser placement, deep drainage and runoff. Measurements and monitoring Trials were conducted in each cotton valley by CottonInfo’s team of regional extension officers during the 2017–18 season were set up with two irrigation treatments (single siphons and double siphons) and four replicates of each treatment. Soil cores were taken to measure soil nitrogen at the start of the season and again after the second irrigation. Cores were taken from the head ditch and the tail drain ends of the field, and split in two, based on depth: 0–30 cm and 30–60 cm. The first two irrigations (watering-up/pre-irrigation and the first in-crop irrigation) were monitored to determine how much nitrogen went on in the irrigation water and how much ran off in the tailwater. Water samples were collected, however to determine the amount of nitrogen applied and lost as runoff, we also needed to know the volume of water applied. The Irrimate suite of tools was used for the irrigation evaluations. A siphon flow meter was used to measure flow rate and total volume of water applied; and advance sensors measured the advance time as the water moved down the length of the field. The data collected was sent to National Centre for Engineering in Agriculture, where Malcolm Gillies and Joe Foley ran it through a model they developed called SISCO. This model simulates the irrigation event based on the data collected in the field. The output of the model included the volume of irrigation water applied, volume of tailwaters, volume of water that infiltrated into the soil profile and the volume of water that was lost as deep drainage (i.e. moved out of the root zone). This information allowed us to calculate the amount of nitrogen that ran off in the tailwater. Using SISCO we could also evaluate the performance of the irrigation event and compare the performance of the single siphons and double siphons in terms of: l application efficiency, which relates the amount of water applied in an irrigation to the amount of water available to the crop. A high efficiency means that most of the water applied has been retained in the root zone (not lost as deep drainage or runoff). l requirement efficiency, which describes how well the soil deficit is met. This measure need not be 100%, but anything less means deficit irrigation is occurring, i.e. the profile is not being filled completely along the length of the field. l distribution uniformity, which is a measure of how evenly the water has been applied. The aim is to have distribution uniformity as high as possible. Flow rate, advance time and total volume of water applied were measured in CottoInfo trials set up to measure nitrogen loss in irrigation runoff. — 23 IREC Farmers' Newsletter No. 200 — Spring 2018 Save water and nitrogen on cotton