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

Targeting certain soils and cropping systems may be necessary in consideration of regional water quality protection policies. However, little information is available relating soils and cropping practices to regional water quality problems. This study evaluates crop yield and NO3-N movement to surface and groundwater on four soils and nine principal cropping systems in the High Plains region of Oklahoma. The cropping systems involve wheat (Triticum aestivum L), grain sorghum [Sorghum bicolor (L.) Moench], and corn (Zea mays L.), and are part of a regional data base also containing soils and chemical management information. For each combination of crop, soil, cropping system, and chemical alternative, a 20-yr simulation was made. The simulation was based on a modeling system that includes EPIC-PST (crop growth/chemical movement model) 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 compared with continuous irrigated wheat and grain sorghum cropping systems, double cropped wheat-grain sorghum resulted in greater NO3-N loss in percolation. Compared with sprinkler and LEPA (low energy precision application) irrigation systems, furrow irrigation resulted in high NO3-N loss on both fine-textured and coarse-textured soils, with significantly greater loss on the coarser textured soils. The modeling framework 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 furrow irrigation.