Comparison of scales of climate and soil data for aggregating simulated yields of winter wheat in Denmark

Crop growth models are essentially site-based, and use of such models for assessing regional productivity of crops requires methods for aggregating over space. Different method for aggregating simulated county and national crop yields for winter wheat (Triticum aestivum L.) in Denmark were tested using a crop simulation model (CLIMCROP), which was run with and without irrigation for a range of soil types and climatic conditions. The aggregated county or national yield was calculated by summing simulated yield of each category multiplied by the area, they represent. Ten different combinations of scales of climate and soil data were used. The wheat area was distributed between the different soil types using either a uniform distribution or a distribution that gave preference to soils with high water-holding capacity. The simulated results were compared with Danish county and national yield statistics for winter wheat from the period 1971-1997. There was, in general, a poor relationship between simulated and observed yields when the observed yields had been detrended to remove the technology effect. A larger fraction of the inter-annual variability was captured by the model on the loamy soils compared with the sandy soils. The model was able to capture most of the spatial variation in observed yields, except at the coarsest resolutions of the soil data. The finest resolution of soil and climate data gave a better fit of simulated to observed spatial autocorrelation in yield. The results indicate that upscaling of simulated productivity of crops for Danish conditions requires a spatial resolution of soil data of 10 x 10 km(2) or finer. A single climate station may be sufficient if only national yields are estimated, but more stations are required, if regional yields are to be estimated. Consideration should also be given to the distribution of crop area on the different soil types, (C) 2000 Elsevier Science B.V. All rights reserved.