Spatial distribution of available nitrogen under center pivot sprinklers

Abstract

The water and nitrogen application from center pivot irrigation systems, in most situations, is assumed to be uniform; however, significant changes in the water and N distribution can occur with both location and time. This study was conducted to determine whether water and N spatial distribution, along with physical and chemical conditions of soil can be used to determine spatial application patterns of nutrients that will minimize leaching and maintain an economic yield under two farmer-operated fields irrigated by center pivot. Pivot position, irrigation mainline pressures and flows, and depth of fertilizer in the supply tank were recorded throughout the irrigation seasons (1998 and 1999) so that spatial distribution of water and N application could be estimated by using USDA's Center Pivot Evaluation and Design (CPED). Opus simulation model was used to simulate spatial distribution of leaching for both fields, using CPED output as input data in conjunction with plant, soil and weather data required by the model. Although farmers historically assumed that each component of fertilizer is uniformly distributed at some average value, the results of my research show considerable variability in N potentially available to the crop. Fertigation was the largest single source of N applied across each field. For field 6 the fertigated amount varied from 70 to 180 kg-N/ha in 1998 and from 150 to 225 kg-N/ha in 1999, and for field 39 varied from 73 to 210 kg-N/ha. Estimated seasonal potentially available N showed a high overapplication of N in certain areas and a low overapplication under other areas of the field, which explains the variability of leaching obtained in the simulations, ranging for more significant areas from 20 to 40 kg-N/ha on field 6 and from 3 to 20 kg-N/ha on field 39 in 1999. A cluster analysis was done with the input and output variables in order to identify areas that can be managed similarly in both fields. The variability of N applied by fertigation suggests that uniformity of irrigation is important to optimize N fertilizer application and to reduce potential leaching. In addition, improved irrigation uniformity will help to make the management decisions easier.