IMPORTANCE OF SOIL AND CROPPING SYSTEMS IN THE DEVELOPMENT OF REGIONAL WATER-QUALITY POLICIES

Targeting certain soils and cropping systems may be necessary in consi deration of regional water quality protection policies. However, littl e information is available relating soils and cropping practices to re gional water quality problems. This study evaluates crop yield and NO3 -N movement to surface and groundwater on four soils and nine principa l cropping systems in the High Plains region of Oklahoma. The cropping systems involve wheat (Triticum aestivum L), grain sorghum [Sorghum b icolor (L.) Moench], and corn (Zea mays L.), and are part of a regiona l data base also containing soils and chemical management information. For each combination of crop, soil, cropping system, and chemical alt ernative, a 20-yr simulation was made. The simulation was based on a m odeling system that includes EPIC-PST (crop growth/chemical movement m odel) interfaced with ii Geographic Information System (GIS), Earthone . Results of each simulation included crop yield and NO3-N movement in runoff and percolation. Results show wide variations in NO3-N losses for different soils, irrigation systems, and cropping systems. When co mpared with continuous irrigated wheat and grain sorghum cropping syst ems, double cropped wheat-grain sorghum resulted in greater NO3-N loss in percolation. Compared with sprinkler and LEPA (low energy precisio n application) irrigation systems, furrow irrigation resulted in high NO3-N loss on both fine-textured and coarse-textured soils, with signi ficantly greater loss on the coarser textured soils. The modeling fram ework can be used to compare alternative water quality policies. Broad policies such as a restriction on the amount of N that can be applied per hectare can be compared with targeted policies, such as Limiting N applications or irrigation water use on coarser soils or under furro w irrigation.