Irrigation and nitrate movement evaluation in conventional and alternate-furrow irrigated cotton

One of the important criteria in determining best management practices for irrigated agriculture is understanding the interaction between irrigation s ystem performance and the movement of water and solutes through the soil. T he objective of this four-year study was to evaluate the irrigation efficie ncy (IE) and the potential movement of applied nitrogen for a commercial co tton field in Arizona. In the first two cotton seasons, the grower used a c onventional furrow irrigation practice consisting of 1.0-m row spacing and irrigation in every furrow. To reduce the amount of surface water runoff (R O) observed during the first two seasons, the grower changed to alternate-f urrow irrigation and narrowed the row spacing to 0.8 m in the third and fou rth seasons. Measurements of irrigation inflow and outflow and estimates of daily crop evapotranspiration were used to calculate the amount of RO, dee p percolation (DP), and soil water storage during each irrigation event, wh ich in turn were used to estimate the IE attained in each cotton season. Po tassium bromide (KBr) was applied to 15 sample sites at the start of the gr owing season in each year to monitor the movement of bromide (Br-). Prior t o the application of KBr and after harvest for each year, soil samples were taken from the sample plots to a depth of 2.7 m and analyzed for bromide a nd nitrate. Runoff of applied irrigation water decreased from an average of 21% under the conventional practice to 13% after changing to alternate fur rows. However, since the average fraction of irrigation water lost to DP wa s increased from 18% to 22% under the alternate-furrow practice, the overal l gains in IE (3% to 4%) were small. Furthermore, bromide and nitrate leach ing below the cotton root zone appeared to be somewhat increased with the a lternate-furrow practice. Implementing a tailwater recovery system and impr oving irrigation scheduling would potentially increase IE and reduce the ov er-irrigation and nitrate leaching observed for the commercial cotton produ ction system.