Issues of spatial and temporal scale in modeling the effects of field operations on soil properties

Tillage is an important procedure for modifying the soil environment to enhance crop growth and conserve soil and water resources. Process-based models of crop production are widely used in decision support, but few explicitly simulate tillage. The Cropping Systems Model (CSM) was modified to simulate tillage and related field operations for single seasons or multiple years. This paper provides an overview of how the new routines were implemented and discusses issues of spatial and temporal scaling that influenced the underlying strategy. The processes considered included effects of crop residues on the soil surface and on chemical and physical properties that vary with soil depth. Each event is described by date and implement used. The implement is characterized by its effects on soil properties, including mixing of soil layers and crop residues and changes in soil bulk density. The modeled responses are illustrated with a hypothetical case comparing effects of four implements (mold board plow, tandem disk, tine harrow, and planking) and a field experiment where winter wheat (Triticum aestivum L.) was grown with different tillage and residue management practices. From a modeling viewpoint, a key issue was how to manage different spatial and time scales. The soil is simulated as varying only with depth but in reality, the thickness of the soil is affected by tillage. This poses challenges for ensuring that the masses for water, nutrients, residues and the soil per se are conserved as soil layers are mixed and the density of each layer is altered. The model runs on a daily time step, but events such as tillage, application of residue, and irrigation can all happen within a single day and the sequence/timing can influence simulations. The new routines for field operations improve representation of tillage and residue management in the CSM model, but they are best viewed as providing a framework for future work that explicitly considers effects of residue type, soil type and distribution, and soil moisture on tillage effects and that deal with effects of rainfall kinetic energy in more detail.