GIS and Cluster Analysis of NO3-N Leaching Losses to Subsurface Drainage Water

The spatio-temporal patterns of NO₃-N leaching losses from a watershed is an outcome of the complex interaction among soil and landscape attributes, management practices and the changing climatic patterns from year to year. A long-term field study was conducted from 1993 through 2003 to investigate the impact of climate, soil and landscape attributes and management practices on the export of NO-N leaching losses from agricultural fields having subsurface drainage ´tile´ systems. The study area is located at the Iowa State University´s northeastern research center near Nashua, Iowa, USA, on the glacial till derived soils. The 36 field experimental plots, each of 0.4 ha in size, have been under various experimental treatments during the study period and each treatment was replicated three times in a randomized complete block design. The field data collected on subsurface drainage and NO-N leaching losses were normalized on a yearly basis using median and interquartile range as the robust estimates to mean and standard deviation. The normalized data were used in cluster and discriminant analysis and building GIS data layers to determine the integrated effects of the soil and landscape attributes on spatial NO-N leaching losses clusters. The spatial clusters of subsurface drainage and NO-N leaching losses were correlated with soil type and digital terrain model (DTM) derived data layers of slope, aspect and curvature. The stepwise discriminant analysis identified the soil and curvature as the significant variables discriminating the subsurface drainage clusters. Similarly, the topographic attributes of elevation, slope, aspect and curvature contributed significantly in discriminating the NO-N leaching losses clusters. Map overlay of the identified soil and topographic data layers on NO-N leaching losses clusters showed the spatial relationships of the soil and topography of the area with NO-N leaching losses to the flow. The results of the study suggest th- at delineation of the spatial management zones across the watershed based on the soil and landscape attributes can promote long-term sustainability of the production agriculture