Submitted to: Biological Systems Simulation Group Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/15/2006
Publication Date: 4/13/2006
Citation: White, J.W., Hoogenboom, G., Hunt, L.A., Mcmaster, G.S. 2006. Structured Procedures for Assessing Model Responsees to Water and Nitrogen Deficits. Biological Systems Simulation Group Proceedings, Fort Collins, Colorado, April 11-13, 2006. Interpretive Summary:
Technical Abstract: Potential end-users of crop simulation models for decision support require an accurate impression of the utility of a given model. A comparison of simulation outputs with observed data is usually central to model evaluations, but this approach has limitations. For example, evaluation data sets usually cover only a limited spectrum of production conditions and measured system responses. Furthermore, in preparing such comparisons, model inputs sometimes include data that were used in calibrations, especially soil profile descriptions and weather data, thus obscuring whether the simulations truly provide an independent assessment of expected model performance. Sensitivity analyses provide a complementary approach for model assessment that can be adapted to diverse needs, yet the approach often is under-used in the crop simulation community. A structured procedure for examining crop model responses to temperature was recently proposed (White et al., 2005). Key features of the procedure include the use of constant weather conditions and an analysis of responses that is based on simple parameters of growth and development. This basic approach appears applicable to evaluating model responses to water and nitrogen deficits, with the qualification that initial soil and other conditions and temporal variation in deficits may have potentially large effects on plant responses. This paper describes initial work to develop structured procedures for assessing modeled responses to water and nitrogen deficits. The CSM-CROPSIM-CERES-Wheat model as released in DSSAT4.0 was used to simulate growth of a spring wheat (cv. Yecora 70) grown under a 12 h daylength, with a constant temperature regime of 21°C daily maximum and 11°C minimum, and solar radiation of 20 MJ m-2 d-1. The soil was a sandy loam, assumed to allow root growth to a depth of 1.5 m. An initial irrigation of 50 mm was given five days before sowing. For initial tests, the simulated crop was irrigated every eight days after seedling emergence. Full irrigation was 36 mm, and deficits were produced by reducing either the amount or timing of the post-emergence irrigations. Nitrogen was applied as ammonium nitrate one day before sowing. The maximum rate used was 95 kg N ha-1, which appeared to exceed the crop nitrogen demand for maximum grain yields of 4300 kg ha-1. In preliminary analyses, the model CSM-CROPSIM-CERES-Wheat showed no effect of water or nitrogen deficits on time to anthesis or maturity. Following the approach of French and Schultz (1984), effects of irrigation and fertilizer level were viewed in relation to cumulative evapotranspiration (ET, Fig. 1). The results suggested that crop weight at maturity increased linearly with ET when the nitrogen requirement was met, while nitrogen deficits caused a disproportionate reduction in growth. With low ET, the crop nitrogen requirement was also low, so there was no response to additional nitrogen. Further analyses will be conducted to determine the effects of water and nitrogen deficits on grain yield, unit grain weight, partitioning, and environmental impacts with a view to establishing a compact set of comparisons that efficiently summarize the responses of a given model to water and nitrogen deficits.