IRRIGATION QUANTITY AND UNIFORMITY AND NITROGEN APPLICATION EFFECTS ON CROP YIELD AND NITROGEN LEACHING

The combined effects of irrigation and N management on crop yield and NO3- leaching have not been extensively investigated. The objective of this study was to quantify the relationships between irrigation manag ement (including uniformity) and N management on corn (Zea mays L(v)) yield and NO3- leaching. Yield and N leaching were simulated using the CERES-Maize (version 2.10) model for various combinations of irrigati on amounts and uniformity and N amount and timing of split N applicati ons for semiarid conditions typical of Tulare County in California. Si mulated grain yield increased, reached a plateau, and then decreased w ith increase in applied water under uniform irrigation. The amount of applied water above which yield decreased was higher for the higher N application rate and the later simulated split N application. The simu lated amounts of N leached were consistent with the yield results. The higher water applications that lead to reduced yields were associated with higher N leaching for a given N application amount. The effects of irrigation were simulated assuming Christensen's Uniformity Coeffic ient (CUC) of 100, 90, and 75. The results were only slightly affected by CUC = 90 compared with 100. A CUC of 75 caused a reduction in yiel d and increase in N leaching compared with uniform irrigation. The low est CUC required a higher N application to achieve the same yield as u niform irrigation. Under nonuniform irrigation, it is impossible to ma nage either water or N application in a manner to achieve high yields without considerable NO3- leaching. High yield and low NO3- leaching a re compatible goals and can be achieved by appropriate irrigation and fertilizer management for irrigation systems that have a CUC of 90 or greater.