| Analysis for the required degree of soil spatial variability is lacking to justify the development of center pivot systems (CPSs) for variable rate applications. This study developed an analytical system to examine whether a cornfield under CPS could be justified for variable applications of water and nitrogen for spatially varied physical properties of soil. The model also helps to identify optimum irrigation-nitrogen (I&N) management strategy for a field.; The solution was a numerical simulation of center pivot movements for I&N applications based on predominant soils of discretized blocks. The optimum block system was determined based on a sustainable yield goal and an allowable maximum N leaching from the root zone. The analytical tool, CPCERES, was developed to integrate a crop model, CERES-Maize, and a CPS model, CPSM. CERES-Maize is good for point analysis and CPSM has power for spatial analysis. CERES-Maize was modified for integration, and a routine was added to CERES-Maize for automatic N scheduling. CPSM was developed based on several movement-control rules and constraints of a commercially available CPS.; The study examined 24 cases, combining six irrigation and N management strategies and five soils with different degrees of spatial variability. Results showed that divisions of a field (of 50 ha in area) into a maximum of four sectors produced maximum profit, based on a partial cost analysis. This outcome suggests no special enhancement of a CPS for soil-specific resource management.; Simulated N leaching (NL) was negligibly small even when I&N was applied at a high rate, allowing minimum plant stresses due to N and water deficiencies. Four controlling measures for water and nitrogen applications may be the reasons for this small NL: each N application was limited to 25 to 40 kgN/ha. Advanced N application from the day plants needed nitrogen most was limited to one or two days. Water added with each N application was equivalent to that required to fill the first 30 cm of soil layer to field capacity. The depth of water, in no case, was allowed to exceed 35 mm. |
