IRRIGATION AND NITRATE MOVEMENT EVALUATI ON IN CONVENTIONAL AND ALTERNATE?FURROW IRRIGATED COTTON

Abstract

One of the important criteria in determining best management practices for irrigated agriculture is understandingthe interaction between irrigation system performance and the movement of water and solutes through the soil. The objectiveof this fouryear study was to evaluate the irrigation efficiency (IE) and the potential movement of applied nitrogen for acommercial cotton field in Arizona. In the first two cotton seasons, the grower used a conventional furrow irrigation practiceconsisting of 1.0m row spacing and irrigation in every furrow. To reduce the amount of surface water runoff (RO) observedduring the first two seasons, the grower changed to alternatefurrow irrigation and narrowed the row spacing to 0.8 m in thethird and fourth seasons. Measurements of irrigation inflow and outflow and estimates of daily crop evapotranspiration wereused to calculate the amount of RO, deep percolation (DP), and soil water storage during each irrigation event, which in turnwere used to estimate the IE attained in each cotton season. Potassium bromide (KBr) was applied to 15 sample sites at thestart of the growing season in each year to monitor the movement of bromide (Br ). Prior to the application of KBr and afterharvest for each year, soil samples were taken from the sample plots to a depth of 2.7 m and analyzed for bromide and nitrate.Runoff of applied irrigation water decreased from an average of 21% under the conventional practice to 13% after changingto alternate furrows. However, since the average fraction of irrigation water lost to DP was increased from 18% to 22% underthe alternatefurrow practice, the overall gains in IE (3% to 4%) were small. Furthermore, bromide and nitrate leachingbelow the cotton root zone appeared to be somewhat increased with the alternatefurrow practice. Implementing a tailwaterrecovery system and improving irrigation scheduling would potentially increase IE and reduce the overirrigation and nitrateleaching observed for the commercial cotton production system.