A spatially referenced model for identifying optimal strategies for managing water and fertiliser nitrogen under intensive cropping in the North China Plain

In this model, dynamic water flow was governed by the one-dimensional Richards' equation in the Kirchhoff form and solved numerically by using a finite difference scheme with the adoption of a Newton-Raphson approach. Solute transport (nitrate only) was governed by the standard convection-dispersion equation (CDE) and solved numerically by the same scheme used in the water flow. WNMM simulated the transformations of several N species in agricultural fields, including mineralisation of' fresh crop residue and soil organic N, formation of soil organic N, immobilisation in biomass, nitrification, ammonia volatilisation, and denitrification (including N20 emission). The model also simulated the agricultural management including crop rotation, tillage, irrigation, and fertilizer N applications.

Data required by the model were categorized as GIS layer information (soil type, land cover, and village administrative boundary); database-formatted source data (soil properties, land use types, and agricultural management survey based on a village unit); referenced data (climatic reference data and crop biological data); and control data (starting date, period of simulation, initial land surface and soil conditions, agricultural management scenarios). The first two data categories were converted to ARC GRID ASCII format from other formats and sources in the GIS environment. The soil hydraulic parameters were derived from the pedo-transfer functions locally developed in Fengqiu County, and the spatial distribution of time-series precipitation was interpolated using Inverse Weighted Distance method based on limited observation in this county.

Computer simulation models are widely used in studies to address the problem that agriculture has been increasingly identified as the largest contributor of non-point source (NPS) pollution of N and P to surface water and groundwater. In this dissertation, a GIS-based water and N management model (WNMM) was developed and demonstrated to simulate dynamic soil water movement and soil-crop N cycling under a given agricultural management, for the purpose of identifying the best agricultural practices under intensive cropping systems for Fengqiu County in the North China Plain.

This spatially referenced water and N management model was tested and calibrated using the time-scries datasets measured in a one-hectare experimental plot at Fengqiu Agricultural Experimental Station (the Chinese Academy or Sciences, P.R.China). The parameter sensitivity analysis of the model for lumped and spatial parameters was also carried out.

When applied for the intensive wheat-maize cropping system at the Fengqiu County scale in the North China Plain, fifteen agricultural scenarios differing in fertilizer N applications and irrigation operations, including the current practices, were simulated to look at the outputs in crop yield, irrigation water use efficiency, fertiliser N use efficiency, and N losses from nitrate leaching and emission of greenhouse gases. The results showed that the current agricultural practices in some areas of Fengqiu County significantly influenced groundwater quality by nitrate contamination and had relatively high rates of gaseous N losses, especially when excess N fertilisers were applied in some areas. The theoretical and realistic best management practices for improving crop production, reducing adverse environmental impacts, and optimising water and fertiliser N efficiencies were also recommended.

The WNMM simulated the key processes of water and N dynamics in the surface and subsurface of soils, including evapotranspiration, canopy interception, soil water movement, fluctuations of groundwater table, heat flow, solute transport, crop growth, N cycling in soil-crop system, and agricultural management practices (crop rotation, irrigation, fertilizer N application, harvest, and tillage). It ran at a daily time step at a county scale, and was fed with lumped parameters (climatic data and crop biological data) in text format, and spatial parameters (soil and agricultural management) in ARC GRID ASCII format.

The strategy or coupling GIS with an environmental model was to use a uni form data structure, ARC GRID ASCII format for its simplicity, easy handling in WNMM simulation, and GIS operation.